The interaction effect trigger (virtual vs regular AddHRVr) by time (before intervention vs after intervention) was highly significant (b=0.08; P <.001; Multimedia Appendix 4 ), indicating that RMSSD increased more strongly following AddHRVr triggers with the intervention (relative to before intervention) than following virtual AddHRVr triggers.
Notably, the HRV-enhancing effect of the slow breathing intervention to an AddHRVr trigger relative to a random prompt was also significant for lnHF-HRV (trigger by time [before intervention vs intervention]: b=0.16; P =.01 and trigger by time [before intervention vs after intervention]: b =0.21; P <.001) and partially significant for lnSDNN (trigger by time [before intervention vs intervention]: b=0.05; P =.07 and trigger by time [before intervention vs after intervention]: b=0.06; P <.001), thus suggesting general effectiveness of the JITAI on HRV.