Heart rate variability in critical illness and critical care.

Although the rhythm of a healthy heart is clinically described as regular, the rate is variable. Studies of diverse populations have led to several generalizations about heart rate variability (HRV): (1) HRV is physiologic and normally declines with age, (2) acute changes in HRV are associated with several disease processes that require critical care, (3) measures of HRV can be used to describe the status of critically ill patients, and (4) measures of HRV can be used to predict events subsequent to at least one type of critical illness, myocardial infarction. This brief review considers the mechanisms underlying HRV, the measures that are used to describe HRV, and recent information regarding the use of HRV measures as predictive tools in critical care. The reviewers' opinion is that real-time analysis of HRV in critical illness may provide caregivers with additional information about patient status, effects of intervention, and prognosis.