Factors influencing MAC reduction after cardiopulmonary bypass in dogs

Feasibility of Anesthesia Maintenance With Sevoflurane During Cardiopulmonary Bypass: A Pilot Pharmacokinetics Study

OBJECTIVE

Adequate maintenance of hypnosis during anesthesia throughout surgery using sevoflurane alone was investigated. In addition, sevoflurane pharmacokinetics during cardiopulmonary bypass were analyzed.

DESIGN

This was a pilot pharmacokinetic study.

SETTING

Tertiary care university hospital.

PARTICIPANTS

The study comprised 10 patients aged between 18 and 75 years who underwent elective mitral valve surgery.

INTERVENTIONS

The end-tidal and sevoflurane plasma concentrations were measured throughout cardiac surgery procedures involving cardiopulmonary bypass. The sevoflurane plasma concentration was measured using gas chromatography. In addition, the ratio between sevoflurane alveolar concentration and inspired concentration over time (F_A/F_I) was analyzed to describe wash-in and wash-out curves.

MEASUREMENTS AND MAIN RESULTS

Hypnosis was maintained adequately throughout surgery using sevoflurane alone. The bispectral index was maintained between 40 and 60 during cardiopulmonary bypass. The end-tidal sevoflurane was significantly different before and during cardiopulmonary bypass (1.86%±0.54% v 1.30%±0.58%, respectively; p<0.001). However, the sevoflurane plasma concentration was not significantly different before and after cardiopulmonary bypass start-up (40.55 µg/mL [76.62-125.33] before cardiopulmonary bypass and 36.24 µg/mL [56.49-81-42] during cardiopulmonary bypass). This mismatch possibly can be explained by changes that occured after cardiopulmonary bypass start-up, such as reductions of body temperature (36.33°C±0.46°C v 32.98°C±2.38°C, respectively; p<0.001) and hematocrit (35.62%±3.98% v 25.5%±3.08%, respectively; p<0.001). The sevoflurane alveolar concentration varied according to sevoflurane plasma concentration and bispectral index values. No adverse events regarding sevoflurane administration during cardiopulmonary bypass were observed.

CONCLUSIONS

Sevoflurane end-tidal values were reliable indicators of adequate anesthesia during all cardiac surgery procedures involving cardiopulmonary bypass.

Effects of cardiopulmonary bypass on propofol pharmacokinetics and bispectral index during coronary surgery

PURPOSE

Cardiopulmonary bypass is known to alter propofol pharmacokinetics in patients undergoing cardiac surgery. However, few studies have evaluated the impact of these alterations on postoperative pharmacodynamics. This study was designed to test the hypothesis that changes in propofol pharmacokinetics increase hypnotic effects after cardiopulmonary bypass.

METHODS

Twenty patients scheduled for on-pump coronary artery bypass graft (group, n=10) or off-pump coronary artery bypass graft (group, n=10) coronary artery bypass grafts were anesthetized with sufentanil and a propofol target controlled infusion (2.0 microg/mL). Depth of hypnosis was monitored using the bispectral index. Blood samples were collected from the induction of anesthesia up to 12 hours after the end of propofol infusion, at predetermined intervals. Plasma propofol concentrations were measured using high-performance liquid chromatography, followed by a non-compartmental propofol pharmacokinetic analysis. Data were analyzed using ANOVA, considering p<0.05 as significant.

RESULTS

After cardiopulmonary bypass, despite similar plasma propofol concentrations in both groups, bispectral index values were lower in the on-pump coronary artery bypass graft group. Time to extubation after the end of propofol infusion was greater in the on-pump coronary artery bypass graft group (334 +/- 117 vs. 216 +/- 85 min, p = 0.04). Patients undergoing cardiopulmonary bypass had shorter biological (1.82 +/- 0.5 vs. 3.67 +/- 1.15 h, p < 0.01) and terminal elimination (6.27 +/- 1.29 vs. 10.5h +/- 2.18, p < 0.01) half-life values, as well as higher total plasma clearance (28.36 +/- 11.40 vs.18.29 +/- 7.67 mL/kg/min, p = 0.03), compared to patients in the off-pump coronary artery bypass graft group.

CONCLUSION

Aside from the increased sensitivity of the brain to anesthetics after cardiopulmonary bypass, changes in propofol pharmacokinetics may contribute to its central nervous system effects.

Less isoflurane is required after than before cardiopulmonary bypass to maintain a constant bispectral index value

OBJECTIVE

To test whether patients require less volatile anesthetic after cardiopulmonary bypass (CPB).

DESIGN

Prospective, observational clinical study.

SETTING

Cardiovascular operating rooms of a large teaching hospital.

PARTICIPANTS

Twenty adult patients undergoing surgery with CPB.

INTERVENTIONS

Subjects received a computer-controlled fentanyl infusion designed to maintain effect site concentrations of 3 ng/mL, combined with a variable amount of isoflurane.

MEASUREMENTS AND MAIN RESULTS

The end-tidal isoflurane concentration associated with a target bispectral index of 55 was recorded during skin preparation, after sternotomy, during rewarming, and after separation from CPB. Adjusted, geometric mean (95% confidence intervals), end-tidal isoflurane concentrations associated with a bispectral index of 55 were 0.46% (0.38% to 0.58%) during skin preparation, 0.47% (0.39% to 0.58%) after sternotomy, 0.35% (0.29% to 0.42%) during rewarming, and 0.36% (0.31% to 0.43%) after separation from CPB. The last 2 concentrations (recorded near the end and after CPB) were significantly (p < 0.05) less than the first 2 concentrations (recorded before CPB).

CONCLUSION

Because the level of surgical stimulation was relatively constant and minimal at the times of the measurements, these results are consistent with a reduced need for isoflurane after compared with before CPB.

Duration of nonresponse to noxious stimulation after intramuscular administration of butorphanol, medetomidine, or a butorphanol-medetomidine combination during isoflurane administration in dogs

OBJECTIVE

To assess duration of actions of butorphanol, medetomidine, and a butorphanol-medetomidine combination in dogs given subanesthetic doses of isoflurane (ISO).

ANIMALS

6 healthy dogs.

PROCEDURE

Minimum alveolar concentration (MAC) values for ISO were determined. for each dog. Subsequently, 4 treatments were administered to each dog (saline [0.9% NaCl] solution, butorphanol [0.2 mg/kg of body weight], medetomidine [5.0 microg/kg], and a combination of butorphanol [0.2 mg/kg] and medetomidine [5.0 microg/kg]). All treatments were administered IM to dogs concurrent with isoflurane; treatment order was determined, using a randomized crossover design. Treatments were given at 7-day intervals. After mask induction with ISO and instrumentation with a rectal temperature probe, end-tidal CO2 and anesthetic gas concentrations were analyzed. End-tidal ISO concentration was reduced to 90% MAC for each dog. A tail clamp was applied 15 minutes later. After a positive response, 1 of the treatments was administered. Response to application of the tail clamp was assessed at 15-minute intervals until a positive response again was detected.

RESULTS

Duration of nonresponse after administration of saline solution, butorphanol, medetomidine, and butorphanol-medetomidine (mean +/- SD) was 0.0+/-0.0, 1.5+/-1.5, 2.63+/-0.49, and 5.58+/-2.28 hours, respectively. Medetomidine effects were evident significantly longer than those for saline solution, whereas effects for butorphanol-medetomidine were evident significantly longer than for each agent administered alone.

CONCLUSION AND CLINICAL RELEVANCE

During ISO-induced anesthesia, administration of medetomidine, but not butorphanol, provides longer and more consistent analgesia than does saline solution, and the combination of butorphanol-medetomidine appears superior to the use of medetomidine or butorphanol alone.