An Introduction to the Cardiovascular System
An easy way to describe the cardiovascular system is as a closed system comprised of a pump and pipes to transport fluid (blood). As a closed system, the cardiovascular system allows passive filling of the heart which then pumps blood back out to the body. The heart does nothing to pull in blood. It simply allows the passive flow of blood into the chambers. The vast interconnecting network of capillaries, arteries, and veins regulate the circulation of blood. Blood is flowing into the heart constantly yet the heart pumps intermittently with fractional pauses to stop and recover between each beat.
Cardiac Output Measurement
The amount of blood the heart ejects per minute is the cardiac output. Cardiac output in men and women are similar. A physically fit male will have a cardiac output of about 30L/min while less fit men have a lower cardio output ranging between 20-25L/min. Even though women cardiac output starts out the same, women will have a higher cardiac output when working out at similar levels of oxygen consumption as men. One theory for this difference is that the lower hemoglobin levels in women reduce the amount of oxygen the blood can carry during an intense workout.
Background: Heart Anatomy
The heart is made of four distinct chambers. Though the heart is one organ, it can be thought of as two pumps within one. Each side of the heart has two chambers. At the top of the heart on each side is the atrium and below that the ventricle. The right side is smaller and receives blood returning to the heart from all parts of the body. Since the returning blood lacks oxygen, this side of the heart is responsible for moving blood to the lungs and pulmonary system. The larger left side of the heart receives this oxygenated blood from the pulmonary system and pumps it back out into the rest of the body.
Between the upper chambers and the lower chambers on each side of the heart are the atrioventricular valves providing a one-way flow of blood from the top of the atrium into the ventricle. Other valves (semilunar valves) are located outside the heart to prevent blood from reentering the heart between each contraction.
Heart Rate and Pulse Rate
As the heart pumps blood out through the left ventricle, a new surge of blood fills the aorta. Peripheral pulmonary vessels do not allow blood to enter the right atrium as quickly as it leaves the heart. With each contraction, some of the blood pumped out to the body remains in the aorta. The blood that remains creates a pressure that travels from the aorta throughout the arterial system. It is this stretching and contracting of the arterial walls that are felt as a pulse at various points along the arterial system. Under normal conditions, the heart rate and pulse rate are identical.
Blood Pressure History and Theory
Daniel Bernoulli (1700-1782), was a researcher in hydrodynamics. He formulated the Bernoulli equation by inserting a tube into a filled pipeline to measure pressure. This led to the first documented blood pressure reading in 1733. Reverend Stephen Hale used Bernoulli's concept to measure the blood pressure by inserting a long upright tube through an incision into the carotid artery of a horse. As the heart pumped, the pressure forced blood up into the tube, which Hale then measured and recorded. This led to investigation into understanding the physiological rationale and theory behind blood pressure readings in humans.
Blood pressure is the force exerted by blood against the arterial walls. The force exerted helps determine the strength the heart is pumping and depends on total blood volume and resistance within the arterial walls. The brachial artery found along the center of the inner arm is where blood pressure measurements are typically obtained. This artery is easy to access and pressure can be measured with the arm at heart level or relaxed in a downward position.
A stethoscope and a sphygmomanometer (blood pressure cuff) is a simple non-invasive way to monitor blood pressure. Newer electronic or digital sphygmomanometers do not require the use of a stethoscope but may not be readily available and are regarded as less accurate by some professionals. A stethoscope and cuff are also portable and inexpensive making them a good choice in a variety of clinical settings. Many individuals easily monitor their blood pressure at home using these same tools.
Determining blood pressure accurately is based on certain sounds (Korotkoff sounds) made within the vascular walls. You cannot normally hear any sounds simply by placing the stethoscope over the artery on the arm inside your elbow. Without the restriction from the blood pressure cuff, the blood moves freely. When you fully inflate the blood pressure cuff completely blocking the flow of blood, there are also no sounds heard. When you restrict the blood flow by inflating the cuff and gradually deflate the cuff, faint sounds are noticeable. The very first sound detected is the systolic pressure. Continue gradually releasing the pressure on the cuff and note the point where no sound is heard as blood flows unhindered. That point coincides with contractions in the left ventricle and is the diastolic pressure. The Korotkoff sounds are heard in five phases. Each phase has distinct characteristics.
- Phase I – The first clear tapping sounds heard indicating systolic pressure
- Phase II – A softer swishing sound
- Phase III – Sharper sounds that increase in intensity
- Phase IV – Abrupt muffling or damping of sound indicates diastolic pressure
- Phase V – Sounds are no longer heard
Cardiac Output is the measure of blood flow through the pulmonary system and includes the use of the Indirect Fick Equation to determine the rate of carbon dioxide transfer per volume.
Indirect Fick Equation is written as Qt (CvCO2 – CaCO2) = VCO2
In this formula;
Qt represents the pulmonary blood flow
VCO2 represents volumetric carbon dioxide rate
(CvCO2 – CaCO2) represents blood carbon concentration difference between venous and arterial
Measuring Blood Pressure
1. Position the blood pressure cuff on your upper arm approximately two-finger widths above the crease of your elbow.
2. Hold the stethoscope with earpieces facing forward and place the stethoscope earpieces into your ears.
3. Hold the disk of the stethoscope firmly over the brachial artery below the cuff and just above your elbow.
4. Inflating the cuff rapidly, listen and continue inflating the cuff to between 30-40 points higher than the last sounds you heard. If you inflate the cuff too slowly, you may get a false reading.
5. Slowly deflate the cuff using the release valve to control deflating the cuff at 2-3 millimeters per second. Deflating the cuff to quickly you will not accurately hear the progression of Korotkoff sounds.
6. While slowly deflating the cuff, listen carefully. When you hear the first heartbeat, note the reading on the blood pressure cuff dial. This is your systolic reading.
7. Continue listening closely as you continue deflating the blood pressure cuff. The final sound you hear is the diastolic pressure. Make note where that occurred on the cuff dial. Write the numbers you noted with systolic pressure over the diastolic pressure. (For example, 122/72)
8. Wait two or three minutes before repeating the measure holding your arm straight vertically. Your arm must remain vertical during the entire re-testing for an accurate measure.
Measuring Cardiac Output
You will need a partner to help with measuring cardiac output . One of you will do the test and the other will monitor the test using the computer and Breeze Ex software.
1. Set up the testing software with the personal data of the one taking the cardiac output test. After entering name, birthdate, height, weight, ID numbers, race and date, click the 'return' option then click the 'VO2 Test button'.
2. Place the pneumotach into the rubber fitting making sure the honeycomb pattern faces outward.
3. Place the saliva trap on the opposite end of the pneumotach.
4. Place a pink sample wire sticking through the white rebreathing valve. You should be able to see the wire exiting the tubing.
5. Firmly push in the black plunger and crumple bag removing any gas inside. With the black bag crumpled, pull the plunger up to open position.
6. Release the bag allowing CO2 to fill the bag half full.
7. Place the saliva trap in your mouth and hold the breathing equipment steady, then signal the computer operator to click the test START button. Breathe normally for 2-3 minutes then have your partner click the NICO button. When you are ready, have the computer operator again press START while you slowly push the black plunger.
8. Next, breathe deeply and rapidly at a rate of 40 breaths per minute without hyperventilating. (Reminder, normal breathing is approximately 14-20 breaths a minute while hyperventilating is 100 breaths or more per minute.)
9. After twenty seconds, click STOP TEST option and push plunger in.
10. Record Blood value control rate or cardiac output Qt as L/min.