Intermittent Aortic Crossclamping at 32 °C, a Safe Technique for Multiple Aortocoronary Bypass Grafting*

Summary

We studied myocardial preservation using biochemical, ultrastructural and hemodynamic parameters, during multiple aortocoronary bypass operations with intermittent aortic crossclamping at 32 to 35 °C. Forty-two patients were studied, an average of 3.8 grafts per patient was inserted. Mean aortic crossclamp time for one distal anastomosis was 11.1 min, mean total crossclamp time was 49 min. The hearts were empty beating during the reperfusion periods.

We used as biochemical markers of myocardial ischemia and injury: (1) arterial coronary sinus differences in lactate, inorganic phosphate (Pi) and potassium (K+), (2) tissue concentration of ATP, creatine phosphate and glycogen and (3) serial serum determinations of CPK-MB isoenzyme intra- and postoperatively. As an ultrastructural index of ischemic injury, the frequency of mitochondria I damage was determined semiquantitatively in transmural left ventricular biopsies. Before cardiopulmonary bypass (CPB) and 5, 10 and 15 min after CPB cardiac index (CO, the first positive derivative of the left ventricular pressure (LV dp/dt max), left ventricular stroke work index (LVSWI), pulmonary wedge pressure (PWP), aortic pressure (AOP) and heart rate (HR) were monitored.

Myocardial ischemia during aortic crossclamping was reflected by a short release of Pi, K+ and lactate during every reperfusion period. Tissue concentration of glycogen at the end of the operation was significantly reduced to 68% of its initial levels (P < 0.05). The concentrations of ATP and CP were slightly reduced as compared to the pre-ischemic values. This difference, however, did not reach the level of significance.

The post-ischemic biopsies revealed a slight increase in the percentage of severely damaged mitochondria (3% in the subepicardium and 5% in the subendocardium (P<0.01)). Serial determination of CPK-MB showed a small peak at the end of the operation (21±5U/I) with an immediate decay in the next 24 hours. Early post CPB-hemodynamics (5 mint showed a significant (P<0.05) decrease in LV dp/dt max and diastolic AOP and an increase in HR. The other hemodynamic variables were unchanged as compared to pre-CPB values (P>0.05). Later (15 min) LV dp/dt was normalized but LVSWI remained slightly but significantly depressed (P<0.05).

Thus, biochemical morphological and functional evidence, of important ischemic damage was not found. This correlates well with the excellent clinical outcome using this technique: the mortality rate in our whole population of patients receiving 5 or more grafts (n=180) is only 2.2%. None of the patients expired from low cardiac output.

Key Words

Coronary artery bypass surgery - Myocardial protection - Intermittent myocardial ischemia - Myocardial ultrastructure - Myocardial metabolism