Lightfoot, JT, J Weir, H Klein, M Whitehurst, M Lyons. Hemoglobin saturation and ventilation during presyncopal lower body negative pressure . The Physiologist. 33:A126, 1990.
Previous work has shown that lung diffusing capacity decreases during lower body negative pressure (LBNP; Dowell, et al. J. Appl. Physiol. 26:352, 1969). Therefore, the purpose of this study was to determine if the alteration in diffusion capacity caused a change in hemoglobin oxygen saturation (SaO2) before the onset of presyncope. Fifteen subjects (27.1±1.2 yrs, 171.5±1.3 cm, 65.1±2.9 kg; mean±SE), underwent an LBNP test terminated at presyncope. SaO2 was monitored using pulse oximeters (Nellcor) at the left index finger and at the bridge of the nose. At presyncope, expected decreases in heart rate (HR), blood pressure (MAP), and forearm vascular resistance (FVR) occurred. The following values (mean±SE) reflect observations at rest, one min before presyncope (PS-2), 30 sec before presyncope (PS-1), and at the onset of presyncope (PS) (*significantly different from Rest, p<0.05):
Variable Rest PS-2 PS-1 PS FBF (ml/100 ml/min) 5.7±1.0 2.5±0.5* 2.5±0.8* 3.4±1.4* SaO2 nose (%) 96.0±0.6 94.3±1.3 94.1±1.6 93.9±1.1 SaO2 finger (%) 98.7±0.4 98.5±0.5 98.0±0.7 97.5±0.7* Ve (l/min) 8.0±0.4 10.0±0.7* 10.2±0.8* 10.6±0.7* VO2 (ml/min) 208±19 224±26 214±21 238±27
Because FVR increases during LBNP, finger SaO2 might be expected to decrease. However, with an altered diffusing capacity during LBNP, the trend (p=0.07) toward a decreased nose SaO2 might represent a mechanism influencing ventilation and cardiovascular homeostasis immediately before presyncope. It appears that these alterations maintain VO2 at baseline levels.
Supported in part by American Heart Association 90GIA/649.