Hyperventilation during orthostatic challenge in spinal cord-injured humans.

OBJECTIVE

To determine whether arterial hypotension is related to hyperventilation during standing in individuals with complete spinal cord injury (SCI).

METHODS

Sixteen individuals with SCI (thoracic 1 to thoracic level 12, ASIA grade: A) and 18 able-bodied individuals participated in this study. Individuals with SCI were divided into a group with injury level at or above thoracic (T) 7 [higher SCI (HSCI), n = 9] and a group with injury level at or below T10 (lower SCI, n = 7). Subjects carried out 6-min quiet sitting and then 6-min quiet standing using a standing frame.

RESULTS

Significant decrease in mean arterial blood pressure (MAP) and significant increase in minute ventilation (V(E)) by postural change from sitting to standing were observed only in HSCI. In HSCI, MAP decreased from 81 +/- 11 to 75 +/- 16 mmHg (P < 0.05), while V(E) increased from 8.4 +/- 1.1 to 9.9 +/- 1.0 l min(-1) (P < 0.01). There was a significant negative correlation between changes in MAP and V(E) in subjects in the HSCI group (r = -0.72, P < 0.05).

INTERPRETATION: We concluded that the magnitude of hyperventilation during standing is associated with individual difference in the magnitude of hypotension in individuals with SCI. The mechanism(s) for this relationship remains unresolved, but afferent signals from baroreceptors to the brainstem respiratory center and/or action of pressor substances on the central nervous system might have caused hyperventilation.