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

Assessment of cardiac vagal activity in patients with hyperthyroidism

Previous studies suggested that in patients with hyperthyroidism an autonomic imbalance and in particular a lower than normal vagal activity might be present. To verify this hypothesis we have evaluated the respiratory sinus arrhythmia (RSA, a measure of cardiac vagal activity) in ten hyperthyroid patients and in ten normal subjects. RSA was calculated from the power of the spectral component of the heart rate variability in high frequency band (HF-RR) during both spontaneous (supine and passive head-up tilt) and controlled breathing (supine). During controlled breathing the phase relation between heart rate and respiratory has been computed. The hyperthyroid patients showed a higher heart rate in all three conditions (P<0.001) and higher spontaneous respiratory rate in supine position (centered frequency of HF-RR: 0.342+/-0.015 vs 0.262+/-0.016 Hz; P<0.001). No difference was found in hyperthyroid patients compared to controls in terms of the HF-RR power in normalized units both during spontaneous breathing (supine, 43+/-8.3 vs 39.7+/-6.7%; tilt 18.8+/-5.9 vs 19.3 vs 4.1%; mean+/-SE) and controlled breathing (45.4+/-7.1 vs 48.9+/-6.9%). No difference was found also in terms of the phase relationship between the heart rate and the respiratory signals (77.5+/-32.3 vs 77.5+/-28.1, degrees). Hyperthyroid patients seem not to have an impaired cardiac vagal activity.

Cardiac baroreceptor sensitivity is impaired after acute stroke

BACKGROUND AND PURPOSE

The blood pressure (BP) fall and increased BP variability after acute stroke have been previously described. The underlying pathophysiological mechanisms producing these findings are unclear but may include abnormalities of cardiac baroreceptor reflex arc and/or changes in sympathetic nervous system activity. To date, evidence of impaired cardiac baroreceptor sensitivity (BRS) after stroke is limited to patients with chronic disease as determined by invasive methodology. Therefore, it was proposed to assess cardiac BRS and sympathovagal balance with the use of novel noninvasive techniques after acute stroke.

METHODS

Thirty-seven acute stroke patients underwent simultaneous surface electrocardiographic and noninvasive beat-to-beat BP recording. Cardiac BRS was assessed by power spectral analysis techniques, and sympathovagal balance was determined from the ratio of the low- to high-frequency powers for pulse interval variability. The responses were compared with a control group matched for age, sex, and BP.

RESULTS

Median cardiac BRS was significantly lower in stroke patients than in control subjects (high-frequency alpha-index, 4.89 versus 6.50 ms/mm Hg; P = .007; combined alpha-index, 4.65 versus 5.46 ms/mm Hg; P = .02). Median normalized high- but not low-frequency power of systolic BP variability was significantly greater in stroke patients (11.0 versus 6.7 normalized units; P < .001), probably reflecting differences in the mechanical effects of respiration on BP in stroke patients. No significant differences were observed in the power spectrum of pulse interval variability between stroke patients and control subjects. Patients with tight hemisphere strokes, however, had a significant reduction in median high-frequency pulse interval power compared with patients with left hemisphere strokes (8 versus 20 normalized units; P = .03), which may reflect a change in sympathovagal balance in favor of increased sympathetic tone in this group.

CONCLUSIONS

The impairment of cardiac BRS may be important in explaining the increased BP variability after stroke. There was no significant difference in surrogate measures of sympathovagal activity between acute stroke patients and control subjects, but right hemisphere stroke patients had a significant alteration in the sympathovagal balance of pulse interval variability compared with left hemisphere stroke patients. This sympathetic predominance in right hemisphere strokes may be important in the development of cardiac arrhythmias after stroke. The prognostic implications of these findings need to be further explored.