Cardiac output is calculated by multiplying stroke volume by heart rate. The result tells you how many liters of blood the heart pumps per minute, and a normal resting value falls between 5 and 6 liters per minute. That simple multiplication is the foundation, but several different methods exist for measuring the inputs, ranging from ultrasound to invasive catheter techniques.
The Basic Formula
The core equation is straightforward:
Cardiac Output = Stroke Volume × Heart Rate
Stroke volume is the amount of blood ejected with each heartbeat, measured in milliliters. Heart rate is the number of beats per minute. If someone’s heart ejects 70 mL per beat and their heart rate is 75 beats per minute, their cardiac output is 5,250 mL per minute, or about 5.25 liters per minute.
This formula works well conceptually, but measuring stroke volume directly isn’t as simple as counting a pulse. That’s where the various clinical measurement techniques come in.
What Determines Stroke Volume
Three factors control how much blood the heart ejects with each beat, and understanding them helps you see why cardiac output changes in different conditions.
Preload is the degree to which the heart muscle stretches before it contracts, determined by how much blood fills the ventricle. More filling means more stretch, and up to an optimal point, more stretch produces a stronger contraction and a larger stroke volume. This is the Frank-Starling principle: the heart automatically pumps out more when it receives more.
Afterload is the pressure the heart has to push against to eject blood. Higher afterload (from high blood pressure or a narrowed aortic valve, for example) makes it harder for the ventricle to empty, which reduces stroke volume.
Contractility is the intrinsic squeezing strength of the heart muscle, independent of how much it’s stretched. When contractility drops, the ventricle can’t empty as completely, and the leftover volume increases while stroke volume falls. Conditions like heart failure reduce contractility; exercise and certain medications increase it.
Stroke volume is essentially the difference between how much blood fills the ventricle and how much remains after contraction. Anything that increases filling or squeezing strength raises stroke volume. Anything that increases resistance or weakens the muscle lowers it.
Measuring Cardiac Output With Ultrasound
Echocardiography is the most common non-invasive method for calculating cardiac output, and it follows a logical chain of measurements. The idea is to figure out stroke volume by measuring how much blood flows through the left ventricular outflow tract (the passage just before the aortic valve) with each beat, then multiply by heart rate.
The process has two steps. First, you measure the diameter of the outflow tract on the ultrasound image, taken in the parasternal long-axis view. That diameter gets plugged into the formula for the area of a circle to calculate the cross-sectional area of the tract. Second, you use Doppler ultrasound to measure the velocity of blood flowing through that same spot. The machine traces the velocity over one heartbeat and calculates the velocity-time integral, which represents the distance a column of blood travels during one ejection.
Multiplying the cross-sectional area by the velocity-time integral gives you stroke volume in cubic centimeters. Multiply that by heart rate, and you have cardiac output. The accuracy depends on getting the diameter measurement right, since even small errors get squared in the area calculation and can shift the result significantly.
The Fick Method
The Fick principle, first described in 1870, takes a completely different approach. Instead of measuring blood flow directly, it calculates cardiac output based on how much oxygen the body consumes.
The logic is elegant: the total oxygen your body uses per minute must equal the amount of oxygen delivered by arterial blood minus the amount returning in venous blood. If you know oxygen consumption and the difference in oxygen concentration between arterial and venous blood, you can solve for how much blood must be flowing to account for that oxygen pickup.
Cardiac Output = Oxygen Consumption ÷ (Arterial Oxygen Content − Venous Oxygen Content)
Oxygen consumption can be measured directly through analysis of inhaled and exhaled gases, similar to what happens during a cardiopulmonary exercise test. Arterial oxygen content comes from a standard arterial blood sample, and venous oxygen content requires a sample from the pulmonary artery (mixed venous blood), which means this method typically requires a catheter. The Fick method is considered highly reliable and serves as a reference standard, especially when other measurements are questionable.
The Thermodilution Technique
Thermodilution is the most widely used invasive method and relies on a catheter positioned in the pulmonary artery. A known volume of cold saline is injected into the right side of the heart, and a temperature sensor downstream tracks how much the blood temperature drops and how quickly it returns to normal.
The underlying principle is that cardiac output is inversely proportional to the temperature change. If the heart is pumping a large volume of blood, the cold saline gets diluted quickly and the temperature barely dips. If cardiac output is low, the cold bolus lingers and produces a larger, longer temperature drop. The area under the temperature-time curve is the key measurement: a smaller area means higher output, and a larger area means lower output.
Modern pulmonary artery catheters automate this calculation. The clinician injects the saline, and the catheter’s computer applies the correction factors (accounting for the volume and temperature of the injectate, the patient’s blood temperature, and catheter-specific constants) to produce a cardiac output number in liters per minute.
Cardiac Index: Adjusting for Body Size
A cardiac output of 5 liters per minute means something different for a person who weighs 50 kg versus one who weighs 100 kg. Cardiac index solves this by dividing cardiac output by body surface area:
Cardiac Index = Cardiac Output ÷ Body Surface Area
Body surface area is calculated from height and weight (most commonly using the DuBois formula), and the result is expressed in square meters. A normal cardiac index typically falls between 2.5 and 4.0 liters per minute per square meter. This standardized value makes it possible to compare heart function across patients of very different sizes and is the preferred metric in critical care settings.
Why Cardiac Output Matters Clinically
Cardiac output is central to understanding how well the body’s organs are being perfused, and it becomes especially important when someone is in shock. Different types of shock produce characteristic cardiac output patterns that guide treatment in opposite directions.
In cardiogenic shock, the heart itself is failing, and cardiac output drops because the ventricle can’t contract effectively. A massive heart attack is a classic cause. In hypovolemic shock (from severe bleeding or dehydration), the problem isn’t the pump but the volume: less blood returns to the heart, filling drops, stroke volume falls, and cardiac output decreases unless the heart rate compensates enough to make up the difference.
Distributive shock, which includes sepsis, flips the pattern. Blood vessels dilate widely, and in many cases cardiac output is actually normal or even elevated. The problem is that blood bypasses the capillary beds where oxygen exchange happens, so tissues starve despite adequate flow. Recognizing these patterns through cardiac output measurement helps clinicians identify the type of shock and choose the right intervention.
Outside of emergencies, cardiac output calculations are used to assess heart failure severity, evaluate valve disease, monitor patients during and after surgery, and track responses to medications that affect heart function.