Cardiac output (CO) is the total volume of blood the heart pumps per minute. It is the product of heart rate (beats per minute) and stroke volume (the volume of blood ejected with each heartbeat): CO = heart rate x stroke volume. In a healthy adult at rest, cardiac output is typically 4 to 6 litres per minute, rising substantially during exercise as both heart rate and stroke volume increase.
Cardiac index (CI) adjusts cardiac output for body size by dividing by body surface area (litres per minute per square metre), allowing comparison between patients of different builds. A cardiac index below 2.2 litres per minute per square metre at rest is generally considered to indicate cardiogenic shock or severely impaired cardiac function. In the intensive care unit following cardiac arrest, cardiac output monitoring helps guide fluid therapy, vasopressor doses and the use of inotropic drugs to support the heart.
Cardiac output can be measured in several ways. Pulmonary artery catheterisation (Swan-Ganz catheter) uses the thermodilution technique to measure output directly, and was historically the gold standard in critical care. Minimally invasive systems such as PiCCO (pulse contour cardiac output) and LiDCO use arterial waveform analysis to estimate continuous cardiac output. Echocardiography can estimate cardiac output non-invasively by measuring stroke volume from the aortic outflow tract and multiplying by heart rate. Each method has specific advantages and limitations in the post-arrest setting.
After cardiac arrest, the heart may be stunned and temporarily unable to generate normal cardiac output even after the rhythm is restored, a phenomenon called myocardial stunning. This transient impairment of cardiac output can cause hypotension, reduced organ perfusion and, if severe, cardiogenic shock requiring mechanical circulatory support. Monitoring and optimising cardiac output is therefore a central goal of post-cardiac arrest intensive care during the first 24 to 72 hours.
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