The Effects of a Respiratory Acidosis on Human Heart Rate Variability

Human heart rate variability (HRV) was examined during a mild respiratory acidosis induced by inhalation of a normoxic hypercapnic gas mixture. On two separate occasions, separated by 72 but not more than 120 hr, ECG was recorded from 9 normotensive subjects during supine rest. ECG was recorded for 20 min breathing room air or a 5% CO₂ in normoxic air mixture. Expired V'E, O₂ and CO₂ were measured with breath-by-breath mass spectrometry. HRV spectra were calculated using a Welch averaged periodogram method and banded as Very Low Frequency (VLF: 0 – 0.04 Hz), Low Frequency (LF: 0.04 – 0.15 Hz), and High Frequency (HF: 0.15 – 0.4 Hz). Student paired samples t-tests were used to compare room air (RA) versus inhaled 5% CO₂ in air (5% CO₂) data. All results reported as mean +/− SD. In the HRV time domain, hypercapnic normoxia reduced mean r-r intervals (5% CO₂: 956.1 ± 149.2 vs. RA: 1035 ± 146.8 ms, p = 0.022) and median r-r intervals (5% CO₂: 942.6 ± 153.1 vs. RA: 1047.8 ± 157.3 ms, p = 0.010), and increased heart rates (5% CO₂: 64.4 ± 12 vs. RA: 59.3 ± 10.1 bpm, p = 0.019). In the HRV frequency domain, hypercapnic normoxia increased the high frequency component of HRV (5% CO₂: 9799 ± 7649 vs. RA: 4399 ± 3857 ms⁴) and reduced the LF/HF ratio (5% CO₂: 0.243 ± 0.145 vs. RA: 0.906 ± 0.672, p = 0.017). An incomplete ventilatory compensation probably accounts for the increased HF contribution.