Cardiac Output FAQs: Your Questions Answered by Experts

Cardiac output is the volume of blood pumped by the heart (specifically, by either the left or right ventricle) in one minute. It’s a key indicator of how well your heart is functioning to supply blood to the body. You can read a detailed explanation on our “What is Cardiac Output?” page.

Cardiac output is best defined as the total volume of blood ejected by a ventricle per minute. It represents the heart’s pumping capacity and is calculated as the product of heart rate and stroke volume (CO = HR × SV). This measurement indicates how effectively the heart delivers oxygenated blood to meet the body’s metabolic demands.

The basic formula is CO = Heart Rate (HR) × Stroke Volume (SV). Clinically, it can be measured or estimated using various methods like the Fick principle, thermodilution, or Doppler echocardiography. We provide calculators for these methods and a guide on how to calculate CO.

To calculate cardiac output in mL/min, multiply heart rate (beats/min) by stroke volume (mL/beat). For example: if HR = 70 beats/min and SV = 70 mL/beat, then CO = 70 × 70 = 4,900 mL/min (or 4.9 L/min). This unit is particularly useful in pediatrics where smaller volumes are involved.

Echocardiographic cardiac output calculation uses the formula: CO = SV × HR, where stroke volume is determined by measuring the left ventricular outflow tract (LVOT) area and velocity-time integral (VTI). SV = LVOT area × VTI. The LVOT area is calculated as π × (LVOT diameter/2)². Our echo-based calculator provides step-by-step guidance.

For a resting adult, normal cardiac output is typically between 4.0 to 8.0 liters per minute (L/min). However, this varies with body size, age, and activity level. Cardiac Index (CI), which normalizes CO to body surface area, has a normal range of 2.5 to 4.0 L/min/m². See our normal values page for more details.

The normal range of cardiac output in mL/min is 4,000 to 8,000 mL/min (equivalent to 4-8 L/min) for resting adults. This can increase significantly during exercise, potentially reaching 15,000-25,000 mL/min in trained athletes. Values below 4,000 mL/min may indicate cardiac dysfunction, while values above 8,000 mL/min at rest may suggest conditions like hyperthyroidism or anemia.

Stroke volume is the amount of blood ejected by a ventricle with each heartbeat. It’s typically 60-100 mL in resting adults. Our Stroke Volume Calculator provides more information.

Heart Rate (HR) is measured by counting heartbeats per minute, either manually (pulse check) or using monitoring equipment. Stroke Volume (SV) can be calculated using echocardiography (SV = LVOT area × VTI), thermodilution, or derived from CO if HR is known (SV = CO ÷ HR). SV can also be estimated as End-Diastolic Volume minus End-Systolic Volume.

Cardiac Index (CI) is cardiac output divided by body surface area (BSA). It’s used to normalize CO for differences in body size, allowing for more accurate comparisons of heart function between individuals. Learn the differences between CO and CI here.

Cardiac output is typically measured in liters per minute (L/min) for adults, or milliliters per minute (mL/min) for pediatric patients or when precision is needed. In some contexts, it may be expressed as mL/kg/min (indexed to body weight) or L/min/m² (cardiac index, indexed to body surface area).

Cardiac output is commonly measured in liters (L/min) for adult patients as it provides convenient numbers (typically 4-8 L/min). However, it can also be expressed in milliliters (mL/min), especially in pediatric medicine, research settings, or when precise calculations are needed. Both units are correct: 1 L/min = 1,000 mL/min.

The most appropriate unit depends on the clinical context. L/min is standard for adult clinical practice, mL/min for pediatrics and research, L/min/m² for cardiac index (normalized to body surface area), and mL/kg/min when indexing to body weight, particularly in pediatric patients.

A cardiac output of 4.7 L/min falls within the normal range (4.0-8.0 L/min) for a resting adult. This means the heart pumps 4.7 liters of blood per minute, which is adequate for meeting the body’s metabolic needs at rest. However, interpretation should always consider the patient’s body size, age, and clinical condition.

Abnormal cardiac output is typically defined as values outside the normal range of 4.0-8.0 L/min for adults at rest. Low CO (<4.0 L/min) may indicate heart failure, cardiogenic shock, or severe dehydration. High CO (>8.0 L/min) at rest may suggest hyperthyroidism, anemia, sepsis, or arteriovenous fistulas. Clinical context is crucial for interpretation.

LV output refers to Left Ventricular output, which is the volume of blood pumped by the left ventricle per minute. In healthy individuals, LV output equals right ventricular output and represents the systemic cardiac output. LV output is what’s typically measured when assessing cardiac output, as it delivers oxygenated blood to the body’s organs and tissues.

Cardiac power output (CPO) measures the rate of energy transfer by the heart, calculated as: CPO = (Mean Arterial Pressure × Cardiac Output) ÷ 451. Normal CPO is approximately 1.0-2.0 watts. It represents the heart’s hydraulic work and is a strong predictor of outcomes in heart failure and cardiogenic shock. CPO provides insights into both flow (CO) and pressure (MAP) components of cardiac performance.

The four main factors are preload (ventricular filling), afterload (resistance to ejection), myocardial contractility (heart muscle strength), and heart rate. Our page on factors affecting cardiac output explains these in detail.

Low CO means inadequate blood supply to organs, leading to symptoms like fatigue, dizziness, organ dysfunction, or shock. High CO can occur in conditions like anemia, sepsis, or hyperthyroidism, and may strain the heart over time. See CO in various medical conditions for more.

Common methods include:

• Invasive: Thermodilution (using a pulmonary artery catheter), Fick method.
• Non-invasive/Minimally Invasive: Doppler echocardiography, arterial pulse contour analysis, esophageal Doppler, bioreactance.

You can find a comparison of methods here.

No single method is “best” for all situations. The choice depends on the clinical need, patient condition, desired accuracy, invasiveness, and available resources. The Fick method (direct) is often considered a research gold standard, while thermodilution is a clinical gold standard in ICUs. Doppler echo is excellent for non-invasive assessment. Our method comparison guide helps clarify choices.

Stroke Volume (SV) is the amount of blood ejected by the ventricle with each heartbeat (typically 60-100 mL). Cardiac Output (CO) is the total volume pumped per minute. The relationship is: CO = SV × HR. SV represents the volume per beat, while CO represents the total flow rate. Changes in either SV or HR will affect CO.

Ejection Fraction (EF) is the percentage of blood ejected from the ventricle with each heartbeat (normal: 50-70%). Stroke Volume (SV) is the actual volume of blood ejected (normal: 60-100 mL). EF = (SV ÷ End-Diastolic Volume) × 100%. EF indicates the efficiency of ventricular emptying, while SV indicates the actual volume pumped per beat.

Yes, an increase in stroke volume will increase cardiac output if heart rate remains constant, since CO = SV × HR. For example, if SV increases from 70 mL to 90 mL while HR stays at 70 bpm, CO increases from 4.9 L/min to 6.3 L/min. However, the body may compensate by adjusting heart rate, so the net effect on CO depends on both variables.

At GCSE level, cardiac output is defined as the volume of blood pumped by the heart in one minute. The simple formula taught is: Cardiac Output = Heart Rate × Stroke Volume. Students learn that normal resting CO is about 5 L/min, and that it increases during exercise to meet higher oxygen demands. This concept helps explain how the circulatory system adapts to different physiological needs.

The GCSE formula for cardiac output is: CO = HR × SV, where CO is cardiac output (L/min), HR is heart rate (beats/min), and SV is stroke volume (L/beat or mL/beat). Students typically use this formula with example values like: HR = 70 beats/min, SV = 0.07 L/beat, therefore CO = 70 × 0.07 = 4.9 L/min.

Children, and especially neonates, have higher resting heart rates and are more dependent on heart rate to modulate CO. Their stroke volumes are smaller. CO is often indexed to body weight (mL/kg/min) or BSA. Normal values are different and change rapidly with growth. We have dedicated calculators for pediatric CO and neonatal CO.

False. Blood pressure is determined by CO and systemic vascular resistance (SVR). A patient can have a normal BP with low CO if SVR is very high (e.g., in severe heart failure or hypovolemic shock with intense vasoconstriction). Conversely, BP can be low with high CO if SVR is very low (e.g., early septic shock).

False. While critical in the ICU, CO assessment is valuable in many settings: cardiology clinics (heart failure, valve disease), operating rooms, emergency departments, and even in sports physiology. The clinical applications are broad.