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.