Long-Term Control of Blood Pressure and Sodium Balance: Is The Baseline Nocturnal?

LONG-TERM CONTROL OF BLOOD PRESSURE AND SODIUM BALANCE: IS THE BASELINE NOCTURNAL? ALASTAIR MICHELL * What happens in our bodies is directed toward a useful end. —W. B. Cannon [1] Homeostasis centers on the physiological ability to maintain the constancy of some set point, and arterial pressure has always been presented as one of the clearest examples. Yet that constancy has been exaggerated by the shortcomings ofrecording systems and the intermittency ofthe readings . The very earliest direct readings of arterial pressure revealed its variability : ''The force ofthe blood in its vessels is continually varying according to . . . the various distances of time after taking food . . . also from exercise, rest, different states of vigour or vivacity of the animal and many other circumstances" [2]. The first studies of 24-hour pressure records in humans and the subsequent development of noninvasive ambulatory blood pressure monitoring have amplified and confirmed our perception of arterial pressure as highly labile, to the extent that an individual's diastolic pressure may exceed systolic pressure during a different part of the same 24 hours [3-5]. Broadly, pressure is low during nocturnal inactivity, i.e., undisturbed sleep. While departures from this pattern are regarded as abnormal ("non-dippers") and may correlate with increased risk of cardiovascular damage, an excessively low nocturnal pressure may also compromise perfusion [6-8]. The normal nocturnal fall depends on a reduction in both peripheral resistance and cardiac output (both stroke volume and heart rate), and the reduction The author wishes to thank his secretary, Rosemary Forster, for her assistance in preparing the manuscript. *Animal Health Trust, PO Box 5, Newmarket, Suffolk, CB8 8JH England.© 1997 by The University of Chicago. All rights reserved. 0031-5982/97/4003-1006$01.00 516 Alastair Micheli ¦ Is the Baseline Nocturnal? in cardiac output is lost in cardiac transplant patients, attenuating the nocturnal fall in pressure [9, 10]. The rhythm of the daily variation in arterial pressure is more sensitive to the pattern of activity than that of heart rate, which is closer to an inherent diurnal rhythm [H]. Loss of the nocturnal fall is not exclusively associated with cardiovascular disease, as it also occurs with severe cognitive disorders such as Alzheimer's disease [2]. Regulation of Blood Pressure THE TEXTBOOK CONCEPT Physiology textbooks during the last 50 years emphasize the role of baroreceptors as the guardians of an adequate and constant arterial pressure, but it has become increasingly clear that while they prevent dangerous fluctuations, caused, for example, by sudden change of posture, the longterm regulation of arterial pressure is renal [13]. This hinges on "pressure natriuresis," the marked acceleration of sodium excretion by even slight increases in arterial pressure so that salt loads are swiftly excreted without any lasting effect on pressure [13] . Obviously, if the slope of pressure natriuresis becomes less steep, as in chronic renal failure, it will take a greater and persistent rise in pressure to maintain the same daily sodium excretion. SHORTCOMINGS OF THE CONCEPT One problem with this concept is the fact that the observations relate to extremely high levels of sodium intake and are suspect at more normal intakes [14]. More importantly, while the kidney may stabilize pressure month by month, the data which it receives must be profoundly affected by minute-to-minute—let alone hour-to-hour—changes in arterial pressure. How does the system know which pressure should provide the baseline for long-term regulation and, accordingly, for sodium excretion? Clearly, the latter will also be influenced by changes in plasma volume associated with activity, recumbency, feeding, etc. The purpose of this article is to explore the hypothesis that nocturnal pressure (and perhaps volume) provides a stable baseline, undistorted by mental or physical activity, posture, etc. Thus, when we talk of a nocturnal fall in pressure, it is because our attitude is that of a diurnal observer: what actually happens is that during the day blood pressure rises from baseline according to the demands of everyday life. It is therefore, perhaps, not surprising that while most sodium excretion occurs during the day, the best correlation with arterial pressure occurs during the night [15]. The risks are becoming increasingly clear for patients whose failure to achieve a...