Commentary

Measuring caregiver HRV in the acute pain context: Methodological considerations

Kaytlin Constantin, Heidi N. Bailey, and C. Meghan McMurtry

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In pediatric pain contexts, caregivers often experience distress and increases in physiological response when viewing their child in pain (Smith et al., 2007; Goubert et al., 2008). Caregivers must regulate their own emotional experience to accurately interpret and effectively respond to their child’s needs (Vervoort & Trost, 2017). Increasingly, researchers have utilized heart rate variability (HRV) or the variation in time between consecutive heartbeats, as an objective index of emotion regulation (Appelhans & Luecken, 2006; Balzarotti et al., 2017). Numerous physiological measures, including heart rate and electrodermal activity, are available to assess general autonomic arousal associated with the experience of emotions. In contrast, HRV can inform an individual’s regulation of emotions given neural connections between the heart and the brain (Balzarotti et al., 2017). HRV at the high or respiratory frequency (i.e. respiratory sinus arrhythmia) is a noninvasive index of autonomic flexibility or the body’s ability to rapidly adjust physiological states via the vagus nerve by increasing or decreasing heart rate (Porges, 2007). HRV is also posited to distally index cortical systems that support regulated emotional responses (e.g. captures the activity between the prefrontal cortex and the vagus nerve; Thayer & Lane, 2000).

Physiological research in the field of pediatric pain has focused on the child in pain; recently, it has been recognized that examining HRV in individuals beyond those experiencing pain (e.g. caregivers) is also relevant to understanding the child’s pain-related experience (Vervoort et al., 2014; Constantin et al., 2016). Examining caregiver HRV can provide complementary information regarding their emotional response in the context of their child’s pain. Such information may be useful in determining whether caregivers may benefit from interventions focused on emotion regulation strategies. Nevertheless, only two investigations to date have explored parent HRV in the context of acute pediatric pain (Vervoort et al., 2014; Constantin et al., 2017).

Laborde and colleagues (2017) provide an in depth review of HRV methodology and recommendations; however, it is not specific to the pain context and may be less accessible to readers who are new to HRV methodology. In contrast, the aim of our paper is to provide succinct and explicit guidance for professionals who are less familiar with HRV methodology. Specifically, we provide a brief overview of the methodological considerations when examining caregiver high frequency HRV in the pediatric pain context, with regard to: (1) study design, (2) selecting an HRV parameter, (3) apparatus and subject preparation, and (4) data cleaning and analysis. Table 1 presents general methodological recommendations. In the text, we explore general and pain-specific (when applicable) methodological considerations.

Table 1
General methodological recommendations

*Indicates pain-specific citation. 

Study design: pain stimulus and time blocks

Prospective studies should include at least three time blocks, allowing for comparison of HRV during: baseline, event, and post-event blocks (see Table 1). This provides three indicators of HRV: resting, reactivity (i.e. change score between baseline and event), and recovery (change score between event and post-event; Laborde et al., 2017). Depending on the research question, an additional index may be calculated by examining the changes between baseline and post-event. Optimally, each block must be the same length, which will vary by the metric used (see Shaffer & Ginsberg, 2017 for an overview of HRV metrics), with a minimum of one-minute and preferably at least two-minute time windows (European Society of Cardiology, 1996; Grant et al., 2011).

In the pediatric pain context, further delineation regarding the experience captured within each time block is needed; research could benefit from more sensitive temporal measurement of HRV as seen in existing behavioral observations of infant and child acute pain responses (e.g. Waxman et al., 2017)¹. For example, two types of baseline blocks are possible: a resting and immediate pre-pain baseline. Pure resting baseline measures of HRV are typically obtained while an individual is relaxed and sitting comfortably, and acclimatized to the environment with nothing specifically happening (Laborde et al., 2017). This leads to the question of whether it is possible to acquire a true resting baseline measure if caregivers arrive to the study anticipating child pain, which may be accompanied by a heightened state of arousal (Smith et al., 2007). A pre-pain baseline occurs in preparation for the event. In the case of a needle, this may include the health care provider entering the room, swabbing, and handling the materials. HRV during the painful procedure (i.e. event block) requires the use of a pain anchor, or the exact timing of the beginning and ending of the painful procedure to ensure each time block captures the same experience for each individual. Acquiring HRV during child pain may prove difficult logistically in clinical settings and the researcher must decide whether it is possible to consistently obtain a minimum recording of one minute during the child’s experience of pain, or whether measuring caregiver HRV immediately before child pain (e.g. anticipation) is more feasible. Longer time blocks are more reliable; therefore, feasibility concerns must be balanced with reliability. However, longer time blocks may compromise validity and the specificity of the emotional experience being captured. Ultimately, the ability to capture caregiver HRV during child pain will depend on the pain stimulus or the type of painful procedure. Caregiver HRV during child pain could be examined during longer painful procedures, such as lumbar punctures and bone marrow aspirations, as well as laboratory pain, heel lances, and venipunctures that are typically longer in duration compared to a vaccination. For shorter procedures, examining HRV immediately before (pain anticipation) and/or after (pain reactivity) the procedure may be more likely. The age of the child is relevant to consider when examining pain reactivity, with infants having longer peak distress responses than preschool and school-aged children (Pillai Riddell et al., 2013; Waxman et al., 2017). Post-event recordings should be acquired following child pain to provide a recovery measure of HRV. Moreover, ideally, multiple post-event recordings could be applied to track the progression of HRV recovery; specifically, to examine early regulation immediately after the procedure, later regulation two or three minutes after the procedure, and determine when caregiver HRV returns to resting levels. A final consideration is that movement should be minimized during each recording as it complicates the interpretation of HRV (European Society of Cardiology, 1996). Parents can be instructed to minimize their movement to reduce potential artifacts, although this may interfere with their typical behaviors and interactions with their child.

Choosing a parameter

HRV parameters can be categorized under time-domain (i.e. statistical calculation), frequency-domain (i.e. filtering ECG signal into different frequency bands), and non-linear indices (Smith et al., 2013). HRV in the high (vs. low) frequency band, between 0.15 to 0.40 Hz, is of most interest as it is posited to index vagal influence on the heart (Porges, 2007). The most common parameters are reported in Table 1. All measures are highly correlated and thought to be comparable indices of vagal tone when respiration frequency is within 9 and 24 cycles per minute, equivalent to between 0.15 and 0.40 Hz (Grossman et al., 1990; Berntson et al., 1997). Therefore, it is recommended that respiration rate be measured to ensure breathing is within this normal range; these data can be acquired online during the experiment or offline utilizing post hoc analyses (e.g. Kubios HRV; see Laborde et al., 2017). HRV may also be affected by other extraneous variables (e.g. caffeine and nicotine consumption; Quintana & Heathers, 2014); Laborde and colleagues (2017) provide a demographic form that may be used to consider potential sources of variability.

Apparatus and skin preparation

In the clinical setting, which may involve either small rooms (e.g. for venipuncture) or transitioning from room to room (perioperative), a wireless ECG system will offer the most flexibility for both patients and health care providers (e.g. less equipment is attached to the caregiver). Numerous ECG configurations with varying numbers of leads (e.g. three, five, twelve) are available to acquire HRV data, and the placement of the electrodes is dependent on the number of leads (see Kligfield et al., 2007, for details). However, as described in Table 1, a consistent approach to preparing the skin for HRV recording is recommended. Dry electrodes, caregiver perspiration, and hair may prevent the electrodes from sticking well. Gel may be applied if electrodes are dry, and storing electrodes in an airtight package will prevent drying. Respiration rate can be acquired by respiratory inductance plethysmography, which utilizes a thoracic and/or abdominal band to monitor chest and abdominal movement (see Al-Khalidi et al., 2011, for a review).

Data cleaning and analysis

An electrocardiogram (ECG) is commonly used to acquire HRV data as it allows precise inspection and cleaning of the data (Shaffer et al., 2014). The data should first be visually inspected for technical and physiological artifacts before using software correction programs (see Peltola, 2012, for a review). The data can be manually edited by using the deletion correction method (i.e. removing abnormal interbeat intervals and shifting subsequent intervals to replace the deleted ones), although the absolute number of samples in the segment is decreased; thus, a time-block of longer duration is desirable (e.g. record for an additional 15 seconds for a 1-minute time block). Alternately, other correction methods involving interpolation preserve the continuous HRV sample.

Software programs are available to acquire and edit the ECG signal (e.g. AcqKnowledge 5.0 [Biopac Systems Inc.]), and existing specialized HRV programs are user-friendly and freely available (e.g. ARTiiFACT; Kaufmann et al., 2011; Kubios HRV Standard; Tarvainen et al., 2017) or can be purchased (e.g. Kubios HRV Premium; Tarvainen et al., 2017²) to edit and analyze the ECG recording (see Table 1). Specialized programs compute HRV parameters using both time-domain methods (reported in units of milliseconds [ms]) and frequency-domain methods (reported in units of squared milliseconds [ms²]). It is a widely held standard among physiological researchers to report the natural logarithm of an HRV parameter (e.g. Goedhart et al., 2007).

Conclusions

This commentary sought to provide clinician-scientists with concrete introductory knowledge on measuring caregiver HRV in the pediatric pain context. The ultimate goal is to foster research examining caregiver physiology to clarify the associations between caregiver physiological, subjective, and behavioral responses, and child pain experience. Incorporating HRV measures into pediatric pain research has theoretical implications (e.g. contributes to existing biopsychosocial models of pain) and may lead to downstream clinical implications (e.g. applying interventions that target HRV).

Cite as: Constantin K, Bailey HN, McMurtry CM. Measuring caregiver HRV in the acute pain context: Methodological considerations. Pediatric Pain Letter 2018;20(1):5-10. www.childpain.org/ppl 

End Notes

¹ We thank a reviewer for raising this important issue and his/her helpful discussion points.

² Of relevance, Kubios HRV Standard only reads data files based on RR intervals; Kubios HRV Premium is required to read ECG files.

Acknowledgement

The authors gratefully acknowledge funding by the Ontario Graduate Scholarship to K Constantin.

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