Which of the following heart rates is considered abnormal for a 3-month-old infant?

In a 3-month-old infant, the normal resting heart rate typically ranges from 100 to 160 beats per minute. This range reflects the higher metabolic demands and activity levels of infants as they are developing rapidly.

A heart rate of 80 beats per minute is considered abnormal for this age group as it falls significantly below the normal range. A resting heart rate this low could indicate bradycardia, which can be concerning and may require further evaluation by a healthcare professional. In contrast, heart rates of 120, 150, and 160 beats per minute are within the expected normal range for a 3-month-old, demonstrating appropriate cardiovascular responsiveness to both rest and activity levels. Therefore, identifying 80 beats per minute as abnormal is based on understanding the normal physiological parameters for infants.