Study objectives: We evaluate the hypothesis that pulse rate increases linearly with increased body temperature in infants and determine how much tachycardia in infants can be explained by a 1 degrees C (1.8 degrees F) increase in body temperature.
Methods: Infants younger than 1 year and presenting to a pediatric emergency department were prospectively enrolled. Rectal temperature and pulse rate were measured. Research personnel rated behavioral state as sleeping, awake and quiet, fussy, or crying. Patients were excluded if they were fussy or crying or if they had any medical condition expected to cause tachycardia. The remaining patients were divided into 6 age-based groups. Linear regression analysis of pulse rate and temperature was performed for each group.
Results: Four hundred ninety patients were enrolled. Pulse rate increased linearly with temperature in all age groups older than 2 months (adjusted r2=0.102 to 0.376) but not in infants younger than 2 months (adjusted r2=0.004). In infants aged 2 months or older, a multivariate linear regression model adjusted for age showed that pulse rate increased an average of 9.6 beats/min (95% confidence interval 7.7 to 11.5) per 1 degrees C (1.8 degrees F) increase in temperature (adjusted r2=0.225). At any given temperature, the prediction interval for an individual's pulse rate had a span of approximately 64 beats/min.
Conclusion: In infants 2 to 12 months of age, pulse rate increases linearly with body temperature, with a mean increase of 9.6 beats/min for each 1 degrees C (1.8 degrees F) increase in body temperature. Pulse rates of individual infants vary greatly, however, with a broad range of pulse rates observed at any given temperature.