In Pediatric Advanced Life Support (PALS), cardiac output is the total volume of blood the heart pumps per minute, measured in liters per minute (L/min). It’s calculated with a simple formula: cardiac output equals heart rate multiplied by stroke volume (CO = HR × SV). Stroke volume is the amount of blood ejected with each heartbeat. This concept is central to PALS because nearly every intervention in pediatric resuscitation aims to improve one or both of those variables.
Why Cardiac Output Matters More in Children
Children depend on heart rate far more than adults do to maintain adequate cardiac output. An adult heart can significantly increase the amount of blood it pumps per beat (stroke volume) when the body needs more oxygen. A child’s heart is smaller and less compliant, so it has limited ability to increase stroke volume. That means a child’s primary way of boosting cardiac output is to beat faster.
This is why tachycardia, a faster-than-normal heart rate, is one of the earliest signs that a child’s body is under stress. It’s also why bradycardia in a pediatric patient is treated as an emergency in PALS. A slow heart rate in a child can cause cardiac output to plummet quickly, leading to hypotension, decreased consciousness, and shock. Symptomatic bradycardia is often the last warning sign before cardiac arrest in infants and children.
The Three Factors That Control Stroke Volume
Since cardiac output depends on both heart rate and stroke volume, PALS providers need to understand what determines stroke volume. Three factors control it: preload, afterload, and contractility.
Preload is the amount of blood filling the heart before it contracts. Think of it as how much the heart muscle stretches before it squeezes. More filling generally means a stronger contraction and more blood ejected, up to a point. This relationship, described by the Frank-Starling principle, explains why giving IV fluids to a dehydrated child can rapidly improve cardiac output. The extra volume increases preload, which increases stroke volume.
Afterload is the resistance the heart has to push against to eject blood. It’s essentially the pressure in the blood vessels that the heart must overcome with every beat. When afterload increases, stroke volume drops because the heart has to work harder to push the same amount of blood out. When afterload decreases, stroke volume rises. In children with certain types of shock, the blood vessels constrict tightly, raising afterload and making the heart’s job harder.
Contractility is the strength of the heart muscle’s squeeze, independent of how much blood is in the chamber. A heart that contracts weakly, whether from infection, toxins, or injury, will pump less blood per beat even if preload and afterload are normal.
How Children Compensate for Falling Cardiac Output
A child’s body has built-in mechanisms to maintain blood flow when cardiac output starts to drop. The two main responses are increasing the heart rate and constricting blood vessels. Tachycardia directly raises cardiac output by increasing the HR side of the equation. Vasoconstriction (tightening of blood vessels) redirects available blood flow to vital organs like the brain and heart, while also maintaining blood pressure.
These compensatory mechanisms can mask a serious problem. A child in early shock may have a normal blood pressure because vasoconstriction is keeping it up, even though cardiac output is already dangerously low. This is called compensated shock, and it’s a critical concept in PALS. By the time blood pressure actually drops (decompensated shock), the child has often lost a significant percentage of their blood volume or cardiac function. That’s why PALS training emphasizes recognizing the subtle early signs: fast heart rate, cool or mottled extremities, delayed capillary refill, and changes in alertness or responsiveness.
Normal Cardiac Output in Children
Because children vary enormously in size, raw cardiac output in liters per minute isn’t very useful for comparison. Instead, clinicians use cardiac index, which adjusts cardiac output for body surface area. The normal cardiac index in children under 18 is roughly 4.0 L/min per square meter of body surface area. This value naturally decreases with age, dropping by about 0.11 L/min per square meter for each year of life during childhood.
Younger children have a higher cardiac index relative to their size because their metabolic demands are proportionally greater. A newborn’s tissues consume oxygen at a higher rate per kilogram than an adolescent’s, so the heart needs to deliver proportionally more blood.
How Cardiac Output Guides PALS Interventions
Almost every algorithm in PALS connects back to cardiac output. When a child is in bradycardia with poor perfusion, the immediate goal is to raise heart rate, because that’s the fastest way to restore cardiac output. When a child is in shock from dehydration, fluid boluses increase preload and therefore stroke volume. When the heart is failing after resuscitation, medications that improve contractility or reduce afterload are used to support stroke volume.
During CPR itself, chest compressions are a manual substitute for cardiac output. The 2025 AHA guidelines for PALS recommend using measurable targets to assess whether compressions are generating enough blood flow. For children with arterial blood pressure monitoring already in place, diastolic pressure targets of at least 25 mmHg in infants and 30 mmHg in children over one year serve as benchmarks for compression quality. End-tidal CO2 monitoring also reflects cardiac output during CPR, since the amount of carbon dioxide detected in exhaled breath correlates with how much blood is circulating through the lungs.
After a child is resuscitated from cardiac arrest, maintaining adequate cardiac output remains a priority. Post-cardiac arrest syndrome commonly includes myocardial dysfunction with low cardiac output, and the current guidelines recommend keeping systolic and mean arterial blood pressure above the 10th percentile for the child’s age and sex to ensure adequate organ perfusion during recovery.