Volatile anesthetic preconditioning (APC) has been shown to attenuate myocardial ischemia reperfusion injury (1–3). However, there may be limitations to its use if the signal transduction pathways required are impaired (e.g., diabetes) (2) or the duration of global ischemia is prolonged during cardiac surgery. Therefore, there is a need to develop alternative approaches to cardioprotection for use in these clinical settings.
Oxidant stress has been implicated in myocardial ischemia-reperfusion injury (4). One study showed that systemic production of reactive oxygen species occurs during cardiopulmonary bypass (CPB) (5). This systemic oxidative stress is likely to participate in secondary myocardial damage and adversely affect postoperative myocardial functional recovery by increasing lipid peroxidation (6). Based on our previous studies in animal models (7,8) and in patients (6,9–11), we postulate that effective antioxidant intervention during ischemia-reperfusion may play a crucial role in preserving postoperative myocardial function in clinical situations such as prolonged global ischemia.
A widely used intravenous anesthetic, propofol, confers an antioxidant effect (12). Propofol reduced oxidative stress-induced lipid peroxidation and attenuated myocardial ischemia-reperfusion injury in the isolated rat heart (7,8,13–15) in a dose-dependent manner (13). Propofol, when given at a dose up to 100 μg · kg−1 · min−1 (50–100 μg · kg−1 · min−1) (16), significantly attenuated myocardial levels of malondialdehyde (MDA), a lipid peroxidation product, in humans undergoing coronary artery bypass graft (CABG) surgery using CPB (16). However, propofol, when administered at 100 μg · kg−1 · min−1 before and then decreased to 50 μg · kg−1 · min−1 during CPB, did not significantly enhance the red blood cell antioxidant capacity in patients as compared with the volatile anesthetic isoflurane (9). Hence, despite its demonstrated antioxidant properties, propofol, when given at a small, clinically relevant doses, has not been shown to have advantages over volatile anesthetics in protecting against postischemic myocardial injury in patients (17,18).
We have observed that propofol can enhance myocardial tissue antioxidant capacity and attenuate postischemic myocardial dysfunction when administered at a clinically achievable large concentration in an experimental model of global myocardial ischemia-reperfusion (8). We hypothesized that the administration of large-dose propofol during CPB may suppress systemic oxidative stress and attenuate myocardial reperfusion injury in coronary surgery patients. The effects of propofol were compared with that of isoflurane, which has been shown to be cardioprotective in CABG patients using CPB through the mechanism of APC (19).
After institutional ethics review board approval and written informed patient consent, 54 ASA class II to III patients, aged 55–73 yr, presenting for scheduled CABG surgery were assigned (according to randomization envelopes) to the small-dose propofol (Group P; n = 18), large-dose propofol (Group HiP; n = 18), or isoflurane (Group I; n = 18) groups. The randomization scheme provided an equal number of patients from each study group for two surgical teams.
Patient exclusion criteria included: 1) a preoperative history of liver or kidney dysfunction; 2) recent intake of vitamin C or E; 3) age younger than 18 or more than 80 yr; and 4) a history of an acute or evolving myocardial infarction.
Systemic arterial blood pressure was measured via radial artery catheterization. A Swan-Ganz catheter (Edwards Life Sciences, Irvine, CA) was inserted for central venous pressure, pulmonary artery pressures, and cardiac output determinations. Hemodynamics were continuously monitored for 48 h after CPB.
Anesthesia was induced with midazolam (0.1 mg/kg) and fentanyl (15 μg/kg) in all groups, and the muscle relaxation for tracheal intubation was facilitated with pancuronium (0.1 mg/kg). Each group received a continuous infusion of fentanyl at 0.2–0.3 μg · kg−1 · min−1 during surgery.
Anesthesia was maintained with isoflurane at an inhaled concentration of 1%–1.5% (except for 2 patients, in whom isoflurane 3.5% was required temporarily at sternotomy) in Group I. Group P received propofol 60 μg · kg−1 · min−1 throughout surgery. In Group HiP, this dose of propofol was increased to 120 μg · kg−1 · min−1 for 10 min before the onset of CPB until 15 min after aortic declamping and then decreased to 60 μg · kg−1 · min−1 until the end of surgery.
Surgeons working in the operation room and intensive care unit (ICU) were blinded to treatment protocols, facilitated by covering the drug infusion pump and lines and shielding the isoflurane vaporizer from view.
Aprotinin (1 × 106 KIU initial loading dose and 2 × 106 KIU added to CPB prime solution) was administered to all patients except those with a preoperative history of drug allergy (1 of 18 in Group I; 2 of 18 in Group P; and 1 of 18 in Group HiP). Porcine heparin was administered at a dose of 300 IU/kg and supplemented when required to maintain activated coagulation time more than 450 s during CPB. Heparin was neutralized with 1 mg of protamine/100 IU of heparin administered after separation from CPB.
Surgery was conducted under conditions of normothermic or tepid CPB (patient esophageal temperature of 34°C–37°C) using a nonpulsatile flow rate of 2 L · min−1 · m−2 and a membrane oxygenator. Mean arterial blood pressure was maintained between 55 and 75 mm Hg during CPB. All patients were treated with intermittent antegrade infusion of warm high-potassium blood cardioplegia during continuous aortic cross-clamping (ACC).
Central venous blood was sampled before CPB (Pre-CPB) and at 8 (post-CPB 8 h), 24 (post-CPB 24 h), and 48 (post-CPB 48 h) h after CPB for the measurement of cardiac troponin (cTn) I, cTnT, cardiac-specific creatine kinase (CK-MB) (enzyme immunoassay), and plasma levels of MDA, a marker of lipid peroxidation. Postoperative inotropic support was defined as the use of dopamine (≥5 μg · kg−1 · min−1) with or without concomitant use of epinephrine (0.01–0.02 μg · kg−1 · min−1) for a duration of 30 min or longer during the first 12 h after surgery. Inotropic therapy was standardized to a treatment algorithm instituted to treat a mean radial arterial blood pressure <60 mm Hg, despite optimization of preload, afterload, and heart rate. Nitroglycerin (0.3–0.5 μg · kg−1 · min−1) was administered IV to all patients during CPB and ICU stay. The patients were tracheally extubated when they were able to sustain adequate spontaneous respiration and required minimal oxygen support, as reflected by normal arterial blood gas levels. The patients were then discharged from ICU when they were hemodynamically stable with blood gas variables within normal range without the need of inotropic or oxygen support.
Plasma levels of MDA were measured by chemical analysis (commercial kits; Nanjing Jiangzheng Biological Engine Institute, China), as previously described (10). The lower detection limit for plasma MDA was 0.11 μM/L. The serum levels of cTnI, cTnT, and CK-MB were measured using commercial enzyme-linked immunosorbent assay kits (Beckman, Fullerton, CA) with an automated analyzer (Boehringer, Mannheim, Germany). The lower detection limits were 1.0 U/L, 0.012 μg/L, and 0.007 μg/L, respectively, for plasma CK-MB, cTnT, and cTnI.
Plasma samples were coded, and the laboratory investigator was blinded regarding treatment regimen. Similarly, all hemodynamic data were collected by trained observers who were not authors of this study and who were blinded to the anesthetic regimen used.
Group sample size was calculated based on differences in cTnI concentration measured at 8 h after CPB in a pilot study of patients who received large-dose propofol (1.4 ± 0.36 ng/mL) and small-dose propofol (1.79 ± 0.4 ng/mL) anesthesia. The formula: n = 15.7/ES2 + 1, where ES = effect size = (difference between groups)/(mean of the standard deviation between groups), with α = 0.05 and power = 0.8 used to determine that the study would be adequately powered with n = 16 per group (20).
All continuous data were expressed as mean ± sd. Statistical evaluation of patients' files and perioperative data was performed by unpaired Student's t-test or χ2 test when appropriate. Between-groups and within-group differences of bio-assay data were analyzed using two-way analysis of variance with repeated measures and Bonferroni corrections (GraphPad Prism) when appropriate. Values were considered to be statistically significant when P was <0.05.
Patient characteristics, preoperative hemodynamic data, preoperative medication, dosage of fentanyl used, duration of CPB, and ACC were similar among groups (Table 1). Support of mean arterial blood pressure with phenylephrine (10–30 μg; IV bolus) was required in 2 patients in Group HiP, 1 in Group P, and none in Group I during CPB.
Mean arterial blood pressure and heart rate did not differ among groups throughout the observation interval (Table 2). There were no significant intra- or intergroup differences in central venous or pulmonary artery pressure over time (data not shown).
Pre-CPB cardiac index (CI) was similar in the study groups, averaging 2 L · min−1 · m−2. CI significantly increased at post-CPB 8 h from pre-CPB values in both Group HiP and Group P (P < 0.01 or P < 0.05). At post-CPB 24 h, values of CI in Group HiP were significantly higher than that in both Group I and Group P. Improvement in CI did not occur in Group I until 48 h after CPB.
Systemic vascular resistance in the propofol anesthesia groups, but not in Group I, significantly decreased at post-CPB 8 h (P < 0.05; Table 2). Systemic vascular resistance in Group HiP was significantly lower than that in Group I at post-CPB 48 h. Pulmonary vascular resistance decreased significantly in all groups after CPB, but there were no significant intergroup differences (Table 2).
Postoperative inotrope requirements are presented in Table 1. Nine patients in Group I, five in Group P, and two in Group HiP required inotrope support for separation from CPB (P < 0.05 Group HiP versus Group I). Four patients in Group I, four in Group P, and two in Group HiP required transient (<30 min duration) inotrope (dopamine) support before the ICU transfer. Two patients in Group I required inotrope support for longer than 30 min after surgery. The length of ICU stay and the time to tracheal extubation in Group HiP were significantly shorter than that in Group I and Group P (P < 0.05). Hospital length of stay was similar for all groups.
Baseline plasma levels of MDA did not differ among groups (Fig. 1). Plasma MDA increased significantly after CPB in all groups. Application of large-dose propofol during CPB significantly attenuated the increase in plasma MDA as compared with Group I or Group P (P < 0.05; Group HiP versus Groups I or P). Plasma levels of MDA returned to baseline levels only in Group HiP, 48 h after CPB.
Baseline plasma levels of cTnI, cTnT, and CK-MB did not differ among groups (Fig. 2). cTnI and cTnT increased significantly after CPB and remained increased up to 48 h after CPB in all groups (Fig. 2). Plasma cTnI exceeded a value of 2 ng/mL in 10 of 18 (Group I), 4 of 18 (Group P), and 1 of 18 (Group HiP) patients at post-CPB 8 h and in 5 of 18 (Group I), 1 of 18 (Group P), and 0 of 18 (Group HiP) patients at post-CPB 24 h (Fig. 2A). Administration of large-dose propofol during CPB significantly attenuated the increase of cTnI and cTnT, as compared with Group I (P < 0.05; Group HiP versus Group I). Plasma levels of cTnI and cTnT in Group P were significantly lower than that in Group I but higher than that in Group HiP 24 h after CPB. The changes of plasma levels of CK-MB basically mirrored the changes of cTnI and cTnT, except that no statistical difference between Group I and Group P was identified. No group had evidence of postoperative myocardial infarction by electrocardiogram criteria (21).
The principal finding of this clinical study is that the application of propofol in a small-large-small dosing regimen during ischemia and early reperfusion attenuated indices of oxidant stress, myocardial injury. It was also inotrope sparing compared with a continuous small dose of propofol or isoflurane in elective CABG involving a low-risk patient population. The length of patient ICU stay was shorter in patients receiving large-dose propofol treatment. Recovery in postoperative cardiac function was delayed in patients receiving isoflurane in this study.
Our current findings are at odds with the results of clinical preconditioning studies comparing volatile anesthetics to total intravenous anesthesia with propofol (17,22). These results can be explained at least in part by differing approaches to the timing and dose of propofol being used. An experimental study has shown that the protective effect of propofol on myocardial function after global myocardial ischemia and reperfusion is dose dependent. Propofol is effective at concentrations of 30 μM (approximately 5 μg/mL) or more and not effective at concentrations of 10 μM (approximately 2 μg/mL) or less (14). This is probably because propofol at relatively small concentrations does not counteract oxygen-free radicals production that is greatly increased during postischemic reperfusion or CPB (22).
Plasma propofol concentrations were not measured in this study. However, we have confirmed, in another study, that propofol 120 μg · kg−1 · min−1 can achieve an average plasma total propofol concentration of 4.2 μg/mL at 15 minutes of reperfusion, doubling the concentration achieved with propofol 50 μg · kg−1 · min−1 (2.1 μg/mL) and about 40% higher than that achieved with propofol 100 μg · kg−1 · min−1 (2.9 μg/mL) (Ansley et al., unpublished data). This concentration would be consistent with levels we have used to confer cardioprotection in animal models of myocardial ischemia-reperfusion injury (13). By contrast, current clinical preconditioning studies report the use of target-controlled infusions to provide for an intraoperative concentration of only 2–4 μg/mL, which we would not consider to be cardioprotective.
The therapeutic timeframe for APC against postischemic contractile dysfunction and infarction has been reported to be approximately 25–40 minutes (1). The protection is maximal when ischemic duration is between 30 and 35 minutes (1). It is therefore reasonable to assume that APC may be a useful therapy if the typical duration of ischemia during CABG is within this range. By contrast, APC is unlikely to be beneficial to patients who undergo more prolonged ischemia (23). Indirectly, this finding is supported clinically in reports of potential beneficial APC where the duration of ACC time was approximately 30 minutes (22). In our study, the average duration of ACC time exceeded 80 minutes in all groups (Table 1), which is well beyond the range when APC would have been effective (1).
Ischemic preconditioning and APC rely on the generation of small amounts of reactive oxygen-free radicals to mediate their cardioprotective effect (24–26). Theoretically, the application of small doses of propofol before and during cardiac surgery, as current clinical studies describe (22), may have blocked any potential preconditioning effects on this basis. This approach, if not followed by adequate antioxidant intervention during CPB and early reperfusion, may prove to be detrimental to ischemic-reperfused hearts.
It is of interest that the value of CI in the propofol groups, but not in Group I, is significantly increased at post-CPB 8 h (Table 2) as compared to pre-CPB values. This is coincident with a reduction in systemic vascular resistance seen in the propofol groups at post-CPB 8 h. The mechanism(s) whereby propofol reduced post-CPB systemic vascular resistance is unclear and might involve multiple factors. It has been shown that levels of systemic cytokines, such as tumor necrosis factor-α, increased after CPB (27). We have recently shown that propofol can attenuate tumor necrosis factor α–induced endothelial cell apoptotic cell death and enhance the bioavailability of nitric oxide (28), a potent vasodilator. We speculate that propofol may preserve post-CPB vascular endothelial cell viability and function, in part, by attenuating tumor necrosis factor α–induced vascular endothelial cell apoptosis. Coronary endothelial cell apoptosis precedes myocyte apoptosis (29). Hence, attenuation of vascular endothelial cell apoptosis by propofol could have resulted in reduced cardiomyocyte apoptosis during myocardial ischemia and reperfusion. This would be associated with a decrease in cardiac enzymatic release and preservation of function but requires further study.
Myocardial necrosis can be recognized by the appearance in the blood of cardiac specific proteins released into the circulation because of cardiomyocyte damage. The biochemical markers cTnI, cTnT, and CK-MB have high, to nearly absolute, myocardial tissue specificity and a high sensitivity. The significant reduction in plasma cTnI, cTnT, and CK-MB seen in Group HiP augurs a better patient prognosis. Our findings are consistent with the results of Lim et al. (30) who demonstrated that propofol is cardioprotective in a clinically relevant model of normothermic cardioplegic arrest and CPB.
Although there were significant intergroup differences in plasma cTn levels, clinical outcome and hospital stay did not differ among groups. Further larger studies are merited to determine the long-term clinical relevance of the changes in biochemical markers observed in the related studies. Nevertheless, the benefits of earlier tracheal extubation and shorter ICU stay seen in the large-dose propofol group are obvious and include cost savings, patient comfort, early patient mobilization, and improvement of cardiac function. In contrast to other studies, our findings suggest that propofol in a large dose is not inferior to APC.
In conclusion, we have shown that propofol's cardioprotection, as determined by the surrogate measures of myocardial injury and function in this study, is dose dependent. The application of propofol in a small-large-small dosing regimen during ischemia and early reperfusion attenuated post-CPB myocardial injury compared with a continuous small dose of propofol or isoflurane and may offer a novel potential therapeutic approach to cardioprotection. Large-dose propofol offers the possibility of an alternative to APC for use in high-risk patient populations where the potential beneficial effects of APC may be limited.
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