Pediatrics
How much tachycardia in infants can be attributed to fever?☆

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Abstract

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°C (1.8°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°C (1.8°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°C (1.8°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.

Introduction

It is widely accepted that fever is associated with tachycardia. Many studies since the early 1900s have shown a linear relationship between pulse rate and body temperature, with pulse rate increasing 4.1 to 17 beats/min for every 1°C (1.8°F) increase in body temperature.1, 2, 3, 4, 5, 6, 7

Editor's Capsule Summary

What is already known on this topic

Children have large ranges of normal pulse rates that vary inversely with age. Although many clinicians believe that elevated temperature is associated with increased pulse rate, there are few empiric data quantifying this relationship.

What question this study addressed

Researchers measured rectal temperature and pulse rate in infants younger than 12 months to determine the effect of naturally occurring fever on pulse rate.

What this study adds to our knowledge

In infants between 2 and 12 months of age, there was a 9.6 beat/min increase in average heart rate for every 1°C increase in temperature. In infants younger than 2 months, a linear relationship between pulse rate and body temperature could not be established. Individual pulse rates varied widely, however, at any given temperature.

How this might change clinical practice

Infants' pulse rates outside the confidence intervals reported by the authors are likely to be abnormal. However, substantial variation exists between individual patients. In addition to fever, clinicians must still look for the presence of other medical conditions that may contribute to tachycardia.

Because increased temperature is only 1 of many conditions that may contribute to tachycardia, clinicians need to know how much tachycardia can be attributed to fever to allow for other potential causes of tachycardia to be recognized. Most previous studies have measured the effect of artificially induced fever in adult populations, with fever induced by hot baths,1 peptone injections,5 or short radio waves.2 The few studies that have evaluated the effect of naturally occurring fever on pulse rate have involved adult subjects,5, 6, 7 often with small sample size.5, 6 We could find no previous studies evaluating the relationship between naturally occurring fever and pulse rate in young children.

This study was designed to evaluate the effect of naturally occurring fever on pulse rate in infants. Our specific objectives were to evaluate the hypothesis that pulse rate increases linearly with increased body temperature in infants and to determine how much tachycardia can be explained by a 1°C (1.8°F) increase in body temperature.

Section snippets

Theoretical model of the problem

According to previously available adult data, we theorized that there would be a linear relationship between pulse rate and temperature in infants. We recognized that several other clinical factors, such as sepsis, toxins related to serious bacterial infection, anemia, dehydration, hypoxemia, medication effects, and pain or agitation, might also contribute to tachycardia and confound the relationship between pulse rate and temperature. To attempt to evaluate more purely the relationship between

Characteristics of study subjects

A total of 773 infants were enrolled. Of these infants, 113 (15%) patients were excluded from analysis because of the presence of 1 or more medical conditions expected to cause tachycardia (Table 1). Hypovolemia, serious bacterial infection, cardiomyopathy, and previous administration of albuterol were the leading medical conditions warranting exclusion. An additional 170 (22%) infants were excluded because they had a behavioral state ranking of 3 or 4 (fussy or crying). The remaining 490 (63%)

Limitations

One limitation is that we had limited information about the clinical status of each patient from which to determine exclusion criteria. For example, the clinician's decision to administer intravenous fluids may or may not be an accurate indication of hypovolemia. Another issue is that we were unable to control for baseline variation between individuals when evaluating the effect of temperature on pulse rate. Previously published data indicate that normal pulse rates can vary widely among

Discussion

We have shown that pulse rate is increased in infants with fever and that pulse rate increases linearly with temperature in infants between 2 and 12 months of age. These results are consistent with previous studies from adult populations that also demonstrate a linear relationship between pulse rate and body temperature.1, 2, 3, 4, 5, 6, 7

In our population of infants between 2 and 12 months of age, mean pulse rate increased about 9.6 beats/min for each 1°C (1.8°F) increase in rectal

Acknowledgements

We thank Kelly Morgan, DMD, and Savita Dandapani, BS, for their invaluable help with data collection, Leslie Kalish, PhD, for his assistance with statistical analysis, and Gary Fleisher, MD, for his support and mentoring.

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Author contributions: DSG and CMH jointly developed the concept and design of the research. CMH undertook recruitment of patients and managed the data. DSG provided statistical advice on study design. DSG and CMH jointly participated in the analysis and interpretation of data, drafted and revised this manuscript, and approved this manuscript as submitted. DSG and CMH take responsibility for the paper as a whole.

Presented as an abstract at the Pediatric Academic Societies annual meeting, Baltimore, MD, May 2002.

Supported in part by the Exergen Corporation, Watertown, MA.

Reprints not available from the authors.

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Copyright © 2004 American College of Emergency Physicians. Published by Mosby, Inc. All rights reserved.