Perception of dyspnea in patients with neuromuscular disease

Background: The perception of dyspnea is not a prominent complaint of resti ng patients with neuromuscular disease (NMD). To our knowledge, no study ha s been addressed at evaluating the interrelationships among lung mechanics, respiratory motor output, and the perception of dyspnea in patients with N MD receiving ventilatory stimulation. Material: Eleven patients with NMD (mean +/- SD age, 44 +/- 11.8 years; 5 m en) of different etiology and a group of normal subjects matched for age an d sex (control subjects). Methods: While patients were breathing room air, lung volumes, arterial blo od gases, the pattern of breathing (minute ventilation [(V) over dot E], ti dal volume [VT], respiratory frequency, inspiratory time), and maximal (les s negatives esophageal pressure during a sniff maneuver (Pessn), as an inde x of inspiratory muscle strength, were measured. Then we evaluated the resp onse to hypercapnic-hyperoxic stimulation (hypercapnic-hyperoxic rebreathin g test [RT]) in terms of breathing pattern, inspiratory swing of pleural pr essure (Pessw), and inspiratory effort (Pessw[%Pessn]). During the RT, dysp nea was assessed every 30 s using a modified Borg scale (0 to 10). Results: Pulmonary volumes were reduced in seven patients, and Pco(2) was o ut of proportion to (V) over dot E in four patients. Group Pessn was 42.8 /- 23.6 cm H2O in patients and 107 +/- 20.4 cur H2O in control subjects (p < 0.001). Dynamic elastance (Eldyn) [p = 0.0016] and Pessw(%Pessn) [p < 0.0 005] were higher in patients. During the RT, Borg/CO2, Pessw(%Pessn)/CO2, a nd Borg/Pessw(%Pessn) were similar in the two groups, while (V) over dot E/ CO2 and VT/CO2 were lower in patients (p < 0.0002 for both). As a consequen ce, for unit change in VT (Percentage of predicted vital capacity [%VC]), g reater changes in Pessw(%Pessn) were associated with greater Borg scores in patients. Baseline Eldyn related to Pessw(%Pessn)/VT(%VC) during hypercapn ia (r(2) = 0.85), an index of neuroventilatory coupling of the ventilatory pump (NVC). NVC predicted a good amount of the variability in Borg/(V) over dot E (r(2) = 0.46, p < 0.02). Conclusions: In this subset of NMD patients during hypercapnic stimulation, a normal inspiratory motor output per unit change in PCO2 results in a sha llow breathing pattern. The consequent impairment of NVC underlies the high er scoring of dyspnea in these patients.