Cardiac output (CO) is a measure of the volume of blood pumped by the heart per unit of time, typically expressed in liters per minute (L/min). It represents the overall effectiveness of the heart in delivering oxygenated blood to the body's tissues and organs.
Cardiac output is calculated by multiplying the stroke volume (SV), which is the volume of blood pumped out of the heart with each contraction, by the heart rate (HR), which is the number of times the heart beats per minute:
CO = SV x HR
The stroke volume is determined by factors such as the preload (the volume of blood returning to the heart), the contractility of the heart muscle, and the afterload (the resistance the heart must overcome to pump blood into the arterial system).
The cardiac output can vary depending on the body's metabolic demands. During exercise or situations requiring increased oxygen delivery, the heart can increase its output by increasing the heart rate, stroke volume, or both. Conversely, in conditions such as heart failure or shock, cardiac output may be reduced.
Measuring cardiac output directly is typically done using specialized techniques, such as thermodilution, where a cold saline solution is injected into a central vein, and the temperature change is measured as it reaches the pulmonary artery. This technique allows for the calculation of cardiac output based on the thermal dilution principle.
Cardiac output is an important parameter in evaluating cardiovascular function, assessing tissue perfusion, and guiding treatment decisions in various clinical settings, including critical care, cardiology, and anesthesia. It helps clinicians determine the need for fluids, inotropic medications, or other interventions to optimize cardiac function and maintain hemodynamic stability.