Morning increase in whole blood viscosity: a consequence of a homeostatic nocturnal haemodynamic pattern

Abstract

In a series of studies, we have shown that in non-human primates there is a consistent overnight fall in cardiac output and central venous pressure, and a rise in total peripheral resistance. This haemodynamic pattern is associated with a higher haematocrit level in the morning suggesting that these changes in the circulation are homeostatic adjustments to a nighttime fall in plasma volume. The present study was designed to test the hypothesis that in the morning whole blood viscosity also is higher. Whole blood viscosity was measured at shear rates of 450, 225, 90, 45, and 22.5 s-1 in each of six monkeys, on four occasions, at 2-week intervals, at 17.00 and 09.00 h the next morning. The average haematocrit was 4.2% higher in the morning than in the previous evening (P < 0.01). Viscosity decreased monotonically at progressively higher shear rates but was always significantly higher in the morning than in the evening (P < 0.01 at all shear rates). When viscosity was adjusted by covarying for haematocrit level, the morning/evening differences became non-significant. However, the morning/evening differences in linear trend of shear stress as a function of shear rate persisted. These findings add further support to our hypothesis that the nocturnal haemodynamic pattern in non-human primates is related to a reduction in plasma volume, and they also suggest that the morning rise in haematocrit is a major contributing factor to the elevated viscosity.

Relevance of blood pressure variation in the circadian onset of cardiovascular events

“…3) [34]. Other contributory factors are increases in vascular tone [35], platelet aggregability [36,37] and blood viscosity [38], together with decreases in protective factors, including fibrinolytic and vagal activity [39].…”

Importance of Blood Pressure Control Over a 24-Hour Period

“…[6][7][8][9][10] This early morning surge is associated with other important hemodynamic and neurohormonal changes, including increases in heart rate, vascular tone and blood viscosity, and decreases in vagal activity. 6,[11][12][13] The activity of the sympathetic nervous system is quiescent during sleep, whereas awakening selectively increases epinephrine levels. 8 heart rate are controlled by direct sympathetic neural input into the heart and vasculature in response to increases in activity and upright posture, rather than by an endogenous surge of plasma catecholamines.…”

Circadian blood pressure: Clinical implications based on the pathophysiology of its variability

“…associated with other important hemodynamic and neurohormonal changes, such as increase in heart rate, vascular tone and blood viscosity, and decrease in vagal activity. 6,[11][12][13] The activity of the sympathetic nervous system appears to be downregulated during the rapid eye movement period of sleep, whereas awakening selectively stimulates the sympathoadrenal branch of the sympathetic nervous system and increases epinephrine levels. 8 However, the increases in BP and heart rate are controlled by direct sympathetic neural input into the heart and vasculature in response to changes in activity and posture, rather than by an endogenous surge of plasma catecholamines.…”