Physiologic strategies to prevent fainting responses during or after whole blood donation.

Vasovagal syncope (VVS) is a consistent, but infrequent (0.1%-0.3%) complication of volunteer, whole blood donation. Given the large number of blood donations, a significant number of donors is involved. Syncope occasionally leads to injury. Recent rigorous data collection and analysis have led to the association of a small number of donor and donation factors with the risk of syncope. An analysis of the time course of syncope reactions among approximately 500,000 whole blood donors suggests that there are three distinct periods of risk for vasovagal reactions before, during, and after phlebotomy. This review examines the physiologic mechanisms that contribute to these periods of increased risk including the direct effects of removal of approximately 500 mL of whole blood, the psychological stress of instrumentation and giving blood (i.e., fear of needles, pain, and the sight of blood), and the orthostatic effects superimposed on a hypovolemic state after the donation. Specifically, we describe interventions that have been useful in controlling VVS in patients with fainting syndromes and we examine the potential of these interventions in the blood donation context, based on the physiologic principles involved. Finally, we propose an intervention (dietary replacement of salt lost with blood donation) that has not been applied in transfusion medicine previously but which has the potential to reduce risk.