Crystalloid hemodilution, hypothermia, and halothane blood solubility during cardiopulmonary bypass

Blood Concentrations of Enflurane Before, During, and After Hypothermic Cardiopulmonary Bypass

OBJECTIVE

The purpose of this study was to determine blood concentrations of enflurane delivered via a membrane oxygenator during hypothermic cardiopulmonary bypass (CPB) with changes in the input enflurane concentration and temperature and to characterize the pharmacokinetics of enflurane washout during and after CPB.

DESIGN

Blood enflurane concentrations were measured by gas chromatography before, during, and after CPB by using mean delivered enflurane concentrations of 0.5% v/v (group 1, n = 5), 0.8% (group 2, n = 7), and 1% (group 3, n = 14).

SETTING

The investigation was performed in a teaching hospital setting.

PARTICIPANTS

Twenty-six patients undergoing cardiac surgery requiring hypothermic CPB.

INTERVENTIONS

Variations in input enflurane concentration in different patients plus blood sampling from the arterial side of the circuit for enflurane assay.

MEASUREMENTS AND MAIN RESULTS

Median (25th and 75th percentiles) pre-CPB blood enflurane concentrations were 48 (25-50) mg/L, 52 (47-56) mg/L, and 115 (90-143) mg/L in groups 1 (0.5% v/v), 2 (0.8% v/v), and 3 (1% v/v), respectively. During hypothermia (28 degrees C) corresponding enflurane concentrations were 44 (31-53) mg/L, 56 (45-62) mg/L, and 145 (109-203) mg/L, respectively. For groups 1 and 2, there were no significant changes in blood enflurane compared with the corresponding pre-CPB value. However, for group 3, cooling resulted in a significant increase (p = 0.006) in blood enflurane. In all groups, enflurane concentrations after rewarming were similar to those in the pre-CPB period.

CONCLUSIONS

It is concluded that exposure to enflurane concentrations greater than 0.8% during CPB can result in high blood concentrations.

Predictive performance of a physiological model for enflurane closed-circuit anaesthesia: effects of continuous cardiac output measurements and age-related solubility data

BACKGROUND

The disposition of inhalation anaesthetics is governed by the factors described in the Fick principle.

METHODS

We have recalibrated a previously validated physiological model for enflurane closed-circuit inhalation anaesthesia, using individual continuous cardiac output measurements as well as age-related enflurane solubility coefficients as inputs to the model. Two model versions using 'calculated' (Brody's formula) or 'measured' (thoracic electrical bioimpedance) cardiac output values, and two versions with 'standard' (fixed) or 'age-related' solubility coefficients were formulated.

RESULTS

Data from 62 ophthalmic surgical patients were used to validate the predictive performance of the four model versions. The root mean squared errors (total error) and scatters (error variation) were similar with the extended model versions, but the group biases (systematic error component) were significantly less with the model versions that included age-related solubility compared with the versions using standard solubility coefficients (bias -0.76/-0.78% vs -3.44/-3.60%).

CONCLUSION

The inclusion of age-related solubility coefficients but not of continuous cardiac output measurements improves the predictive performance of the physiological model for closed-circuit inhalation anaesthetic conditions in routine clinical practice.

Dynamic changes in blood solubility of desflurane, isoflurane, and halothane during cardiac surgery

OBJECTIVE

To determine an estimate of blood/gas partition coefficients of volatile anesthetics during cardiac surgery.

DESIGN

Descriptive

SETTING

University hospital

PARTICIPANTS

Six adult patients undergoing valvular replacement with hypothermic cardiopulmonary bypass.

MEASUREMENTS AND MAIN RESULTS

Blood samples were obtained from patients at 6 time points: before induction, at skin incision, at aortic cannulation, at rewarming during bypass, at weaning off bypass, and at skin suture. Measured blood/gas partition coefficients were plotted against corresponding solubilities estimated according to the combined effects of hypothermia and hemodilution. Significant differences were found in blood/gas partition coefficients of the 3 anesthetics at different times during surgery (p < 0.05). Blood/gas partition coefficients at weaning off bypass were the lowest, about 75% of that before anesthetic induction. A direct linear relationship for estimated solubility against measured solubility was found (r2 = 0.94; p < 0.05).

CONCLUSION

Dynamic changes in blood/gas partition coefficients of volatile anesthetics were found during cardiac surgery. They could be estimated by using multiple linear regression equations reflecting the combined effects of hypothermia and hemodilution.

Prediction of volatile anaesthetic solubility in blood and priming fluids for extracorporeal circulation

Volatile anaesthetics are often used during cardiopulmonary bypass (CPB). To understand the kinetics of inhaled anaesthetics during CPB, anaesthetists should understand changes in blood solubility caused by fluid use. We set out to predict the solubility of three volatile anaesthetics, desflurane, isoflurane and halothane, during CPB by determining: (i) their solubility in fresh whole blood and eight CPB priming fluids at 37 degrees C; (ii) the effect of temperature on the solubility of these anaesthetics in lactated Ringer's, gelofusin, banked blood and plasma; (iii) their solubility in different mixtures of these four priming fluids at different temperatures; and (iv) their estimated and actual solubility in blood during hypothermic CPB. We calculated solubility using a concept of volume fraction partition coefficient and compared estimated and measured solubilities. For the three anaesthetics tested, solubilities are in the order: fresh whole blood approximately = plasma > banked blood > normal saline approximately = lactated Ringer's approximately = gelofusin approximately = Haemaccel approximately = hydroxyethyl starch > mannitol. The solubilities of the anaesthetics in all priming fluids increased logarithmically at lower temperatures (P<0.05). The volume-fraction estimates of the partition coefficients were within approximately +/-20% of the measured values for all values of solubility. The corresponding estimates of solubility for CPB blood samples were between -36% and +24% of the measured values. During normothermic CPB, blood solubility of volatile anaesthetics would be unchanged when using plasma, slightly reduced when using banked blood and markedly reduced when using crystalloids and colloids.

Blood/gas solubility coefficient and blood concentration of enflurane during normothermic and hypothermic cardiopulmonary bypass

The blood/gas solubility coefficient and blood concentration of enflurane were measured at intervals in 10 patients undergoing coronary artery revascularization with cardiopulmonary bypass (CPB) and moderate hypothermia. A constant end-tidal concentration of enflurane was maintained throughout the study. Blood/gas solubility coefficient was determined at 37 degrees C, which when combined with an initial single-step equilibration of the blood sample with air, permitted the accurate measurement of blood concentration. Blood/gas solubility coefficient and blood concentration both decreased significantly with the onset of CPB. During the period of hypothermia, blood/gas solubility as measured at 37 degrees C showed little change; however, there was a progressive, marked increase in blood concentration with a mean increase of 80% prior to rewarming. Therefore, the level of anesthesia provided by enflurane may lighten with the onset of CPB, and a deeper level will accompany any decrease in blood temperature. On rewarming, blood concentration levels rapidly returned to levels similar to those measured before cooling. The increased uptake and accumulation of volatile anesthetic agent that occurred as a result of the period of hypothermic CPB was rapidly cleared. The rapidity with which blood concentration responded to the changes occurring during CPB make it unlikely that there was any significant increase in myocardial depression in response to the raised blood concentration secondary to the hypothermia.

[Pharmacokinetics of propofol injected after deliberate preoperative hemodilution]

The effects of acute isovolaemic haemodilution (AIH) on propofol pharmacokinetics were studied in 16 male patients scheduled for prostatectomy. They were all ranked ASA 1, and were randomly allocated to two groups, group I (n = 8), who did not undergo any haemodilution, and group II (n = 8), in whom AIH was carried out. Anaesthesia was induced with a single 2.5 mg.kg-1 propofol bolus given in 30 s; maintenance was achieved with fentanyl 2 micrograms.kg-1, atracurium 1 mg.kg-1, and a ventilation with a mixture of nitrous oxide in oxygen 50 %, with enflurane 1 %. Those patients due to be haemodiluted had blood withdrawn before surgery (1,387.5 +/- 423.3 ml), at the same time as they were given the same volume of modified fluid gelatin (Plasmion). The volume of blood to be withdrawn was calculated according to the initial haematocrit, and that required. Haematocrit was decreased to 32.3 +/- 3.9 % (extremes 27 and 37 %). Thereafter blood samples were then collected over a 24 h period, which included surgery. Propofol was assayed in whole blood using high performance liquid chromatography. Analysis with a three-compartment model was carried out. The AIH only altered the central compartment volume (65.5 +/- 15.6 l in the control group vs 83.6 +/- 13.3 l in group II, p less than 0.01). Initial concentrations were not significantly different in the two groups (2,892 +/- 762 ng.ml-1 in controls vs 2,373 +/- 589 ng.ml-1 in the others). Clinically, anaesthesia and recovery were uneventful. It is concluded that the induction dose of propofol in patients scheduled for haemodilution does not require any alteration.

Aneurysms of the basilar artery treated with circulatory arrest, hypothermia, and barbiturate cerebral protection

Complete circulatory arrest, deep hypothermia, and barbiturate cerebral protection are efficacious adjuncts in the surgical treatment of selected giant intracranial aneurysms. These techniques were utilized in seven patients, one with a large and six with giant basilar artery aneurysms; four had excellent results, one had a good result, one had a fair outcome, and one died. The rationale for the use of complete cardiac arrest with extracorporeal circulation, hypothermia, and barbiturate cerebral protection is outlined. The surgical and anesthetic considerations are reviewed. The perioperative morbidity and long-term results support the use of these techniques in selected patients with complex intracranial vascular lesions.

Isoflurane elimination via a bubble oxygenator during extracorporeal circulation

It has been suggested that inhalational anesthetics should be discontinued at least 15 minutes prior to termination of extracorporeal circulation (ECC) to avoid myocardial depression. However, data regarding elimination of inhalation agents via a bubble oxygenator from hypothermic, hemodiluted patients have not been previously reported. The washout of isoflurane (ISF) from ten cardiac surgical patients using mass spectrometry was studied. The mean baseline oxygenator exhaust concentration of ISF was 0.85% prior to termination of ECC. Oxygenator concentration of ISF decreased to less than 0.05% in 8.8 +/- 2.5 minutes. Eight of ten patients had ISF washout curves best characterized by a one-compartment model, with a mean time constant of 1.94 minutes. Therefore, 95% washout of ISF should occur in 5.8 minutes (three time constants). It is suggested that ISF may be used closer to the termination of ECC than previously recommended without fear of significant myocardial depression.

Halothane solubility in blood during cardiopulmonary bypass: the effect of haemodilution and hypothermia

The influence of haemodilution and cooling on the solubility of halothane in blood during cardiopulmonary bypass was studied in six normothermic and six hypothermic patients undergoing open-heart operations. The solubility of halothane in blood was 2.17 at 30 degrees C and 1.34 at 37 degrees C. There was also an increase in the blood/gas partition coefficient of about 8.7 per cent for each degree celsius of temperature fall. These changes in haemodiluted blood explain the different anaesthetic requirements observed in patients undergoing cardiopulmonary bypass, either at normal temperature or at moderate hypothermia.