Note. Colored lines indicate individual participants. Black lines indicate the predicted interaction effect based on the mixed effect model depicted in Table 5. The waitlist period occurred between sessions W1 and T1, while the heart rate variability biofeedback intervention period took place between T1 and T5. DASS – Depression Anxiety and Stress Scales; *** – p<.001; ns – not significant.
3.5 Heart rate variability
The RMSSD was log-transformed to approximate a normal distribution, aligning it with the methodology used in other vmHRV research studies. Two participants had to be excluded from the post measurement, as they had acute respiratory tract infections.
The final model predicting the log(RMSSD) included only TREATMENT, TIME and the TREATMENT * TIME interaction as fixed effects (see Table 6).
Value ~ treatment*time + (1|vpn) + (1|item).
None of the fixed effects were significant predictors. A post-hoc Tukey test of the interaction confirmed no significant improvement in the intervention group, d = 0.11, tratio(44) = 0.66, p = .51, and no significant pre-post difference in the waitlist group, d = -0.14, tratio(43) = 0.61, p = .55 (see Figure 6).
Figure 6. Course of vagally mediated heart rate variability