Respiratory muscle function and control of breathing in patients with acromegaly

Increase in lung size has been described in acromegalic patients, but data on respiratory muscle function and control of breathing are relatively scarce. Lung volumes, arterial blood gas tensions, and respiratory muscle strength and activation during chemical stimulation were investigated in a group of 10 patients with acromegaly, and compared with age- and sex-matched normal controls. Inspiratory muscle force was evaluated by measuring pleural (Ppl,sn) and transdiaphragmatic (Pdi,sn) pressures during maximal sniffs. Dynamic pleural pressure swing (Ppl,sw) was expressed both as absolute value and as percentage of Ppl,sn. Expiratory muscle force was assessed in terms of maximal expiratory pressure (MEP). In 8 of the 10 patients, ventilatory and respiratory muscle responses to hyperoxic progressive hypercapnia and to isocapnic progressive hypoxia were also evaluated. Large lungs, defined as total lung capacity (TLC) greater than predicted (above 95% confidence limits), were found in five patients. Inspiratory or expiratory muscle force was below normal limits in all but three patients. During unstimulated tidal breathing, respiratory frequency (fR) and mean inspiratory flow (tidal volume/inspiratory time (VT/tI)) were greater, while inspiratory time (tI) was shorter than in controls. Minute ventilation (V'E) and mean inspiratory flow response slopes to hypercapnia were normal In contrast, four patients had reduced delta(VT/tI)/arterial oxygen saturation (Sa,O2) and three had reduced deltaV'E/Sa,O2. Ppl,sw(%Ppl,sn) response slopes to increasing end-tidal carbon dioxide tension (PET,CO2) and decreasing Sa,O2 did not differ from the responses of the normal subjects, suggesting normal central chemoresponsiveness. At a PET,CO2 of 8 kPa or an Sa,O2 of 80%, patients had greater fR and lower tI compared with controls. Pdi,sn and Ppl,sn related both to deltaV'E/deltaSa,O2 (r=0.729 and r=0.776, respectively) and delta(VT/tI)/deltaSa,O2 (r=0.860 and r=0.90, respectively). Pdi,sn also related both to deltaV'E/deltaPET,CO2 (r=0.8) and delta(VT/tI)/deltaPET,CO2 (r=0.76). In conclusion, the data suggest the relative independence of pneumomegaly and respiratory muscle strength. Peripheral (muscular) factors appear to modulate a normal central motor output to give a more rapid pattern of breathing.