Cardiac output (CO) can be defined in words, units of measurement or equations. In short it is the volume of blood being pumped from the left ventricle of the heart in one minute, also expressed as liters per minute (L/min). In the average adult at rest the cardiac output will typically measure between 4.5 and 5.5 L/min. Several factors can affect cardiac output indirectly by affecting the heart rate (HR) and stroke volume (SV), the primary components of cardiac output determination often expressed by the equation CO = HR x SV.
The resting heart rate on average is between 60 and 100 beats per minute. This number is established by the heart's inherent electrical system, which acts as a pacemaker, thereby ensuring that the heart rate can be kept within range. An increase in heart rate due to stimulation or excitation by activity, drugs, medications or other sources, will result in an increase in cardiac output. A decrease in the heart rate due to an electrical abnormality and some medications can result in a decreased cardiac output. This is often true as the cardiac output is directly proportional to changes in heart rate. When heart rates become excessively fast, the heart may not have enough time to adequately fill with blood between beats; this can result in a decreased cardiac output. Medical intervention through electrical or intravenous techniques are sometimes needed to decrease the heart rate to an adequate range. Implanted defibrillators may be surgically placed to ensure that if heart rates become too fast they can be quickly reset to a normal range. When heart rates are too slow the cardiac output can also decrease significantly, and interventions may be required to increase the heart rate. Implanted pacemakers may be surgically placed to ensure a consistent adequate heart rate.
Stroke volume is the measurement of the volume of blood being forced from the left ventricle in one cardiac contraction. This is often determined by measuring the volume of blood present within the left ventricle just prior to contraction and measuring the volume of blood present after the full contraction is complete. These measurements are often referred to as end diastolic volume (EDV) and end systolic volume (ESV) respectively. Therefore, SV = EDV - ESV. This measurement can be affected by changes in the heart's ability to contract, the force of contraction, the volume of blood available to be pumped or other variables such as resistance within the circulatory system that can affect or alter these factors. Severe hemorrhage or shock, heart damage or extreme infections can change the heart's ability to pump effectively.
Factors Indirectly Affecting Cardiac Output
Catecholamines, the chemicals produced during periods of excitement such as activity, exercise or work, can cause an increase in both heart rate and stroke volume thereby greatly increasing cardiac output. Medications or poisons that slow the heart rate or decrease the heart's ability to forcibly contract will often decrease the cardiac output, often to critically low levels.
During a myocardial infarction, or heart attack, there can be significant variations in cardiac output. Often in the beginning phase of a heart attack the body responds by increasing catecholamine production, which can increase cardiac output. This can cause increased pressures against which the heart has to pump, further increasing the workload of a heart that is already experiencing muscle tissue death due to the blockage of an artery. During or even after a heart attack, the muscle tissue that is damaged can have devastating effects on the heart's ability to pump. Often the results can be significantly reduced left ventricle contraction and subsequently heart failure. Once the heart begins to fail, fluid can back up into the lungs leading to congestion or wet lung sounds. This is often referred to as congestive heart failure.
Chronic high blood pressure, smoking, drug use, kidney disease, high cholesterol, lack of exercise, poor diet and other genetic and lifestyle factors can affect the heart's ability to contract and pump blood.