Postural change alters autonomic responses to breath-holding.
OBJECTIVE
We used breath-holding during inspiration as a model to study the effect of pulmonary stretch on sympathetic nerve activity.


METHODS
Twelve healthy subjects (7 females, 5 males; 19-27 years) were tested while they performed an inspiratory breath-hold, both supine and during a 60 degrees head-up tilt (HUT 60). Heart rate (HR), mean arterial blood pressure (MAP), respiration, muscle sympathetic nerve activity (MSNA), oxygen saturation (SaO(2)) and end tidal carbon dioxide (ETCO(2)) were recorded. Cardiac output (CO) and total peripheral resistance (TPR) were calculated.


RESULTS
While breath-holding, ETCO(2) increased significantly from 41 +/- 2 to 60 +/- 2 Torr during supine (p < 0.05) and 38 +/- 2 Torr to 58 +/- 2 during HUT60 (p < 0.05); SaO(2) decreased from 98 +/- 1.5% to 95 +/- 1.4% supine, and from 97 +/- 1.5% to 94 +/- 1.7% during HUT60 (p = NS). MSNA showed three distinctive phases, a quiescent phase due to pulmonary stretch associated with decreased MAP, HR, CO, and TPR; a second phase of baroreflex-mediated elevated MSNA which was associated with recovery of MAP and HR only during HUT60; CO and peripheral resistance returned to baseline while supine and HUT60; a third phase of further increased MSNA activity related to hypercapnia and associated with increased TPR.


CONCLUSIONS
Breath-holding results in initial reductions of MSNA, MAP, and HR by the pulmonary stretch reflex followed by increased sympathetic activity related to the arterial baroreflex and chemoreflex.
Taneja
Indu
I
Department of Pediatrics, New York Medical College, Valhalla, NY, USA. indu_taneja@nymc.edu
Medow
Marvin S
MS
Clarke
Debbie A
DA
Ocon
Anthony J
AJ
Stewart
Julian M
JM
D000328:Adult; D001049:Apnea / Q000503:physiopathology*; D001341:Autonomic Nervous System / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D001239:Inhalation / Q000502:physiology; D008297:Male; D011187:Posture / Q000502:physiology*; D011661:Pulmonary Stretch Receptors / Q000502:physiology; D012119:Respiration; D016683:Supine Position / Q000502:physiology; D018667:Tilt-Table Test; D014655:Vascular Resistance / Q000502:physiology