Coronary haemodynamics and myocardial metabolism during weaning from mechanical ventilation in cardiac surgical patients

Lactate Elevation During and After Major Cardiac Surgery in Adults: A Review of Etiology, Prognostic Value, and Management

Elevated lactate is a common occurrence after cardiac surgery. This review summarizes the literature on the complex etiology of lactate elevation during and after cardiac surgery, including considerations of oxygen delivery, oxygen utilization, increased metabolism, lactate clearance, medications and fluids, and postoperative complications. Second, the association between lactate and a variety of outcomes are described, and the prognostic role of lactate is critically assessed. Despite the fact that elevated lactate is strongly associated with many important outcomes, including postoperative complications, length of stay, and mortality, little is known about the optimal management of postoperative patients with lactate elevations. This review ends with an assessment of the limited literature on this subject.

Role of Hematocrit Concentration on Successful Extubation in Critically Ill Patients in the Intensive Care Units

Background:

Hematocrit (Hct) is an important parameter for optimal oxygenation during discontinuation from ventilator, but there is no consensus about its concentration and effectiveness on successful extubation.

Objectives:

The current study aimed to determine the role of Hct concentration on extubation failure in critically ill patients.

Patients and Methods:

The current prospective cohort study investigated the effect of age, gender and Hct level on successful extubation of 163 mechanically ventilated patients in Imam Khomeini hospital intensive care units (ICUs), Tehran, Iran. Following successful weaning process, the patients were classified into two groups on the basis of Hct level; 62 with an Hct level of 21% - 27% and the other 101 patients with Hct levels above 27%. The data were analyzed by chi-square test and multiple logistic regressions. A probability value of less than 0.05 was considered significant.

Results:

There was no significant association between the level of Hct concentration and extubation failure (8.9% vs. 9.2%, P = 0.507). Gender and age were significantly associated with extubation failure (OR = 9.1, P = 0.034, OR = 12.5, P = 0.014, respectively). Although the differences between, before and after extubation of PaO₂ and P/F ratio, were of significant values between the two different groups of Hct (P = 0.001, P = 0.004 respectively), they had no effect on the failure of extubation (P= 0.259, P = 0.403, respectively).

Conclusions:

Although some studies showed association between anemia and extubation failure, the current study could not confirm it. The study showed that males, regardless of the Hct level, had a better extubation success rate than those of females.

Simultaneous transdermal extraction of glucose and lactate from human subjects by reverse iontophoresis

This study investigated the possibility of simultaneously extracting glucose and lactate from human subjects, at the same skin location, using transdermal reverse iontophoresis. Transdermal monitoring using iontophoresis is made possible by the skin’s permeability to small molecules and the nanoporous and microporous nature of the structure of skin. The study was intended to provide information which could be used to develop a full, biosensor-based, monitoring system for multiple parameters from transdermal extraction. As a precursor to the human study, in vitro reverse iontophoresis experiments were performed in an artificial skin system to establish the optimum current waveforms to be applied during iontophoresis. In the human study, a bipolar DC current waveform (with reversal of the electrode current direction every 15 minutes) was applied to ten healthy volunteers via skin electrodes and utilized for simultaneous glucose and lactate transdermal extraction at an applied current density of 300 μA/cm². Glucose and lactate were successfully extracted through each subject’s skin into the conducting gel that formed part of each iontophoresis electrode. The results suggest that it will be possible to noninvasively and simultaneously monitor glucose and lactate levels in patients using this approach and this could have future applications in diagnostic monitoring for a variety of medical conditions.

Cardiovascular effects of mechanical ventilation and weaning

Because of their anatomic position in the closed thoracic cavity, the heart and lungs interact during each ventilation cycle. The application of mechanical ventilation and subsequent removal changes normal ventilatory mechanics and produces alterations in cardiac preload and afterload that influence global hemodynamic state and delivery of oxygen and nutrients. Adverse cardiovascular responses to mechanical ventilation and weaning from ventilation include hemodynamic alterations and instability, myocardial ischemia, autonomic dysfunction, and cardiac dysrhythmias. Clinicians must have a clear understanding of the cardiovascular effects of mechanical ventilation and weaning so they may anticipate, recognize, and effectively manage negative effects and improve patient outcomes.

Prevalence of myocardial ischemia during mechanical ventilation and weaning and its effects on weaning success

BACKGROUND

Myocardial ischemia may be considered both a consequence of weaning from ventilation and a potential cause of weaning failure. A limited number of investigations have evaluated myocardial ischemia during mechanical ventilation and weaning and its effect on weaning success. The purpose of this pilot investigation was to determine the prevalence of myocardial ischemia in a diverse group of medical intensive care unit (MICU) patients during baseline mechanical ventilation, during weaning using continuous positive airway pressure, and up to 24 hours after extubation and to evaluate the relationship between ischemia and weaning failure.

METHODS

This study was a prospective, repeated-measure, descriptive investigation that studied 43 ventilated MICU patients. Slightly more than half of participants were male (53%), and participants had a mean age of 51.1 +/- 14.6 years and were ventilated an average of 11.7 +/- 11.3 days. Myocardial ischemia was evaluated by examination of plasma cardiac troponin I, creatine phosphokinase-myocardial band (CK-MB), and ST-segment changes on electrocardiogram. Continuous electrocardiographic data were obtained by a calibrated, frequency-modulated, continuous 3-channel electrocardiographic recorder using leads I, II, and V2.

RESULTS

Seventy percent of these participants (n = 30) exhibited ST-segment deviation at some point during data collection (baseline mechanical ventilation, during weaning, after extubation). Twenty-four participants exhibited ST-segment deviation at baseline, 7 during weaning, and 8 after extubation. Nine participants exhibited ST-segment deviation at >1 data collection time point. None had ST-segment deviation at all 3 time points. Cardiac enzyme concentrations were highly variable; five participants demonstrated clinically important increases in either CK-MB and/or troponin I. Thirty-five percent of participants required >1 weaning trial. Participants who exhibited ST-segment deviation during baseline ventilation were 60% more likely to fail their initial weaning trial. There were no significant differences in CK-MB or troponin I levels between those participants who were successfully weaned with 1 trial and those who failed to wean during that first trial.

CONCLUSIONS

Silent myocardial ischemia was a common occurrence in this diverse group of MICU patients, although only 21% had previously diagnosed coronary disease. Clinicians must be aware of the potential for silent ischemia, monitor and evaluate their patients for such, and intervene to promote optimal cardiovascular function, particularly during the stress of ventilator weaning.

Anemia in the long-term ventilator-dependent patient with respiratory failure

Anemia occurs in virtually all critically ill patients receiving long-term mechanical ventilation and has been associated with increased mortality and poor outcomes. Allogeneic RBC transfusions are routinely administered to critically ill anemic patients, especially during lengthy stays in ICUs or in long-term acute care facilities. Although RBC transfusions are a physiologically rational approach to raising hemoglobin levels, they may increase the risk of complications and have been associated with higher mortality in critically ill patients. Treatment with epoetin alfa, an erythropoiesis-stimulating agent, as a means of reducing transfusion requirements has been studied in the critically ill and in patients receiving long-term mechanical ventilation. Promising results have been reported, including a potential survival benefit, although larger and more definitive studies are needed in order to establish whether raising hemoglobin levels affects clinical outcomes in patients receiving mechanical ventilation.

Do blood transfusions improve outcomes related to mechanical ventilation?

BACKGROUND

Correcting the decrease in oxygen delivery from anemia using allogeneic RBC transfusions has been hypothesized to help with increased oxygen demands during weaning from mechanical ventilation. However, it is also possible that transfusions hinder the process because RBCs may not be able to adequately increase oxygen delivery. In this study, we determined whether a liberal RBC transfusion strategy improved outcomes related to mechanical ventilation.

METHODS

Seven hundred thirteen patients receiving mechanical ventilation, representing a subgroup of patients from a larger trial, were randomized to either a restrictive transfusion strategy, receiving allogeneic RBC transfusions at a hemoglobin concentration of 7.0 g/dL (and maintained between 7.0 g/dL and to 9.0 g/dL), or to a liberal transfusion strategy, receiving RBCs at 10.0 g/dL (and maintained between 10.0 g/dL and 12.0 g/dL). The larger trial was designed to evaluate transfusion practice rather than weaning per se.

RESULTS

Baseline characteristics in the restrictive-strategy group (n = 357) and the liberal-strategy group (n = 356) were comparable. The average durations of mechanical ventilation were 8.3 +/- 8.1 days and 8.3 +/- 8.1 days (95% confidence interval [CI] around difference, - 0.79 to 1.68; p = 0.48), while ventilator-free days were 17.5 +/- 10.9 days and 16.1 +/- 11.4 days (95% CI around difference, - 3.07 to 0.21; p = 0.09) in the restrictive-strategy group vs the liberal-strategy group, respectively. Eighty-two percent of the patients in the restrictive-strategy group were considered successfully weaned and extubated for at least 24 h, compared to 78% for the liberal-strategy group (p = 0.19). The relative risk (RR) of extubation success in the restrictive-strategy group compared to the liberal-strategy group, adjusted for the confounding effects of age, APACHE (acute physiology and chronic health evaluation) II score, and comorbid illness, was 1.07 (95% CI, 0.96 to 1.26; p = 0.43). The adjusted RR of extubation success associated with restrictive transfusion in the 219 patients who received mechanical ventilation for > 7 days was 1.1 (95% CI, 0.84 to 1.45; p = 0.47).

CONCLUSION

In this study, there was no evidence that a liberal RBC transfusion strategy decreased the duration of mechanical ventilation in a heterogeneous population of critically ill patients.

Cardiac power output during transition from mechanical to spontaneous ventilation in canines

Hemodynamic instability during weaning from mechanical ventilation is one proposed cause of weaning failure. This study evaluated cardiac power output (CPO) as an indicator of significant hemodynamic alteration and cardiac reserve during the transition from controlled mechanical ventilation to spontaneous ventilation using three clinical weaning modes. It also compared CPO with commonly used indicators of hemodynamic instability during weaning. The data suggest that CPO is a sensitive indicator of significant hemodynamic alteration and could be used to optimize cardiovascular function during weaning from mechanical ventilation to increase the likelihood of weaning success. Key words: cardiac output, cardiac power output, hemodynamic monitoring, mechanical ventilation

Weaning from mechanical ventilation

Patients that require positive pressure ventilation to maintain sufficient alveolar ventilation or pulmonary gas exchange may eventually reach a point in the course of their care wherein mechanical ventilation is no longer necessary. This process of transferring the work of breathing from the ventilator back to the patient is referred to as ventilator weaning. The term "ventilator weaning" may be used to refer to all methods by which this transfer of workload may be accomplished. In many patients, particularly those with short-lasting or readily correctable causes of respiratory insufficiency (e.g., general anesthesia), the discontinuation of positive pressure ventilation may be easily achieved. Indeed, in patients awakening from general anesthesia, the axiom "awake enough to blink, awake enough to breath" may prove to be a sufficient guideline. However, in those patients requiring long-term mechanical ventilatory support, the process can prove to be both frustrating and exceptionally challenging. It is of crucial importance to identify those patients that may be successfully weaned because of both the financial impact of prolonged intensive care unit hospitalization and the risks imposed on the patient by the process of positive pressure ventilation. To be able to predict which patients may be ready to be weaned from the ventilator requires an understanding of the balance between the work of breathing (ventilatory load) and the ability of the patient's respiratory pump to meet those needs (ventilatory capacity). The management of patients experiencing difficulty during the weaning process requires that the clinician recognize imbalances between ventilatory load and capacity and to correct these imbalances once identified.

Proposed entry criteria for postoperative cardiac extracorporeal membrane oxygenation after pediatric open heart surgery

While extracorporeal membrane oxygenation (ECMO) is being used increasingly after pediatric cardiac surgery, criteria are lacking for initiating ECMO after bypass weaning. To develop clinically useful ECMO entry criteria based on parameters readily available, children were examined at postoperative pediatric intensive care unit (PICU) admission. Using hospital mortality as the primary outcome, univariate and multiple logistic regressions were performed to estimate the predictive value of clinical (age, weight, and diagnosis) and laboratory (arterial blood pressure, pH, lactate, creatine kinase, and arterial and central venous oxygen saturation [ScvO2]) variables. Data from 218 children over a 2 year period were analyzed retrospectively. Univariate regression demonstrated that age, weight, diagnosis, blood pressure, venous and arterial saturation, and lactate were significantly associated with postoperative mortality (p < 0.05). In multiple regression, ScvO2 and lactate level were found to be independent predictors and were used in a predictive model (ScvO2 odds ratio: 2.03-828.6, p = 0.016) (lactate odds ratio: 1.58 -4.20, p = 0.0002) (R2 = 0.70). Applying an 80% risk of mortality to establish entry criteria as in neonatal ECMO, PICU admission values of lactate > 70 mg/dl if ScvO2 < 60% or lactate >163 mg/dl if ScvO2 > 60% are proposed to serve as postoperative ECMO entry criteria if bypass weaning has been possible but is followed by low cardiac output.

Changes in intrathoracic fluid volumes during weaning from mechanical ventilation in patients after coronary artery bypass grafting

PURPOSE

Although it is known that weaning from mechanical ventilation is associated with alterations in intrathoracic pressure, lung volume, and venous return, changes in intrathoracic fluid volumes during weaning are not reported. Especially in patients with impaired cardiac function, the development of pulmonary edema during weaning has been described. Thus, we investigated changes in intrathoracic fluid volumes in patients after coronary artery bypass grafting after changing the ventilatory pattern from mechanical to spontaneous ventilation.

MATERIALS AND METHODS

Intrathoracic blood volume index (ITBVI), pulmonary blood volume index (PBVI), and extravascular lung water (EVLW) were calculated during mechanical ventilation (T1), T-piece breathing (T2), and spontaneous breathing after extubation of the trachea (T3) in 72 consecutive patients after coronary artery bypass grafting using a combined dye-thermal dilution method.

RESULTS

Changing from mechanical ventilation to T-piece breathing resulted in an increase in ITBVI from 880 +/- 22 mL/m2 to 970 +/- 22 mL/m2 (P < .01), and in PBVI from 162 +/- 6 mL/m2 to 173 +/- 6 mL/m2 (P < .01). After extubation of the trachea, both parameters decreased again (ITBVI, 879 +/- 20 mL/m2; PBVI, 160 +/- 7 mL/m2). EVLW remained unchanged after transition to T-piece breathing (T1, 5.8 +/- 0.3 mL/kg; T2, 6.0 +/- 0.3 mL/kg), but increased to 6.6 +/- 0.5 mL/kg (P < .01) after extubation of the trachea. However, pathological values of EVLW were not observed.

CONCLUSIONS

In patients after coronary artery bypass grafting, changes in intrathoracic intravascular fluid volumes during weaning are restricted to the period of T-piece breathing and reflect an increased venous return. The maintenance of EVLW in the normal range during weaning indicates that cardiac function was matched to this elevated preload.

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.

The effect of preoperative beta-blocker therapy on cardiovascular responses to weaning from mechanical ventilation and extubation after coronary artery bypass grafting

The hemodynamic and electrocardiographic changes during weaning from mechanical ventilation and tracheal extubation were studied in 75 patients after elective coronary artery bypass surgery. Transfer from synchronized intermittent mandatory ventilation to spontaneous respiration through a T-piece was associated with an increase greater than 20% over baseline in systolic (SBP) and diastolic (DBP) blood pressure in 27% of patients, and in heart rate (HR) in 5% of patients. Although baseline SBP, DBP, and HR differed significantly between the patients taking chronic beta-blocker therapy and those not on beta-blockers (P values all < 0.003), there were no differences between these groups in their response to transfer to the T-piece. (P values: SBP = 0.98; DBP = 0.46; HR = 0.20). Tracheal extubation was associated with an increase greater than 20% of baseline in SBP in 18.9%, DBP in 16.2%, and HR in 5% of patients. However, there were significant differences between the chronically beta-blocked and non-beta-blocked groups, both in baseline values for SBP, DBP, and HR (P values all < 0.001), and also in the SBP response (P = 0.007) and HR response (P = 0.02) to extubation. Extubation was associated with a greater than 20% increase in SBP in 8.2% and DBP in 12.2% of chronically beta-blocked patients, compared to 40% and 23% of non-beta-blocked patients, although the DBP response was not statistically different (P = 0.14) between the groups. Similar proportions of patients in both groups increased their HR more than 20% above baseline, but the increase was much greater in the non-beta-blocked group (P = 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Infusion of propofol versus midazolam for sedation in the intensive care unit following coronary artery surgery

The use and the hemodynamic effects of propofol and midazolam were studied during titrated continuous infusions to deep sedation (sedation level 5: asleep, sluggish response to light glabellar tap or loud auditory stimulus) following coronary artery surgery. The drugs were compared in 30 ventilated patients in an open randomized study. The duration of infusion was approximately 570 minutes in both groups. After a loading dose of propofol (1 mg/kg) or midazolam (0.07 mg/kg), the infusion rates were 2.71 +/- 1.13 mg/kg/h and 0.092 +/- 0.028 mg/kg/h, respectively. An analgesic infusion of sufentanil was also given in both groups. In the midazolam group, to maintain the predetermined level of sedation, more frequent additional bolus doses (4.7 +/- 1.8; P < 0.001) and infusion rate adjustments (5.3 +/- 1.6; P < 0.001) were required than for similar sedation in the propofol group (2.3 +/- 1.0 bolus doses and 3.3 +/- 1.2 adjustments). The time from stopping sedation to patient responsiveness was 11 +/- 8 minutes in the propofol group and 72 +/- 70 minutes in the midazolam group (P < 0.001), and the time from stopping sedation to extubation was 250 +/- 135 minutes and 391 +/- 128 minutes (P < 0.014), respectively. Following the loading dose of propofol, there was a fall in blood pressure (BP) (mean from 80 +/- 11 mmHg to 67.5 +/- 10 mmHg; P < 0.05). After approximately 15 minutes, BP started to rise but remained below pretreatment level throughout sedation.(ABSTRACT TRUNCATED AT 250 WORDS)