This study investigated the effect of different breathing rates on cardiac autonomic tone and blood pressure control. The influence of nose and mouth-only breathing was also assessed. After obtaining ethical approval and informed consent, sixteen healthy subjects (8 males, 8 females, aged 33.9 ± 2.5 years, mean ± S.E.M.) breathed spontaneously for 5 min and then randomly at 6, 9 and 12 cycles per min (cpm) whilst having continuous measures of heart rate (MP100, BIOPAC Systems Inc., USA) and blood pressure (Portapres, TNO-Netherlands) performed. Cardiac autonomic tone was assessed by heart rate variability (HRV) in the time domain and by power spectral analysis (PSA) in the frequency domain. Blood pressure control was evaluated by baroreflex sensitivity (BRS) derived from a transfer function analysis of the PSA data. ANOVA and Bonferroni-adjusted t tests were performed. Data are given as means ± S.E.M. Time domain measures showed that compared to spontaneous breathing, standard deviation of normal RR intervals (SDNN) (ms) 81.5 ± 9.3 vs. 52.2 ± 5.2 (a primary index of HRV), root mean square of successive differences (RMSSD) (ms) 51.2 ± 6.5 vs. 36.3 ± 4.7 and percentage of normal RR intervals greater than 50 ms from the previous beat (pNN50) (%) 17.0 ± 3.8 vs. 9.8 ± 3.0 (both measures of parasympathetic activity) were all significantly increased at 6 cpm (P < 0.01), and SDNN (ms) 63.4 ± 7.3 vs. 52.2 ± 5.2 and pNN50 (%) 14.0 ± 4.1 vs. 9.8 ± 3.0 at 9 cpm (P < 0.05). Total power (ms2) (a frequency domain measure of HRV) was increased at 6 cpm 7813.0 ± 1959.0 vs. 3045.0 ± 591.0, as was BRS (ms mmHg-1) 10.6 ± 1.1 vs. 8.7 ± 1.0 (P < 0.05). When compared with baseline, nose-only breathing demonstrated a decrease in LF/HF ratio (a measure of sympathovagal balance) 1.0 ± 0.3 vs. 3.1 ± 1.0 (P < 0.01), normalised low frequency power (nLF, an index of sympathetic activity) 42.0 ± 5.0 vs. 62.0 ± 4.6 (P < 0.05) and an increase in normalised high frequency power (nHF, a measure of parasympathetic activity) 58.0 ± 5.0 vs. 38.0 ± 4.6 (P < 0.05). Compared with mouth breathing, nose-only breathing decreased nLF 42.0 ± 5.0 vs. 50.0 ± 5.0 and increased nHF 58.0 ± 5.0 vs. 50.0 ± 5.0 (P < 0.05). These data suggest that reduced breathing frequency (particularly 6 cpm) is associated with increased parasympathetic activity, cardiac autonomic tone and blood pressure control, and that nose-only breathing is associated with altered sympathovagal balance, predominantly reflecting increased cardiac vagal activity.
All procedures accord with current UK guidelines and the Declaration of Helsinki.