Cardiac Output Measurement

Cardiac output is the volume of blood the heart ejects each minute, the product of stroke volume and heart rate. It is measured to direct fluid and vasoactive therapy in shock, heart failure, sepsis and major surgery.

Normal resting cardiac output is about 4 to 8 L/min. Cardiac index, which is cardiac output divided by body surface area, is normally 2.5 to 4.0 L/min/m².

Methods. Pulmonary artery thermodilution, transpulmonary thermodilution, pulse contour analysis, oesophageal Doppler, echocardiography, bioimpedance and bioreactance, and the Fick method. The choice depends on invasiveness, setting and available devices.

Thermodilution. A bolus of cold fluid is injected into the right atrium through a pulmonary artery catheter. The temperature change in the pulmonary artery gives cardiac output. Transpulmonary thermodilution uses a central venous line and an arterial thermistor line and also estimates extravascular lung water.

Pulse contour analysis. Arterial waveform is analysed to estimate stroke volume continuously. Calibration against thermodilution improves accuracy.

Echocardiography. Stroke volume is calculated from the left ventricular outflow tract diameter and the velocity time integral, multiplied by heart rate.

Contraindications and precautions. Pulmonary artery catheterisation is avoided in some right heart conditions and coagulopathy. Complications include arrhythmia, pulmonary artery rupture and infection.

Interpretation.

• A low cardiac index (under 2.2 L/min/m²) with raised filling pressures suggests cardiogenic shock.
• A high cardiac output with low systemic vascular resistance suggests distributive shock such as sepsis.
• Stroke volume variation of over 10 to 13% in a mechanically ventilated patient in sinus rhythm suggests fluid responsiveness.
• A rise in stroke volume of 10 to 15% after a passive leg raise predicts a response to fluid.

Causes of misleading results. Tricuspid regurgitation and intracardiac shunts affect thermodilution. Arrhythmia, aortic regurgitation and vasoactive drugs affect pulse contour methods. Spontaneous breathing invalidates variation indices.

Critical values and findings needing immediate action. A cardiac index under 2.2 L/min/m² with hypotension and hypoperfusion requires immediate haemodynamic support.

Limitations and artefacts. Absolute values differ between devices, so trends are more reliable than single values. Operator skill affects echocardiographic and Doppler methods.

Follow up tests. Lactate, central venous oxygen saturation, echocardiography, troponin, and BNP or NT-proBNP.

Special groups. Children and neonates use devices and normal ranges suited to body size. Pregnancy raises resting cardiac output.