30 healthy male participants (25 ±5years) were randomly allocated to either the HRV biofeedback group (treatment) or the no-intervention group (control). Participants attended a pre and post intervention session (spaced 30 days apart), consisting of two stressors (5 minutes each): an orthostatic stressor (head up tilt table test) and a mental arithmetic test. Vagal modulation was indexed by root mean square of successive differences (RMSSD) from 5-minute ECG recordings. The treatment group practised HRV biofeedback for 10 minutes per day during the 30 days between the pre and post intervention sessions.
No significant difference existed between the groups at baseline (age, sex, BMI, vagal modulation). Post HRV biofeedback intervention, the treatment group showed a significant increase in vagal modulation at baseline (14.4 ±6.5ms, p = 0.032) and during orthostatic stress (7.4 ±2.1ms, p = 0. 028). The results suggest that HRV biofeedback increases vagal modulation. HRV biofeedback is safe for testing in a clinical setting in conditions characterised by poor vagal modulation such as hypertension and depression.