INFLUENCE OF AUTONOMIC NEUROPATHY OF DIFFERENT SEVERITIES ON THE HYPERCAPNIC DRIVE TO BREATHING IN DIABETIC-PATIENTS

To investigate the effects of the autonomic nervous system on control of breathing, the neuromuscular (mouth occlusion pressure at 0.1 s aft er onset of inspiration [P-0.1]) and ventilatory (minute ventilation [ (V) over dot E]) response to progressive hyperoxic hypercapnia was ass essed in diabetic patients with autonomic dysfunction of different sev erity. Eighteen diabetics with autonomic neuropathy, nine with parasym pathetic damage (DANp), and nine with parasympathetic and sympathetic damage (DANp+s), as indicated by marked postural hypotension, low incr ement of diastolic BP during sustained handgrip, and lowest resting ca techolamine plasma levels, were studied together with a group of 10 di abetic patients without autonomic neuropathy (D) and a group of 10 nor mal subjects (C). All subjects had pulmonary function tests, including maximal voluntary ventilation and diffusion of carbon monoxide, measu rements of respiratory muscle strength as maximal inspiratory mouth pr essure (MIP) and maximal expiratory mouth pressure (MEP), and a CO2 re breathing test (Read's method). Although in the normal range, lung vol umes and FEV1 and forced expiratory flows were lower in the DANp and D ANp+s groups than in the D and C groups, MIP and MEP were similar amon g C and diabetic groups, as well as resting P-0.1, (V) over dot E, tid al volume (VT), and respiratory rate (RR). The slope of the linear rel ationship between P-0.1 and end-tidal PCO2 (PETCO2) was higher in DANp +s (0.63+/-0.07 cm H2O/mm Hg) than in C (0.45+/-0.06 cm H2O/mm Hg; p<0 .05) and three times greater in DANp+s than in D (0.26+/-0.03 cm H2O/m m Hg; p<0.001) and DANp (0.24+/-0.03 cm H2O/mm Hg; p<0.001), who in tu rn showed a lower Delta P-0.1/Delta PETCO2 than C. The (V) over dot E increase with increasing PETCO2 was greater in DANp+S (3.70+/-0.85 L/m in/mm Hg) than in DANp (2.13+/-0.20 L/min/mm Hg; p<0.05) and D (2.37+/ -0.40 L/min/mm Hg; p=0.07), but not significantly higher from that of C (3.17+/-0.36 L/min/mm Hg). No differences were found for Delta VT/De lta PETCO2 among the groups, whereas the Delta RR/Delta PETCO2 relatio nship was steeper in DANp+s than in DANp (p<0.05) and D (p=0.055). The se data reflect a depressed CO2 response both in D and DANp. The presu mable decrease of the sympathetic nerve traffic in DANp+s appears to r everse this abnormality. DANp+s, however, exhibit an enhanced CO2 neur omuscular response even in respect to C, suggesting that the sympathet ic nervous system might modulate the output of the respiratory centers to hypercapnic stimulus.