Also known as heart output, cardiac output describes the volume of blood which is being pumped by the heart; this is done by the left ventricle and the right ventricle per one compression or unit time.
An excellent example of the human heart to exhibit its efficiency is cardiac output which is related to the amount of blood pumped by the heart per minute. Normally, a cardiac output results in pumping of about 5-6 litres of blood per minute at resting position; it can be said that it’s the normal cardiac output. We will get this figure with the help of calculation mentioned further in the article.
Cardiac Output Definition
Amount of blood ejected by each ventricle in one minute is the cardiac output. It (CO) is measured as ml/minute.
How to Calculate Cardiac Output
Cardiac output can be calculated using two variables-
Heart Rate (HR) - It is measured by knowing the number of beats in one minute. And cardiac output also can be related to the heart rate.
Stroke Volume (SV) - It is the volume of blood pumped by each ventricle with each beat. It is measured in ml/beat.
So, the Cardiac Output formula goes like this-
CO = HR * SV
(ml/min) (beats/min) (ml/beat)
The average Heart Rate for a person is 75 beats/min and Stroke Volume is 70 ml/beat; with this we get Cardiac Output as 5,250 ml/Min or we can say 5 L/Min.
So, the normal cardiac output range for a person i.e. the amount of blood passing through the heart each minute is 5-6 L.
Let’s Know About the Terms Heart Rate and Stroke Volume in More Detail.
Heart Rate - It is how many beats per minute and factors that positively influence the heart rate (to increase the heart rate) are called positive chronotropic factors. We get over 100 beats per minute with a fast heart rate called tachycardia. Examples of positive chronotropic factors include sympathetic stimulation secreting adrenaline/noradrenaline or epinephrine/norepinephrine to increase heart rate. Other examples are hypercapnia which results in increased CO2 and drugs like atropine.
The negatively influencing factors are called negative chronotropic factors that are responsible for below 60 beats per minute called bradycardia. Examples are parasympathetic stimulation whose main neurotransmitter is acetylcholine, hypoxi that decreases oxygen and then heart rate and hypercalcemia that increases Calcium and decreases heart rate.
Stroke Volume - It is influenced by three factors namely Preload, Afterload and Contractility.
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Determinants of Cardiac Output
As stroke volume influences the cardiac output, we can say that there are four determinants of cardiac output.
Heart Rate - As we mentioned above, increase in heart rate increases the cardiac output.
Preload - It is the volume of blood entering the ventricles. Increased volume of blood entering the heart results in stretch of ventricle and increase in preload takes place. It ultimately affects stroke volume and then increases cardiac output.
According to the Frank Starling mechanism, the amount of blood that enters into the ventricles will be the amount of blood ejected from the ventricles to the body. So, end diastolic volume (the volume of blood in the ventricles prior to ejection) will be proportional to the stroke volume.
Afterload - It is the resistance ventricles must overcome to circulate blood around the body. Increase in resistance means increase in afterload. Therefore, increase in afterload means decrease in stroke volume and vice versa. All this influences cardiac output.
Contractility - It is basically how hard the myocardium contracts for a given preload. Increase in contractility means increase in cardiac output and vice versa.
Factors that increase contractility are positive inotropic factors like sympathetic stimulation, caffeine, hypercalcemia. Negative inotropic factors are parasympathetic stimulation, hypocalcemia, increase in potassium and myocardial hypoxia that decrease contractility.
Measurement for Cardiac Output
Cardiac output can be measured using various invasive and non-invasive methods. However, there can be merits and demerits of measuring cardiac output.
Various invasive methods are available but may not be accurate or effective and these include doppler ultrasound, echocardiography, transcutaneous and transesophageal echocardiography, partial CO2 rebreathing, etc.
Cardiac Index
It is a haemodynamic parameter and relates the heart performance to the size of an individual. It relates the cardiac output (CO) from the left ventricle in one minute to the BSA called body surface area. So, its measurement is litres per minute per square meter and written as L/min/m2.
The normal range for cardiac index (CI) is 2.5 to 4 L/min/m2.
A lower cardiac index indicates a disturbance in cardiovascular performance which is less than 2.5 L/min/m2.
FAQs on Cardiac Output
1. What is cardiac output and how is it defined in physiology?
In physiology, cardiac output (CO) is defined as the total volume of blood pumped by one ventricle of the heart in a single minute. It is a critical measure of the heart's efficiency and the circulatory system's ability to deliver oxygenated blood to the body's tissues. It is typically measured in litres per minute (L/min).
2. How do you calculate cardiac output using the standard formula?
Cardiac output is calculated by multiplying the stroke volume (SV) by the heart rate (HR). The formula is: CO = SV x HR. Here, stroke volume is the volume of blood pumped from the left ventricle per beat, and heart rate is the number of heartbeats per minute. For example, if SV is 70 mL and HR is 72 beats/minute, the CO is 5040 mL/minute or approximately 5 L/min.
3. What is the normal range for cardiac output in a healthy adult?
For a healthy adult at rest, the normal cardiac output range is approximately 5 to 6 litres per minute (L/min), which is equivalent to 5000 to 6000 millilitres per minute (mL/min). This value can change significantly depending on the body's metabolic demands, such as during exercise or in response to stress.
4. What are the main factors that regulate or affect cardiac output?
Cardiac output is primarily regulated by factors that influence heart rate and stroke volume. Key factors include:
- Heart Rate: The speed at which the heart beats, controlled by the autonomic nervous system.
- Stroke Volume: This is influenced by three main variables: preload (the degree of stretch on the heart muscle before contraction), afterload (the pressure the heart must work against to eject blood), and contractility (the intrinsic strength of the heart muscle's contraction).
5. What is the direct relationship between stroke volume and cardiac output?
The relationship is directly proportional. Since cardiac output is the product of stroke volume and heart rate (CO = SV x HR), any increase or decrease in stroke volume will cause a corresponding increase or decrease in cardiac output, assuming the heart rate remains constant. For instance, stronger heart contractions increase stroke volume, which in turn raises cardiac output to meet the body's demands.
6. Why does cardiac output increase during physical exercise?
During physical exercise, the body's muscles require significantly more oxygen and nutrients to produce energy. To meet this increased metabolic demand, the circulatory system responds by increasing cardiac output. This is achieved through two mechanisms: the heart beats faster (increased heart rate) and it pumps more forcefully with each beat (increased stroke volume). This ensures a greater volume of oxygenated blood is delivered to the working muscles per minute.
7. Can a consistently high cardiac output be a sign of a health problem?
Yes, while a temporarily high cardiac output is normal during exercise, a persistently high output at rest can indicate an underlying medical condition. Pathological conditions like severe anaemia (low red blood cell count) or hyperthyroidism (overactive thyroid) can cause a high-output state. In these cases, the heart works harder to compensate for the body's needs, which can lead to long-term cardiac strain if left untreated.
8. How does the nervous system regulate cardiac activity and output?
The autonomic nervous system plays a crucial role in regulating cardiac output by controlling heart rate and contractility. The sympathetic nervous system (the 'fight-or-flight' response) releases hormones that increase heart rate and the force of contraction, thereby increasing cardiac output. Conversely, the parasympathetic nervous system (the 'rest-and-digest' response) acts through the vagus nerve to slow the heart rate, which decreases cardiac output.
