Influence of posture on cardiorespiratory reflexes in normal subjects and patients with diabetes mellitus

This study was designed to assess the effect of patient posture, either sitting or supine, on the results and diagnostic yield of tests of cardiorespiratory reflexes on 25 normal subject controls and 128 patients with diabetes mellitus. The heart rate changes during slow breathing at 6 per minute (R6) and during a Valsalva manoeuvre (highest heart rate during manoeuvre divided by lowest heart rate within 30 s after manoeuvre; Valsalva ratio) were measured from the electrocardiographic signal and tachometry. In control subjects, heart rate changes during slow breathing averaged 25.0 when sitting and 21.3 when supine (p < 0.01); Valsalva ratio averaged 1.90 and 1.83 (p > 0.2) respectively. In diabetic patients, mean values for heart rate changes during slow breathing were 6.1 and 5.1, and mean values for Valsalva ratio were 1.23 and 1.28 in sitting and supine postures respectively. According to our normal limits, heart rate changes during slow breathing was below normal in 109 patients tested when sitting and in 113 tested when supine, and Valsalva ratio was abnormal in 103 and 95 respectively. These results indicate that subject posture is an important variable of cardiorespiratory reflex testing, and should be standardized to allow comparisons of individual results.