Effects of Adding Facial Immersion to Chest-Level Water Immersion on Vagally-Mediated Heart Rate Variability.

Recent studies have shown that both facial immersion and head-out water immersion up to the chest (HOIC) positively influence cardiac vagal activity, as indexed non-invasively through vagally mediated heart rate variability (vmHRV). While facial immersion activates the diving reflex, HOIC induces effects via hydrostatic pressure, each engaging distinct physiological mechanisms. This study aims to investigate whether combining facial immersion with HOIC results in an additional increase in vmHRV. In total, the vmHRV [log10RMSSD] of 37 participants (14 females, M_age = 23.8; SD_age = 4.4 years) was assessed under two conditions, with resting and recovery measurements taken before and after each condition. The first condition involved HOIC alone (M = 1.97, SD = 0.27), followed by HOIC combined with facial immersion (M = 1.87, SD = 0.29). HOIC alone significantly increased RMSSD compared to baseline (p < 0.001); however, no additional increase was observed when facial immersion was added (p = 0.436). This suggests that, while HOIC effectively increases vmHRV, the addition of facial immersion does not provide any further enhancement under the conditions tested. Potential methodological limitations, such as the absence of breath holding, variability in immersion depth, and the use of thermoneutral water temperatures, may have influenced the outcomes and warrant further investigation.