Cardiac Output – The Complete Overview
Cardiac output is the amount of blood pumped out of the heart per minute. This value is especially important for individuals who suffer from cardiovascular disease or who suffer from any cardiovascular risk factors in general. Cardiac output often gets confused with stroke volume, which is the amount of blood pumped out of either ventricle of the heart per beat.
The cardiac output (Q) formula is Q = Heart rate (HR) x Stroke volume (SV). The product of this equation is a very important indication of how capable your heart is at providing your working tissues with oxygen. Other factors come into play with this as well, such as your blood oxygen saturation levels (SpO2), which is an estimate of the amount of oxygen in the blood. Nevertheless, cardiac output is still a very viable and reliable indication of heart efficiency.
Cardiac output says a lot about an individuals cardiovascular health, as well as their cardiovascular fitness level. Cardiac output itself is contingent on many different processes and phenomena that occur in the heart. From the neural impulses at the SA node in the right atria to the end diastolic volume of the left ventricle, all of these things and more factor into what someone’s cardiac output would be.
An average resting cardiac output is 5.6 L/min for males and 4.9 L/min for females. The larger volume of blood for men is mainly due to more girth, especially in the myocardium (heart-muscle tissue). That’s a lot of blood flowing through your cardiovascular system. Thus, why it’s so important to eat cholesterol lowering foods and exercise consistently so that the valves in your cardiovascular system stay clean and clear of any harmful plaque that could disrupt the flow of blood rushing throughout.
Cardiac Output correlation with VO2
What use would blood be if if didn’t contain any oxygen in it? Well, not much. See, you need oxygen in your blood to help re-energize working tissues. VO2 stands for oxygen consumption. And as you can imagine, an increase in cardiac output would correlate to an increase in oxygen consumption as well. Oxygen is contained in the blood via hemoglobin.
Hemoglobin is a protein molecule found in red blood cells that carries oxygen away from the lungs and back to the body’s tissues. It then returns carbon dioxide from the tissues back to the lungs . Essentially, an increase in heart rate will result in an increase in cardiac output, as well as VO2. So, as you can see, there is a clear correlation with cardiac output and oxygen consumption.
Total peripheral resistance and cardiac output
Total peripheral resistance (TPR) is the sum of the resistance of all peripheral vasculature in systemic circulation. Systemic circulation is where oxygenated blood flows from the heart throughout the cardiovascular system to various tissues in the body. The formula for total peripheral resistance is TPR = Mean Arterial pressure / Cardiac Output.
Cardiac output will be hindered if your total peripheral resistance is higher than normal. The same can be said for those who suffer from high blood pressure. Your blood pressure should be 80-120/60-80. Anything above this range can negatively affect your cardiac output. This can become a huge problem as you need a certain amount of blood continuously flowing through systemic circulation to ensure that oxygen is saturated in working tissues, among many other very important benefits.
How preload and afterload affect cardiac output
Essentially, preload is the pressure in the myocardium (muscle portion of the heart) due to the amount of blood filling in either ventricle (left or right) immediately before contraction. A greater preload means a greater amount of blood filling the ventricle. Thus, a greater cardiac output. When preload is at its greatest, this increases myocardial stretch, myocardial contraction, and stroke volume. All of which are contributing factors to a healthy and efficient cardiac output.
Afterload is related to the compliance of the aortic or pulmonary artery. The greater compliance of these vessels, the more easily the walls of these vessels can be stretched to accommodate the influx of blood during systole (contraction). Both preload and afterload contribute to cardiac output. Theoretically, a greater preload means a stronger myocardial contraction and greater compliance (afterload) means that the aortic or pulmonary arteries can receive blood with little to no hindrances with regards to resistance.
Is cardiac output influenced by exercise?
As you exercise, your heart rate and your stroke volume increase. Thus, causing cardiac output to increase as well. See, during exercise there’s a decrease in peripheral resistance mainly due to the dilation of vessels supplying skeletal muscles.
It’s important to note that the effective reduction in resistance is not as great as the effective increase in cardiac output. This is essentially why mean arterial pressure increases during exercise.
Both aerobic and anaerobic exercise are very helpful for improving heart health. They both affect the heart differently. For instance, aerobic or endurance exercise will actually increase the size of the left ventricle in the heart. This increased size of the ventricle will allow it to fill up with more blood during diastole. Anaerobic exercise will actually increase the thickness of the left ventricle, allowing the heart to forcefully pump out more blood.
How can diet affect cardiac output?
If your arteries are full of plaque, then your red blood cells can’t flow through at their normal rate, which can hinder cardiac output. This can be a huge problem for you if you’re engaging in any type of vigorous exercise or stressful activity where your heart rate will significantly increase. Cleaning up your diet is one of the first things that your doctor or dietitian would most likely encourage you to do.
Decreasing your salt intake, as well as saturated/trans fats should greatly improve your cardiac output. Eating less processed foods and more fruits, vegetables, and fish is the right way to go! Also, the dash diet is one of the best ways to go about keeping your blood pressure in a healthy range. DASH stands for Dietary Approaches to Stop Hypertension, and you can find out more about the dash diet here.
Making even small improvements to your exercise routine and diet can acquire a lot of goodwill toward your health, well being, and cardiac output. You’ll definitely want to talk to your doctor first to ensure that you’re put on the right path to success early on.
1. “What is Hemoglobin?” Medicine Net. http://www.medicinenet.com/hemoglobin/article.htm. Accessed on Sept 2017.