When adequate hypothermic perfusion combined with a total hemodilution technic is use for cardiovascular surgery, several advantages over the conventional method are offered. These are reduction of flow rate, simplification of the heart-lung machine, economy in operation without need for large amount of fresh heparinized blood and elimination of the complication of blood priming, i.e., homologou blood syndrome, blood incomoatibility, serum hepatitis and sludging. There are many reports of favorable clinical results using this combined technique (Zuhdi et al., 1962; Sealy et al., 1959). However, one of the still unsettled problems in this combined approach is the flow rate which can meet the reduced oxygen demands of the organism. Another controversial subject is the ideal type of non-hemic solution for priming the machine. According to some authors, Ringer´s lactate solution caused the least over-all physiologic disturbance, including changes in blood vlonme, acid-base balance, electrolyte concentration and hemolysis (Neville et al., 1966, 1967).
In the following studies, attempts were made to define the adequate flow rate in cardiopnlmonary bypass combined with moderate hypothermia and also to observe the effect of perfusates on the acid-base equilibrium, which has been one of the primery concerns since the combined bypass technic has been adopted.
Sixty cases with congenital or acquired heart disease were subjected to the open cardiac repairs under cardio pulmonary bypass at Severance Hospital from Nov. of 1963 through Sept. of 1967. The metabolic changes including oxygen consumption, blood pH, CO₂tension and arterial oxygen saturation were observed in the last 40 cases consecutively.
Thirty patients on whom the data for this study was available were divided into four groups according to the flow rates on total bypass and the types of priming solution. Group Ⅰ, Ⅱ and Ⅲ were all primed with 5% dextrose or 5% dextrose in 0.2% saline and Group Ⅳ with Ringer´s lactate solution. The flow rates in each groups were as follows; less than 1,500 ml./min./M² in Group Ⅰ, 1,500 to 1,800 ml./min./M² in Group Ⅱ, more than 1,800 ml./min./M² in Group Ⅲ and an average of 1,699 ml./min./M² in Group Ⅳ. Blood samples were obtained before anesthesia, 10 min., 20 min., and 30 min. after onset of total bypass, in the immediate postoperative period and 3 hours after termination of surgery and once a day for 5 to 7 days following surgery.
A Zuhdi modification (1961) of the DeWall bubble oxygenator was used, with a stainless steel coil incorporated into the helix for the purpose of blood stream heat exchange. The flow rate was regulated manually during the bypass according to the amount of venous return to the oxygenator and this tended to produce higher flow rates than previously reported (Zuhdi et al., 1964).
The results are summarized as follows:
1) Group Ⅰ perfused with a flow rate of less than 1,500 ml./min./M² showed evidence of mild hypoxia during bypass in terms of decreased oxygen consumption below 50% of the control value, significant increase of (A-V) O₂ difference and metabolic acidosis developing during and subsequent to bypass for 4 days.
2) In Group Ⅱ and Ⅲ whose flow sates were above 1,500 ml./min./M² (an average flow rate of 1,664 ml./min./M² in Group Ⅱ and 2,218 ml./min./M² in Group Ⅲ respectively) the oxygen consumption rate on total bypass was more than 52% of the preoperative value in Group Ⅱ and more than 73% in Group Ⅲ. There was also no increase of (A-V)O₂ difference on the bypass and the decreased arterial pH which developed during perfusion was restored to normal range within the first postoperative day in both groups. Accordingly the flow rates employed in Group Ⅱ and Ⅲ were considered to satisfy the oxygen needs of the tissue.
3) In Group Ⅳ, primed with Ringer´s lactate solution and perfused with an average flow rate similar to Group Ⅱ, the arterial pH remained within normal limit throughout the bypass and postoperative period despite a longer perfusion time than the one of Group Ⅱ. This result was thought to be striking evidence of superiority Ringer´s lactate solution as a priming solution to combat the metabolic acidosis which tends to develop during and subsequent to low flow perfusion.
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