Diurnal variations in central venous pressure

We analysed the haemodynamic patterns of seven monkeys (Macaca mulatta) during a one-month period during which we monitored beat-to-beat levels of heart rate, stroke volume, intra-arterial pressure and central venous pressure 7 days/week, 18 h/day. We replicated an earlier finding that cardiac output falls and total peripheral resistance rises throughout the night: the fall in cardiac output was mediated by a fall in heart rate since stroke volume did not change; and the rise in peripheral resistance reflected a homeostatic adjustment to the fall in cardiac output since blood pressure was stable throughout the night although it fell early in the evening. In addition, we have shown for the first time that central venous pressure also falls throughout the night; this occurs even in the presence of alpha- and beta-adrenergic sympathetic blockade. Since stroke volume does not change overnight this indicates that the nocturnal fall in cardiac output is not due to a shift of blood volume into the venous compartment. These findings support our hypothesis that there is a nocturnal decline in plasma volume.