The current study aimed to investigate the acute effects of MIM sessions, an MBI, on respiration rate, heart rate, and HRV. In agreement with the original hypothesis based on self-reported breath rates from previous MIM cohorts (Merrigan et al. 2023), respiration rates (measured through physiological signals) were reduced during the Practice and Post-MIM timepoints compared to Baseline-MIM. Several HRV metrics considered to be susceptible to fluctuations in respirations were also significantly improved during Practice and/or Post-MIM (SDNN, Low Frequency Power, L:H Power Ratio, Total Power). However, mean heart rate was elevated and High Frequency Absolute Power was decreased during Practice, while Total Power was increased during post-MIM and rMSSD was not significantly different at any timepoint. The gentle yogic movements that occurred during the Practice segment of MIM likely contributed to the increased mean heart rate and decreased High Frequency Absolute Power. These findings also indicate that the impact of mindfulness training on HRV may be dependent on the metric being analyzed. Regardless, the current findings bare ecological validity considering the generally positive influence of the MIM sessions on respiration rates and HRV (showing a more relaxed mind-body system at the end of MIM) within real world health care environments.
In general, variations of MBIs are aimed to provoke improvements in habitual reactions to stressors including combating unnecessary sympathetic nervous system responses with parasympathetic nervous system modulation (e.g., lowering blood pressure, heart rate, and respiration rate). By bringing more awareness to the breath during mindfulness practices (Kabat-Zinn 2003a), respiration rates expectedly decrease (Ahani et al. 2014; Kodituwakku et al. 2012; Wahbeh et al. 2016; Wielgosz et al. 2016) and often do so in similar magnitudes to the level of intent of the controlled slow breathing practices (Kirk and Axelsen 2020). In the current study, participants learned about the potential impact of breathing exercises, followed examples during the mindfulness practices, and drew their attention to counting their own breath at the start and end of each session to evoke further awareness to their breath. Previous research on the efficacy of MIM has found slowed respiration rates, measured via self-counted exhalations, immediately after completing the MIM intervention as well as during weeks 3–8 compared to week 1 (Klatt et al. 2015; Merrigan et al. 2023). The aforementioned findings are further validated with current results showing slowed respiration rates, according to ECG analysis using a validated algorithm,(Lipponen and Tarvainen 2021; Rogers et al. 2022), from Baseline-MIM to the Practice and Post-MIM measures. Still, these results alone may not be indicative of attaining more parasympathetic modulation without also considering the effects of MIM on additional parameters and timepoints.
Heart rate variability has been proposed as a non-invasive marker of autonomic nervous system (ANS) function, with higher HRV indicating greater ANS flexibility and adaptability while subsequently improving a range of positive health outcomes, including cardiovascular disease, anxiety, depression, and chronic pain (Berntson et al. 1997; Rajendra Acharya et al. 2006). Resultantly, researchers may seek to use HRV to understand the efficacy of mindfulness programs as others have also interpreted changes in HRV as a marker of effective cardiac vagal activation during slow-paced breathing (Kromenacker et al. 2018). According to a recent review, there has been strong evidence on the positive influence of mindfulness interventions on heart rate while results on HRV as the outcome measure remain uncertain (Pascoe et al. 2017). However, Italian workers did not experience lower heart rate after sessions of mindfulness training compared to relaxing music on a mobile application (Carissoli et al. 2015). Similarly, heart rate values were not significantly altered immediately after MIM, which may have been due to the significantly elevated heart rate during the Practice section of MIM that included subtle physical movement (i.e., yogic poses) in the current study. These findings are important to note as they allude to potential considerations of certain MBIs that may involve salient discussions or standing with yogic practices which may induce more arousal within the session and affect heart rate and HRV results (Bernardi et al. 2000; Lumma et al. 2015).
Some researchers have found trait mindfulness to be associated with one’s ability to maintain their HRV levels but not directly with absolute HRV values (Sun et al. 2019), while others found mindfulness traits to be positively associated with absolute HRV values (Burg et al. 2012). Interestingly, one study showed that rMSSD and SDNN did not differ across their phases of mindfulness training (calming, mental arithmetic, recovery, mindfulness practice), whereas low and high frequency and total power HRV were improved (Sun et al. 2019). Similarly, prior research has found that mindfulness practices significantly increased high frequency HRV after a cognitively stressful event (Azam et al. 2016). Although similar findings from log transformed high frequency HRV were noted acutely during baseline testing (Krygier et al. 2013), a 10 day intensive Vipassana meditation intervention resulted in an increase in normalized high frequency HRV and a decrease in absolute LF power HRV (Krygier et al. 2013). Meanwhile, in the current study, several HRV measures improved from Baseline-MIM to during Practice and/or Post-MIM (SDNN, Low Frequency Power, L:H Power Ratio, Total Power) but not rMSSD. Thus, it is clear by prior and current results that HRV responses to mindfulness are dependent on the metric being analyzed (Krygier et al. 2013; Sun et al. 2019).
As mentioned, paced and relaxed breathing has been historically found to alter HRV (Bernardi et al. 2000; Peng et al. 2004; Soer et al. 2021; Yasuma and Hayano 2004). However, it is important to understand that certain HRV metrics are more influenced and representative of changes in respiration (Shaffer and Ginsberg 2017; Stephenson et al. 2021). The respiratory sinus arrhythmia (RSA) describes the speeding and slowing of the heart rate by the vagus nerve due to respiration changes (Karemaker 2009). The power within LF and HF bands are thought to be affected by breathing from ~ 3–9 bpm and 9–24 bpm, respectively (Camm et al. 1996; Shaffer and Ginsberg 2017). Whereas total power encompasses very low frequency, LF, and HF bands in short term recordings. The LF to HF ratio attempts to estimate the SNS and PNS balance, but there are concerns with use of this metric (Billman 2013; Shaffer and Ginsberg 2017; Stephenson et al. 2021), particularly since the participants in the current study were seated and the LF measures may be more altered by PNS and baroreflex activity compared to normally being influenced by SNS activity (Eckberg 1983). For SDNN, the primary source of the variation is parasympathetically-mediated RSA, especially with slow, paced breathing protocols (Shaffer and Ginsberg 2017), such as MIM. Yet, the influence of respiration rates on rMSSD is uncertain (Penttilä et al. 2001; Schipke et al. 1999), suggesting rMSSD may be more representative of overall PNS modulation. Overall, respiration rates have little influence on time domain metrics while having clear effects to frequency domain measurements of HRV via shifts in the RSA (Schipke et al. 1999). Thus, since major changes in respiration rate were expected and found from Baseline- to Post-MIM, the changes in HRV metrics should be confirmed with future research by including additional Post-session timepoints.
It is also necessary to consider that evidence of slowed respiration rates is greater during mindfulness practices than when evaluating long-term mindfulness effects on respiration and HRV measures (Reive 2019). In some instances, such as after a 6-week mindfulness intervention in combat veterans, respiration rates were slowed, but there were no differences in heart rate or HRV (Wahbeh et al. 2016). Unfortunately at this time, due to inconsistencies in participation and missing data points causing inadequate power, we were unable to examine the weekly effects of MIM on respiration rates and HRV in the current sample. However, one strength of the current findings is the potential implications of including biofeedback in MBIs, such as counting of one’s own breaths at the start and end of each session and receiving analysis of heart rate data. This biofeedback may increase awareness of physical sensations associated with emotions which is likely to evoke greater affect regulation and self-awareness (Bechara and Naqvi 2004; Damasio 2003). As such, researchers have found promise for combined use of biofeedback and mindfulness in stress management programs (Gevirtz 2015). One limitation of the current study is a lack of physiological data at additional timepoints to evaluate the time-course of responses following MIM sessions (i.e., that night or following morning). Another limitation is the derivation of respiration rates from ECG, although considered accurate estimations (Lipponen and Tarvainen 2021; Rogers et al. 2022). The ECG devices were used to capture all participants using one centrally located recording device that enabled syncing time recordings of all participants in a given session instead of trying to align multiple devices (i.e., ECG and respiration transducer) on each participant. Because the MIM program is built to occur during work hours, the use of these procedures also provided the ability to rapidly capture a variety of physiological data in a fast-paced setting without requiring further delays from equipment set up. Another limitation is that we do not currently have enough data to pinpoint the effects of specific practices during the practice segments of MIM. For example, some practice themes required slight yogic movement while others focused on stillness and slow breathing which would have differing effects on HRV. However, a purpose of the current pilot research was to identify general responses to the segments within MIM to best set up the delivery of similar interventions. As such, it may be beneficial to begin sessions with discussion and practice to account for potential arousal and then finish the sessions with a lecture and quite reflection to help participants reach more relaxed states at the session completion.