Cardiovascular dynamics during orthostasis and the influence of intravascular instrumentation.

Abstract 1. 1. Orthostatic tolerance of normal subjects is markedly diminished by the use of intravascular instrumentation such as venous catheters and intraarterial needles. 2. 2. Psychic stimulation due to discomfort or unconscious fear and anxiety associated with such instrumentation may interfere with compensatory reflexes activated by the upright posture and mediated through the central nervous system. 3. 3. Individual reactions to orthostasis vary greatly. Within three minutes following upright tilting, late fainters who were then asymptomatic showed the same decrease in cardiac index and stroke volume as nonfainters. With the onset of symptoms of presyncope, the cardiac index further decreased to 39 per cent of baseline, but nonfainters manifested only a 19 per cent decrease after 20 min. of orthostasis. 4. 4. Recumbent systolic pressure was found to be significantly lower in early fainters than in nonfainters. After three minutes of orthostasis, asymptomatic late fainters had a significantly lower systolic and narrower pulse pressure than nonfainters at the same time. This finding suggests an incomplete compensatory response to the orthostatic stress. 5. 5. Effective cerebral arterial pressure was maintained in nonfainters, but markedly decreased in fainters prior to syncope. 6. 6. The 19 per cent decrease in cardiac index found after 20 min. of orthostasis is associated with a 36 per cent increase in peripheral resistance and a 7 per cent increase in mean arterial pressure. A similar decrease in cardiac index in the recumbent position was associated with only a 14 per cent increase in peripheral resistance and a 5 per cent decrease in mean arterial pressure. This discrepancy in response to central cardiac changes is probably related to the relative effects of the upright posture on cerebral arterial pressure or peripheral vascular receptors.

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