Wijeysundera D N, Butler G C, Ando S, Pollard M, Picton P, Floras J S
Division of Cardiology of the Toronto General and Mount Sinai Hospitals and the University of Toronto, Suite 1614, 600 University Avenue, Toronto, Ontario, Canada M5G1X5.
Clin Sci (Lond). 2001 Mar;100(3):303-9.
Mechanisms responsible for presyncope during lower body negative pressure (LBNP) in otherwise healthy subjects are poorly understood. Muscle sympathetic nerve activity (MSNA), blood pressure, heart rate (HR), HR power spectra, central venous pressure (CVP) and stroke volume were determined in 14 healthy men subjected to incremental LBNP. Of these, seven experienced presyncope at LBNP >-15 mmHg. Subjects who tolerated LBNP >-15 mmHg had significantly lower CVP (2.6+/-1.0 versus 7.2+/-1.2 mmHg; means+/-S.E.M., P<0.02), HR (59+/-2 versus 66+/-3 beats/min, P<0.05) and MSNA burst frequency (29.0+/-2.4 versus 39.0+/-3.5 bursts/min, P<0.05) during supine rest. LBNP at -15 mmHg had no effect on blood pressure, but caused similar and significant reductions in stroke volume and cardiac output in both groups. Subjects who tolerated LBNP had significant reflex increases in HR, MSNA burst frequency and burst amplitude with LBNP of -15 mmHg. These responses were absent in those who experienced presyncope. The gain of the cardiac baroreflex regulation of MSNA was markedly attenuated in pre-syncopal subjects (1.2+/-0.6 versus 8.8+/-1.4 bursts/100 heart beats per mmHg; P<0.001). Healthy subjects who experience presyncope in response to LBNP appear more dependent, when supine, upon MSNA to maintain preload, and less able to increase sympathetic vasoconstrictor discharge to skeletal muscle reflexively in response to orthostatic stimuli.
在健康受试者中，下体负压（LBNP）期间发生前驱晕厥的机制尚不清楚。对14名健康男性进行递增式LBNP试验，测定其肌肉交感神经活动（MSNA）、血压、心率（HR）、HR功率谱、中心静脉压（CVP）和每搏输出量。其中，7人在LBNP＞-15 mmHg时出现前驱晕厥。耐受LBNP＞-15 mmHg的受试者在仰卧休息时CVP显著较低（2.6±1.0对7.2±1.2 mmHg；均值±标准误，P＜0.02）、HR较低（59±2对66±3次/分钟，P＜0.05）以及MSNA爆发频率较低（29.0±2.4对39.0±3.5次/分钟，P＜0.05）。-15 mmHg的LBNP对血压无影响，但两组的每搏输出量和心输出量均出现相似且显著的降低。耐受LBNP的受试者在-15 mmHg的LBNP时HR、MSNA爆发频率和爆发幅度有显著的反射性增加。而出现前驱晕厥的受试者则无这些反应。在前驱晕厥受试者中，MSNA的心脏压力反射调节增益明显减弱（1.2±0.6对8.8±-1.4次/100次心跳/mmHg；P＜0.001）。因LBNP而出现前驱晕厥的健康受试者在仰卧时似乎更依赖MSNA来维持前负荷，且对体位性刺激反射性增加骨骼肌交感缩血管神经放电的能力较弱。
