Acceptability will be assessed at post-intervention (T2) using a structured 7-item survey evaluating perceived relevance, helpfulness, usability, satisfaction, and likelihood of recommending the intervention. Items are rated on a 5-point Likert scale (1 = strongly disagree/not helpful to 5 = strongly agree/very helpful).

Metric: Number of participants rating items as 4 or 5 ÷ total number of respondents Unit of Measure: Percentage (%) of participants endorsing the acceptability threshold Benchmark: ≥ 80% of participants rate the intervention as helpful or very helpful (scores 4-5).

Qualitative acceptability will be assessed using open-ended questions examining perceived benefits, challenges, and recommendations for improvement of the HRVB intervention.

Metric: Thematic saturation and frequency of key themes Unit of Measure: Narrative themes categorized as benefits, barriers, and recommendations Benchmark: Identification of actionable themes supporting feasibility and refinement of implementation.

Affect will be measured using the Positive and Negative Affect Schedule (PANAS-SF, 20 items), a validated self-report measure that assesses the extent of positive affect (e.g., enthusiasm, alertness) and negative affect (e.g., distress, irritability) experienced over a specified time period. Higher scores indicate greater levels of the respective affective state. In addition, affect will be assessed through a brief daily diary using 12 PANAS-derived items to capture day-to-day fluctuations in positive and negative affect, providing a more nuanced understanding of affect over the 3-week study period.

Perceived difficulties in emotional regulation will be measured using the Difficulties in Emotion Regulation Scale-Short Form (DERS-16), which is a validated self-report instrument that assesses global impairments in emotion regulation across multiple domains. It evaluates the frequency with which individuals experience problems such as heightened emotional reactivity, nonacceptance of emotional responses, difficulties maintaining goal-directed behavior when distressed, challenges controlling impulses during negative affect, and limited perceived access to effective emotion regulation strategies. Higher total scores reflect greater overall difficulty in regulating emotions.

Cravings for illicit opioids will be measured using the Obsessive Compulsive Drug Use Scale (OCDUS). This tool will be used to assess craving severity, capturing obsessive thoughts about substance use, compulsive urges, and perceived control over craving-related cognitions and behaviors. Higher scores indicate greater craving intensity and preoccupation with drug use. In addition, craving will be monitored through a brief daily diary item assessing subjective craving severity, allowing examination of day-to-day fluctuations and temporal patterns in craving experiences across the study period.

Perceived social support will be measured using The Multidimensional Scale of Perceived Social Support (MSPSS), which is a validated self-report instrument that assesses perceived adequacy of social support from three primary sources: family, friends, and significant others. It evaluates individuals' subjective sense of being valued, understood, and supported emotionally and practically by their social networks. Higher scores indicate greater perceived availability and quality of social support across these domains.

The Drug-Taking Confidence Questionnaire-8 (DTCQ-8) is a brief, validated self-report measure of self-efficacy to resist substance use across high-risk situations. It assesses an individual's perceived confidence in their ability to refrain from using drugs when experiencing common triggers such as negative emotions, social pressure, craving, or interpersonal conflict. Higher scores indicate greater confidence in resisting drug use and a lower likelihood of relapse in these situations.

Heart rate variability (HRV) metrics, including SDNN, RMSSD, mean heart rate, mean interbeat interval (IBI), total power, LF, HF, and LF/HF ratio, will be used as objective indicators of autonomic nervous system functioning, reflecting overall variability, parasympathetic activity, and sympathetic-parasympathetic balance. In addition, normalized coherence scores from daily HeartMath Inner Balance practice will assess the degree of physiological synchronization and autonomic efficiency. Together, these measures provide complementary indices of autonomic regulation, stress resilience, and emotional regulation capacity, with higher variability and coherence generally indicating healthier autonomic functioning.