Most of us give little thought to how consistent and reliable the human heart is. Without an effectively pumping heart, the human body ceases to function and can die physiologically within minutes. Some organs can tolerate a lack of blood flow better than others. Vital organs such as the brain, the kidneys, the heart and the liver are the most rapidly affected by a lack or decrease in blood flow while other organs of the body can tolerate a lack perfusion for a longer period of time. The heart is considered “the pump’ of the circulatory system and without a consistently functioning pump, the human body cannot perform adequately to sustain life.
Lets put the heart’s reliability into perspective. It is widely accepted that a normal adult heart rate range is between 60-100 beats per minute. So lets find the midpoint of that range and we come up with 80 beats per minute. Multiply 80 x 60 as there are 60 minutes in every hour. This gives us a total of 4800 beats per minute. Multiply 4800 x 24 and we come up with 115 200 beats per day. Multiply 115, 200 x 7 as there are 7 days in a week and we arrive at 806,400 beats per week. Multiply 806,400 x 52 because there are 52 weeks in a year and the grand total of average heart beats annually is a staggering 41,932,800. If you really want to scare yourself, multiply this number by your age in years and you come up with the average total times your heart has pumped blood effectively in your lifetime. The heart is just one part of the body’s circulatory system albeit and important one.
Without an effectively functioning pump, the billions of cells in the human body cannot function effectively and maintain aerobic metabolism. Heart rate is an important part of this equation. At the basic level it would seem that a heart that beats faster would pump more blood to the body and to a point this is true. So why is it when a patient has an extremely fast heart rate do they become symptomatic with complaints such as dizziness, weakness, chest pain or shortness of breath? On average, every time the heart beats, it pumps out between 60 and 100 ml of blood. This is called stroke volume and is a very important part of total cardiac output of the heart and overall tissue perfusion. Think of the heart as a 4 chambered pump with 2 atria and 2 ventricles. If the heart rate is too fast, the atria and the ventricles do not have adequate time to fill between contractions (heart beats). This causes less blood to be pumped with every contraction and less blood being pumped by the heart means less blood being delivered to the end consumer: the body’s cells. This in turn can cause the cells of the body to convert from aerobic respiration, which is very efficient, to anaerobic metabolism which is much less efficient and produces a large amount of pyruvic acid and lactic acid. On the other end of the spectrum, a heart rate that is too slow is also going to cause our patient to possibly become symptomatic since once again not enough blood is being supplied at the cellular level.
Think of it as a fuel pump on a vehicle: if it is pumping too slow then not enough fuel will delivered to the engine and the engine will not run as effectively and efficiently as it should. As stated earlier, stroke volume is the amount of blood being pumped by there heart with every contraction. This is another important part of the hearts overall performance. While heart rate is important, if stroke volume is not adequate then overall tissue perfusion will suffer. Regardless of how fast or slow the heart is beating, if there is not enough blood being pumped with every beat, then not enough blood will be supplied to the patient’s body. Stroke volume is dictated by a couple of factors:
Preload: the amount of stretch caused by the volume of blood in the ventricle at the end of the filling phase or diastole
Contractility: How effectively and efficiently the atria and more importantly the ventricles contract
Afterload: the resistance against which the ventricle has to overcome to eject blood
Going back to the vehicle engine analogy, if the fuel tank is empty or the fuel line between the tank and the fuel pump is kinked then not enough fuel will reach the pump. Think of this as preload. If enough fuel is reaching the fuel pump from the fuel tank but the pump isn’t performing as it should, then again not enough fuel is being pumped to the engine. Think of this as contractility. And finally, if the fuel supply line between the fuel pump and the engine is narrowed or kinked then again not enough fuel will reach the engine. Think of this as after load.
So as you can see, there are many factors that go into how well or how poorly the human heart will perform. These are just a few of the factors that can affect overall cardiac performance. We have talked about heart rate and stroke volume as these are the two factors that make up cardiac output. Normal cardiac output is in the range of 4-8 liters per minute and varies according to many factors including activity level, health of the patient and many other variables. The formula for Cardiac Output is Stroke Volume x Heart Rate. In future installments, we will unpack the other two parts of overall perfusion status: Blood Vessel Integrity and Blood Volume and Components.
