Cardiac output is a measure of the overall functioning of the heart. Cardiac output is the volume of blood pumped by the heart in a minute or it is the amount of blood pumped by the each ventricle of the heart in one minute. Cardiac output is equal to the heart rate multiplied by the stroke volume. The typical cardiac output for an average adult at rest is 4900 ml / minute. Cardiac output increases according to the needs and requirements of the body and may reach up to 30 liters / minute during vigorous exercise.

Stroke volume depends on several factors including the rate at which blood returns to the heart through veins; how vigorously the heart contracts; and the pressure of the blood in the arteries. If either the heart rate or stroke volume or both increase, it results in increase of cardiac output. Most of the increase in cardiac output in a healthy but untrained individual is attributed to increase in heart rate. Cardiac output may also be increased by change in posture, increased sympathetic nervous system activity and decreased parasympathetic nervous system activity. During exercise, sympathetic nerve fibers increase heart rate. At the same time, stroke volume increases, primarily because venous blood returns to the heart more quickly and the heart contracts more vigorously. Many of the factors that increase heart rate also increase stroke volume.

Cardiac output is measured using several invasive and non-invasive methods. Non-invasive methods take into consideration that the pressure in the heart increases as blood is forced into the aorta. As the aorta gets stretched, the pulse pressure also increases. For every two 2ml increase in blood volume, the pressure increases by 1mm Hg. Therefore stroke volume = 2ml X Pulse Pressure. As cardiac output is the product of stroke volume and heart rate, cardiac output = 2ml X Pulse Pressure X heart rate.

The pulmonary artery catheter (PAC) also known as the Swan-Ganz thermodilution catheter provides right heart blood pressures. Using the PAC thermodilution cardiac output can be measured. Modern catheters are fitted with a distal heated filament, which allows automatic thermodilution measurement via heating the blood and measuring the resultant thermodilution trace. This provides near continuous cardiac output monitoring. The PAC is used in assessment of hemodynamic status and direct intracardiac and pulmonary artery pressures. The distal (pulmonary artery) port allows sampling of mixed venous blood for the assessment of oxygen transport and the calculation of derived parameters such as oxygen consumption, oxygen utilization coefficient, and intrapulmonary shunt fraction.

The PAC is balloon tipped which can be inflated to occlude the pulmonary artery, the subsequence back pressure is a reflection of the left atrial filling pressure and until recently was considered a good indicator of preload.

The pulmonary artery wedge pressure (PAWP) has been superseded by more reliable techniques such as intrathoracic blood volume or stroke volume variation as indicators of volume status. The PAC also allows sampling of mixed venous blood, the oxygen content of which can be used to indicate the adequacy of overall oxygen delivery. The PAC has fallen out of common use as clinicians favour less invasive, less hazardous technologies for monitoring haemodynamic status. Considerable controversy exists over whether the PAC increases mortality; recent studies suggest it neither increases nor improves mortality. Complications such as cardiac tamponade, pulmonary artery rupture and air emboli are a danger.