In its simplest form, the solution for tachycardia in children has been the same since the inception of our specialty—find the source. We routinely recognize the source or have a good suspicion of what that source may be. We find the pieces, put together the puzzle, develop differential diagnoses, and risk stratify the potential outcomes. But sometimes it doesn’t all quite make sense. The goal of this discussion is to explore some short topics about pediatric tachycardia for those moments when we think, “Is this kiddo a little too tachycardic?” or “Am I missing something?”

“How much tachycardia can we attribute to fever?”

After taking into account a child’s agitation and anxiety when recording a heart rate, is there a particular amount of tachycardia that we can expect relative to each degree of fever?

A 2004 prospective study enrolled 490 children younger than one year of age.¹ Rectal temperatures were the gold standard, and children who were fussy or crying were excluded. Other known causes of tachycardia were excluded, such as dehydration warranting IV fluid rehydration, hypoxemia, albuterol within four hours, cardiomyopathy, dysrhythmia, sepsis, known endocrine diseases, and anemia. The authors were investigating the relationship between heart rate and temperature increases of 1°C. The mean temperatures in infants with and without fever were 37.2°C and 38.8°C, respectively. In infants younger than two months of age, there was no association between heart rate and temperature. In infants older than two months, the authors found that the mean increase in pulse rate per 1°C temperature increase was 9.6 beats/min (95 percent CI 7.7-11.5 beats/min).

Another retrospective study of 21,033 children evaluated heart rate via the pulse oximeter and temperature via tympanostomy thermometer.² The authors identified a 10.52 bpm/1°C increase broadly across all ages. Although the increase in heart rate (approximately 10 bpm/1°C rise) is consistent with the earlier study and the data set is large, the data analysis and exclusion criteria are very broad, making this a potentially significant limitation of the latter study’s results. Although earlier studies suggest 10 bpm/1°C, more recent studies may suggest otherwise. A recent retrospective study evaluated 61,321 children with temperatures ranging from 36°C to 40.5°C.³ Children were divided into six pre-determined age groups (zero to less than three months, zero to younger than three months, three months to younger than one year, one to younger than two years, two to younger than five years, five to younger than 10 years, and older than or equal to 10 years). In an effort to exclude any additional factors that may lead to tachycardia, the authors had an extensive exclusion criteria list. Examples included, but were not limited to, any child who required serum labs (including a serum dextrose), children with hypoxia, agitated or crying children, and those with suspected anemia, orthopedic complaints, trauma, environmental factors, overdoses, or acute abdominal complaints that would cause “intense internal pain.” The temperatures were all digital axillary readings rather than rectal or oral temperatures that are typically considered gold standards. For all groups except the zero to three-month age group, the biggest increase in heart rate per 1°C was when the temperature was rising from 37 to 38°C and was about 20 bpm. The range of increase was similar among all age groups when rising from 38 to 39°C and was approximately 10-15 bpm/1°C. The range of variation across all groups from 39 to 40°C was wider at three to 10 bpm/1°C. This study would suggest that children’s heart rates seemed to increase the most just before spiking a fever at 38°C. The 10 bpm/1°C is similar to the prior studies in the 38 to 39°C temperature range but did not seem to hold true in the 39 to 40°C range. A potential limitation of this study’s findings, though, may be the digital axillary temperature readings, which other studies have shown may not be routinely consistent with rectal temperatures.⁴ A separate 2020 retrospective study found a greater increase in heart rate at 21.5 bpm/1°C in their local subjects and 18.3 bpm/1°C when they analyzed a national database.⁵ This latter study’s primary goal was to evaluate this same topic in adults, so no exclusion criteria were used, and no summary pediatric data were published.