Which vital sign may indicate pain in a patient?

An elevated heart rate, or tachycardia, is often an indication of pain in a patient. When a person experiences pain, the body releases stress hormones like adrenaline, which can lead to an increase in heart rate as part of the sympathetic nervous system's response to stress. This physiological reaction serves to prepare the body for a "fight or flight" response, which is characterized by various changes including increased heart rate to enhance blood flow to essential organs and muscles.

In contrast, a lower than normal blood pressure can occur for a variety of reasons, including dehydration or bleeding, and doesn't specifically signal pain. A decreased respiratory rate may indicate pain in cases where breathing becomes shallow due to discomfort, but typically, pain actually causes an increase in respiratory rate as well due to anxiety or distress. A stable temperature does not provide any indication of pain, as pain can occur regardless of whether the body temperature is within the normal range, elevated, or decreased. Thus, the best indicator among the listed vital signs that may indicate pain is an elevated heart rate.