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What is “normal” pump flow?

The concept of a “normal” pump flow during cardiopulmonary bypass (CPB) meets the same requirements of the concept of “normal” cardiac output in the physiologic circulation. In both cases, the concept of “normality” is misleading. The normal blood flow (either if generated by the heart or a pump) simply does not exist: much more correct is the notation of “adequate” blood flow, that means a flow adequate to support the metabolic needs of the various organs. As a matter of fact, metabolic needs are represented by oxygen consumption (VO₂), that under normal conditions is about 250 ml/min in a subject weighing 70 kg, awake and at rest. This value may dramatically increase during physical exercise (up to 5 L/min), or decrease if the patient is under anaesthesia. Moreover, temperature plays a very important role in determining the VO₂ of the patient during cardiac operations with CPB. Not by chance, the usual way for determining the correct pump flow is based on the body surface area (BSA) and the temperature. Usually, the pump flow is settled between 2.0 and 3.0 L . min-1 . m-2 according to the temperature during CPB. However, pump flow alone is insufficient to define “adequacy” of perfusion with respect to metabolic needs. The right physiological notation for this is the oxygen delivery (DO₂), that is the arterial oxygen content (ml/dL) times the pump flow. The arterial oxygen content depends on the Hb concentration and on its saturation; however, this last term is usually amendable, since modern oxygenators are usually able to provide a total (99.9%) Hb saturation throughout the CPB duration. Therefore, the value of Hb concentration and of its surrogate, the hematocrit (HCT) value, is very important in determining the DO₂. Since 1994, Ranucci et al. [1] demonstrated that in a series of 300 consecutive patients who had undergone myocardial revascularization with CPB, the presence of severe hemodilution was an independent risk factor for postoperative acute renal failure (ARF). In particular, the cut-off value was identified at HCT <25%. More recently, the lowest hematocrit on CPB has been recognized as an independent risk factor for postoperative low cardiac output and hospital mortality by Fang et al. [2], and for an impressive series of postoperative adverse events by Habib et al. in 2003 [3]. The relationship between hemodilution and ARF has been...


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