Cardiac Output

Introduction

Place your hand at the centre of your chest and move left by an inch or two. Can you feel the beat? That is your heartbeat, and it is caused by the human heart pumping blood in and out. The whole process is known as the cardiac cycle. The volume of blood each ventricle pumps out each minute is known as your cardiac output.

A healthy heart has a normal cardiac output of about 5 to 6 litres each minute. Isn’t that astounding? The human body is borderline magical in its functioning; medical science, however, has managed to unravel the magic.

Understanding Cardiac Cycle

Before jumping into cardiac output, one must understand the Cardiac Cycle. The series of steps that allow the heart to pump in and pump out blood is known as a cardiac cycle. This consists of two phases – the muscle relaxation phase or diastole and the muscle contraction phase or systole.

Understanding Cardiac Output

By strict definition, Cardiac Output refers to the volume of blood pumped out by each ventricle per minute. A healthy heart’s cardiac output is about 5 to 6 litres of blood each minute in a resting position. This is known as normal cardiac output. It is measured in mL/minute. Low output can lead to health complications. Similarly, high output heart failure is also a common cause of worry.

Calculating Cardiac Output

Two variables determine cardiac output or CO –

• • Heart rate or HR – Cardiac output can be determined using the heart rate, which is essentially the number of heart beats per minute (beats/min)

• Stroke volume or SV – Stroke volume refers to the amount of blood pumped by each ventricle on each beat and is measured in mL/beat

Thus,

Normal Cardiac Output

The average human heart beats 75 times in a minute with each beat pumping roughly 70 mL of blood. Thus, we can conclude that normal cardiac output for a person is about 5.25 L.

Cardiac Output Is About 4 Determinants

Given that stroke volume is a factor that helps determine cardiac output, the following variables are direct factors that help us understand the matter –

• Heart rate: As already discussed, heart rate is directly proportional to the value of cardiac output

• Preload: As the name suggests, this is the volume of blood that enters each ventricle during diastole. An increase in input volume directly increases the stroke volume, thus affecting the cardiac output proportionally 

• Afterload: The name in this case is a little misleading. This does not refer to the blood output, instead, it refers to the resistance the blood must overcome to circulate throughout the body. Thus, more the afterload, the less is the stroke volume. Cardiac Output is therefore inversely proportional to afterload

• Contractility: The force with which the myocardium contracts for a given preload is known as contractility, and it is directly proportional to cardiac output

Measuring Cardiac Output

There are multiple invasive as well as non-invasive methods of measuring cardiac output. There are merits and demerits of each method that one must keep in mind. Although the invasive methods are fairly common, they may not be entirely in numerous instances.

Some common methods of measuring cardiac output are as follows:

• Echocardiography
• Doppler Ultrasound
• Transoesophageal
• Transcutaneous

Interestingly, both low and high cardiac output is equally problematic. Low output may be indicative of severe heart infection, whereas high output may be a symptom of blood infections. 

High Output Heart Failure

High output heart failure occurs when the heart cannot seem to keep up with very high demands of blood supply from one or more organs. Even a healthy heart might fail to try to work this hard. If the body suddenly feels the need for high amounts of oxygen in one or more organs, that is when this condition occurs.

Symptoms for high output heart failure include sweating and shortness of breath. Conditions that can cause this are –

• Severe anaemia causes the blood to have too little oxygen-carrying red blood cells
• Hyperthyroidism  where the thyroid glands produce a little too much hormones leading to an abnormal increase in the body’s metabolism rate
• Arteriovenous fistula where an abnormal connection between an artery and a vein short circuits the circulation which means the heart has to work harder to even pump out the normal amount of oxygenated blood
• Beriberi which can cause an increase in metabolic demand

And many more.

Cardiac Index

Cardiac index is a haemodynamic parameter, much like heart rate, blood pressure, etc. It determines the relation between the performance of a heart to the size of the individual. Breaking it down to variables, it simply compares the cardiac output of the left ventricle to the body surface area of the person.

Thus, the unit of measurement for cardiac index becomes litres per minute per square meter or L/min/m2. The normal cardiac index value is in the range of 2.5 to 4 L/min/m2. Any lower than that is indicative of abnormal cardiovascular performance.

Conclusion

Although we strive to maintain a healthy cardiac output, being a stickler for a perfect value is not wise. The body can adjust its requirements as per its needs. For example, during exercising, the body needs a lot more oxygen and thus, the cardiac output measured at that time may be very high. However, given the circumstances, that is perfectly normal.

However, this does not mean that one should ignore any abnormal value. A lower or higher cardiac output without any external stimulus can be indicative of serious health complications. The right range of values is essential for maintaining oxygen-rich blood in the system and maintaining healthy organs.