The voluntary drive to breathe is not decreased in hypercapnic patients with severe COPD

How do the respiratory centres of patients with chronic obstructive pulmona ry disease (COPD) and hypercapnia respond to acute increases in inspiratory load? A depressed respiratory motor output has long been postulated, but s tudies on this issue have yielded inconsistent results, partly due to limit ations of investigative techniques. Many of these limitations can be overco me by the twitch interpolation technique, which is capable of accurately qu antifying the degree of diaphragmatic activation, termed the voluntary driv e to breathe. The hypothesis that patients with COPD and hypercapnia compen sate for an acute increase in mechanical load on the inspiratory muscles wi th a lower voluntary drive to breathe than is the case with normocapnic pat ients was tested. Measurements were obtained in 15 patients with COPD, six of whom displayed hypercapnia and nine normocapnia. The maximum degree of diaphragmatic activ ation, expressed as a voluntary activation index (mean +/- SEM), was higher in hypercapnic than in normocapnic patients (98.7 +/- 0.7 versus 94.5 +/- 0.9% (p = 0.006)), as was the mean value (94.5 +/- 0.7 versus 88.5 +/- 1.9% (p = 0.01)). Within-patient values of the index were also less variable in the hypercapnic patients (coefficients of variation, 3.4 +/- 0.3 versus 6. 1 +/- 0.9%, p=0.01). Multiple regression analysis revealed the ratio of dyn amic elastance to maximum transdiaphragmatic pressure, an index of inspirat ory muscle loading, and pH as the only variables that correlated with maxim um voluntary activation index (r(2) = 0.69, p = 0.02 for each variable). Contrary to the hypothesis, it was concluded that voluntary activation of t he diaphragm was greater and less variable in hypercapnic patients than nor mocapnic patients with severe chronic obstructive pulmonary disease during an acute increase in inspiratory mechanical load. Whether greater diaphragm atic recruitment during episodes of a severe exacerbation of chronic obstru ctive pulmonary disease provides a survival advantage for hypercapnic patie nts with chronic obstructive pulmonary disease remains to be determined.