Hypermetabolic response after hypothermic cardiopulmonary bypass

Respiratory Changes in Ventilated and Not-Ventilated Neonates During and After Whole-Body Hypothermia: A Multicenter Retrospective Study

The aim of this study was to describe whether whole-body hypothermia induced different respiratory changes in both invasively and noninvasively ventilated newborns and spontaneously breathing asphyxiated newborns during the course and after therapeutic hypothermia (TH). Data of 44 asphyxiated newborns undergoing TH at five different neonatal intensive care units in southern Italy were collected retrospectively between January 2018 and January 2021. For each type of ventilation, patient data on pH, partial pressure of Carbon Dioxide (pCO2), base excess, lactate, and heart rate were recorded before cooling was started and at 24, 48, 72, and 96 hours from its initiation. Patients were later subgrouped into spontaneously breathing, noninvasively ventilated, and mechanically ventilated groups. The average trend of each parameter was reported, and a nonparametric statistical analysis of differences among groups before initiation and at 96 hours was performed using the Kruskal-Wallis test. Our results confirmed previous findings (supported by a small amount of literature) that no increase in requests for respiratory support is recorded in asphyxiated newborns undergoing TH during and after the rewarming phase. Furthermore, no statistically significant differences in the analyzed parameters were found among spontaneously breathing, noninvasively ventilated, and mechanically ventilated newborns, suggesting that changes in parameters might be attributable to TH itself rather than to an improvement in the respiratory condition over time; otherwise, a difference between spontaneously breathing patients, by definition "stable" from a respiratory point of view, and those requiring any type of respiratory support would have been expected.

Physiological relationship between cardiorespiratory fitness and fitness for surgery: a narrative review

Epidemiological evidence has highlighted a strong relationship between cardiorespiratory fitness and surgical outcomes; specifically, fitter patients possess heightened resilience to withstand the surgical stress response. This narrative review draws on exercise and surgical physiology research to discuss and hypothesise the potential mechanisms by which higher fitness affords perioperative benefit. A higher fitness, as indicated by higher peak rate of oxygen consumption and ability to sustain metabolic homeostasis (i.e. higher anaerobic threshold) is beneficial postoperatively when metabolic demands are increased. However, the associated adaptations with higher fitness, and the related participation in regular exercise or physical activity, might also underpin the observed perioperative benefit through a process of hormesis, a protective adaptive response to the moderate and intermittent stress of exercise. Potential mediators discussed include greater antioxidant capacity, metabolic flexibility, glycaemic control, lean body mass, and improved mood.

Resting Energy Expenditure Early after Cardiac Surgery and Validity of Predictive Equations: A Prospective Observational Study

OBJECTIVE

Several predictive equations have been used to estimate patients' energy expenditure. The study aimed to describe the characteristics of resting energy expenditure (REE) in patients undergoing mechanical ventilation during early postoperative stage after cardiac surgery and evaluate the validity of 9 REE predictive equations.

METHODS

This was a prospective observational study. Patients aged 18-80 years old, undergone open-heart surgery, were enrolled between January 2017 and 2018. The measured REE (mREE) was evaluated via indirect calorimetry (IC). The predictive resting energy expenditure (pREE) was suggested by 9 predictive equations, including Harris-Benedict (HB), HB coefficient method, Ireton-Jones, Owen, Mifflin, Liu, 25 × body weight (BW), 30 × BW, and 35 × BW. The association between mREE and pREE was assessed by Pearson's correlation, paired t test, Bland-Altman method, and the limits of agreement (LOA).

RESULTS

mREE was related to gender, BMI, age, and body temperature. mREE was significantly correlated with pREE, as calculated by 9 equations (all p < 0.05). There was no significant difference between pREE and mREE, as calculated by 30 × BW kcal/kg/day (t = 0.782, p = 0.435), while significant differences were noted between mREE and pREE calculated by other equations (all p < 0.05). Taking the 30 × BW equation as a suitable candidate, most of the data points were within LOA, and the percentage was 95.6% (129/135). Considering the rationality of clinical use, accurate predictions (%) were calculated, and only 40.74% was acceptable.

CONCLUSIONS

The 30 × BW equation is relatively acceptable for estimating REE in 9 predictive equations in the early stage after heart surgery. However, the IC method should be the first choice if it is feasible.

Respiratory and Hemodynamic Changes in Neonates with Hypoxic-Ischemic Encephalopathy during and after Whole-Body Hypothermia

OBJECTIVE

This study aimed to determine the degree to which whole-body hypothermia (WBH) impacts hemodynamic and respiratory status during hypothermia and the subsequent rewarming period in neonates with hypoxic-ischemic encephalopathy (HIE).

STUDY DESIGN

This is a retrospective study reviewing the medical records of infants treated with WBH. Data including oxygenation index (OI), ventilator efficiency index (VEI), fraction of inspired oxygen (FiO₂), blood lactate level, heart rate (HR), and mean blood pressure (MBP) were collected from defined time points from the beginning, middle, and end of WBH and then every 2 hours from the beginning of rewarming for 14 hours thereafter. The analysis included 65 infants. Data were analyzed using a piecewise linear regression with a mixed-effect model.

RESULTS

HR decreased during WBH and significantly increased during rewarming. Lactate level, OI, VEI, FiO₂, and MBP all decreased during WBH but showed no significant change during and after rewarming.

CONCLUSION

There was a decrease in metabolic demand as measured by oxygen requirement, OI, HR, and MBP during WBH, but only HR increased during rewarming, with no significant change in the other parameters. Some of this effect may be explained by improvement in the respiratory condition over time.

Evolution of the concept of oxygen transport in the critically ill, with a focus on children after cardiopulmonary bypass

The concept of oxygen transport, defined as the relation between oxygen consumption (VO2) and delivery (DO2), is of fundamental importance in critically ill patients. The past 200 years have witnessed a stepwise progressive improvement in the understanding of pathophysiological disturbances in the balance of DO2 and VO2 in critically ill patients including those after cardiopulmonary bypass surgery. Intermittent spectacular technological achievements have accelerated the rate of progress. Therapeutic advances have been particularly impressive during the recent decades. Examination of the relation between DO2 and VO2 provides a useful framework around which the care of the critically ill may be developed. Until now, only a few groups have used this framework to examine children after cardiopulmonary bypass. The key topics that will be covered in this review article are the evolution of the concept from its early development to its present, increasingly sophisticated, role in the management of critically ill patients, with a focus on children after cardiopulmonary bypass surgery.

Challenge of predicting resting energy expenditure in children undergoing surgery for congenital heart disease

OBJECTIVES

To determine pre- and postoperative predictors of energy expenditure in children with congenital heart disease requiring open heart surgery; and to compare measured resting energy expenditure with current predictive equations.

DESIGN

Prospective resting energy expenditure data were collected, using indirect calorimetry, for ventilated children admitted consecutively to the pediatric intensive care unit after surgery for congenital heart disease. A 30-min steady-state measurement was performed in suitable patients. Resting energy expenditure was compared to pre- and postoperative clinical variables, and to predicted energy expenditure, using currently used predictive equations.

SETTING

Pediatric intensive care unit at the Royal Brompton Hospital, London.

PATIENTS

Children ventilated in the pediatric intensive care unit post surgery for congenital heart disease.

INTERVENTIONS

Measurement of energy expenditure by indirect calorimetry.

MEASUREMENTS AND MAIN RESULTS

Twenty-one mechanically ventilated children (n = 17 boys, 4 girls) were enrolled in the study. Mean +/- sd measured resting energy expenditure was 67.8 +/- 15.4 kcal/kg/day. Most children had inadequate delivery of nutrients compared with actual requirements. Cardiopulmonary bypass had a significant influence on energy expenditure after surgery; in patients who underwent cardiopulmonary bypass during surgery, mean resting energy expenditure was 73.6 +/- 14.45 kcal/kg/day vs. 58.3 +/- 10.29 kcal/kg/day in patients undergoing nonbypass surgery. Children who were malnourished preoperatively had greater resting energy expenditure postoperatively. There was also a significant difference between measured energy expenditure and the Schofield (p = .006), World Health Organization (p = .002), and pediatric intensive care unit-specific formula (p < .0001). However, energy expenditure or a relative energy deficit in the early postoperative period was not associated with severity or duration of organ dysfunction.

CONCLUSIONS

Poor nutritional status preoperatively and cardiopulmonary bypass were associated with a greater energy expenditure post cardiac surgery. None of the current predictive equations predicted energy requirements within acceptable clinical accuracy.

Postoperative Care of the Cardiac Surgical Patient

The subspecialty of interventional cardiology began in 1977. Since then, the discipline of interventional cardiology has matured rapidly, particularly with regards to ischemic heart disease. As a result, more patients are undergoing percutaneous catheter interventional therapy for ischemic heart disease and fewer patients are undergoing surgical myocardial revascularization. Those patients referred for surgical revascularization are generally older and have more complex problems. Furthermore, as the population ages more patients are referred to surgery for valvular heart disease. The result of these changes is a population of surgical patients older and sicker than previously treated.

Comparison of the Profiles of Postoperative Systemic Hemodynamics and Oxygen Transport in Neonates After the Hybrid or the Norwood Procedure

Background— After the Norwood procedure, early postoperative neonatal physiology is characterized by hemodynamic instability and imbalance of oxygen transport that is commonly attributed to surgical myocardial injury and a systemic inflammatory response to cardiopulmonary bypass (CPB). Because the Hybrid procedure (arterial duct stenting and bilateral pulmonary artery banding) avoids CPB, cardioplegic arrest, and circulatory arrest, we hypothesized that the Hybrid procedure is associated with superior postoperative hemodynamics and oxygen transport.

Methods and Results— Oxygen consumption (VO 2 ) was continuously measured using respiratory mass spectrometry for 72 hours after Hybrid (n=6) and Norwood (n=13) procedures. Arterial, superior vena cava, and pulmonary venous blood gases and pressures were measured at 2- to 4-hour intervals to calculate systemic and pulmonary blood flows (Qs, Qp), and systemic vascular resistance (SVR), total pulmonary vascular resistance including pulmonary arterial band or B-T shunt (tPVR), cardiac output (CO), oxygen delivery (DO 2 ), and oxygen extraction ratio (ERO 2 ). Rate-pressure product was calculated as heart rate×systolic arterial pressure. When compared with the Norwood procedure, the early postoperative Hybrid patients had lower CO, higher SVR, and higher Qp:Qs ratios. In addition, the DO 2 and VO 2 were both lower in the Hybrids with higher ERO 2 and lactate levels. This early postoperative pattern reversed after 48 hours.

Conclusions— Although Hybrid procedure avoids CPB and cardioplegic arrest, the early hemodynamic profile is not superior to the Norwood in terms of cardiac output and control of pulmonary blood flow. These data strongly suggest that a “hands off” approach to postoperative care in Hybrid patients may not be appropriate in patients with preoperative diminished myocardial function; and in such patients a Norwood-derived management strategy (afterload reduction and inotropic support) should be considered.

Profiles of hemodynamics and oxygen transport derived by using continuous measured oxygen consumption after the Norwood procedure

OBJECTIVES

The lack of accurate measurement of hemodynamics and oxygen transport has limited our understanding of Norwood physiology and postoperative management. We used measured oxygen consumption to characterize hemodynamics and oxygen transport after the classic Norwood procedure.

METHODS

Fourteen neonates had continuous respiratory mass spectrometry to measure oxygen consumption (VO2). Arterial, superior vena caval, and pulmonary venous saturations were measured at 2- to 4-hour intervals for 72 hours postoperatively. Systemic (Qs) and pulmonary (Qp) blood flows, systemic vascular resistance (SVR) and pulmonary vascular resistance inclusive of the Blalock-Taussig shunt (BT-PVR), systemic oxygen delivery (DO2), and the oxygen extraction ratio (ERO2) were calculated.

RESULTS

Qs and DO2 were low during the first 12 hours (1.8 +/- 0.6 L x min(-1) x m(-2) and 281 +/- 86 mL x min(-1) x m(-2) at the 12th hour, respectively) and increased over the study period (P < .05 for both). VO2 decreased markedly during the first 24 hours (101 +/- 26 to 86 +/- 16 mL x min(-1) x m(-2), P < .0001). Consequently, ERO2 decreased significantly over the study, most rapidly during the first 24 hours (0.44 +/- 0.11 to 0.28 +/- 0.09, P < .0001). There was a close correlation of DO2 to SVR and to Qs (P < .0001 for both). There was no correlation of DO2 to BT-PVR (P = .14) or to Qp (P = .67). DO2 was closely correlated with hemoglobin value (P < .0001), weakly correlated with PaO2 (P = .0002), and not correlated with arterial oxygen saturation (P = .32).

CONCLUSIONS

There is wide variability of hemodynamics and oxygen transport after the Norwood procedure. The decrease in VO2 during the first 24 hours is the main contributor to improving the balance of oxygen transport. DO2 is most closely correlated to SVR and hemoglobin and weakly correlated to PaO2. It is not correlated to Qp. Postoperative management strategies to decrease VO2 and maintain a high hemoglobin level and a low SVR appear to be rational.

An analysis of oxygen consumption and oxygen delivery in euthermic infants after cardiopulmonary bypass with modified ultrafiltration

BACKGROUND

The balance between systemic oxygen consumption (VO2) and delivery (DO2) is impaired after cardiopulmonary bypass (CPB) and is related to systemic inflammatory response syndrome. We sought to assess VO2 and DO2 and their relationship with proinflammatory cytokines after CPB with the use of modified ultrafiltration (MUF) in infants.

METHODS

Sixteen infants, aged 1-11.5 months (median, 6.3 months), undergoing hypothermic CPB with MUF were studied during the first 12 hours after arrival in the intensive care unit (ICU). The central temperature was maintained at 36.8-37.1 degrees C using external cooling or warming. VO2 was continuously measured using respiratory mass spectrometry. Arterial blood samples for the tumor necrosis factor (TNF), interleukin-6 (IL-6), and interleukin-8 (IL-8) were taken and DO2 was calculated using the Fick principle on arrival at the ICU, and 2, 4, 8, and 12 hours postoperatively. Cytokines were additionally measured after induction of anesthesia and at the end of MUF.

RESULTS

VO2 significantly decreased by 18.8% during the study period. DO2 was depressed throughout this period and reached a nadir at 8 hours (357.1 +/- 136.2 ml x min(-1) x m(-2)). The decrease in cytokines was accompanied with the decrease in VO2 despite varied relationships between the levels of each of the cytokines and VO2 measurements.

CONCLUSIONS

Our data indicate an unusual continuous decrease in VO2 during the first 12 hours after CPB in infants. Control of body temperature to maintain euthermia in addition to the use of MUF may be beneficial to the balance between VO2 and DO2 in the early postoperative period.

Effect of carbon dioxide on systemic oxygenation, oxygen consumption, and blood lactate levels after bidirectional superior cavopulmonary anastomosis

OBJECTIVE

We sought to assess the effects of four different CO2 tensions on systemic oxygenation, oxygen consumption, and arterial blood lactate levels early after bidirectional superior cavopulmonary anastomosis.

DESIGN

Prospective study.

SETTING

Quaternary pediatric cardiac critical care unit.

PATIENTS

Nine children aged 2-23 months (median, 7 months).

INTERVENTIONS

All patients were sedated, muscle relaxed, and mechanically ventilated. Baseline Paco2 was adjusted to 35 mm Hg by changing tidal volume. CO2 was added via the inlet port of the ventilator to maintain the Paco2 at 45 and 55 mm Hg. Measurements were repeated after discontinuing additional CO2 gas at a Paco2 of 40 mm Hg. Arterial blood gases and lactate were measured at each level of Paco2. We measured oxygen consumption continuously by respiratory mass spectrometry.

MEASUREMENTS AND MAIN RESULTS

Mean (95% confidence interval) Paco2 increased from 35 (34-36) to 45 (44-46) to 55 (54-56) mm Hg (4.7 [4.5-4.9] to 6 [5.7-6.3] to 7.3 [7.2-7.4] kPa), arterial pH decreased from 7.43 (7.39-7.47) to 7.35 (7.31-7.39) to 7.28 (7.24-7.32). Pao2 increased from 36 (32-40) to 44 (40-48) to 50 (45-55) mm Hg (4.8 [4.3-5.3] to 5.9 [5.4-6.4] to 6.7 [6.2-7.2] kPa), and oxygen saturation increased from 72% (67-79%) to 77% (73-81%) to 80% (76-84%). Oxygen consumption decreased significantly, with each increase in Paco2, from 146 (125-167) to 132 (112-152) to 126 (107-145) mL.min.m (p = .0001), and lactate decreased from 1.5 (1-2.0) to 1.2 (0.8-1.6) to 0.8 (0.5-1.1) mmol/L (p < .01). These changes returned toward baseline at a Paco2 of 40 mm Hg.

CONCLUSIONS

Moderate hypercapnia with respiratory acidosis improved arterial oxygenation and reduced oxygen consumption and arterial lactate levels, thus improving overall oxygen transport in children after bidirectional superior cavopulmonary anastomosis.

Insulin-like growth factor 1 improves the relationship between systemic oxygen consumption and delivery in piglets after cardiopulmonary bypass

OBJECTIVE

We sought to assess the effects of insulin-like growth factor 1 on the balance between systemic oxygen consumption and oxygen delivery after cardiopulmonary bypass in piglets.

METHODS

Twelve piglets weighing 4.5 to 8.3 kg undergoing hypothermic (28 degrees C) cardiopulmonary bypass for 70 to 120 minutes with 40 minutes of aortic crossclamping were studied before and during the first 6 hours after cardiopulmonary bypass. Oxygen consumption was continuously measured by an indirect calorimeter, Deltatrac II MBM-200 Metabolic Monitor (Datex Division Instrumentarium, Helsinki, Finland). Oxygen delivery and cardiac output were calculated from oxygen consumption and the arterial and mixed venous oxygen contents sampled before and every 30 minutes after cardiopulmonary bypass. Oxygen extraction ratio was derived by the ratio of oxygen consumption to oxygen delivery. Arterial blood lactate was measured before and every 30 minutes after cardiopulmonary bypass. Six animals were randomly assigned to receive an intravenous infusion of insulinlike growth factor 1 at 1.2 mg/h from 1 to 6 hours after cardiopulmonary bypass; the remaining 6 served as a control group.

RESULTS

Relative to the control group, intravenous infusion of insulin-like growth factor 1 significantly reduced oxygen consumption (P =.02) and increased cardiac output (P =.016) and oxygen delivery (P =.049) during the first 6 hours after surgery with hypothermic cardiopulmonary bypass. As a result, oxygen extraction was significantly decreased (P =.012).

CONCLUSIONS

Intravenous infusion of insulin-like growth factor 1 improved oxygen transport by reducing oxygen consumption as well as increasing cardiac output and oxygen delivery during the first 6 hours after cardiopulmonary bypass in piglets. This may have important clinical implications for the care of critically ill children after surgery with cardiopulmonary bypass.

Oxygen metabolism during and after cardiac surgery: role of CPB

BACKGROUND

Cardiopulmonary bypass (CPB) has been reported to increase oxygen metabolism and to influence the relation between oxygen consumption (VO(2)) and delivery (DO(2)) in the early hours after cardiac surgery. To investigate the role of CPB, we studied oxygen metabolism in coronary artery bypass procedures performed on-pump (CABG) and off-pump (OPCAB).

METHODS

Twenty-five patients were randomized to undergo CABG (n = 14) or OPCAB (n = 11). All patients received the same anesthetic management. Oxygen metabolism variables were assessed before induction of anesthesia and up to 18-hours after surgery.

RESULTS

At baseline, before induction of anesthesia, there were no differences between CABG and OPCAB in oxygen consumption (VO(2)), delivery (DO(2)), or extraction (ExO(2)). After surgery VO(2) and ExO(2) increased in both groups, while DO(2) decreased. No significant differences between CABG and OPCAB were detected in postoperative VO(2), DO(2), and ExO(2) levels. The relation between VO(2) and DO(2) was very similar in CABG and OPCAB patients throughout the study, and no significant differences were detected in slopes and intercepts of the regression lines between CABG and OPCAB at all time points. There was, however, a significant effect of time on the relation between VO(2) and DO(2): this relation was stronger in the postoperative period, and the slope of this relation increased over time as well.

CONCLUSIONS

A hypermetabolic state and progressive and significant increases in the strength of the relationship between VO(2) and DO(2) and in the slope of this relationship occur after both CABG and OPCAB. Cardiopulmonary bypass is not responsible for these changes in oxygen metabolism.

Measured versus estimated oxygen consumption in ventilated patients with congenital heart disease: the validity of predictive equations

OBJECTIVE

To determine the validity of predictive equations in calculating oxygen consumption (Vo(2)) in ventilated patients with congenital heart disease.

DESIGN

Prospective study.

SETTING

Cardiac catheterization laboratories and intensive care units of two university teaching hospitals.

PATIENTS

A total of 126 patients with congenital heart disease were studied. Of these, 75 patients received anesthesia in the pediatric cardiac catheterization laboratory, and 51 were deeply sedated in the intensive care unit after open heart surgery.

MEASUREMENTS AND MAIN RESULTS

Vo(2) was measured directly in all patients using respiratory mass spectrometry. Estimated values for absolute Vo(2) (mL/min) and indexed Vo(2) (mL.min-1.m-2) were calculated from the four predictive equations published by LaFarge and Miettinen, Lundell et al., Wessel et al., and Lindahl. The agreement between measured and estimated Vo(2) was evaluated by calculating their bias and limits of agreement. A failure of agreement between measured and estimated Vo(2) was noted in both groups of patients, irrespective the equation used, and the agreement was poorer in patients in the intensive care unit. The equation by LaFarge and Miettinen produced the closest estimation in patients at cardiac catheterization with a bias of 4.5 mL/min for absolute Vo(2) and 6.9 mL.min-1.m-2 for indexed Vo(2). A systematic error of overestimating lower and underestimating higher indexed Vo(2) mL.min-1.m-2 was introduced in both groups.

CONCLUSION

Predictive equations do not accurately estimate Vo(2) in ventilated patients with congenital heart disease.

The emerging role of induced hypothermia in the management of acute stroke

Current treatment of acute stroke remains unsatisfactory. This review presents experimental and clinical data which suggest that mild induced hypothermia could be a potent and practicable neuroprotective treatment of acute ischaemic stroke and intracerebral haemorrhage. Hypothermia, if proven to be safe, effective and widely practicable in patients with acute stroke, could have an enormous positive impact on reducing the burden of stroke worldwide. Critical issues that will need to be considered in a well designed randomised controlled trial of induced hypothermia in acute stroke patients are discussed.

The oxygen debt during routine cardiac surgery: illusion or reality?

BACKGROUND

Patients undergoing cardiac surgery with the use of cardiopulmonary bypass (CPB) are often thought to have tissue hypoxia and intraoperative oxygen debt accumulation despite the lack of sufficient data to support this assumption.

METHODS AND RESULTS

Oxygen uptake and related parameters, including the plasma lactate and pyruvate concentrations, were studied during the perioperative period in a group of 15 consecutive patients who underwent coronary artery bypass graft surgery. The actual oxygen uptake (VO2) and delivery (DO2) were compared with the individual expected (computed) oxygen transport values. The mean values of DO2 and VO2 were in the range of the expected values. Our results demonstrate a leading role for body temperature in perioperative changes of oxygen consumption rate (r2=0.65, p<0.001). Plasma lactate and pyruvate did not exceed the physiological range in any patient. However, with initiation of CPB, the lactate to pyruvate (LA/PVA) ratio increased (from 9.87 +/- 2.43 at T1 to 12.08 +/- 1.51 at T2, p<0.05). The mean value of the LA/ PVA ratio was elevated during surgery. Later, upon lowering of the plasma lactate concentration in the postoperative period, the LA/PVA ratio decreased to normal values. Without any other evidence of hypoxia, this increase in the LA/PVA ratio could be explained by washout of lactate from previously hypoperfused tissues and intraoperative decrease of lactate clearance.

CONCLUSION

Systemic oxygenation was not impaired during CPB, or during 18 h after surgery in the studied group of patients.

Outcome with high blood lactate levels during cardiopulmonary bypass in adult cardiac operation

BACKGROUND

High blood lactate levels during cardiopulmonary bypass (CPB) are associated with tissue hypoperfusion and may contribute to postoperative complications or death. The objective of this study was to determine an association between blood lactate levels during CPB and perioperative morbidity and mortality.

METHODS

We reviewed 1,376 patients who underwent cardiac operation with CPB. Patients with abnormal preoperative blood lactate levels were excluded (n = 101). Blood lactate concentration during CPB, clinical data, and perioperative events were recorded.

RESULTS

Peak blood lactate levels of 4.0 mmol/L or higher during CPB were present in 227 patients (18.0%). Postoperative mortality was higher in this group than in the patients who had peak blood lactate levels of less than 4.0 mmol/L during CPB (11.0% versus 1.4%; p < 0.001, relative risk [RR] = 9.0). Postoperative hemodynamic instability occurred in 29.5% of patients with elevated levels of lactate during CPB compared with 10.9% of patients with lower lactate levels (p < 0.001, RR = 3.4). Overall, major postoperative complications occurred in 43.2% and 21.8% of patients in each group, respectively (p < 0.001, RR = 2.7). Logistic regression analysis revealed that peak blood lactate levels of 4.0 mmol/L or higher during CPB were strongly associated with postoperative mortality (p = 0.0001) and morbidity (p = 0.013).

CONCLUSIONS

Blood lactate concentration of 4.0 mmol/L or higher during CPB identifies a subgroup of patients with increased risk of postoperative morbidity and mortality.

Impaired carbon dioxide transport during and after cardiopulmonary bypass

Changes in the CO2 carrying power of blood were evaluated during and after cardiopulmonary bypass (CPB) by calculating the equation of the whole blood CO2 dissociation curve and the ratio between the arterial-venous differences of CO2 content and CO2 tension (Ra-v). Sixteen patients undergoing normothermic CPB for coronary revascularization were studied; arterial and mixed venous blood gas analyses were performed prior to CPB, at the end of first cardioplegia infusion, 25 and 45 min after CPB commencement and 10 min after the termination of CPB. After CPB commencement, the whole blood CO2 dissociation curve became flatter and did not further change during or after CPB. Ra-v decreased from 1.06 +/- 0.16 to 0.72 +/- 0.12 ml/mmHg after the start of CPB and did not change significantly during CPB; it was still 0.73 +/- 0.13 ml/mmHg after CPB. The data indicate that during CPB the amount of CO2 removed from tissues by 1 litre of blood decreases by about 30% and that impairment in CO2 transport persists after the restoration of physiological circulation. Impairment in CO2 transport is mainly caused by haemodilution, but it could be worsened by acidosis.

The effect of sedation on weaning following coronary artery bypass grafting: propofol versus oxycodone-thiopental

BACKGROUND

Propofol has been advocated for sedation in intensive care because of superior recovery characteristics. We hypothesised that the use of two totally different sedation methods after coronary artery bypass grafting should result in differences not only in extubation time, but also in breathing pattern and gas exchange during weaning and after extubation.

METHODS

Thirty patients participated in this randomised and controlled study. We used propofol infusion and oxycodone-thiopental bolus dosage, titrated to sedation level 4 or 5 according to Ramsey. Weaning was performed using protocol-based pressure support trials.

RESULTS

Total (SD) fentanyl dose during operation was 33 (6) microg x kg(-1) for propofol and 34 (6) microg x kg(-1) for oxycodone-thiopental (ns). The target sedation was achieved equally with both methods. The time from admission to intensive care unit to extubation was 494 (100) min for propofol and 521 (98) min for oxycodone-thiopental (ns). Weaning times were 63 (24) min and 112 (63) min in the propofol and oxycodone-thiopental groups, respectively (P<0.05). Breathing frequency increased and tidal volume decreased from weaning to 2 h postextubation.

CONCLUSION

Propofol infusion and oxycodone-thiopental bolus dosages, titrated to the same sedation end point, resulted in similar time from admission to extubation, although the weaning period was shorter in the propofol group. In terms of breathing pattern, gas exchange, blood gases and haemodynamics, the methods were similar. Propofol, despite its attractive pharmacological profile, may offer no clinical benefit in short-term sedation after a moderate dose fentanyl anaesthesia in cardiac surgery.

Oxygen consumption after cardiopulmonary bypass surgery in children: determinants and implications

OBJECTIVE

We sought to assess oxygen consumption and its determinants in children shortly after undergoing cardiopulmonary bypass operations.

METHODS

Twenty children, aged 2 months to 15 years (median, 3.75 years), undergoing hypothermic cardiopulmonary bypass operations were studied during the first 4 hours after arrival in the intensive care unit. Central and peripheral temperatures were monitored. Oxygen consumption was continuously measured by using respiratory mass spectrometry. Oxygen delivery was calculated from oxygen consumption and arterial and mixed venous oxygen contents, which were sampled every 30 minutes. Oxygen extraction was derived by the ratio of oxygen consumption and oxygen delivery. Arterial blood lactate levels were measured every 30 minutes.

RESULTS

There was a correlation between oxygen consumption and age in patients older than 3 months (r = -0.76). Mean oxygen consumption increased by 14.7% during the study. The increase in oxygen consumption was correlated with the increase in central temperature (r = 0.73). Nine patients had an arterial lactate level above 2 mmol/L on arrival. There were no significant differences in oxygen consumption, oxygen delivery, and oxygen extraction between the group with lactate levels between 2 and 3 mmol/L and the groups with normal lactate levels both on arrival and at 2 hours. One patient with a peak lactate level of 6.8 mmol/L had initially low oxygen delivery (241.3 mL. min(-1). m(-2)).

CONCLUSIONS

During the early hours after a pediatric cardiac operation, the increase in oxygen consumption is mainly attributed to the increase in central temperature. Oxygen consumption is negatively related to age. Mild lactatemia is common and does not appear to reflect oxygen delivery or oxygen consumption or a more complicated recovery.

Cardiopulmonary bypass and oxygen consumption: oxygen delivery and hemodynamics

BACKGROUND

This study was undertaken to investigate the relations between whole body oxygen consumption (VO2), oxygen delivery (DO2), and hemodynamic variables during cardiopulmonary bypass.

METHODS

One hundred one patients were studied during cooling, hypothermia, and rewarming. Oxygen consumption, DO2, hemodynamics, and DO2crit were measured at these times.

RESULTS

There was a direct linear relation between DO2 and VO2 during all three times. No relation between VO2 and hemodynamics was detected during cooling; during hypothermia, an inverse linear relation with peripheral arterial resistance was found. Finally, during rewarming, there was a direct relation with pump flow rate, and an inverse relation with arterial pressure and arterial resistance. The same relations among the variables were found at delivery levels above or below DO2crit.

CONCLUSIONS

During cardiopulmonary bypass there is a direct linear relation between DO2 and VO2; the relations with hemodynamic variables depend on the phases of cardiopulmonary bypass. This suggests that increasing delivery levels may recruit and perfuse more vascular beds, and higher delivery levels are advisable during perfusion. During rewarming and hypothermia, lower arterial resistances are also desirable to optimize VO2.

Effect of low-dose propofol infusion on total-body oxygen consumption after coronary artery surgery

OBJECTIVE

To investigate the effect of low-dose propofol infusion on total-body oxygen consumption (VO2) after coronary artery bypass grafting (CABG) surgery.

DESIGN

A prospective, randomized, placebo-controlled, double-blind study.

SETTING

Cardiovascular intensive care unit in a university hospital.

PARTICIPANTS

Thirty patients after elective, uncomplicated CABG surgery.

INTERVENTION

Patients were administered a continuous infusion of propofol with a fixed rate of 1 mg/kg/h (n = 15) or placebo (n = 15) during the spontaneous rewarming period of approximately 5 hours after surgery. A light level of sedation (Ramsay sedation score > or =2) was maintained by administering small doses of diazepam, 0.1 mg/kg, as required. Morphine, 0.05 mg/kg, was administered for analgesia as required.

MEASUREMENTS AND MAIN RESULTS

Total-body VO2 was measured by indirect calorimetry. In addition, shivering (on a five-grade scale), hemodynamics, and plasma catecholamine and serum cortisol concentrations were measured. Diazepam, 5.6+/-7.4 mg (mean +/- standard deviation), was administered to the patients receiving propofol, and 16.1+/-12.2 mg was administered to the patients receiving placebo (p < 0.05). There was no difference in the dose of morphine between the groups (3.2+/-3.9 v 4.2+/-5.5 mg in the propofol and placebo groups, respectively). At any time during the study, VO2 was not different between the groups. VO2 increased from 130+/-29 to 172+/-29 mL/min/m2 in the propofol group and from 118+/-24 to 167+/-27 mL/min/m2 in the placebo group. Mean arterial pressure and heart rate were lower in the propofol group (p < 0.05). Stress hormone levels did not differ between the groups.

CONCLUSION

Low-dose propofol infusion and additional diazepam as required does not decrease total-body VO2 compared with a pure diazepam bolus-dose technique when administered for light sedation during the immediate recovery period after CABG surgery.

Postoperative pattern of various hormonal and metabolic variables. A pilot study in patients without complications following cardiac surgery

The use of single predictors for threatening postoperative complications are widely accepted. However, a typical pattern of multiple parameters could be more helpful than a single predictor. To study this hypothesis, various variables of normal postoperative changes in patients without postoperative complications were investigated. Secondly, this pattern needs to be compared in the future with those findings in patients with postoperative complications. Blood parameters of 13 patients undergoing cardiovascular surgery without postoperative complications for 24 hours were evaluated. Samples were obtained on the afternoon before the operation and 1, 3, 6, 12 and 24 hours after the end of surgery. At one hour postoperation increased levels of the following parameters were noted: growth hormone (p < 0.0001), glucose (p < 0.0001), insulin (p < 0.001), c-peptide (p < 0.001), lactate (p < 0.002), glutamate (p < 0.0001), aspartate (p < 0.001) and total amino acids (p < 0.05), although the concentration of some amino acids decreased. Three hours postoperatively free fatty acids (p < 0.05) were increased. Total-T3 concentrations were reduced postoperatively. Other parameters were not altered. Most of the parameters returned to normal values during the period of observation.

Moderate hypothermia in the treatment of patients with severe middle cerebral artery infarction

BACKGROUND AND PURPOSE

Animal research and clinical studies in head trauma patients suggest that moderate hypothermia may improve outcome by attenuating the deleterious metabolic processes in neuronal injury. Clinical studies on moderate hypothermia in the treatment of acute ischemic stroke patients are still lacking.

METHODS

Moderate hypothermia was induced in 25 patients with severe ischemic stroke in the middle cerebral artery (MCA) territory for therapy of postischemic brain edema. Hypothermia was induced within 14+/-7 hours after stroke onset and achieved by external cooling with cooling blankets, cold infusions, and cold washing. Patients were kept at 33 degreesC body-core temperature for 48 to 72 hours, and intracranial pressure (ICP), cerebral perfusion pressure, and brain temperature were monitored continuously. Outcome at 4 weeks and 3 months after the stroke was analyzed with the Scandinavian Stroke Scale (SSS) and Barthel index. The side effects of induced moderate hypothermia were analyzed.

RESULTS

Fourteen patients survived the hemispheric stroke (56%). Neurological outcome according to the SSS score was 29 (range, 25 to 37) 4 weeks after stroke and 38 (range 28 to 48) 3 months after stroke. During hypothermia, elevated ICP values could be significantly reduced. Herniation caused by a secondary rise in ICP after rewarming was the cause of death in all remaining patients. The most frequent complication of moderate hypothermia was pneumonia in 10 of the 25 patients (40%). Other severe side effects of hypothermia could not be detected.

CONCLUSIONS

Moderate hypothermia in the treatment of severe cerebral ischemia is not associated with severe side effects. Moderate hypothermia can help to control critically elevated ICP values in severe space-occupying edema after MCA stroke and may improve clinical outcome in these patients.

Impact of autotransfusion after coronary artery bypass grafting on oxygen transport

BACKGROUND

Autotransfusion of shed mediastinal blood after coronary artery bypass grafting (CABG) has been shown to reduce the requirement for allogeneic blood. We have previously demonstrated in non-randomized studies that the oxygen capacity of shed mediastinal blood is similar to the patient's circulating blood and better than stored allogeneic blood. Therefore, we wanted to examine the influence of autotransfusion of shed mediastinal blood on oxygen transport capacity in patients undergoing CABG.

METHODS

A prospective, randomized, controlled study involving 120 patients having elective, uncomplicated CABG was performed. The autotransfusion group received transfusion of shed mediastinal blood for 18 h. Both groups received allogeneic red cells if their hemoglobin concentration decreased below 5 mmol/L. Red blood cell 2,3-diphosphoglycerate (2,3-DPG) was measured preoperatively and at intervals up to the hospital discharged. Hemodynamic measurements as well as blood gas and hemoglobin measurements from samples of arterial and mixed venous blood were used for calculation of oxygen transport capacity.

RESULTS

During the autotransfusion period only 2 patients (4%) in the autotransfusion group required allogeneic blood compared to 11 patients (20%) in the control group. The 2,3-DPG levels in the autotransfusion group were unchanged before and after autotransfusion (4.4 vs. 4.3 mumol/ml erythrocyte). In the control group, 2,3-DPG levels decreased from 4.3 to 3.9 mumol/ml erythrocyte during the same period. There were no differences in the other measured parameters for oxygen transport capacity between the groups.

CONCLUSION

Autotransfusion of shed mediastinal blood conserves the 2,3-DPG level of the red blood cells, while transfusion of stored blood leads to a decrease in 2,3-DPG levels. Autotransfusion had no effect on hemodynamic parameters, oxygen delivery or oxygen extraction.

Relationship between intra- and postoperative oxygen transport and prolonged intensive care after cardiac surgery: a prospective study

BACKGROUND

Prolonged intensive care is a rare but serious complication of cardiac surgery. It is required in less than 10% of operated patients but they use more than 30% of all the intensive care resources needed for cardiac surgery. The aim of our study was to describe the clinical course of the patients who need prolonged intensive care following cardiac surgery and to assess whether the intra- and postoperative oxygen transport variables are different in these patients as compared to patients with an uncomplicated course.

METHODS

The study patients were divided into two groups according to the length of stay in the intensive care unit (ICU) after the operation: Group I, n = 241, ICU-stay < 5 days and Group II, n 20, ICU-stay > or = 5 days. Hemodynamic and oxygen transport data were prospectively obtained intra- and postoperatively and postoperative organ dysfunctions were recorded.

RESULTS

The patients in the prolonged intensive care group tended to be older, have lower ejection fraction and longer cardiopulmonary bypass time. Postoperatively, this group had significantly increased oxygen extraction rate (P = 0.035, repeated measures for ANOVA). In the logistic regression analysis, increased oxygen extraction (31% in Group I vs. 36% in Group II, P < 0.005) at 6 hours after arrival at the intensive care unit had the strongest independent association with the need for prolonged intensive care.

CONCLUSIONS

There was no significant relationship between the factors conventionally assumed to be risk factors for prolonged intensive care. Instead, an increase in whole body oxygen extraction, reflecting a mismatch between the whole body oxygen demand and supply, was associated with the need for prolonged intensive care. Oxygen extraction increased to compensate for the reduced oxygen delivery, which in turn was caused by a lower arterial oxygen content.

Early vs conventional extubation after cardiac surgery with cardiopulmonary bypass

OBJECTIVES

Sedation and ventilation overnight after cardiac surgery is common practice. However, early extubation may be feasible with no increase in postoperative complications. This study examines (1) if early extubation is possible in a significant number of patients, (2) if it reduces ICU stay, and (3) if this practice increases postoperative complications.

DESIGN

Prospective, controlled, randomized clinical trial.

PATIENTS AND METHODS

We randomized 404 consecutive patients to early extubation (7 to 11 h postoperatively) (group A, 201 patients) or conventional extubation (between 8 and 12 AM the following day) (group B, 203 patients). Variables included type and severity of the disease, surgical risk, type of operation, operative incidences, postoperative complications, duration of mechanical ventilation, intubation and ICU stay, bleeding, reoperation, vasoactive drugs, and mortality.

RESULTS

Groups were comparable. Extubation within the preestablished time was successful in 60.2% of patients in group A and 74.4% in group B. Median ICU stay was 27 h in group A and 44 h in group B (p=0.008). Discharge from ICU within the first 24 h postoperatively was 44.3% in group A and 30.5% in group B (p=0.006). There was no significant difference in complications between groups. Successfully extubated patients in group A had more reintubation and prolonged ventilation than in group B.

CONCLUSIONS

(1) Sixty percent of our patients were extubated within 11 h of operation. (2) As a result, the length of stay in ICU was reduced and the percentage of patients discharged within 24 h was increased. (3) There was no increase in clinically important postoperative complications.

Evaluation of oxygen availability with oxygen status algorithm in patients undergoing open heart surgery treated with epoetin beta

We evaluated in a double-blind randomized study the effect of epoetin beta (recombinant human erythropoietin) therapy on oxygen status in patients undergoing cardiac surgery who were contraindicated for autologous blood donation. All 76 patients enrolled in this study were randomized to the two treatment groups (5 x 500 U epoetin beta or placebo/kg body weight intravenously over a 14-day period before surgery) and received 300 mg Fe2+ per day orally before surgery. Before and after surgery the lactate level and the following parameters according to the oxygen status algorithm by Siggaard-Andersen were evaluated: arterial oxygen tension (PaO2), effective hemoglobin concentration (ceHb), arterial oxygen saturation (SaO2), oxygen half saturation tension (p50), red cell 2.3 diphosphoglycerate (2.3 DPG), arterial total oxygen concentration (ctO2), concentration of extractable oxygen (cx), and oxygen compensation factor (Qx). Therapy with epoetin beta led to increases in ceHb, PaO2, ctO2, and cx and to a decrease in Qx before surgery (p < 0.05 for PaO2, p < 0.0001 for the other parameters vs placebo). The cx in patients who received epoetin beta rose by approximately 20%, thus indicating a considerable improvement in O2 delivery. In patients receiving placebo the hemoximetric parameters remained outside the normal limits at all times after surgery, but in the epoetin beta group PaO2, ctO2, cx, and Qx returned almost to their baseline values by the second or fifth postoperative day, even though the frequency of transfusions was significantly higher in the placebo group. Whereas p50 and 2.3 DPG fell in the placebo group after surgery, these two parameters were significantly higher in the epoetin beta group and led to a further increase in cx (from 24% to 38%) versus the placebo group as a result of the right shift in the hemoglobin O2-binding curve. The postoperative incidence and severity of lactic acidosis were higher in the placebo group. Preoperative epoetin beta therapy is a safe way of providing increased extractable O2 (by 24% to 38%) and decreasing the risk of lactic acidosis after surgery. This therapy has a more favorable effect on the O2 binding curve than the transfusion of erythrocyte concentrate and enhances the effect of epoetin beta therapy on the postoperative oxygen status.

Association between oxygen delivery and consumption in patients undergoing cardiac surgery. Is there supply dependence?

We studied the relationship between oxygen delivery (DO2) and consumption (VO2) in twenty patients undergoing cardiac surgery, in order to determine if VO2 was dependent on DO2 (pathological oxygen supply dependence). We measured VO2 from expired gas analysis (VO2G) and compared this to that calculated using the reverse Fick method (VO2F). Both VO2G and VO2F increased after cardiopulmonary bypass (P < 0.001), without change in DO2 (i.e. oxygen extraction ration increased). There was a significant relationship between changes in DO2 and VO2F, both before bypass (r = 0.74, P < 0.001) and after bypass (r = 0.69, P < 0.001), while changes in DO2 and VO2G had no such relationship (pre-bypass: r = 0.38, P = 0.094; post-bypass: r = 0.10, P = 0.68). There was poor agreement between VO2F and VO2G perioperatively. We could not demonstrate supply dependence in elective cardiac surgical patients.

Analysis of P50 and oxygen transport in patients after cardiac surgery

OBJECTIVE

To determine whether standard P50 after cardiac surgery decreases and whether decreased P50 is related to the transfusion of red blood cells (RBCs), acid-base changes, body temperature, oxygen parameters and/or duration of cardiopulmonary bypass (CPB).

DESIGN

Pilot study in cardiac surgery patients.

SETTING

University hospital.

PATIENTS

12 Consecutive elective cardiac surgery patients.

INTERVENTIONS

Blood was taken before surgery, after CPB and in the intensive care unit until 18 h post-operatively. Cardiac output and oxygen consumption were measured. Buffy coat-poor RBCs were transfused, anticoagulated with citrate-phosphate-dextrose buffer and stored in saline-adenine-glucose-mannitol at 4 degrees C, when haemoglobin was < 5.6 mmol.l-1.

MEASUREMENTS AND RESULTS

Standard P50 was calculated from measured partial pressure of oxygen and of carbon dioxide, pH and oxygen saturation in mixed venous blood (SvO2) using the Severinghaus formula. Median length of RBC storage was 25 days. Standard P50 after surgery was significantly lower than baseline value (p = 0.0001). The number of RBC units transfused and duration of CPB were conjointly associated with P50 (R2 = 0.72). Patients who received more RBCs consumed more oxygen.

CONCLUSION

Cardiac surgery patients receiving more RBC units have lower standard P50 and consume more oxygen. P50 decreased more when the CPB took longer. Because a decrease in P50 implies a low ratio of mixed venous oxygen tension (PvO2) to SvO2, a shift in P50 should be taken into account when using SvO2 as a measure of global oxygen availability. When a direct measurement of SvO2 is not available, PvO2 should be used instead of calculated SvO2.

Energy metabolism of thoracic surgical patients in the early postoperative period. Effect of posture

STUDY OBJECTIVE

To determine the effect of elective thoracic surgery on energy metabolism and gas exchange and to evaluate whether the 30-degree sitting position would affect these variables.

DESIGN

Prospective, unblinded, controlled study.

SETTING

Surgical ICU in a university hospital.

PATIENTS

Twenty-two adult patients undergoing elective pulmonary resection.

INTERVENTIONS

Posture change from supine to 30-degree sitting position.

MEASUREMENTS AND RESULTS

Oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory quotient (RQ), and energy expenditure (EE) were measured by means of computerized indirect calorimetry before and after surgery. Heart rate and respiratory frequency were measured continuously during gas exchange measurement. Blood gases were analyzed with an automated blood gas analyzer. Preoperatively, altering position did not affect energy metabolism, gas exchange, and cardiopulmonary variables. Postoperatively, the measured EE was 116% of the expected value. Mean EE and VO2 values for each position were higher than the preoperative values for the corresponding postures (p<0.05 for each position), while VCO2 increased only in the supine position (p<0.05). Mean percent increases in EE, VO2, and VCO2 were significantly lower in the 30-degree sitting position than in the supine position (EE: 7.9+/-2.7% vs 14.4+/-2.3%; p<0.001; VO2: 9.0+/-3.0% vs 16.4+/- 2.6%; p<0.001; VCO2: 3.2+/-2.1% vs 6.5+/-1.4%: p<0.05). Arterial oxygen tension and all the physiologic indexes of gas exchange for each position were worse than the preoperative values for the corresponding postures (p<0.05 for each position). Mean arterial pressure, heart rate, and respiratory frequency for each position were higher than the preoperative values for the corresponding postures (p<0.05 for each position). No changes in mean values of these variables occurred between the two positions postoperatively.

CONCLUSIONS

The early postoperative period of patients undergoing elective thoracic surgery is characterized by a condition of impaired gas exchange and by a hypermetabolic state. Hypermetabolism can be partly mitigated by assuming the 30-degree sitting position.

Splanchnic oxygen transport after cardiac surgery: evidence for inadequate tissue perfusion after stabilization of hemodynamics

OBJECTIVE

To evaluate the adequacy of visceral oxygen transport and gastric pHi after open heart surgery in patients with stable hemodynamics.

DESIGN

Nonrandomized control trial.

SETTING

A general intensive care unit in a tertiary care center.

PATIENTS

Sixteen postoperative cardiac surgery patients were studied after stabilization of systemic hemodynamics.

INTERVENTIONS

The effect of dobutamine infusion (6 mu g kg-1 min-1) on systemic and regional oxygen transport was studied in ten patients, with six patients serving as controls. Systemic oxygen consumption was measured by indirect calorimetry and splanchnic and femoral blood flow, by continuous infusion of indocyanine green using regional catheters and gastric mucosal pHi by gastric tonometer.

MEASUREMENTS AND RESULTS

Gastric mucosal acidosis was observed in half of the patients. Dobutamine increased cardiac output (3.2 +/- 0.6 vs 4.4 +/- 0.7 l x min-1 x min-2; P <0.05), splanchnic blood flow (0.68 +/- 0.28 vs 0.91 +/- 0.28 l x min-1 x m-2; p <0.05) and femoral blood flow (0.25 +/- 0.08 vs 0.32 +/- 0.11 l x min-1 x m-2; p <0.05). Changes in splanchnic oxygen delivery and consumption were parallel in the two study groups. In response to dobutamine, gastric pHi did not change (7.30 +/- 0.08 vs 7.31 +/- 0.06; NS), while in the control group, gastric pHi tended to decrease (7.32 +/- 0.04 vs 7.28 +/- 0.06; NS). Systemic oxygen consumption increased in response to dobutamine (141 +/- 11 vs 149 +/- 11 ml x min-1 x m-2; P <0.05) but did not change in the control group.

CONCLUSIONS

We conclude that a mismatch between splanchnic oxygen delivery and demand may be present despite stabilization of systemic hemodynamics after cardiac surgery. This is suggested by the parallel changes in splanchnic oxygen delivery and consumption. Dobutamine is likely to improve splanchnic tissue perfusion at this phase.

Gut mucosal ischemia during normothermic cardiopulmonary bypass results from blood flow redistribution and increased oxygen demand

Impaired gut mucosal perfusion has been reported during cardiopulmonary bypass. To better define the adequacy of gut blood flow and oxygenation during cardiopulmonary bypass, we measured overall gut blood flow and ileal mucosal flow and their relationship to mucosal pH, mesenteric oxygen delivery and oxygen consumption in immature pigs (n = 8). Normothermic, noncross-clamped, right atrium-to-aorta cardiopulmonary bypass was maintained at 100 ml/kg per minute for 120 minutes. Animals were instrumented with an ultrasonic Doppler flow probe on the superior mesenteric artery, a mucosal laser Doppler flow probe in the ileum, and pH tonometers in the stomach, ileum, and rectum. Radioactive microspheres were injected before and at 5, 60, and 120 minutes of cardiopulmonary bypass for tissue blood flow measurements. Overall gut blood flow significantly increased during cardiopulmonary bypass as evidenced by increases in superior mesenteric arterial flow to 134.1% +/- 8.0%, 137.1% +/- 7.5%, 130.3% +/- 11.2%, and 130.2% +/- 12.7% of baseline values at 30, 60, 90, and 120 minutes of bypass, respectively. Conversely, ileal mucosal blood flow significantly decreased to 53.6% +/- 6.4%, 49.5% +/- 6.8%, 58.9% +/- 11.6%, and 47.8% +/- 10.0% of baseline values, respectively. Blood flow measured with microspheres was significantly increased to proximal portions of the gut, duodenum and jejunum, during cardiopulmonary bypass, whereas blood flow to distal portions, ileum and colon, was unchanged. Gut mucosal pH decreased progressively during cardiopulmonary bypass and paralleled the decrease in ileal mucosal blood flow. Mesenteric oxygen delivery decreased significantly from 67.0 +/- 10.0 ml/min per square meter at baseline to 42.4 +/- 4.6, 44.9 +/- 3.5, 46.0 +/- 3.6, and 42.9 +/- 3.9 ml/min per square meter at 30, 60, 90, and 120 minutes of bypass. Despite the decrease in mesenteric oxygen delivery, mesenteric oxygen consumption increased progressively from 10.8 +/- 1.4 ml/min per square meter at baseline to 13.4 +/- 1.2, 15.9 +/- 1.2, 16.7 +/- 1.4, and 16.6 +/- 1.54 ml/min per square meter, respectively. We conclude that gut mucosal ischemia during normothermic cardiopulmonary bypass results from a combination of redistribution of blood flow away from mucosa and an increased oxygen demand.

Extraction of discriminant features in post-cardiosurgical intensive care units

A linear transformation, based on the Karhunen-Loève expansion, is applied to 13 physiological variables, measured in 200 surgical patients, in order to extract a limited number of features well representative of the differences between normal and high-risk classes of subjects. This transformation may be considered as a mapping from the primitive 13-dimensional space to a lower dimensional one, without severely reducing class separability. The efficacy of both transformed and primitive variables in the separation of normal and high-risk subjects is compared using the error probability, i.e. the probability that a patient is assigned to the wrong class. In particular, its upper bound is evaluated through the Kullback divergence and its estimate is computed, from the available samples, by applying a quadratic classifier. The results obtained show that only two transformed variables are able to present a divergence better than the most effective set of eight primitive variables. In agreement with the divergence criterion, the classifier provides a recognition error lower than 5% and greater than 13% when using the two best transformed and the two best primitive variables, respectively. Even though the new variables do not have a direct physiological meaning, this limitation has been partially overcome by calculating the correlation matrix between transformed and primitive variables. The results presented show that the first two transformed variables are strongly related to the most discriminant primitive ones (i.e. cardiac index, oxygen delivery and arterio-venous oxygen difference). In conclusion, the transformation of variables proposed appears to be extremely attractive for practical applications, since it allows recognition systems to be designed which exhibit both high performance and great simplicity.

Oxygen consumption following pediatric cardiac surgery

Metabolic responses during recovery from cardiac operations for various congenital heart defects were studied in 30 mechanically ventilated pediatric patients in two groups: infants 1 year or less (group I) and children more than 1 year old (group II). Oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured using a pediatric metabolic monitor intermittently after induction of anesthesia, after skin closure, 2 to 4 hours postoperatively, and on the first postoperative morning in the pediatric intensive care unit. Energy expenditure and respiratory quotient were determined from respiratory gas measurements. Rectal and skin temperatures and hemodynamic variables were recorded at the same time. VO2 increased during rewarming 2 to 4 hours after the operation by 12 +/- 15% in group I and by 24 +/- 19% in group II, while rectal temperature increased by 2.0 +/- 1.2 degrees C and 1.8 +/- 1.4 degrees C, respectively. No further increase in VO2 occurred until the first postoperative morning. A hypermetabolic response was not seen in all cases despite marked thermal changes. High-dose fentanyl anesthesia partly explains the low responses. On the other hand, low cardiac output may also compromise oxygen supply. Sixty-three percent of infants were treated for cardiac failure before surgery and 75% needed inotropic support immediately after the operation. Low central venous oxyhemoglobin saturation values (ScvO2 < 60%) were observed during rewarming, indicating an increase in oxygen extraction secondary to an increased oxygen demand in the brain during recovery from anesthesia, and a low cardiac output or delayed restoration of cerebral blood flow after CPB and deep hypothermia.

A four-parameter linear model for analysing cardiorespiratory data in post-operative cardiac patients

This paper investigates the possibility of characterizing the differences between normal- and high-risk postoperative cardiac patients on the basis of four parameters related to a simple linear model of cardiorespiratory performances. The model comprises three subsystems representing cardiac, vascular and respiratory functions, respectively. These parameters, determined from physiological variables measured in the Intensive Care Unit, seem useful for clinical evaluation of patient status. In fact, their values quantify the improved cardiovascular and respiratory response that normal-risk patients exhibit to increasing metabolic needs after hypothermic treatment, with less utilization of blood oxygen reserve. In addition, a set of three parameters derived from the proposed four allows a prediction of patient class membership with an error lower than 7% when used with a Bayes quadratic classifier.

Splanchnic ischaemia and its role in multiple organ failure

Multiple organ failure remains the leading cause of death in the intensive care unit. Increasing numbers of investigators have focused their attention on the role of gastrointestinal tract in the pathogenesis of this syndrome. Their data indicate that inadequate gut perfusion leads to a measurable imbalance between oxygen delivery and the needs of the tissues, i.e., ischaemia. Gut ischaemia of sufficient duration impairs gastrointestinal tract barrier function, facilitating the passage of enteric bacterial endotoxin into the circulation. It has been hypothesized that production of tumor necrosis factor alpha, and other biologic mediators by endotoxin-stimulated macrophages, triggers a generalized and uncontrolled inflammatory response that ultimately leads to multiple organ failure. Preliminary evidence suggests that survival can be improved significantly if gut ischaemia is promptly identified and aggressively treated by administration of fluids and inotropic drugs, using gastric intramucosal pH as the therapeutic endpoint. Future studies are needed to determine whether additional treatment modalities can improve outcome once the inflammatory response has fully developed.

Anaesthesia for coronary artery surgery--a plea for a goal-directed approach

The purpose of the current literature review was to examine whether changes in current anaesthetic techniques are warranted for patients undergoing coronary artery surgery in light of recent information presented in the literature. The objectives of a cardiac anaesthetic technique are to maintain haemodynamic stability and myocardial oxygen balance, minimize the incidence and severity of ischaemic episodes, be aware of cardiopulmonary bypass-induced pharmacokinetic changes, and facilitate early tracheal extubation if appropriate. Many techniques have been utilized. Provided attention is paid to the details of managing myocardial oxygen supply and demand, none has emerged as superior in preventing intraoperative myocardial ischaemia. Silent myocardial ischaemia (i.e., ischaemia occurring in the absence of haemodynamic aberrations) is common throughout the perioperative period and may occur even in the presence of an appropriately used anaesthetic technique. The incidence and severity appear to be greatest in the postoperative period when the effects of anaesthesia are dissipating. The use of high-dose opioid anaesthesia may no longer be the most appropriate technique to facilitate the anaesthetic objectives. The role of pain management in altering the incidence of ischaemia requires further study. Increased waiting lists for cardiac surgery and ever-diminishing resources should prompt a re-evaluation of early extubation (i.e., within eight hours) as a method of improving utilization of scarce ICU resources. It is suggested that this should be possible with currently available agents to achieve the anaesthetic objectives. Future suggestions for research in this area are made.

Metabolic profile of patients after elective open heart surgery

To evaluate the surgical stress of open heart surgery with moderate hypothermic cardiopulmonary bypass (CPB), oxygen consumption (VO2), carbon dioxide production (VCO2), resting energy expenditure (REE), respiratory quotient (RQ), 24 hour-urinary urea nitrogen excretion (UUN), and glucose, fat and protein utilization were determined in 20 patients before and after open heart surgery. Proteins (albumin, prealbumin and transferin) and body weight were measured preoperatively and on 6th postoperative day (POD). Preoperative predicted EE as determined by the Harris-Benedict equation was correlated with measured REE. No significant alteration in VO2, VCO2, REE, 24 hour UUN and protein utilization was observed on the first 6 PODs. RQ decreased significantly on the 1st, 3rd and 4th POD. This was attributed to greater fat utilization due to reduced calorie intake during the early postoperative period. Transport proteins reduced slightly but insignificantly. There was a significant reduction in body weight at the end of the study period due probably to loss of body water. We conclude that patients in the early postoperative period after uneventful open heart surgery are neither hypermetabolic nor hypercatabolic when compared with their stable state before operation.

Oxygen consumption after cardiopulmonary bypass--implications of different measuring methods

OBJECTIVE

To determine whether intra-pulmonary oxygen consumption or whole body oxygen consumption is the main determinant of the hypermetabolic response after cardiopulmonary bypass. Secondly, which method of measuring oxygen consumption best quantifies this hyperdynamic response.

DESIGN

We measured oxygen consumption by analysing respiratory gas (VO2-gas), carbon dioxide excretion (VCO2), and respiratory exchange ratio (RER = VCO2/VO2), and calculated oxygen consumption using the Fick-method (VO2-Fick) and intra-pulmonary oxygen consumption (VO2-gas - VO2-Fick) in patients at fixed times before and after elective cardiac surgery. Next, comparisons were made between methods and also between measurements at different times before and after bypass.

SETTING

University hospital.

PATIENTS

10 elective cardiac surgical patients.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

VO2-gas, VCO2 and RER were measured with an open circuit indirect calorimeter. VO2-Fick was calculated: VO2-Fick = cardiac index x (arterial - mixed venous oxygen content). Intrapulmonary oxygen consumption was calculated as the difference between VO2-gas and VO2-Fick. Both VO2-gas and VO2-Fick were about 20% higher after bypass than after induction of anaesthesia. Absolute values of VO2-gas were about 30% higher than VO2-Fick. Intra-pulmonary oxygen consumption accounted for 32% of whole body oxygen consumption after induction of anaesthesia and did not increase after bypass.

CONCLUSION

Whole body oxygen consumption and not intra-pulmonary oxygen consumption is the main determinant of the hypermetabolic response after bypass. Increased intra-pulmonary oxygen consumption is not related to bypass. VO2-gas best quantifies this hypermetabolic response directly after bypass, and not VO2-Fick, VCO2 or intra-pulmonary oxygen consumption, since VO2-Fick excludes intra-pulmonary oxygen consumption and VCO2 does not reflect metabolism directly after bypass.

Total parenteral nutrition in patients with intra-aortic balloon counterpulsation

The effects of total parenteral nutrition (TPN) were studied in nine of 19 patients with intra-aortic balloon pumping TPN (c. 27 kcal/kg/day) was begun 3 hours after the start of pumping. The non-protein caloric source was composed of hypertonic dextrose and fat emulsion (60% and 40%). The nitrogen intake was 1 g/150-200 kcal/day. The ten control patients received 5% dextrose in corresponding volume/hour. Hemodynamic studies were performed before and 24, 48 and 72 hours after the start of counterpulsation. The predicted and the observed resting energy expenditure were recorded in both patient groups during counter-pulsation. Systemic and pulmonary vascular resistance differed significantly between the groups. Cardiac function improved in both groups. In the TPN group the measured resting energy expenditure increased by 33% more than predicted on day 2 and by 56% on day 3 and in the controls the figures were 31% and 40%--all rises significant. Total parenteral nutrition with low fat content thus alters the hemodynamic equilibrium without clinically significant effects in patients undergoing intra-aortic balloon pumping. These patients are hypermetabolic and should receive artificial nutrition as soon as possible.

Different effects of prostacyclin and phentolamine on delivery-dependent O2 consumption and skin microcirculation after cardiac surgery

Inadequate tissue oxygen uptake autoregulation has been reported during the first hours after extracorporeal circulation for cardiac surgery. In the present study, we examined whether a dependence of oxygen consumption (VO2) on oxygen delivery (DO2) can be detected 24 hr after cardiac surgery using two different vasodilating agents. Cardiac output in triplicate was measured by thermodilution. Oxygen saturation of arterial and mixed venous blood was measured using a CO-oximeter. Oxygen consumption was assessed from the reverse Fick equation. In addition skin blood flow was assessed continuously by laser Doppler flowmetry. To investigate the VO2/DO2 relationship in 15 patients an increase in cardiac output and DO2 of at least 15% was achieved by systemic vasodilatation with iv prostacyclin (5-10 ng.kg-1.min-1) or phentolamine (5-10 g.kg-1.min-1). Infusion of phentolamine produced a 29 +/- 2% (mean +/- SE) increase in DO2 which was associated with a 20 +/- 6% increase in VO2. In contrast, prostacyclin produced a 22 +/- 3% increase in DO2 without change in VO2. Phentolamine did not alter skin microvascular blood flow, whereas prostacyclin increased skin microvascular blood flow by 33 +/- 3%. The results of the present study demonstrate a supply-dependency of VO2 in clinically stable patients 24 hr after cardiac surgery, suggesting the presence of an inadequate tissue O2 uptake autoregulation. The type of the vasodilator used to increase DO2 seems to play an important role in detecting such a supply-dependency of VO2, as well as changes of skin blood flow.

Pro: early endotracheal extubation is preferable to late extubation in patients following coronary artery surgery

Prolonged mechanical ventilation following CABG should not be uncritically considered "routine," and should only be used where indicated. A thorough physiologic and clinical evaluation with attention to hemodynamics, neurologic status, temperature and metabolism, hemostasis, and respiratory reserve should precede extubation. Continued post-operative ventilation is indicated in patients at high risk for complications, and it is possible to identify this subset preoperatively and upon arrival in the postoperative ICU. Early extubation (within 8 hours of arrival) should otherwise be the goal. The benefits of early extubation include improved cardiac function and patient comfort, reduction in respiratory complications, ease in management, and cost savings as the result of shortened length-of-stay in expensive postoperative units. More research is needed to clarify unanswered questions regarding ablating the stress response and avoiding myocardial ischemia.

Classification of postoperative cardiac patients: comparative evaluation of four algorithms

Four classification algorithms based on Bayes' rule for minimum error are compared by evaluating their ability to recognize high- and normal-risk cardio-surgical patients. These algorithms differ in the modelling of the probability density function (pdf) for each class and include: (a) two parametric algorithms based on the assumption of normal pdf; (b) two non-parametric algorithms using Parzen multidimensional approximation of pdf with normal kernels. In each case, classes with both equal and different covariance matrices were considered. A set of 200 patients in the 6 h immediately following cardiac surgery has been used to test the performance of the algorithms. For each patient the three measured variables most effective in representing the difference between the two classes were considered. We found that the two algorithms which explicitly incorporate the information on the different sample covariance between the physiological variables existing in the two classes generally provide better recognition of high- and normal-risk patients. Of these two algorithms the parametric one appears extremely attractive for practical applications, since it exhibits slightly better performance in spite of its great simplicity.