Six juvenile diabetics [age, 31 ± 2 yr (mean and SEM); duration of diabetes, 15 ± 4 yr] with signs of autonomic neuropathy (decreased beat-to-beat variation in heart rate during deep breathing) and seven control patients of similar age (27 ± 1 yr) and duration of diabetes (14 ± 2 yr) performed graded exercise oh an ergometer cycle. Resting heart rate was higher and the increase in heart rate at the lowest work load (50 W) was diminished in patients with autonomic neuropathy compared with control patients (P < 0.001), indicating a vagal defect.
The relationships in autonomic neuropathy between heart rate and systolic blood pressure, respectively, and relative work load (expressed as oxygen uptake — in percent—of individual maximal oxygen uptake) were identical with previous findings in normal subjects during beta-adrenergic receptor blockade, indicating impaired sympathetic activity. Maximal heart rate was 157 ± 9 min−1 in autonomic neuropathy and 181 ± 4 in controls, P < 0.05; maximal systolic blood pressure was 179 ± 11 mm Hg and 197 ± 5, respectively.
The greatest tolerable work load was significantly less in patients with autonomic neuropathy (125 ± 13 vs. 161 ± 9 w, P < 0.05). Similarly, maximal oxygen uptake was reduced (1.68 ± 0.21 vs. 2.78 ± 0.18 L/min, 25 ± 3 vs. 38 ± 2 ml/min/kg, P < 0.005).
In conclusion, diabetics with decreased beat-to-beat variation in heart rate displayed signs of cardiovascular dysfunction of parasympathetic as well as sympathetic origin during graded exercise.