Cardiac output can be simply defined as the amount of blood that the heart is able to pump in one minute, and it is based on the heart rate, preload, afterload and contractility. When interpreting the cardiac output values, it is important to understand the applicability and practical relevance of each of the four components. Cardiac output is measured in liters per minute, and is logically equal to the product of the stroke volume and also the number of beats per minute, also known as the heart rate.
Heart rate is one of the factors that can affect cardiac output. The faster the heart beat, the more the amount of blood that can be pumped over a given period of time. For example, if a cyclist is pedaling a bike, it can be said that the faster he or she pedals, the faster the bike will be able to move. However, it should also be noted that if the cyclist continues peddling too fast for too long, he or she will get tired and unable to maintain the speed, thus, the bike will slow down. In a similar way, if the heart rate is too slow or too fast, the cardiac output can be impaired
With regards to contractility, we can still use the example of a cyclist to be able to understand the concept. In case the cyclist flexes his or her muscles more and pedals a little faster, the bicycle will move faster. The can be compared to an increase in contractility of the muscles of the heart that can result into increased cardiac output. When the contractility is impaired such that it moves slowly, cardiac output will be significantly reduced. However, you should keep in mind that too much effort can result into fatigue, an occurrence that may further lad to complete collapse.
Preload greatly depends on the amount of ventricular filling. An increase in the distension of the ventricle results into an increase in the contraction force that is similarly reflected in the cardiac output. Preload can be compared with a tailwind that allows the cyclist to move faster without additional muscular effort. However, it gets to a point when the preload value in the heart is eventually reached and there is no longer an increase in cardiac output.
Afterload is defined as the force against which the ventricles must act so as to discharge blood, and is based on arterial blood pressure and vascular tone. Still with reference to the example of a cyclist, cycling on a large smooth road compared to a narrow and bumpy one with a gentle down slope can significantly increase the speed at which the bicycle moves. Reducing afterload can increase cardiac output, especially in conditions whereby contractility is hindered.
Apart from the above mentioned, there are also some factors that have also been cited to impact cardiac output like chemicals produced during times of excitement, medications, chronic high blood pressure, kidney diseases among several others. However, these can affect cardiac output indirectly.
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