Cardiac output (CO) is the volume of blood pumped by the left ventricle (or right ventricle) per minute. CO pumped by the RV is also called the pulmonary blood flow. The volume of blood pumped per beat is called the stroke volume (SV).
The output of the two ventricles is essentially equal over a period of time (minutes), whereas the SV for any one beat can be different for the two ventricles. Inspiration increases return of blood to the right ventricle (RV) resulting in an increased SV for a few beats. This results in an increased filling of the left ventricle (LV) and increased SV by the LV for a few beats. CO, during resting conditions, can be estimated as 80 ml per kilogram of body weight or 8% of the body weight in kilograms. For a man weighing 70 kg, the CO is approximately 5600 ml/min or 5.6 liters/min under resting conditions. CO can increase four- to sixfold depending on the metabolic requirements of the body. As O2 consumption by the body increases, as with exercise, CO increases proportionally in order to deliver more O2 to the active tissues (Figure 14-1). As O2 consumption by the body decreases, as with sleep, CO also decreases.
Cardiac output is linearly related to whole body work and whole body oxygen consumption. Cardiac output delivers the oxygen in the blood to support the work performed by the body and the oxygen is consumed by the body to produce the ATP to perform the work.
In order to normalize the CO in patients of different body sizes, CO is divided by the body surface area (BSA). This value is called the cardiac index (CI), and its units are liters per minute per square meter or BSA. BSA can be estimated from the height (cm) and weight (kg) of an individual. The CI for the man weighing 70 kg with a BSA of 1.7 m2 equals 3.3 liters/min/m2.
Measurement of Cardiac-Output
CO can be measured either invasively or noninvasively. One common method in animal research is to place a flow probe around the ascending aorta (or the main pulmonary artery). Ultrasonic and electromagnetic flow probes generally measure the velocity of flow (centimeter per second, cm/s). Volume flow is calculated by multiplying the measured velocity (cm/s) by the blood vessel cross-sectional area (cm2):