Cardiac Function - Introduction
The primary function of the heart is to impart energy to blood in order to generate and sustain arterial blood pressure at a level to provide adequate perfusion of organs. The heart achieves this by contracting its muscular walls around a closed chamber to generate pressure to propel blood from the left ventricle, across the aortic valve and into the aorta. Simultaneously, the right ventricle contracts and propels blood across the pulmonic valve and into the pulmonary artery to perfuse the lungs.
Calculation of Cardiac Output
Averaged over time, the amount of blood ejected per beat into the aorta (stroke volume) 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. If the SV is 70 mL/beat and the HR is 70 beats/min, then the CO is 4,900 mL/min, or 4.9 L/min.
Calculation of Cardiac Index
Normal resting cardiac output differs among individuals of different sizes. Obviously, the resting cardiac output of someone who weighs 240 lbs is greater than the cardiac output found in a person who weighs 120 lbs. Therefore, 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" (CI) 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