A lesson about the heart: Cardiac Output


Here is some information that is useful to know. It is a little in-depth when it comes to heart function, but I believe education is power and/or a sense of control when it comes to managing a chronic disease.  The more you know, the better you can work with your health care provider to help to manage it. 

Why are certain tests performed? An example is an echo is often performed 3-6 months following a heart attack. This allows the heart time for recovery and remodeling, and gives the healthcare provider a good idea of your cardiac output. Is your ejection fraction diminished, is the stroke volume lower thus decreasing the output? Is the cardiac output lower because the heart chamber is stretched, and weaker – inotropic effect, or due to medications? Does this place additional risk to you? When the ejection fraction is below 30% the risk for arrhythmia increases, thus precautionary measures such as Implanted Cardiac Defibrillators become an issue. Or vise versa if the heart function improved back to safe levels since the event there may no longer be the need, or those wearing external defibrillator vests may no longer be necessary.

The following came from Jewels of Clinical Medicine

What Is Cardiac Output?
Cardiac output is defined as the amount of blood pumped by the ventricles, the lower chambers of the heart, in one minute. Two factors determine cardiac output: stroke volume and heart rate. The equation used is: heart rate x stroke volume = cardiac output. A normal adult heart will have a cardiac output of approximately 4.7 litre( 5 quart) of blood per minute. Exercise will increase cardiac output, since it increases heart rate.

Heart rate
• The number of times the heart beats in one minute is the heart rate. In adults, the average heart rate is 60 to 100 beats per minute. Heart rates are usually higher in children and women. Differences in gender, size, age and fitness can affect the heart rate, as can some medications and conditions. Very fit people have lower resting heart rates. Heart rates increase when people are upset or excited.
Stroke Volume
• Stroke volume is the amount of blood pumped by the ventricles with each heartbeat. An average resting stroke volume is about 2 ounces (60 to 80 milliliters) per beat. Usually only 65 percent of the blood is pumped out of the ventricles during each beat. This is the normal ejection fraction value. Starling’s law of the heart and the inotropic effect are two things that can alter the force of the contraction, causing more of the blood to be expelled with each beat.

Starling’s Law of the Heart
• Starling’s law depends on the amount of stretch in the cardiac muscle fibers. If there is an increase in the volume of the blood pumped into the heart, that increase causes the ventricle to stretch, which in turn increases the force of contraction and the cardiac output. If less blood volume enters the heart, the ventricle does not stretch as much, the contraction is less forceful, and the cardiac output is decreased. This is important to ensure that the heart pumps out only what it receives at a given time.

Inotropic Effect
• If the strength of the contraction is increased without increasing the stretch of the cardiac fibers, cardiac output will be increased. Certain hormones and medications can cause this to happen. Sympathetic nerve stimulation of the heart, for example when a person is scared or excited, is another mechanism of the inotropic effect. Some drugs may also cause a negative inotropic effect and will decrease the cardiac output, which can lead to heart failure. It is extremely important to closely monitor the usage of any medication that has a negative inotropic effect on the heart.

Interesting Fact
• The amount of blood pumping though the body of the average adult is about 5 liters. That is equivalent to the average cardiac output. That means that the heart pumps the total amount of blood in the body every minute.

Heart Disease Health Centre- Dr.Yaseer SK

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