Does Inhalation-to-Exhalation Ratio Matter in Heart Rate Variability Biofeedback?

This randomized controlled study examined whether inhalation-to-exhalation ratio affects HRV frequency domain and time domain measures. Twenty-six undergraduates (10 female and 16 male) participated in this study. A Thought Technology ProComp Infiniti[TM] system monitored ECG, respiration, temperature, and skin conductance. Active ECG electrodes were placed on the upper chest below the sternum and the reference centered on the xyphoid process. A respirometer was positioned over the naval in order to measure abdominal excursion and respiration rate. A skin conductance sensor was placed on the palmar aspect of the second phalange of both the index and ring fingers. Finally, a thermistor was taped to the web dorsum of the nondominant hand. Participants were randomly assigned to begin with one of two conditions, either a 1:2 inhalation-exhalation ratio or a 1:1 inhalation-exhalation ratio. All participants were monitored during both ratios and compliance with breathing ratio instructions was confirmed. Participants were instructed to sit quietly with their back straight and their hands on their thighs, and to breathe following a visual pacer for each 10-min recording session. Between conditions, participants sat quietly without breathing ratio instructions, a visual pacer, or feedback for 5 min to minimize carryover. Data were analyzed using a GLM analysis with family wise correction. Participants successfully followed the visual pacer and breathed at 6 bpm in both inhalation-to-exhalation ratio conditions. The breathing ratio did not affect heart rate, skin conductance, temperature, or HRV time domain or frequency domain measurements. Two nonlinear measurements, DFA alpha1 and sample entropy were affected by breathing ratio. DFA alpha1 was greater during a 1:1 ratio, F(1, 24) = 6.06, p = 0.02, [[eta].sup.2] = .20, Cohen's d = 1.00, while SampEn was greater during a 1:2 ratio, F(1, 23) = 1157.40, p = 0.001, [[eta].sup.2] = .98, Cohen's d = 14.00. Overall, inhalation-to-exhalation ratio does not appear to influence the time and frequency domain measurements that clinicians typically use during HRV biofeedback, and should be chosen based on client preference. Future researchers should replicate these findings with clinical populations. Keywords * Heart rate variability * Breathing ratio * Biofeedback