Tachycardia is defined as an abnormally fast heart rate. In pediatric patients, this definition is entirely dependent on the child’s age, not a single number. A heart rate normal for an infant would be dangerously fast for an adolescent, reflecting significant physiological changes during growth. Because a child’s baseline heart rate is naturally much higher than an adult’s, understanding the expected rate for a child’s specific age is necessary before determining if the rhythm is truly accelerated.
Defining Tachycardia by Age Group
The numerical threshold for defining tachycardia decreases progressively as a child gets older. In clinical practice, a heart rate is considered accelerated when it consistently exceeds the upper limit of the normal range for that specific age group while the child is at rest.
For a newborn or infant up to one month old, a heart rate above 180 beats per minute (bpm) may be considered tachycardia, though some pathological rhythms exceed 200 bpm. For an infant between one month and two years of age, the threshold is often set at over 160 bpm.
This rate continues its decline into early childhood; a child between two and five years old is considered tachycardic if their rate is above 140 bpm. By school age (six to eleven years old), a heart rate exceeding 130 to 140 bpm generally crosses the threshold for concern. Adolescents over twelve years old are typically held to the adult standard, with a heart rate above 100 bpm defined as tachycardia. These figures refer to a sustained, rapid rate rather than a brief increase due to activity or excitement.
Physiological Reasons for Higher Pediatric Rates
The primary reason children have significantly higher baseline heart rates than adults is their reliance on rate to maintain adequate cardiac output. Cardiac output, the volume of blood pumped per minute, is calculated by multiplying the heart rate by the stroke volume (the amount of blood ejected with each beat).
Because a child’s heart is smaller and less muscular, its stroke volume is relatively fixed and cannot increase substantially to meet metabolic demands. To compensate, the heart must beat faster—a mechanism known as chronotropy—to ensure growing tissues receive sufficient oxygen and nutrients.
Children also have a faster overall metabolic rate compared to adults due to the energy demands of rapid growth and development. As children age, their heart muscle matures and enlarges, allowing for a greater stroke volume. This, combined with increased influence from the parasympathetic nervous system, leads to a gradual slowing of the resting heart rate.
Common Triggers for Rapid Heart Rate
Most instances of a rapid heart rate in children are classified as sinus tachycardia, a normal physiological response to an underlying temporary stressor. A common trigger is fever, where the heart rate typically increases by approximately ten beats per minute for every degree Celsius rise in body temperature. Other expected causes include pain, anxiety, or excitement, which stimulate the body’s sympathetic nervous system (“fight or flight” response).
Dehydration (hypovolemia) is another frequent non-cardiac cause, as the body attempts to circulate a lower volume of blood more rapidly to maintain blood pressure. Less common non-cardiac causes include anemia, where the heart speeds up to compensate for reduced oxygen-carrying capacity, and the side effects of stimulant medications or excessive caffeine intake. This temporary, physiological response must be distinguished from pathological tachycardias, such as Supraventricular Tachycardia (SVT), which are caused by an electrical short-circuit in the heart itself.
Recognizing Signs of Distress
While a rapid heart rate can be a normal response, parents should watch for signs indicating the child is not tolerating the acceleration well. In infants, who cannot verbally express discomfort, these signs are often subtle, including unusual lethargy, poor feeding, or excessive irritability. Pale or grayish-blue skin color (pallor or cyanosis) is a serious sign of poor blood circulation.
Older children may report symptoms such as dizziness, a sensation of their heart “pounding” in their chest, or chest pain. Any episode of fainting (syncope) that accompanies a rapid heart rate requires immediate medical attention. Difficulty breathing, shortness of breath, or a noticeable decrease in urine output are also indicators that the heart is struggling to pump effectively.