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1 Department of Aviation Medicine, The Heart Centre, Copenhagen University Hospital 7522; 2 Department of Clinical Physiology, Copenhagen University Hospital 4012; 4 Department of Medical Physiology, The Panum Institute, DK-2100 Copenhagen; and 3 Department of Internal Medicine and Endocrinology, Herlev Hospital, University of Copenhagen, DK-2730 Herlev, Denmark
The hypothesis was tested that changing
the direction of the transverse gravitational stress in horizontal
humans modulates cardiovascular and renal variables. On different study
days, 14 healthy males were placed for 6 h in either the
horizontal supine or prone position following 3 h of being supine.
Eight of the subjects were in addition investigated in the horizontal
left lateral position. Compared with supine, the prone position
slightly increased free water clearance (349 ± 38 vs. 447 ± 39 ml/6 h, P = 0.05) and urine output (1,387 ± 55 vs. 1,533 ± 52 ml/6 h, P = 0.06) with no
statistically significant effect on renal sodium excretion (69 ± 3 vs. 76 ± 5 mmol/6 h, P = 0.21). Mean arterial pressure and left atrial diameter were similar comparing effects of
supine with prone. The prone position induced an increase in heart rate
(54 ± 2 to 58 ± 2 beats/min, P < 0.05),
total peripheral vascular resistance (13 ± 1 to 16 ± 1 mmHg · min
1 · l1,
P < 0.05), forearm venous plasma concentration of
norepinephrine (97 ± 9 to 123 ± 16 pg/ml, P < 0.05), and atrial natriuretic peptide (49 ± 4 to 79 ± 12 pg/ml, P < 0.05), whereas stroke volume decreased (122 ± 5 to 102 ± 3 ml, P < 0.05, n = 6). The left lateral position had no effect on
renal variables, whereas left atrial diameter increased (32 ± 1 to 35 ± 1 mm, P < 0.05) and mean arterial
pressure decreased (90 ± 2 to mean value of 85 ± 2 mmHg,
P < 0.05). In conclusion, the prone position reduced
stroke volume and increased sympathetic nervous activity, possibly
because of mechanical compression of the thorax with slight impediment
of arterial filling. The mechanisms of the slightly augmented urine
output in prone position require further experimentation.
blood pressure; diuresis; gravitation; natriuresis
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