Cardiac output refers to the abstraction of blood that the heart pumps in a minute, depending on the heart rate, preload, afterload and contractility. Understanding the pertinence of these four components is essential when understanding cardiac output. This article will use a very simple analogy for analysing cardiac output – an example of the speed of a bicycle. It will also highlight the effects of numerous diseases that cause cardiac output and its four components and how to measure stroke volume and cardiac output.
What is Cardiac Output?
The cardiac output is in minutes/litres, i.e., the number of litres of blood the heart forms in a minute. The cardiac output is calculated as the product of the number of beats per minute or heart rate and stroke volume. Sometimes the term cardiac output is possibly misleading as to how to interpret the cardiac output.
The heart is an integral part of a larger and more effective cardiovascular system. It is conditional on both extracardiac and cardiac factors depending on the practical relevance and applicability of four components: heart rate, preload, afterload and contractility.
Heart Rate
The heart rate is the simplest characteristic or determinant in understanding cardiac output. The faster the heartbeat, the more blood can be pumped over time intervals. It is similar to how the faster the cyclist paddles, the quicker the bicycle will go. Still, sometimes a cyclist will get tired and unable to maintain that constant pedalling rate, resulting in the slow speed. If the cyclist pedal fast, the cycle will move. If the cyclist pedals slowly, the bicycle will speed very slow and be unable to reach the destination in the given time. Similarly, if the heart rate is too low, usually it’s a disease of severe bradyarrhythmia, and if it is too fast, then the cardiac output is damaged.
Contractility
If the cyclist tries to flex their muscles and at the same time make more effort to paddle faster, the bicycle will move way quicker. Similarly, the increase of contractility of the muscles in the heart leads to an increase in cardiac output.
If the contractility of the heart muscles is low, it will result in a very low cardiac output. If it is too much, it will result in fatigue or complete collapse, and it needs to stop or slow down.
Preload
Preload refers to the degree of myocardial distension preceding shortening. Demonstrated by Ernest Starling and Otto Frank, preload is a built-in property of myocardial cells. The force of their muscular contraction depends solely on the extent to which they are stretched. The greater the force of strength, the larger the contraction force is felt. So when there is an increase in expansion of ventricles, it will result in the force of contraction, which will result in the increment of cardiac output. Preload is generally dependent on the amount of ventricular filling.
Afterload
The force against which the ventricles must act to chuck out blood, and the force that is majorly dependent on the vascular tone and arterial blood pressure, is known as the afterload. The increment in cardiac output is seen by reducing afterload, especially in the condition of contractility damage.
How to Measure Stroke Volume and Cardiac Output
Stroke volume is the amount of blood chucked out from the ventricles with each cycle of cardiac arrest. Stroke volume can be obtained by subtracting end-systolic volume to end-diastolic volume and multiplying the heart rate with the stroke volume, which came as an output resulting in cardiac output, having the unit in litres per minute.
Conclusion:
Cardiac output is the product of heart rate and stroke volume. It is measured in liters per minute. Heart rate is defined as the number of times the heart beats in one minute. Stroke volume is the volume of blood ejected during ventricular contraction or for each stroke of the heart.