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 se ven patients with complete transection of the midthoracic cord (Th6-Th7) an d six normal subjects. Minute ventilation (V-E) and mean inspiratory flow r esponses to hypercapnia were similar in normal subjects and patients with p araplegia, but in the latter, at any given level of end-tidal CO2 partial p ressure (PETCO2), tidal volume (V-T) was reduced and frequency was increase d. In normal subjects during hypercapnia, end-expiratory transpulmonary pre ssure (P-L) and abdominal volume at end expiration decreased markedly, wher eas end-expiratory volume of the rib cage (Vr(C,E)) remained constant, sugg esting progressive recruitment of abdominal muscles. In patients with parap legia compared to normal subjects the decrease in end-expiratory P-L was re duced, and it was associated with a decrease in Vr(C,E), suggesting recruit ment 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 pat ients with paraplegia and normal subjects, respectively. We conclude that t he V-E-CO2 relationship is preserved in patients with paraplegia with the d evelopment of a rapid and shallow pattern of breathing. This suggests that expiratory muscle paralysis elicits adaptation of the ventilatory control s ystem similar to that observed in patients with generalized respiratory mus cle weakness.