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Ann Thorac Surg 1996;61:276-277
© 1996 The Society of Thoracic Surgeons
Section of Cardiothoracic Surgery, Department of Surgery and Department of Anesthesiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905
To the Editor:
We read the recent article by Craver and associates [1] with great interest. Articles such as this and a prior one by the same group [2] can be highly influential, and so require unusual scrutiny. There are three issues requiring attention.
First, we would hesitate to draw such a strong conclusion on the basis of a cohort-comparison study. This is particularly true when management of the two groups may not be equivalent. It has been conjectured that elevations in blood glucose may have contributed to worsened neurologic outcome in the ``warm'' cohort group. The glucose management of the study population differed from the cohort, and blood glucose level during cardiopulmonary bypass (CPB) is not reported. Other potentially relevant physiologic data are also not provided, particularly CPB hematocrit. Normothermic CPB patients may tend toward lower hematocrits because of generally higher fluid requirements. A table providing these basic physiologic values would be helpful.
Second, the suggestion that the warm cohort did worse neurologically because of greater cerebral embolic load is speculative. It is true that cerebral blood flow is much higher with normothermic than hypothermic CPB [3]; however, blood flow to all organs is increased with normothermic hemodilution, so it is unclear if a proportionately greater embolic load is in fact delivered to the central nervous system. This has never been documented. This situation probably differs from pH-stat CO2 management, where a disproportionate increase in CBF, relative to other organ beds, does occur.
Finally, in our experience and that of Craver and associates, hypothermia generally increases CPB time. This is particularly true if rapid rewarming is to be avoided [3]. Increased bypass duration is an independent risk factor for neurologic morbidity [4], and it is often difficult to achieve adequate rewarming (as reflected by post-CPB or intensive care unit core temperature) unless the rewarming period is prolonged.
Although selective, we remain enthusiastic about warm heart surgery. In our experience, hemodynamic performance is superior with the warm technique. Although the hemodynamic advantages can presumably be achieved with CPB temperatures less than 37°C, the ``ideal'' temperature for bypass will optimize both neurologic and hemodynamic results and will be, in part, patient specific. Both temperature management strategies are useful, so we must be careful not to dismiss either technique categorically.
References
Department of Surgery (Cardiothoracic), Emory University School of Medicine, Emory Clinic Inc, 1365 Clifton Rd, NE, Atlanta, GA 30322
To the Editor:
My colleagues and I agree that the techniques used for myocardial and cerebral protection while on cardiopulmonary bypass (CPB) should be individualized. Keeping the myocardium warm and the brain cold appears to offer the optimal functional preservation for those organs. Lowering systemic temperatures on CPB is favored when a patient's potential risk for cerebral injury is thought to be higher (eg, patients greater than 70 years of age, operation for combined aortic stenosis and coronary disease, known cerebrovascular disease). In contrast, when the risk of cerebral injury is thought to be low, perfusion is employed with minimal systemic cooling to optimize myocardial functional recovery (eg, the patient less than 60 years of age, operation for acute myocardial ischemia, no history of cerebral vascular disease). In this latter situation, a patient's temperature on CPB is allowed to drift downward without using any active cooling effort. No longer are any patients maintained at strict normothermia (37°C) by actively warming the arterial blood on CPB. Strict normothermia and hyperglycemia were associated with both an increased incidence and an increased severity of cerebral injury in our initial study [1].
We remain enthusiastic about the hemodynamic benefits of warm heart surgery and apply it to the majority of our patients. In our experience ``tepid'' levels (34° to 33°C) that are usually reached when the patients are allowed to ``drift'' downward on CPB may gain the myocardial hemodynamic benefit without increasing the risk of cerebral injury. However, in patients whose clinical descriptors indicate an increased risk of stroke, we prefer to lower the systemic temperatures on CPB (30° to 28°C) for the majority of the perfusion interval. These patients are then gradually rewarmed to normothermia before separation from bypass.
Data on blood glucose and hematocrit levels were not systematically recorded or analyzed in either earlier study [1, 2]. However, the treatment to correct deviations from normal physiologic parameters was the same for all patients. Elevated blood glucose level greater than 150 mg/dL was treated with a bolus of regular insulin followed by an insulin infusion to maintain the blood glucose level at or just below 200 mg/dL. When a patient's hematocrit dropped to less than 20% on CPB, transfusion with packed red blood cells was administered.
References
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