Cardiac output (CO) is a fundamental measure of the heart’s performance, representing the total volume of blood the heart pumps out to the circulation every minute. A healthy resting adult typically maintains a cardiac output of approximately five liters per minute, which is roughly equivalent to the body’s entire blood volume. Adequate blood flow is necessary for delivering oxygen and nutrients to all tissues and organs.
The Foundational Calculation (Heart Rate and Stroke Volume)
The theoretical calculation for cardiac output is straightforward, defined by multiplying two primary components: Heart Rate (HR) and Stroke Volume (SV). This relationship is expressed as \(CO = HR \times SV\). HR is the number of times the heart beats per minute, typically ranging between 60 and 100 beats per minute for a resting adult.
Stroke Volume (SV) represents the volume of blood ejected from the left ventricle with each single heartbeat, measured in milliliters per beat. Any change in either the heart rate or the stroke volume will directly alter the cardiac output. During exercise, for example, the body’s metabolic demand increases, requiring the heart to raise its output by increasing both the rate and the volume of blood pumped per beat.
Invasive Clinical Measurement Techniques
While the foundational formula is simple, obtaining accurate measurements often requires sophisticated clinical techniques, particularly for critically ill patients. The Fick principle is a foundational concept, historically considered the gold standard for accuracy. It calculates cardiac output based on the body’s total oxygen consumption and the difference in oxygen content between the arterial and mixed venous blood. Although rarely used directly today due to the complexity of collecting samples, the Fick principle provides the conceptual basis for many modern flow measurements.
Thermodilution is a common, highly accurate invasive method performed using a pulmonary artery catheter (Swan-Ganz catheter). The catheter is threaded through a vein into the right side of the heart and positioned in the pulmonary artery. A known volume of cold saline is rapidly injected into the right atrium. A temperature sensor measures the resulting temperature decrease and return to baseline in the pulmonary artery, allowing a computer to calculate the cardiac output based on the rate the indicator is washed away by the blood.
Non-Invasive Estimation Methods
Because invasive procedures carry inherent risks and cannot be used for continuous monitoring, non-invasive methods have become popular for estimating cardiac output. Echocardiography, which uses Doppler ultrasound, is a widely adopted technique. It involves using sound waves to measure the velocity of blood flow through the heart valves or the aorta.
By combining the velocity measurement with the known cross-sectional area of the vessel, clinicians calculate the Stroke Volume, which is then multiplied by the Heart Rate to estimate CO. Newer, entirely non-invasive technologies include electrical bioimpedance and bioreactance. These methods involve placing electrodes on the chest to pass a low-amplitude electrical current across the thoracic cavity, measuring small changes in electrical signals that occur as the volume of blood in the aorta changes with each heartbeat.
Interpreting Cardiac Output Measurements
Calculating cardiac output is only the first step; interpreting the result requires considering the patient’s size and metabolic needs. The Cardiac Index (CI) is a refined measurement that adjusts cardiac output to the individual’s body surface area (BSA). This adjustment allows for accurate comparison between people of different sizes, providing a standardized measure of heart performance. A normal Cardiac Index falls between \(2.5\) and \(4.2\) liters per minute per square meter of BSA.
A persistently low cardiac output, or hypoperfusion, signifies that the heart is not pumping enough blood to meet the body’s demands for oxygen and can be a sign of serious conditions like heart failure or shock. Conversely, a high cardiac output can occur in hyperdynamic states. This may be seen in conditions such as severe infection (sepsis) or in endocrine disorders like hyperthyroidism, where the body’s metabolic rate is elevated. These measurements guide treatments, such as adjusting medications to increase the heart’s contractility or manage vascular resistance.