Ventilatory and respiratory muscle responses to hypercapnia in patients with paraplegia

To evaluate ventilatory and respiratory muscle responses to hypercapnia in patients with paraplegia with paralysis of abdominal muscles, we studied seven patients with complete transection of the midthoracic cord (Th6-Th7) and six normal subjects. Minute ventilation (V-E) and mean inspiratory flow responses to hypercapnia were similar in normal subjects and patients with paraplegia, but in the latter, at any given level of end-tidal CO2 partial pressure (PETCO2), tidal volume (V-T) was reduced and frequency was increased. In normal subjects during hypercapnia, end-expiratory transpulmonary pressure (P-L) and abdominal volume at end expiration decreased markedly, whereas end-expiratory volume of the rib cage (Vr(C,E)) remained constant, suggesting progressive recruitment of abdominal muscles. In patients with paraplegia compared to normal subjects the decrease in end-expiratory P-L was reduced, and it was associated with a decrease in Vr(C,E), suggesting recruitment of rib cage expiratory muscles. For a PETCO2 of 70 mm Hg the estimated expiratory muscle contribution to V-T was 10.3 and 28.4% (p < 0.02) in patients with paraplegia and normal subjects, respectively. We conclude that the V-E-CO2 relationship is preserved in patients with paraplegia with the development of a rapid and shallow pattern of breathing. This suggests that expiratory muscle paralysis elicits adaptation of the ventilatory control system similar to that observed in patients with generalized respiratory muscle weakness.