Ann Thorac Surg 2004;77:1663
© 2004 The Society of Thoracic Surgeons
Invited commentary
John W. Hammon, MD
Department of Cardiothoracic Surgery, Wake Forest University School of Medicine Medical Center Boulevard Winston-Salem, NC, 27157, USA
e-mail: jhammon{at}wfubmc.edu
The concept of higher hematocrit maintained during cardiopulmonary bypass and circulatory arrest is becoming generally accepted, due in part to the innovative basic and clinical research from the group at Boston Children's Hospital, some of which is presented here.
The authors of this manuscript easily convinced me that animals in the low hematocrit group have vasodilatation induced increased cerebral blood flow and pressure with evidence of decreased venous pH, which should signal the onset of cerebral anaerobic metabolism. Conversely, they give data that indicate an increased cerebral metabolic rate for oxygen-a counterintuitive result.
They explain this unexpected finding as, "Perhaps more oxygen is needed to counteract acidosis due to the lower pH buffering capacity of the dilute blood." It is my opinion that the experimental preparation is probably inadequate to obtain an accurate cerebral venous sample. In the pig, the brain is relatively small and has a blood supply of lesser magnitude than the large neck and skull-based muscles when compared to the human. Since the sagittal sinus was not sampled, the jugular venous cannula must be inserted well into the temporal bone to avoid contamination from the multiple systemic venous connections, which enter the jugular vein just below the exit from the skull. The authors do not explain how they verified this position of the catheter, thus leading me to believe that jugular vein cannula position must be at least one factor in explaining this odd result. While it is possible that CMRO2 is elevated in this experimental situation, it is more likely that despite higher flow, O2 delivery is reduced, resulting in cellular acidosis. This assertion has been confirmed by a number of previous studies, albeit using different methodology, as the authors mention in their discussion.
Nevertheless, the main point of the manuscript is that higher hematocrits than have been traditionally accepted are generally safer, particularly in pediatric cardiac surgery. Their point is well taken and adds to the growing body of literature describing "best practices" in cardiopulmonary bypass.
