Vol.3, # 4
February 4, 2006

Q: How can I improve my cardiac output? - Technical

A: This question is usually refering to improving oxygen uptake into the body through the blood supply.The primary function of the heart is to impart energy to blood in order to generate and sustain an arterial blood pressure necessary to provide adequate perfusion of organs with oxygen. The heart achieves this by contracting its muscular walls around a closed chamber to generate sufficient pressure to propel blood from the cardiac chamber (e.g., left ventricle), through the aortic valve and into the aorta.

Each time the heart beats, a volume of blood is ejected. This stroke volume (SV), times the number of beats per minute (heart rate, HR), equals the cardiac output (CO).

CO = SV · HR

Stroke volume is expressed in ml/beat and heart rate in beats/minute. Therefore, cardiac output is in ml/minute. Sometimes, cardiac output is expressed in liters/minute.

Cardiac output, - is the total volume of blood pumped by the ventricle per minute, or simply the product of heart rate (HR) and stroke volume (SV). The stroke volume at rest in the standing position averages between 60 and 80 ml of blood in most adults. Thus at a resting heart rate of 80 beats per minute the resting cardiac output will vary between 4.8 and 6.4 L per min. When cardiac output increases in a healthy but untrained individual, most of the increase can be attributed to increase in heart rate. Change of posture, increased sympathetic nervous system activity, and decreased parasympathetic nervous system activity can also increase cardiac output. Heart rate can vary by a factor of approximately 3, between 60 and 180 beats per minute, whilst stroke volume can vary between 70 and 120 ml, a factor of only 1.5.

However, the cardiac output of Olympic medal winners in cross country skiing increased 8 times above resting cardiac output to approximately 40 liters for one minute of maximal work with an accompanied stroke volume of 210 ml per beat.

Measurement of Cardiac Output

Several direct and indirect techniques for measurement of cardiac output are available. The thermodilution technique uses a special thermistor-tipped catheter (Swan-Ganz) that is inserted from a peripheral vein into the pulmonary artery. Cold saline of known temperature and volume is injected into the right atrium from a proximal catheter port. The saline mixes with the blood as it passes through the ventricle and into the pulmonary artery, thus cooling the blood. The blood temperature is measured at the catheter tip lying within the pulmonary artery and a computer is used to acquired the thermodilution profile and compute flow.

Echocardiographic techniques and radionuclide imaging techniques can be used to estimate real-time changes in ventricular dimensions, thus computing stroke volume, which when multiplied by heart rate, gives cardiac output.

An old technique based upon the  Fick Principle, developed by Adolf Eugen Fick (1829 - 1921),  can be used to compute cardiac output (CO) indirectly from whole body oxygen consumption (VO2) and the mixed venous (O2ven) and arterial oxygen contents (O2art):

                                 CO = VO2/(O2art – O2ven)

To calculate CO, the oxygen contents of arterial and venous blood samples are measured, and at the same time, whole body oxygen consumption is measured by analyzing expired air. The blood contents of oxygen are expressed as ml O2/ml blood, and the VO2 is expressed in units of ml O2/min. If O2art and O2ven contents are 0.2 ml and 0.15 ml O2/ml blood, respectively, and VO2 is 250 ml O2/minute, then CO = 5000 ml/min, or 5 L/min. Ventricular stroke volume would simply be the cardiac output divided by the heart rate.

CARDIAC OUTPUT

Vertebrate circulatory systems consist of blood, which transports materials to and from cells via blood vessels, and a heart to pump the blood. One important role of the circulatory system is to provide oxygen to cells. As a general rule, small animals have a higher rate of oxygen consumption per unit body mass than large animals. Therefore, the heart of a small animal must supply oxygen at a higher rate than the heart of a large animal. Since the oxygen capacity of blood is similar between small and large animals, small animals must have hearts that pump blood at a higher rate, or in other words, have a higher cardiac output.

Importance: We can use a mathematical model to quantify the cardiac output of an individual’s heart. By comparing this equation to data, we can determine methods organisms may use to increase their cardiac output.

Changes in stroke volume and heartbeat frequency affect cardiac output.

Variables:

VO2

volume of oxygen consumed (mL/min)

Q

cardiac output (mL/min)

Ca

oxygen content in arterial blood (mL O2/mL blood)

Cv

oxygen content in venous blood (mL O2/mL blood)

f

heartbeat frequency (min-1)

S

stroke volume (mL)

Methods: Cardiac output can be estimated in mammals and birds from Fick's principle. Fick’s principle states that the rate of diffusion is proportional to the difference in concentration. Similarly, the volume of oxygen consumed per unit time is proportional to the difference in oxygen content between arterial and venous blood. The degree of proportionality depends on the volume of blood pumped per unit time, or cardiac output (Q). Therefore, cardiac output (Q) can be calculated from the equation

VO2 = Q(Ca -Cv)

where VO2 is the volume of oxygen consumed per unit time, Ca and Cv are the arterial and venous oxygen concentrations. Experimentally, the volume of oxygen consumed and oxygen concentration in the blood can be calculated. We can then solve for cardiac output (Q). Unfortunately, using Fick’s principle requires can be misleading in animals where arterial and venous blood may mix, such as reptiles and amphibians, or in animals that uptake a considerable amount of oxygen through their skin.

Cardiac output is the volume of blood pumped by the heart per unit time and depends on the heartbeat frequency as well as the volume of blood ejected in one contraction. We can therefore write the following equation:

Q = f x S

where f is the heartbeat frequency and S is the volume of blood ejected in one contraction, or the stroke volume.

We can quantify the cardiac output of the heart by measuring heart beat frequency, stroke volume, and oxygen concentration in the blood. Organisms can improve their oxygen consumption by manipulating functions that increase their cardiac output.An animal may increase its cardiac output by increasing its heartbeat frequency or the stroke volume or both. 

Cardiac output during exercise

Now that we have discussed both of the components of cardiac output (heart rate and stroke volume), we can put this information together to understand what happens to cardiac output during exercise. Changes in cardiac output, because it is the product of both heart rate and stroke volume, are predictable with increasing work levels.

Because:

cardiac output = heart rate x stroke volume (CO = HR x SV)

changes in either heart rate or stroke volume will have an impact on the other component.

For an untrained person who has a resting heart rate of 72 beats/min and a stroke volume of 70 ml, the resting cardiac output is calculated as follows:
  • Cardiac output = Heart rate x Stroke
  • Cardiac output = 72 x 70
  • Cardiac output = 5040 ml/min
  • Cardiac output = 5.04 L/min

(NOTE: 1000 ml = 1 L)

During the initial stages of exercise, increased cardiac output is due to an increase in both heart rate and stroke volume. When the level of exercise exceeds 40% to 60% of the individual's capacity, stroke volume has either plateaued or begun to increase at a much slower rate. Thus further increases in cardiac output are largely the result of increases in heart rate.  

During Olympic cross country skiing, cardiac output increases primarily to match the need for increased oxygen supply to the working muscles. Compare the Olympic cross country skiers cardiac output with popular sports. You can see that cross country skiers cardiac output (men = 40 liters and women = 35 liters) are much higher than basketball players. Compare your favorite sport with cross country skiing.

While cross country skiing, blood is forced out of the heart and circulation speeds up. Blood is redirected, through the action of the sympathetic nervous system, away from areas where it is not essential, to those areas that are active. This causes constriction of vessels in those areas by reducing blood flow to the kidneys, liver, stomach, abd intestines. Thus, blood flow is concentrated to the skeletal muscles where it is needed. This ensures that adequate supplies of the needed materials oxygen and nutrients reach the tissues and that waste products, which build up much more rapidly during cross-country skiing, are quickly cleared away.

 During exercise, muscles receive 66% of the cardiac output, but the kidneys only receive 3%! At rest the liver has the highest percentage of cardiac output (27%), while the muscles only receive 15%!

The following supplements have been shown to improve cardiac output-the complement of B vitamins, but niacin in particular, L-carnitine, and Coenzyme Q10    Use caution when trying these supplements, especially if you have cardiac problems(do so slowly and under the guidance of a healthcare professional-your condition should improve).



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DISCLAIMER:  The information in this column, is NOT intended to diagnose and/or treat any health related issues and is provided solely for informational purposes only. Consult the appropriate healthcare professional before making any changes to your healthcare regime. Even what may seem like simple changes in the diet for example, can interact with, and alter, the efficiency of medications and/or the body's response to the medications. Many herbs and supplements exert powerful medicinal effects. Neither the author, nor the website designers, assume any responsibility for the reader's use or misuse of this information.

© 2002 Nature's Corner