Factors affecting cardiac output

This is an automatically translated article.

The article is professionally consulted by Master, Doctor Nguyen Tung Hoanh - Interventional Cardiologist - Department of Resuscitation - Emergency - Vinmec Nha Trang International General Hospital.
Cardiac output changes depend on many factors. Changes in cardiac output may be a sign of response to an increase in the body's oxygen demand or are serious diseases affecting cardiac output.

1. What is cardiac output?

Cardiac output is the amount of blood the heart pumps out in a given unit of time, usually in minutes. Normally, cardiac output can vary depending on how active the body is. For healthy men, resting cardiac output averages about 5.6 l/min. For healthy women, it is about 4.9 l/min. In addition, cardiac output also changes based on age factors.
Cardiac output will be influenced by many different factors and individual health status of each person. However it is mainly governed by 4 factors.

2. Factors that can affect cardiac output

The formula for cardiac output is stroke volume times heart rate. In which stroke volume is affected by preload, afterload and myocardial contractility. In healthy adults, stroke volume is about 60-90 ml/swallow. Thus, cardiac output is influenced by four factors: heart rate, preload, afterload, and contractility.

2.1 Heart rate

Heart rate or heart rate is the number of times the heart contracts to pump blood into the arteries in a unit of time, normally the resting heart rate is about 70-80l/min. During activity, the demand for oxygen increases, the heart rate also increases to increase cardiac output to the organ tissues.
However, if this rate is too slow, it may be caused by an arrhythmia or people who play intense sports for a long time, reducing the heart rate. Thus, some erratic changes in heart rate are very easy to cause danger to health and life, even cardiogenic shock.
The heart rate is regulated by the autonomic nervous system; hormones; K+, Ca2+ and Na+ ions and some drugs.

2.2 Contraction of the heart

The heart contracts to push blood into the arteries, so if the force of contraction of the heart muscle is fast and strong, the cardiac output will increase, when the heart's ability to contract is reduced, the output will also decrease to a significant extent. However, in cases where the heart contracts too much, it can cause myocardial ischemia, cause chest pain, heart arrhythmias, which can lead to death.
Agents that increase myocardial contractility include: Sympathetic nerve stimulants, substances that increase intracellular Ca2+ and drugs such as Digitalis.
Factors that decrease myocardial contractility include: inhibition of sympathetic nervous system activity, hypoxia, metabolic acidosis, and increased K+ in the extracellular fluid.

2.3 Charges

Preload is the volume of blood that is filled into the ventricles at the end of diastole, the greater the preload, the greater the force of contraction. Thus, when preload increases, that is, the amount of blood returning to the heart is large, the more blood is pushed into the arteries by the heart, which means an increase in cardiac output. Conversely, when the blood flow to the heart decreases, the preload decreases, which can cause a decrease in cardiac output.

2.4 Afterload

Afterload is the resistance that the ventricles must exert to expel blood. It depends mainly on factors including arterial blood pressure and vascular tone.
When reducing afterload can increase cardiac output. Especially in the case of heart failure, the heart is having a reduced ability to contract. Conversely, an increase in afterload will decrease cardiac output.

3. Changes in cardiac output

Cardiac output can change up and down due to the influence of many factors.

3.1 When does cardiac output increase?

Cardiac output increases with a certain increase in heart rate, increasing myocardial contractility, increasing preload, and decreasing afterload. In some cases, an increase in cardiac output can be seen such as:
When the body is active, the oxygen demand for the organs increases. Septic shock, anaphylactic shock. Afterload reduction: ventricular thickening... Increased preload: When aortic pressure increases, ventricular chambers dilate, varicose veins...

3.2 When does cardiac output decrease?

Reduce preload: Atrioventricular valve stenosis; increased heart rate, arrhythmia; impaired ventricular diastolic function; hypovolemic shock. Increased afterload: Hypertension, aortic and pulmonary stenosis, embolic shock. Thus, cardiac output changes when there are changes in the factors affecting cardiac output. However, in the early stages of cardiovascular disease there is usually not much change in cardiac output because of compensatory mechanisms. But in later stages, this compensation is no longer sufficient to respond, causing changes in cardiac output.
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