Accurate Blood Pressure Measurement
For Medical Instrumentation

     
 

Accurately Measuring Blood Pressure

Blood PressureMeasuring blood pressure is one of the most common measurements performed in medical practice, but seldom is it measured accurately. While blood pressure could apply to the pressure in the arteries, veins, or within the heart, this article refers to the pressure of the blood in the arteries, which is what is commonly referred to as blood pressure or BP.

There are normally two numbers provided for a blood pressure measurement. A common measurement might be 120/80. The higher number is called systole, and is the pressure produced when the heart "beats" or contracts, producing the maximum pressure in the artery. The lower number, diastole, is a measurement of the minimum pressure that is present between beats of the heart. The number is normally given in units of millimeters of mercury (mmHg).

While we talk of blood pressure as if it were a more or less fixed number similar to body weight, it is not. It is much more similar (and related) to heart rate.  Blood pressure must, and does, change in order to meet the body's needs. The body needs oxygen to function. The amount of oxygen needed depends on what is happening. The higher number, systole, is particularly variable with activity. Normal, healthy people can have systoles well above 200 when they are exerting themselves physically or even mentally. Diastole (the lower number) changes less, and has been shown in studies to be less indicative of cardiovascular risk (heart attack or stroke) than systole.

Does Accuracy Matter?

It depends on why BP is being measured. If it is being measured in an emergency or hospital environment such as the operating room, recovery, or critical care, then an error of 10 mmHg is not very important. The reason for measuring blood pressure is to determine if the pressure is within a wide range of acceptable pressures. What is important is obtaining a reading quickly and determining if a sudden change is occurring. On the other hand if it is being measured in a clinical environment, such as your doctor's office, accuracy may be of high importance. Even a few mmHg can make the difference between being prescribed medication or just having your blood pressure monitored. Once being placed on medication, it is usually for a lifetime. Frequently the medication is quite expensive, and has side effects. Also, being diagnosed as hypertensive (a fancy term for high blood pressure) will affect insurance rates, and may, for instance in the case of an airplane pilot, mean the end of a career. Unfortunately some of the least accurate measurements take place in the clinical environment where accuracy is most important.

Measuring Blood Pressure

There are three different ways that are commonly used to measure blood pressure:  invasive, ausculatory, and oscillometric. We will discuss each and review some of the difficulties with each method.

Invasive Measurement

Brachial, Radial & Ulnar ArteriesThis is the only direct measurement of blood pressure. In theory this measurement could be highly accurate. However, in practice, it often is not accurate, principally because of the measurement site, and not paying attention to all of the various details concerning the measurement.

In order to perform this measurement, a tube (catheter) is inserted into an artery of interest. A pressure transducer is connected to the tube (or better, mounted on the tip of the catheter), and the pressure is measured. Because there is risk associated with puncturing an artery, this invasive means of measurement is not commonly used except in cases when it is vitally important to quickly determine changes in blood pressure. Most commonly, the catheter is inserted into the radial artery (located at the wrist). This site is lower risk than tapping into a larger artery. Also, unlike most of the potential sites in the body, there is a second artery supplying blood to the hand (the ulnar artery).

Compared to brachial artery pressure, the blood pressure at the wrist is higher for systole, lower for diastole, and about the same for "mean" or average pressure. The amount of difference depends on the stiffness of the arteries. Although the radial artery is a convenient site, it does not result in the same blood pressure measurement as a site closer to the heart such as the brachial artery. An additional accuracy issue is produced if the wrist is not at the same elevation as the heart. For every inch, the pressure either increases or decreases by 2 mmHg, depending on the elevation being lower or higher. In any event, invasive measurements cannot be routinely obtained, and, because of the many factors that cause the measurement to differ from a brachial artery measurement, may not be a suitable means to determine if a subject has "high" blood pressure even if available.

Ausculatory Measurement

Measuring Blood PressureThis is the way blood pressure is most commonly measured in a clinical environment. A cuff is put on an arm and inflated to a pressure above what is expected to be the systolic pressure. A stethoscope is held against the arm down the arterial stream from the cuff, hopefully directly above the artery. The pressure in the cuff is then gradually reduced until the person listening to the stethoscope hears an indication that blood flow has started to occur at the peak of the blood pressure cycle (systole), and continues to reduce the pressure in the cuff until the blood is flowing throughout the cardiac cycle (diastole). This method takes a minimum of equipment, and is the method used in large population studies to determine what normal blood pressure is for humans. Every medical professional "knows" how to perform this measurement. Unfortunately few know how to perform it correctly.  Perhaps the four most common errors are:

  1. Using the wrong size cuff. 
  2. Applying the cuff incorrectly 
  3. Not positioning the stethoscope directly above the artery. Incorrect placement will result in too low of a systolic and too high of a diastolic pressure measurement.
  4. Incorrect interpretation of the sounds heard.

There are many more ways to obtain inaccurate results such as trying to obtain a measurement in a noisy environment, not supporting the arm at the correct level, wrong definition of systole or diastole, not pumping high enough, releasing the pressure too rapidly, too much (or even too little) finger pressure on the stethoscope, manometer not at eye level, hand not palm up, etc. The point is that it is easy to obtain an inaccurate measurement without realizing it, and a correct measurement is only obtained with a great deal of attention being paid to every detail. This attention is seldom present in a clinical environment.

The American Heart Association publishes recommendations on how to perform this measurement. The author has had his pressure measured many times, and not once has it been in accordance with these recommendations. Studies comparing invasive to ausculatory measurements have been all over the map (see AAMI SP10 for comparison). The only agreement that is normally reached is that these measurements differ. The author believes much of this "differing" is caused by poor measurement techniques, and varying measurement locations on the body. Few people know how to accurately measure blood pressure either invasively or indirectly to the degree needed for a comparison study. Also most of the invasive measurements have been performed using the radial artery for reasons previously stated. An invasive radial artery measurement cannot be expected to be the same as an ausculatory brachial artery measurement except for the "mean" blood pressure. More on this later.

Oscillometric Measurement

This is the method most frequently used by automatic devices to measure blood pressure. A cuff is placed on the arm, and is inflated to a pressure that is above the systolic pressure, blocking the flow of blood. The pressure is then decreased in a controlled fashion. During this deflation, the pressure in the cuff is monitored for minor pressure fluctuations caused by the artery encircled by the cuff opening and closing. This method of blood pressure determination is older than the ausculatory method, but is very difficult to perform accurately without involving a microprocessor.

Some automatic devices use the ausculatory method. In this case, a microphone is used to detect the blood flow sounds (Korotkoff sounds). When using the ausculatory method, placement of the microphone over the brachial artery is quite important to get a correct measurement. The popularly of the oscillometric method can be mostly attributed to the lack of critical cuff placement, and better performance when the subject is in shock. This method has been modified and refined over the years in order to closely match other blood pressure measurements, however the reference standard has been a major subject of disagreement.

One school of thought is that only an invasive measurement can be thought of as being the "real" blood pressure, and therefore it should be the standard of comparison. While this school has much to recommend it, the author strongly believes that for the diagnosis of hypertension the ausculatory method should be the "gold" standard, because all the population studies that give us the knowledge of what is normal and high blood pressure have been based on the ausculatory method. This is not likely to change, as it would be unethical to perform large population studies using an invasive method.  If future studies show that properly performed ausculatory measurements agree closely with properly performed brachial artery invasive measurements, then this becomes a moot point.

There is widespread misconception on how oscillometry works. It does not "determine the mean pressure and then calculate systole and diastole" as many think. Rather it determines the relative volume of the artery surrounded by the cuff, determines when the artery is starting to allow blood to pass (systole) by determining the pressure at which the artery volume is at 50% of the maximum, and determines when blood is flowing throughout the cardiac cycle (diastole). The better implementations are much more accurate than those of almost any clinician, and may be even better in some cases than those of the best clinician. This technique is especially good in the cases where there is a trail-off of Korotkoff phase V sounds such that it is difficult to determine diastole.

What is "Normal" Adult Blood Pressure?

Cardiovascular disease is the leading cause of deaths in the United States. Little was known about the risk factors until a study was instituted in Framingham MA in 1948 (see http://www.nhlbi.nih.gov/about/framingham). Initially 5,209 residents were enrolled into the study, which was about two-thirds of the population of this small town. These subjects were interviewed and tested for various factors in order to better understand the risk factors. Another 5,124 residents were enrolled in 1971. These subjects were the offspring of the original subjects. Every two years the surviving subjects returned for detailed medical history, physical examination and laboratory testing. By testing and keeping track of these subjects, it was possible to determine which of the various risk factors contribute most to cardiovascular events. They determined that blood pressure higher than 120/80 increased the risk for a cardiovascular event. The higher the pressure, the more the risk. The blood pressure was determined using the ausculatory method.

In medical practice, frequently 140/90 becomes the dividing line where patients are put on medication. Why 140/90? Increased risk with increased blood pressure is a gradual curve, and the prognosis of 139/89 is not much different than 140/90, but the decision point must be somewhere.

What is the "Real" Blood Pressure?

While this seems like a simple question, it is quite complex. First the question of what use is to be made of the pressure needs to be addressed. If blood pressure is being measured in order to determine the short-term prognosis of the subject, then it makes little difference what pressure is used. In the operating room, the anesthesiologist is not likely to treat a pressure of 140/90 much differently than a pressure of 115/70. What is required is trend information to assure that the patient is properly maintained at a safe level of blood pressure. However in clinical practice, consistent pressures above 140/90 are usually treated with medication, while a pressure below this figure is not treated.

It is easy to argue that a properly performed invasive measurement can determine the pressure at the measurement site. There are many pitfalls in obtaining accuracy, but in theory, if all are addressed, one could determine the actual pressure at the site of the catheter. The questions now becomes one of what is an appropriate site? It has been shown by many investigators that invasive pressure is greatly influenced by the measurement site. Systole in particular changes as the site in moved from the aorta away from the heart towards the extremity. Usually it is a surprise to the newcomer to this field that systole increases as one moves down the arterial stream. This increase (systolic heightening) is caused by the reflection of the incidence waveform adding to the pressure. The amount of this increase depends on the transmission speed of the artery, which is affected by the arterial compliance. Hard arteries produce more heightening than compliant arteries.

The pressure in the aorta is not much influenced by reflections, and may be the best indication of "true" blood pressure. Unfortunately this is a very difficult place to measure blood pressure except when undergoing cardiac evaluation or surgery, and there are no population studies to indicate the cardiovascular risk factors.

Resources

http://www.nhlbi.nih.gov/about/framingham - This site reviews the history of the Framingham study, and the results. It also offers risk assessment worksheets and much more.

http://www.dableducational.com/index.html - A must visit site if you are interested in blood pressure measurements.

http://marketplace.aami.org - Location to order AAMI SP10 which specifies the requirements for blood pressure measuring equipment.

http://ash-us.org/ - Home site of The American Society of Hypertension

http://smipdx.com Website of a development partner who has designed blood pressure devices and many other types of equipment.

Sources for some of the illustrations on this website include:
http://health.allrefer.com/pictures-images/blood-pressure.html 
http://www.merck.com/mmhe/sec03/ch022/ch022a.html