Cardiac Function - Introduction
The primary function of the heart is to impart energy to blood in order to generate and sustain an arterial blood pressure necessary to provide adequate perfusion of organs. The heart achieves this by contracting its muscular walls around a closed chamber to generate sufficient pressure to propel blood from the left ventricle, through the aortic valve and into the aorta. Simultaneously, the right ventricle contracts and propels blood through the pulmonic valve and into the pulmonary artery to perfuse the lungs.
Each time the heart beats, a volume of blood contained within the left and right ventricles is ejected into the aorta and pulmonary artery, respectively. Averaged over time, the amount of blood ejected per beat into the aorta is essentially the same as the volume of blood ejected per beat into the pulmonary artery. This stroke volume (SV), times the number of beats per minute (heart rate, HR), equals the cardiac output (CO).
CO = SV × HR
Stroke volume is expressed in mL/beat and heart rate in beats/minute. Therefore, cardiac output is in mL/minute. Cardiac output may also be expressed in liters/minute.
Normal, resting cardiac output differs among individuals of different size. Obviously, the resting cardiac output of someone who weighs 240 lbs is greater than the cardiac output found in a person that weighs 120 lbs. Therefore, in clinical practice, measured values for cardiac output are often expressed as a flow (L/min) per body surface area (m2). When cardiac output is expressed in this way, it is termed "cardiac index" and has the units of L/min/m2. The surface area is estimated from calculations based on body weight and height. Cardiac index normally ranges from 2.6 to 4.2 L/min/m2.
Related topic: Measurement of Cardiac Output