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Departments of Integrative Physiology and Internal Medicine, University of North Texas Health Science Center at Fort Worth, Fort Worth, Texas 76107
We tested the
hypothesis that hypotension occurred in older adults at the onset of
orthostatic challenge as a result of vagal dysfunction. Responses of
heart rate (HR) and mean arterial pressure (MAP) were compared between
10 healthy older and younger adults during onset and sustained lower
body negative pressure (LBNP). A younger group was also assessed after
blockade of the parasympathetic nervous system with the use of atropine
or glycopyrrolate and after blockade of the
1-adrenoceptor by use of metoprolol. Baseline HR (older
vs. younger: 59 ± 4 vs. 54 ± 1 beats/min) and MAP (83 ± 2 vs. 89 ± 3 mmHg) were not significantly different between the groups. During 
40 Torr, significant tachycardia occurred at the
first HR response in the younger subjects without hypotension, whereas
significant hypotension [change in MAP (
MAP) 7 ± 2 mmHg] was observed in the elderly without tachycardia. After
the parasympathetic blockade, tachycardiac responses of younger
subjects were diminished and associated with a significant hypotension
at the onset of LBNP. However, MAP was not affected after the cardiac
sympathetic blockade. We concluded that the elderly experienced
orthostatic hypotension at the onset of orthostatic challenge because
of a diminished HR response. However, an augmented
vasoconstriction helped with the maintenance of their blood pressure
during sustained LBNP.
vagal dysfunction; tachycardiac response; lower body negative pressure; atropine; glycopyrrolate; metoprolol
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