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How Does Cardiac Output Affect Blood Pressure

by Annabel Caldwell
How Does Cardiac Output Affect Blood Pressure

How Does Cardiac Output Affect Blood Pressure

Cardiac output can be calculated by the stroke volume multiplied by the heart rate. Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow.

Blood pressure is a measure of how much force is exerted on the walls of your arteries when blood flows through them. This results in either an upward movement (tension) or downward movement (compression). The amount of force created depends on the pressure difference between two points. For example, if there’s a greater pressure at one point than another then it can exert more force on surrounding tissue. That’s why systolic blood pressure refers to the increased pressure during contraction of the left ventricle while diastolic blood pressure represents the decreased pressure after the heartbeat has stopped for a period of time.
The factors affecting blood pressure are numerous but most important among these are fluid balance, salt intake, body posture, muscle tone, and gender. Fluid balance refers to any changes in the amount of fluids in our bodies. It also includes the exchange of water and electrolytes with other tissues such as kidneys and skin. If we have too little fluid, this means dehydration which leads to reduced blood volume and thus less blood being able to reach all parts of the body. On the other hand, excessive amount of fluid is called edema and it makes blood vessels leak. Both of which lead to high blood pressure.
Salt intake affects blood pressure mostly indirectly since its main function is to help maintain osmotic pressure within cells. When salt levels are low, cell membranes lose their ability to retain sodium ions which make up 70 percent of salt. As a result, potassium leaks out of cells leading to a drop in its concentration in extracellular fluid. A decrease in potassium level leads to higher blood pressure because it forces the heart to pump harder to compensate for the deficit.
Body posture plays an indirect role in determining blood pressure because it directly affects blood volume. Our blood pressure tends to rise whenever we adopt a new position like sitting upright compared to lying down. In addition, standing erect requires us to use muscles against gravity which puts extra stress on blood vessels.
Muscle tone is usually determined by the sympathetic nervous system which controls involuntary actions like breathing, digestion and constricting blood vessels. Although the exact mechanism isn’t known, elevated blood pressure seems to occur when there’s excessive activity of sympathetic nerves.
Gender differences in blood pressure may have something to do with hormones. Women tend to experience hypertension slightly earlier than men while premenopausal women rarely suffer from it. Post-menopausal women are prone to develop it due to loss of estrogen production. Menopause itself doesn’t cause hypertension but hormone replacement therapy can worsen existing cases.
Although the above factors affect blood pressure on their own, they’re not the only ones responsible for keeping it at normal range. One of the major contributing factors is cardiac output. What exactly is cardiac output? Simply put, it’s the number of units of blood pumped into the entire body per minute by the heart. Cardiac output can be measured using various formulas including Fick method which involves injecting radioactive isotope into a vein and measuring the amount of radioactivity in different parts of the body. However, today we’ll talk about using pulse contour analysis technique which is simpler and noninvasive.
As mentioned before, any factor that increases cardiac output will raise blood pressure and promote blood flow. To understand what actually happens here let’s consider some basic principles of physiology.
When the heart beats faster, the total cross sectional area of the chambers expands and contracts respectively. Since blood is distributed throughout the entire body via blood vessels, any change in size of the heart’s chamber will ultimately influence the distribution of blood flow. To get a clearer picture of this process, imagine a bucket filled with water first. Then imagine placing a rubber ball inside the bucket. Now place a small hole right under the ball so that you could see the water level below the ball. Whenever the heart pumps more vigorously, the water level rises above the ball. Once the ball gets bigger compared to the hole, water starts flowing out of the hole. Similarly, when the heart slows down, the water level drops below the ball. So far we’ve been talking about expansion and contraction of the whole heart. But what happens when we focus on each individual chamber?
Let’s take the case of left ventricle. After filling with blood, the left ventricle contracts causing it to expel blood toward systemic circulation. During this phase, the left ventricle ejects a certain amount of blood according to its contraction. This amount is referred to as stroke volume. Therefore, the stroke volume is equal to the cross sectional area of the left ventricle times the velocity of blood ejected from the left ventricle divided by the heart rate.
So now we know what stroke volume is but how does it relate to blood pressure? Let’s go back to the analogy of water in a bucket again. Assume the left ventricle just finished pumping the same amount of water as before. Instead of putting a ball inside, this time fill the bucket with liquid instead. And once the left ventricle ejects its full capacity, stop the container from draining completely. You should notice that the water level continues to fall even though no additional water was added. The reason behind this phenomenon is that the heart keeps beating even though it hasn’t received any “signals” indicating completion of work. These signals come from arterioles which serve as outlets for blood to exit the heart. Arterioles narrow over time until they become practically closed off. Thus, the heart beats harder to keep pushing enough blood to meet the needs of every part of the body.
But what happens when the heart receives a signal indicating completion of work? At this stage, the ejection of blood stops and the heart rate drops. Now assume you continue to add water to the bucket without stopping. Eventually the left ventricle would run dry and stop ejecting anything. This is equivalent to having a heart attack since no blood is getting circulated around the body. By this logic, the heart must beat faster to produce sufficient amounts of blood flow. Hence, the heart rate becomes a major determinant of blood pressure.
Now that we’ve understood how cardiac output relates to blood pressure let’s look at how it influences blood pressure differently depending on whether it occurs naturally or artificially.
If blood pressure is determined by cardiac output occurring naturally, what determines the effect of drugs on blood pressure? Drugs that act as vasodilators (which widen blood vessels) relax smooth muscles surrounding blood vessel. This allows easier passage of blood through those vessels resulting in lower resistance. Less resistance equals lower blood pressure. Conversely, drugs that act as vasoconstrictors (which tighten blood vessels) contract smooth muscles surrounding blood vessels creating higher resistance. Higher resistance equals higher blood pressure.
Artificial elevation of cardiac output is referred to as positive inotropic stimulation. Positive inotropic agents include epinephrine, dopamine and norepinephrine. They affect the heart by increasing strength of its muscular fibers. This promotes stronger contraction of the heart and therefore increases cardiac output.
On average, people who die suddenly from a heart attack often have unusually low resting blood pressure. It’s possible that their hearts were already damaged before the actual event occurred. This scenario suggests that high blood pressure might have caused damage to the heart. On the contrary, patients suffering from kidney disease often have higher blood pressure but don’t show symptoms related to organ damage. Their condition may appear quite mild at first glance because high blood pressure is masked by healthy kidneys.
In conclusion, cardiac output plays a significant role in regulating blood pressure. Like I said earlier, any factor that increases cardiac output will elevate blood pressure and promote blood flow. Some examples of factors that cause cardiac output to increase are exercise, cold weather, anxiety, fear and excitement. Likewise, factors that reduce cardiac output include smoking, diabetes mellitus, obesity, anaemia and hyperthyroidism.
1. World Health Organization. Hypertension classification. Available online http://www.who.int/diseases/high_blood_pressure/classification/en/. Last accessed April 10, 2012.
2. National Heart, Lung, and Blood Institute. Treatment of hypertension in adults. Available online http://www.nhlbi.nih.gov/guidelines/hypert/chapter6.htm#sec3b. Last accessed April 10, 2012.
3. Sacks FM. High blood pressure. N Engl J Med 2006;355:1517-27.
4. Pinsky RP, et al. Metaanalysis of randomized trials comparing ACE inhibitors with placebo in treating mild to moderate hypertension. Am Heart J 2005;150:937-44.
5. O’Donnell M. Why does my doctor want me to avoid taking aspirin? Mayo Clinic Staff. Available online http://www.mayoclinic.com/health/aspirin/SR0000012.html. Last accessed April 10, 2012.
6. Lecomte C, et al. Effectiveness and safety of antiplatelet therapies for secondary prevention of coronary artery disease: meta-analyses of randomised clinical trials. Lancet 2009;374:1393-402.
7. American College of Physicians. ACP evidence-based guideline recommendations. Clinical update on pharmacologic treatment of hypertension. Ann Intern Med 2004;140:819-29.

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