In Reply

We welcome Dr. Schwartz’s interest in our article.¹  However, his comments are misleading in relation to cardiac output and cerebral blood flow (CBF) during cardiopulmonary bypass.

Interpretation of data from the relatively small body of literature on the relationship among blood pressure, cardiac output/pump flow, and CBF is confounded by different experimental conditions, species, and CBF measurement methods. Furthermore, hypercarbia (in pH-stat management) and profound hypothermia can cause cerebral vasoplegia, resulting in pressure dependency. We assume that when Dr. Schwartz states that our “explanation is deficient,” he is referring to the discrepancies in the literature that likely stem from these confounders.

With alpha-stat management, CBF was shown to correlate with blood pressure, not with pump flow.²  This fact is supported by Dr. Schwartz’s own study. With pH-stat management, Soma et al.,³  whose study was cited by Dr. Schwartz as well as us, demonstrated that CBF is correlated with pump flow and not with blood pressure in humans. However, this has not been a consistent finding, perhaps reflecting the confounders mentioned. Rogers et al.,⁴  cited by Dr. Schwartz, performed a study in patients randomized to alpha-stat or pH-stat in which the primary aim of the study was to investigate the changes in CBF at two-pump flow rates applied in random order while maintaining a constant blood pressure. Their conclusion was that pump flow exerts no effect on CBF with either management strategy. They did not mention that CBF was dependent on blood pressure because it was kept constant. Hindman et al.⁵  investigated the effect of pH management (alpha- or pH-stat) on cerebral metabolic rate of oxygen during profound hypothermia (17°C) in rabbits. There was an imbalance in blood pressures in the two groups (alpha- or pH-stat), requiring them to perform a substudy to normalize the blood pressure for comparison of cerebral metabolic rate of oxygen. The finding of large changes in CBF by blood pressure was incidental and was likely confounded by the profound hypothermia.

Our assertion, “Organ perfusion is propelled by centrifugal pump,” is a general statement to describe the physiology of patients on cardiopulmonary bypass. Roller pumps are no longer used in most adult surgical centers. The “centrifugal pump” did not refer to any quoted studies in the article. In addition, evidence suggests that centrifugal pump and roller pump have little influence on CBF during cardiopulmonary bypass.⁶