4.3. Physiological indicators
Physiologic indicators of pain refer to measurable changes in the body that occur in response to acute painful stimuli. Some indicators include changes in vital signs (e.g., heart rate and blood pressure)136, skin conductance, pupil dilation137, and neurophysiological activity138. Similar to behavioral observation, these physiological changes have been shown to be useful in patients unable to adequately communicate pain, such as individuals receiving invasive forms of mechanical ventilation in critical care settings139, 140. For populations with OUD, they may be particularly helpful in the acute care setting, as these variables may offer clues into needs for higher opioid dosages in the setting of acute pain, particularly as people with OUD usually experience high opioid tolerance.
Respiratory rate is a commonly used physiological indicator of pain. A large observational study including 19,908 patients who called for emergency medical service due to pain, found that respiratory rate had the strongest correlation with patients’ self-reported pain intensity compared to other vital signs141. This suggests that increased respiratory rate is a useful indicator of acute pain. It should be noted, however, that opioids directly depress respiratory activity by acting on opioid receptors in the brainstem142, therefore, pain-induced increases in respiratory rate might be masked by an underlying opioid-induced respiratory depression. As such, careful interpretation of respiratory rate is required when using it to assess pain in this population, and it may be an unreliable indicator in isolation.
One of the most studied physiological indicators for acute pain assessment is heart rate variability (HRV). HRV refers to the variation in the time interval between consecutive heartbeats143. It is influenced by the autonomic nervous system, which regulates the body’s response to pain and stress144. A study has shown that patients with OUD have lower resting-state high-frequency heart rate variability when compared to patients without OUD, suggesting a disturbed autonomic flexibility in the former145. Another study found that opioid withdrawal might induce a reduction in cardiac vagal tone, resulting in increased systolic blood pressure, heart rate, and decreases in heart rate variability146. Therefore, the autonomic sequelae of OUD might confound the interpretation of HRV for pain assessment. Still considering cardiovascular markers, blood pressure becomes an additional confounder in the assessment of pain among those using opioids chronically. Because of opioids vasodilating effects, we may not observe pain-related arterial hypertension frequently associated with acute pain147, 148.
Thus, physiological indicators may not accurately reflect acute pain in patients with OUD due to opioid-induced physiological alterations. It’s critical, therefore, to consider these potential confounders in pain assessment; integrating these indicators with self-report measures may offer a more comprehensive and precise pain assessment in this unique population.