Ventilatory Response to Hypercapnia in C 5–8 Chronic Tetraplegia: The Effect of Posture

Abstract

Ben-Dov I, Zlobinski R, Segel MJ, Gaides M, Shulimzon T, Zeilig G. Ventilatory response to hypercapnia in C 5–8 chronic tetraplegia: the effect of posture. Objective To study the effect of posture on the hypercapnic ventilatory responses (HCVR). Design Nonrandomized controlled study. Setting Rehabilitation hospital and a pulmonary institute. Participants Patients with neurologically stable C 5–8 tetraplegia (n=12) and healthy control subjects (n=7). Interventions Not applicable. Main Outcome Measures Supine and seated forced vital capacity (FVC) and HCVR, and supine and erect blood pressure. Results 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±0.3L/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±13mmHg vs mean erect blood pressure 61±13mmHg, respectively, P<.0001). The magnitude of the orthostatism correlated with that of the postural change in HCVR (r=.93, P<.0001). Conclusions 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.