Carotid arterial hemodynamics in response to LBNP in normal subjects: methodological aspects.

Pulsatile changes in blood pressure and arterial diameter were studied noninvasively with applanation tonometry and echo-tracking techniques at the sites of the common carotid artery (CCA) and the carotid arterial bulb (CAB) in 12 healthy volunteers. Determinations were performed before and during application of -10 and -40 mmHg lower body negative pressure (LBNP) to investigate noninvasively the tensile forces acting on the CAB. Together with significantly decreased mean arterial pressure, increased heart rate, forearm vascular resistance, and plasma norepinephrine, the -40 mmHg LBNP stimulus produced the following significant changes in CCA and CAB hemodynamics: 1) for the same decrease in mean arterial pressure, a greater decrease in carotid than in brachial pulse pressure was observed (P < 0.01) due to a significant change in pressure wave transmission and in the timing of the carotid backward pressure wave; and 2) a highly significant decrease in pulsatile changes in diameter and tangential tension occurred, with a greater decrease in systolic than in diastolic tangential tension. Subsequently, cyclic tangential tension decreased more substantially than mean tangential tension. The cyclic changes in tension were quite significant after -40 mmHg LBNP but were already observed for mild -10 mmHg LBNP in which mean systemic blood pressure and heart rate were not modified. During -10 and -40 mmHg LBNP, CCA and CAB compliance and distensibility were unchanged. This study provides evidence that the autonomic nervous system activation produced by the LBNP procedure is associated with significant changes in pressure-wave amplification and in cyclic tensile forces acting on the CAB. These changes, which may occur even for mild LBNP, should be taken into account when interpreting results of the LBNP procedure in humans.