Key result
Head-down tilt at 30 degrees reduced muscle sympathetic nerve activity burst rate by 51±5%, a significantly greater reduction than the 31±5% decrease seen at 15 degrees (P<0.05).
Why the study?
Does graded head-down tilt alter muscle sympathetic nerve activity and hemodynamics in healthy males?
Does graded head-down tilt alter muscle sympathetic nerve activity and hemodynamics in healthy males?
p-value: p=< 0.05
Graded head-down tilt progressively increases cardiopulmonary mechanoreceptor loading, leading to a proportional inhibition of muscle sympathetic nerve activity.
Graded head-down tilt should not change clinical practice; leaves open proportional sympathetic inhibition in patients with cardiovascular disease.
These experiments were conducted to examine whether changes in central and peripheral hemodynamics were proportional to muscle sympathetic nerve activity (MSNA) during graded head-down tilt (HDT). Twelve healthy males (19-42 yr old) underwent HDT at 15 degrees and 30 degrees for 10 min each with a 10-min rest period between the trials. MSNA at 15 degrees HDT declined by 31 +/- 5% (P < 0.05) for burst rate and by 37 +/- 3% (P < 0.05) for total activity. At 30 degrees HDT, the reduction in MSNA was 51 +/- 5% for burst rate (P < 0.05 vs. 15 degrees HDT) and 46 +/- 5% for total activity (P < 0.05 vs. 15 degrees HDT). Stroke volume increased (P < 0.05) during both 15 degrees and 30 degrees HDT, but heart rate and blood pressure remained unchanged. A concurrent increase in central venous pressure (P < 0.05) and stroke volume with a reduction of thoracic impedance (P < 0.05) suggests that both pressure and volume in the atrium were elevated during HDT, and the magnitude of these changes was greater (P < 0.05) at 30 degrees HDT than at 15 degrees HDT. Forearm blood flow increased during HDT at both 15 degrees and 30 degrees, and the magnitude of the increase was greater (P < 0.05) at 30 degrees HDT. It is concluded that the magnitude of the loading of the cardiopulmonary mechanoreceptors during HDT was higher at 30 degrees in comparison to 15 degrees. This increased the afferent firing rate by the cardiopulmonary receptors and probably inhibited sympathetic outflow in the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)
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Nagaya et al. (1995) studied Healthy (n=12). Head-down tilt (HDT) vs. Baseline / 15 degrees HDT was evaluated on Muscle sympathetic nerve activity (MSNA) burst rate and total activity (p=< 0.05). Head-down tilt at 30 degrees reduced muscle sympathetic nerve activity burst rate by 51±5%, a significantly greater reduction than the 31±5% decrease seen at 15 degrees (P<0.05).
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