Sequential modulation of cardiac autonomic control induced by cardiopulmonary and arterial baroreflex mechanisms.
BACKGROUND
Nonhypotensive lower body negative pressure (LBNP) induces a reflex increase in forearm vascular resistance and muscle sympathetic neural discharge without affecting mean heart rate. We tested the hypothesis that a reflex change of the autonomic modulation of heartbeat might arise during low intensity LBNP without changes of mean heart rate.


RESULTS
Ten healthy volunteers underwent plasma catecholamine evaluation and a continuous recording of ECG, finger blood pressure, respiratory activity, and central venous pressure (CVP) during increasing levels of LBNP up to -40 mm Hg. Spectrum and cross-spectrum analyses assessed the changes in the spontaneous variability of R-R interval, respiration, systolic arterial pressure (SAP), and CVP and in the gain (alpha(LF)) of arterial baroreflex control of heart rate. Baroreceptor sensitivity was also evaluated by the SAP/R-R spontaneous sequences technique. LBNP began decreasing significantly: CVP at -10, R-R interval at -20, SAP at -40, and the indexes alpha(LF) and baroreceptor sensitivity at -30 and -20 mm Hg, compared with baseline conditions. Plasma norepinephrine increased significantly at -20 mm Hg. The normalized low-frequency component of R-R variability (LF(R-R)) progressively increased and was significantly higher than in the control condition at -15 mm Hg.


CONCLUSIONS
Nonhypotensive LBNP elicits a reflex increase of cardiac sympathetic modulation, as evaluated by LF(R-R), which precedes the changes in the hemodynamics and in the indexes of arterial baroreflex control.
Furlan
R
R
Medicina Interna II, Ospedale L. Sacco, Dipartimento Scienze Precliniche LITA di Vialba, Università degli Studi di Milano, Italy. Raffaellof@fisiopat.sacco.unimi.it
Jacob
G
G
Palazzolo
L
L
Rimoldi
A
A
Diedrich
A
A
Harris
P A
PA
Porta
A
A
Malliani
A
A
Mosqueda-Garcia
R
R
Robertson
D
D
D000328:Adult; D001158:Arteries / Q000502:physiology*; D001341:Autonomic Nervous System / Q000502:physiology*; D017704:Baroreflex / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology; D005260:Female; D006321:Heart / Q000294:innervation / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008150:Lower Body Negative Pressure / Q000379:methods; D008168:Lung / Q000502:physiology*; D008297:Male; D015656:Respiratory Mechanics / Q000502:physiology; D016683:Supine Position / Q000502:physiology