Lower-body suction and cardiovascular reflexes: physiological and applied considerations.

ABSTRACT

Lower body subatmospheric pressure (LBSP) can be applied in a graded manner to a supine subject enclosed in a box upto the level of the iliac crest in order to elicit cardiovascular reflexes without a change of posture, and without a gravity induced shift of the central blood volume into the periphery. The procedure effectively produces a controlled, non-haemorrhagic hypovolaemia. The method may be used to differentiate the reflex cardiovascular effects induced by the deactivation of the low-pressure cardiovascular receptors (LBSP less than 30mmHg), and those produced by the deactivation of the arterial baroreceptors (LBSP greater than 30 mmHg). The former results in an increase in the limb vascular resistance without a change in heart rate and blood pressure, while the latter produces a tachycardia, an increase in the diastolic BP with a fall in the sytolic BP. The CVP and the cardiac output reduce with increasing suction. Cardiovascular reflex effects of -40 mmHg are similar to those produced by a change of posture from supine to standing. Vaso-vagal syncope appears with increasing frequency when LBSP exceeds -60 mmHg even in the normal subjects. The test is useful in the aeromedical assessment of apparently normal individuals with low orthostatic tolerance, in the evaluation of the effects of physiological and pharmacological interventions on cardiovascular reflexes, and in the evaluation of patients of autonomic neuropathies. LBSP is also a means of inducing safe, well controlled syncope in order to study the genesis of this phenonmenon.