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

Indirect Calorimetry Overestimates Oxygen Consumption in Young Children: Caution is Advised Using Direct Fick Method as a Reference Method in Cardiac Output Comparison Studies

Direct Fick method is considered a standard reference method for estimation of cardiac output. It relies on indirect calorimetry to measure oxygen consumption. This is important as only a minor measurement error in oxygen consumption can result in false estimation of cardiac output. A number of studies have shown that indirect calorimetry overestimates oxygen consumption in adults. The aim of this prospective single center observational method comparison study was to compare the determination of oxygen consumption by indirect calorimetry and reverse Fick method in pediatric patients. Forty-two children mean age 352 days (range 30 to 1303 days) and mean weight 7.1 kg (range 2.7-13.6 kg) undergoing corrective cardiac surgery were included in the study. The mean (standard deviation) oxygen consumption by reverse Fick method was 43.5 (16.2) ml/min and by indirect calorimetry 49.9 (18.8) ml/min (p < 0.001). Indirect calorimetry overestimated the reverse Fick oxygen consumption by 14.7%. Bias between methods was 6.5 (11.3) ml/min, limits of agreement (LOA) - 15.7 and 28.7 ml/min and percentage error of 47.7%. A significant bias and large percentage error indicates that the methods are not interchangeable. Indirect calorimetry and the direct Fick method should be used with caution as a reference method in cardiac output comparison studies in young children.

Endpoints of Resuscitation for the Victim of Trauma

Useful resuscitation endpoints must serve both to diagnose the need for and to ensure the ongoing adequacy of resuscitation. To this end, traditional measures of organ perfusion are now widely appreciated to be grossly inadequate. Useful endpoints or milestones range from the global, to the regional, to the cellular specific. Understanding the basic principles of perfusion-related dysoxia in trauma and hemorrhage and its potential rapid transition to involve inflammatory and immune responses on cellular oxygen utilization will aid the clinician in choosing and appropriately interpreting endpoint monitoring data. There also appears to be an optimal window of opportunity for monitoring to help mitigate the development of more complicated inflammatory states. This article reviews the underlying need for endpoint selection (both global and regional, biochemical and functional) and monitoring during resuscitation of the polytrauma patient. At this juncture it appears that early use of a blend of global markers such as lactate and base deficit coupled with an available sensitive regional monitor such as gastric tonometry may offer the best combination of current technology to guard against early perfusion-related dysoxia. Future techniques involving optical spectroscopy offer the exciting potential to assess oxygenation at the cellular level. This may aid in ultra-early detection and resolution of perfusion-related dysoxia in addition to recognizing its transition to more complex inflammatory-mediated circulatory and metabolic failure.

Comparison of Minimally and More Invasive Methods of Determining Mixed Venous Oxygen Saturation

OBJECTIVE

To investigate the accuracy of a minimally invasive, 2-step, lookup method for determining mixed venous oxygen saturation compared with conventional techniques.

DESIGN

Single-center, prospective, nonrandomized, pilot study.

SETTING

Tertiary care hospital, university setting.

PARTICIPANTS

Thirteen elective cardiac and vascular surgery patients.

INTERVENTIONS

All participants received intra-arterial and pulmonary artery catheters. Minimally invasive oxygen consumption and cardiac output were measured using a metabolic module and lithium-calibrated arterial waveform analysis (LiDCO; LiDCO, London), respectively. For the minimally invasive method, Step 1 involved these minimally invasive measurements, and arterial oxygen content was entered into the Fick equation to calculate mixed venous oxygen content. Step 2 used an oxyhemoglobin curve spreadsheet to look up mixed venous oxygen saturation from the calculated mixed venous oxygen content. The conventional "invasive" technique used pulmonary artery intermittent thermodilution cardiac output, direct sampling of mixed venous and arterial blood, and the "reverse-Fick" method of calculating oxygen consumption.

MEASUREMENTS AND MAIN RESULTS

LiDCO overestimated thermodilution cardiac output by 26%. Pulmonary artery catheter-derived oxygen consumption underestimated metabolic module measurements by 27%. Mixed venous oxygen saturation differed between techniques; the calculated values underestimated the direct measurements by between 12% to 26.3%, this difference being statistically significant.

CONCLUSION

The magnitude of the differences between the minimally invasive and invasive techniques was too great for the former to act as a surrogate of the latter and could adversely affect clinical decision making.

Estimation of pulmonary oxygen consumption in the early postoperative period after thoracic surgery

Lung injury, including pneumonia, can occur in the early postoperative period following thoracic surgery. Pulmonary oxygen consumption is thought to increase in patients with pulmonary infection. This study measured oxygen consumption in relationship to lung injury in the early postoperative period after thoracic surgery. Thirty-five patients who underwent thoraco-abdominal oesophagectomy for oesophageal cancer were studied. Measured oxygen-consumption was obtained by indirect calorimetry and calculated oxygen-consumption was simultaneously determined by the reverse Fick method. The difference in oxygen consumption was attributed to pulmonary oxygen consumption. The difference in oxygen consumption increased to 23.1 ml.min(-1).m(-2) on postoperative day 2. In patients with pneumonia the difference in oxygen consumption increased significantly to 39.0 ml.min(-1).m(-2) the day before clinical onset of pneumonia, and it increased further to 65.7 ml.min(-1).m(-2) on the day that pneumonia became clinically apparent. These findings suggest that the difference in oxygen consumption may be useful for estimating the extent of lung injury and for predicting pulmonary complications in the postoperative period.

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.

Measured pulmonary oxygen consumption: difference between systemic oxygen uptake measured by the reverse Fick method and indirect calorimetry in cardiac surgery

Measurement of oxygen uptake by indirect calorimetry was compared with the reverse Fick method in a series of patients undergoing cardiac surgery. Oxygen uptake measurements for indirect calorimetry were made using a calibrated measurement system utilizing the Haldane transformation in a simple semiclosed breathing system based upon a modified Bain circuit. Pulmonary blood flow for the reverse Fick method was measured using bolus thermodilution. The results were (mean [standard deviation]): indirect calorimetry 167[26] ml.min(-1), and reverse Fick 148[27] ml.min(-1). Indirect calorimetry overestimated the reverse Fick value by 11.3% (p < 0.001). These results are consistent with the findings of previous studies in similar patient groups and are explained by lung tissue oxygen consumption.

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.

Comparison of closed circuit and Fick-derived oxygen consumption in patients undergoing simultaneous aortocaval occlusion

Agreement between continuously measured oxygen consumption during quantitative closed system anaesthesia and intermittently Fick-derived calculated oxygen consumption was assessed in 11 patients undergoing simultaneous occlusion of the aorta and inferior vena cava for hypoxic treatment of pancreatic cancer. All patients were mechanically ventilated using a quantitative closed system anaesthesia machine (PhysioFlex) and had pulmonary and radial artery catheters inserted. During the varying haemodynamic conditions that accompany this procedure, 73 paired measurements were obtained. A significant correlation between Fick-derived and closed system-derived oxygen consumption was found (r = 0.78, p = 0.006). Linear regression showed that Fick-derived measure = [(1.19 x closed system derived measure) - 72], with the overall closed circuit-derived values being higher. However, the level of agreement between the two techniques was poor. Bland-Altman analysis found that the bias was 36 ml.min(-1), precision 39 ml.min(-1), difference between 95% limits of agreement 153 ml.min(-1). Therefore, we conclude that the two measurement techniques are not interchangeable in a clinical setting.

Accuracy of feedback-controlled oxygen delivery into a closed anaesthesia circuit for measurement of oxygen consumption

BACKGROUND

Oxygen consumption (V*O2) is rarely measured during anaesthesia, probably because of technical difficulties. Theoretically, oxygen delivery into a closed anaesthesia circuit (V*O2-PF; PhysioFlex Draeger Medical Company, Germany) should measure V*O2. We aimed to measure V*O2-PF in vitro and in vivo.

METHODS

Three sets of experiments were performed. V*O2-PF was assessed with five values of V*O2 (0-300 ml min(-1)) simulated by a calibrated lung model (V*O2-Model) at five values of FIO2 (0.25-0.85). The time taken for V*O2-PF to respond to changes in V*O2-Model gave a measure of dynamic performance. In six healthy anaesthetized dogs we compared V*O2-PF with V*O2 measured by the Fick method (V*O2-Fick) during ventilation with nine values of FIO2 (0.21-1.00). V*O2-PF and V*O2-Fick were also compared in three dogs when V*O2 was changed pharmacologically [102 (SD 14), 121 (17) and 200 (57) ml min(-1)]. In patients during surgery, we measured V*O2-PF and V*O2-Fick simultaneously after induction of anaesthesia (n=21) and during surgery (n=17) (FIO2 0.3-0.5).

RESULTS

Compared with V*O2-Model, V*O2-PF values varied from time to time so that averaging over 10 min is recommended. Furthermore, at an FIO2 >0.8, V*O2-PF always overestimated V*O2. With FIO2 <0.8, averaged V*O2-PF corresponded to V*O2-Model and adapted rapidly to changes. Averaged V*O2-PF also corresponded to V*O2-Fick in dogs at FIO2 <0.8. V*O2 measured by the two methods gave similar results when V*O2 was changed pharmacologically. In contrast, V*O2-PF systematically overestimated V*O2-Fick in patients by 52 (SD 40) ml min-1 and this bias increased with smaller arteriovenous differences in oxygen content.

CONCLUSION

V*O2-PF measures V*O2 adequately within specific conditions.

Stress response during weaning after cardiac surgery

We compared the effects of weaning using synchronized intermittent mandatory ventilation (SIMV) with the use of biphasic positive airway pressure (BIPAP) on the stress response, oxygen uptake (VO2) and work of breathing (WOB) in 10 patients after aortocoronary bypass surgery. All three ventilatory settings were investigated in each patient, for example, volume-controlled mechanical ventilation immediately before weaning was followed, in randomized order, by both SIMV and BIPAP. In addition to routine monitoring of continuous and respiratory state, we measured VO2, WOB, and pressure-time product (PTP) as well as the plasma concentrations of epinephrine, norepinephrine, ACTH, cortisol, vasopressin, and prolactin. Although respiratory rate (f), WOB and PTP were greater with both SIMV and BIPAP when compared with control, other variables did not change with the ventilatory mode. In conclusion, weaning from mechanical ventilation using partial support modes does not affect the postoperative stress response in patients who have had uncomplicated cardiac surgery.

Thoracic epidural anesthesia as an adjunct to general anesthesia for cardiac surgery: effects on ventilation-perfusion relationships

OBJECTIVE

To determine the effects of thoracic epidural anesthesia (TEA) on ventilation-perfusion (VA/Q) relationships, atelectasis, and oxygenation before and after coronary artery bypass graft surgery (CABG).

DESIGN

Prospective, controlled, unblinded, randomized trial.

SETTING

Cardiothoracic clinic at a major university referral center.

PARTICIPANTS

Twenty-eight patients undergoing elective CABG.

INTERVENTIONS

Perioperative and postoperative TEA was added to general anesthesia (GA) in 14 patients, and 14 patients receiving GA alone served as controls.

MEASUREMENTS AND MAIN RESULTS

VA/Q relationships were measured by the multiple inert gas elimination technique, and, 20 hours postoperatively, atelectasis was assessed by computerized tomographic scans. Arterial and mixed venous blood gases and hemodynamic variables were measured by standard techniques. TEA per se caused no change in shunt, VA/Q matching, or oxygenation. Induction of GA in the control group and induction of TEA caused similar reductions in mean arterial pressure. The TEA patients needed less morphine analgesia postoperatively and were extubated earlier. Extubation caused significant improvement in VA/Q matching. On the first postoperative day, a slight reduction in PaCO2 was seen in the TEA group, but no differences in shunt, VA/Q matching, or oxygenation compared with the GA group. Both groups showed extensive bilateral atelectasis.

CONCLUSION

TEA can reduce respirator time and the need for morphine analgesics after CABG without negative effects on VA/Q matching, oxygenation, or atelectasis formation.

A comparison between the Fick method and indirect calorimetry for determining oxygen consumption in patients with fulminant hepatic failure

OBJECTIVE

To compare the Fick method of determining oxygen consumption (VO2) with a gas exchange method in a group of patients in whom the cardiac output and mixed venous oxygen saturation values were consistently high.

DESIGN

A prospective, observational study.

SETTING

A ten-bed intensive therapy unit at a university teaching hospital.

PATIENTS

Seventeen patients suffering from fulminant hepatic failure who required ventilatory support and invasive hemodynamic monitoring. All patients were sedated and paralyzed throughout the study period.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

VO2 was determined simultaneously by indirect calorimetry and by the Fick method five or six times in each patient over a 5-hr period after resuscitation with fluids and, if clinically indicated, norepinephrine infusion. The agreement between the methods was poor (limits of agreement +19 to -101 mL/min/m2) and the Fick method consistently underestimated gas exchange measurements (mean bias 41 mL/min/m2). The bias varied widely, both between and within individual patients. The reproducibility of the Fick-derived VO2 was worse than the indirect calorimetry measurements, indicating that the dispersion of data attributable to measurement error was greater with the Fick method.

CONCLUSIONS

Under clinical conditions, the agreement between Fick calculations and indirect calorimetry measurements of VO2 in hyperdynamic patients with fulminant hepatic failure was extremely poor. The reproducibility of Fick calculations was less than the reproducibility derived by gas exchange measurements because of the large measurement errors that may occur with the Fick method when the cardiac output is large and the arterial-venous oxygen content difference is small. Fick calculations systematically underestimate gas exchange measurements. The Fick method is inaccurate and unreliable when an estimation of VO2 is required in patients with this hemodynamic pattern.

Calculated versus measured oxygen consumption during and after cardiac surgery. Is it possible to estimate lung oxygen consumption?

BACKGROUND

Lung tissue is metabolically active and consumes oxygen. The oxygen content difference between arterial and mixed venous blood does not include the effect of pulmonary tissue oxygen uptake. Thus, oxygen consumption (VO2) of the lung should be reflected as a difference between VO2 measured by gas exchange and VO2 derived by the Fick principle. The purpose of this study was to measure in clinical conditions this difference (taken to represent the VO2 of the lung), and to evaluate the sources of error in lung VO2 estimation.

METHODS

Nine patients undergoing coronary artery bypass grafting were studied. VO2 was measured by indirect calorimetry (VO2gasex) and compared to Fick-derived VO2 (VO2Fick) after induction of anaesthesia, after closure of the chest, at admission to intensive care, after stabilization of haemodynamics and during weaning from mechanical ventilation. The Fick-derived VO2 was calculated from blood samples taken at the beginning and at the end of each 20 min measurement period, and the mean of 12 consecutive thermodilution cardiac output measurements taken during each 20 min measurement period.

RESULTS

VO2gasex was higher than VO2Fick (P < 0.01; in all except 4 of 45 measurements). The difference between the measured and the calculated VO2 was 33 +/- 25 ml/min (mean +/- SD, range -16-100 ml/min). This difference represented 14 +/- 3% (range 11-18%) of the whole body VO2. The VO2-difference was highest after the induction of anaesthesia (50 +/- 19 ml/min; range 20-41 ml/min, P < 0.03) and lowest on arrival at the intensive care unit (10 +/- 16 ml/min; range -16-39 ml/min). Core temperature did not correlate with the oxygen consumption difference.

CONCLUSIONS

A constant difference between measured and calculated VO2 can be detected in carefully controlled clinical conditions. The difference between the two methods is due to both lung oxygen consumption and errors in the measurement of VO2 thermodilution cardiac output, haemoglobin and blood oxygen contents. We suggest that the perioperative changes of the VO2-difference are due not only to variation of the measurements but also to changes in lung metabolic activity.

Prolonged oxygen debt after abdominal aortic surgery

OBJECTIVE

The aim of the study was to investigate whether oxygen consumption and carbon dioxide production changed in the early postoperative period in the same manner in patients who underwent coronary artery bypass graft (CABG) surgery or abdominal aortic surgery (AAS).

DESIGN

Prospective clinical study.

SETTING

Single community hospital.

PARTICIPANTS

Twenty-four patients (ASA II to III) who underwent elective abdominal aortic surgery and 49 patients (NYHA II, III) who underwent elective CABG surgery.

INTERVENTIONS

In the ICU, oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory quotient (RQ) were continuously monitored with an indirect calorimetric device. Hemodynamic and metabolic parameters were measured during the first 3 hours.

MEASUREMENTS AND MAIN RESULTS

In the early postoperative course, VO2 and VCO2 in the AAS group were significantly higher (p < 0.001), and the RQ was significantly lower during the first 90 minutes (p < 0.001). In the AAS patients, VO2 decreased with the postoperative increase of body temperature; whereas in the CABG patients, VO2 slightly increased.

CONCLUSIONS

The high VO2 in the AAS patients observed during the early postoperative course can be explained by an oxygen debt that occurred as a result of clamping the aorta. It was presumed that the repayment of the debt was delayed and extended to the ICU stay because of thermoregulatory vasoconstriction. Abnormally low RQ values are probably a clue to repayment of an oxygen debt.

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.

Relationship between pulmonary oxygen consumption, lung inflammation, and calculated venous admixture in patients with acute lung injury

OBJECTIVE

To determine in patients with acute lung injury whether increased pulmonary oxygen consumption (VO2pulm), computed as the difference between oxygen consumption measured by indirect calorimetry (VO2meas) and calculated by the reverse Fick method (VO2Fick), would: (1) correlate with the degree of lung inflammation assessed by bronchoalveolar lavage (BAL); (2) lead to an overestimation of calculated venous admixture (Qva/Qt).

DESIGN

Prospective study.

SETTING

University hospital, medical intensive care unit.

INTERVENTION

None.

MEASUREMENTS AND RESULTS

In nine mechanically ventilated patients with acute lung injury (Apache II 12 +/- 5, lung injury score 2 +/- 0.6, mean +/- SD), whole-body VO2 (VO2wb) was determined simultaneously by indirect calorimetry and the reverse Fick technique, after which BAL was immediately performed. VO2meas was significantly higher than VO2Fick (128 +/- 24 and 102 +/- 18 ml/min per m2, respectively, p < 0.001). Median VO2pulm was 25.3 ml/min per m2 (range 1.98-51.5), thus representing 19 +/- 11% of VO2wb. Total BAL cellularity was increased in all patients (median 47, range 24-200 x 10(4)/ml), as was the total polymorphonuclear (PMN) count (median 78 range 5-93 x 10(4)/ml). Macrophage counts were in the normal range. There were raised BAL levels of interleukin-6 (IL-6) (median 945, range 23-1800 ng/ml) and elastase (median 391, range 5-949 ng/ml). Median protein levels were 270 micrograms/ml (range 50-505). There was no correlation between VO2pulm and BAL cellularity, PMNs, elastase, IL-6, or protein. Qva/Qt was 31.7 +/- 8%. Qva/Qt, corrected for the presence of VO2pulm, (Qva/Qtcorr), was 30.3 +/- 8% (P < 0.01 vs Qva/Qt), a 4.2% overestimation due to VO2plum. There was no correlation between Qva/Qt or Qva/Qtcorr and VO2pulm.

CONCLUSIONS

In mechanically ventilated patients with acute lung injury, VO2pulm was increased and led to a 19% underestimation of VO2wb determined by the reverse Fick method, as well as to a 4.2% overestimation of calculated Qva/Qt. Lung inflammatory activity was increased, as assessed by BAL cellularity, IL-6 and elastase levels. However, there was no correlation between VO2pulm and the intensity of pulmonary inflammation.

Increased oxygen consumption after cardiac surgery is associated with the inflammatory response to endotoxemia

OBJECTIVE

The aim of this study was to determine whether the increase in post-operative oxygen consumption (delta VO2) in cardiac surgery patients is related to endotoxemia and subsequent cytokine release and whether delta VO2 can be used as a parameter of post-perfusion syndrome.

DESIGN

Prospective study.

SETTING

Operating room and intensive care unit of a university hospital.

PATIENTS

Twenty-one consecutive male patients undergoing elective coronary artery bypass surgery without major organ dysfunction and not receiving corticosteroids.

MEASUREMENTS AND RESULTS

Plasma levels of endotoxin, tumor necrosis factor (TNF) and interleukin-6 (IL-6) were measured before, during and for 18 h after cardiac surgery. Oxygen consumption, haemodynamics, the use of IV fluids and dopamine, body temperature and the time of extubation were also measured. Measurements from patients with high delta VO2 (> or = median value of the entire group) were compared with measurements from patients with low delta VO2 (< median). Patients with high delta VO2 had higher levels of circulating endotoxin (P = 0.004), TNF (P = 0.04) and IL-6 (P = 0.009) received more IV fluids and dopamine while in the ICU, and were extubated later than patients with low delta VO2. Several hours after delta VO2 the patient's body temperature rose. Forward stepwise regression analysis showed that circulating endotoxin and TNF explained 50% of the variability of delta VO2.

CONCLUSIONS

This study demonstrates that patients with high post operative oxygen consumption after elective cardiac surgery have higher circulating levels of endotoxin, TNF and IL-6 and also have more symptoms of post-perfusion syndrome. Early detection of high VO2 might be used as a clinical signal to improve circulation in order to meet the high oxygen demand of inflammation. In addition, continuous measurement of VO2 provides us with a clinical parameter of inflammation in interventional studies aiming at a reduction of endotoxemia or circulating cytokines.

Intestinal permeability, circulating endotoxin, and postoperative systemic responses in cardiac surgery patients

OBJECTIVES

To determine whether intestinal permeability increases during cardiac operations, and whether the degree of endotoxemia is related to this increase. Furthermore, to determine whether intestinal permeability is related to the hemodynamic state during operation and to postoperative systemic responses.

DESIGN

Prospective study.

SETTING

University hospital.

PARTICIPANTS

Twenty-three male patients undergoing elective coronary artery bypass surgery.

INTERVENTIONS

Before surgery and during the fifth postoperative day, 100 mL of a solution containing L-rhamnose and cellobiose were administered orally.

MEASUREMENTS AND MAIN RESULTS

Intestinal permeability was assessed by measuring the urinary excretion of L-rhamnose and cellobiose. Endotoxin concentrations in blood and prime fluid, hemodynamics, oxygen consumption, gas exchange, fluid balance, and the dose of vasoactive drugs were measured. Systemic responses were assessed by measuring hypermetabolism, circulatory support, and gas exchange. Intestinal permeation of cellobiose, reflecting paracellular transport, significantly increased during operation (p < 0.01), and correlated with the amount of circulating endotoxin (r2 = 0.46; p < 0.01). A high dose of ephedrine administered during operation, low baseline central venous pressure, and a less positive fluid balance during operation were associated with high intestinal permeability (r2 = 0.7; p < 0.01). Intestinal permeability was related to postoperative systemic responses (r2 = 0.49; p < 0.01).

CONCLUSIONS

This study shows that during elective coronary artery bypass operations intestinal permeability between cells may increase. The degree of endotoxemia is related to this increase. Increased intestinal permeability is related to the use of ephedrine, especially during hypovolemia, and to postoperative systemic responses. Although a causative relation is not shown, these results might indicate that hypovolemia and vasoconstriction should be avoided during the operation.

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.