Hypoxic and hypercapnic breathlessness in patients with type I diabetes mellitus

Study objectives: The putative role of the performance of inspiratory muscl es and breathing pattern in inducing dyspnea has been recently assessed dur ing hypoxic stimulation in patients with type I diabetes (IDDM). Compared t o a hypoxic stimulus, a hypercapnic stimulus, which may differently affect the pattern of breathing, could therefore modulate the coupling between res piratory effort and ventilatory output, which is involved in dyspnea sensat ion. Subjects: Eight stable patients aged 19 to 48 years old, with IDDM (duratio n of disease, 36 to 240 months) and no smoking history, cardiopulmonary inv olvement, or autonomic neuropathy; and an age- and sex-matched control grou p. Measurements: Pulmonary volumes, diffusing capacity of the lung for carbon monoxide, time and volume components (tidal volume [VT] and respiratory fre quency), dynamic elastance (Eldyn), and swings in pleural pressure (Pessw) were measured. Maximal inspiratory pleural pressure (Pes) during a maximal sniff maneuver (Pessn), respiratory muscle effort or output (Pessw%Pessn), tension time index (TTI) = TI/total breathing cycle time x Pessw(%Pessn), a nd swing in Pes during VT as a percentage of Pessn were also evaluated. Dys pnea sensation was assessed by a modified Borg scale. Subjects were studied at baseline and during hypoxic and hypercapnic rebreathing tests. Results: Compared to control subjects, patients exhibited normal routine sp irometric function and Pessn, but a higher Eldyn, indicating peripheral air way involvement, In patients, but not in control subjects, Eldyn increased during both chemical stimuli and increased more during hypoxia than during hypercapnia. Also, changes in both VT and Pessw(%Pessn) on changes in PCO2 were lower, while changes in Pessw(%Pessn)/VT, an index of neuroventilatory dissociation (NVD) of the ventilatory pump, on changes in PCO2 were greate r. Changes in VT and NVD for unit change in arterial oxygen saturation were lower and higher, respectively, Changes in Borg scale per changes in NSD w ere greater during both stimuli. Furthermore, compared to hypoxic condition s, a greater VT for any level of both minute volume and Pessw(%Pessn), and lower changes in Borg scale on changes in Pessw(%Pessn) and Pessw(%Pessn)/V T were found in hypercapnia. Changes in NVD and Borg scale related to chang es in Eldyn with both chemical stimuli. Conclusions: In IDDM, the greater perception of dyspnea is associated with changes in inspiratory effort being out of proportion to changes in VT. The greater increase in Eldyn and the lower increase in VT may, in part, accou nt for the greater perception of breathlessness during hypoxia.