Ventilatory response to hypercapnia in C(5-8) chronic tetraplegia: the effect of posture.

Abstract

To study the effect of posture on the hypercapnic ventilatory responses (HCVR). Nonrandomized controlled study. Rehabilitation hospital and a pulmonary institute. Patients with neurologically stable C(5-8) tetraplegia (n=12) and healthy control subjects (n=7). Not applicable. Supine and seated forced vital capacity (FVC) and HCVR, and supine and erect blood pressure. FVC in the sitting position was reduced in patients with tetraplegia (52+/-13% predicted); supine FVC was 21% higher (P=.0005). In the sitting position, HCVR was lower in patients than in controls (0.8+/-0.4 vs 2.46+/-0.3 L/min/mmHg, P<.001). Supine HCVR was not significantly different between the groups. When HCVR was normalized to FVC, there was still a significant difference between patients and controls in the sitting position. Patients with tetraplegia were orthostatic (mean supine blood pressure 91+/-13 mmHg vs mean erect blood pressure 61+/-13 mmHg, respectively, P<.0001). The magnitude of the orthostatism correlated with that of the postural change in HCVR (r=.93, P<.0001). Respiratory muscle weakness may contribute to the attenuated HCVR in tetraplegia. However, the observation that supine HCVR is still low even when normalized to FVC suggests a central posture-dependent effect on the HCVR, which may be linked to the postural effect on arterial blood pressure.