Overnight increases in haematocrit: additional evidence for a nocturnal fall in plasma volume

Talan, M. I., Engel, B. T. & Kawate, R. 1992. Overnight increases in haematocrit: additional evidence for a nocturnal fall in plasma volume. Acta Physiol Scand144, 473–476. Received 2 September 1991, accepted 20 November 1991. ISSN 0001–6722. National Institute on Aging, NIH, Gerontology Research Center, Baltimore, MD 21224, USA. Our previously published studies with monkeys describe a characteristic nocturnal haemodynamic pattern consisting of a monotonic decline in cardiac output and central venous pressure, and a concomitant rise in total peripheral resistance. These findings led us to hypothesize that there is a reduction in total plasma volume during the night. Since a fall in plasma volume should cause an increase in haemoglobin and plasma protein concentration, we designed this experiment to test the hypothesis that haematocrit levels would be significantly greater in the morning than in the evening. In a study of five monkeys, the levels of haematocrit were measured at 1700 h and at 0900 h the next morning for 14 days. The average morning haematocrit levels were 6S0h higher than the average evening values. Lower plasma volume and possibly greater blood viscosity could contribute to the pathophysiology of the morning increase in the incidence of ‘silen’ ischaemia and catastrophic vascular events such as sudden cardiac death, myocardial infarctions and strokes in man.