IMPACT OF CHANGES IN RESPIRATORY FREQUENCY AND POSTURE ON POWER SPECTRAL-ANALYSIS OF HEART-RATE AND SYSTOLIC BLOOD-PRESSURE VARIABILITY IN NORMAL SUBJECTS AND PATIENTS WITH HEART-FAILURE

1. Autonomic dysfunction is a major feature of congestive cardiac fail ure and may have an important role in determining progression and prog nosis. The low-frequency/high-frequency ratio derived from power spect ral analysis of heart rate variability has been proposed as a non-inva sive method to assess sympatho-vagal balance. However, the effects of different respiratory rates or posture are rarely accounted for, but m ay be relevant in patients with heart failure in whom clinical improve ment is accompanied by a fall in respiratory rate and an increased pro portion of the day in the upright position. 2. We have assessed the ef fect of controlled respiration at different rates (10, 15, 20 breaths/ min or 0.17, 0.25 and 0.33 Hz), while supine and standing, on power sp ectral analysis of heart rate and blood pressure variability in 11 pat ients with heart failure and 10 normal subjects. 3. Heart rate varianc e and low-frequency power (normalized units) were reduced in patients with heart failure (absent in six). During controlled breathing while supine, the power of the high-frequency component was significantly gr eater at 10 breaths/min than at 20 breaths/min in patients with heart failure, whether expressed in absolute units (P=0.005) or percentage o f total power (P=0.03). 4. On standing, controlled breathing in patien ts with heart failure produced less change in high-frequency power (P= 0.054), but the low-frequency/high-frequency ratio at lower respirator y rates was reduced (P=0.05). In normal subjects, as expected, respira tory rate had a highly significant effect on high-frequency power. Als o, in normal subjects there was the expected increase in heart rate lo w-frequency power (P=0.04) moving from supine to standing with an incr ease in the low-frequency/high-frequency ratio (P=0.003), while in the patients with heart failure this was absent, reflecting blunted cardi ovascular reflexes.