Blood Pressure Measurement

One of the most common diagnostic procedures used in health care is the measurement of arterial blood pressure.

Blood pressure measurement goes back several hundred years—to before the advent of electrocardiograms. It is probably one of the most subjective measurements in use. Blood pressure measurement depends, for starters, on heart rate, cardiac output, vessel elasticity, and volume and thickness of the blood. Add to this the patient’s condition, nervousness, and weight; the time of day; and when the patient last ate. Now add age, sex, race, and assorted medical conditions, and you have multiple factors influencing the accuracy of blood pressure measurement. The techniques themselves, be they direct or indirect, automatic or semi-automatic, also affect the measurement.

The Basics
When the left ventricle contracts, it ejects blood into the aorta. The contraction is called systole. As the blood leaves the heart, it passes through the aortic valve. As the contraction ends, the valve closes, preventing blood from flowing backward into the ventricle. At the end of the contraction, the heart enters its diastole phase. The elasticity of the vessels smooths out the pressure waveform like a capacitor in an electronic circuit. The difference between the systolic and diastolic pressures is called the pulse pressure.

As the blood flows into the arterial tree, some goes to the head via the carotid arteries and some to the arms via the brachial artery. The aorta then carries blood to the abdomen and most of the organs before branching out into the femoral arteries going to the legs. The lowest pressures are measured in the ventricle and the highest at the furthest point from the heart, usually in the foot/ankle region. Systolic pressure increases the further from the heart that the measurement is taken.

Indirect Manual Method
Most blood pressure measurements are done manually by a trained provider who places a cuff on the upper arm of the patient and inflates the cuff to about 180 millimeters of mercury (mm Hg) and slowly bleeds the pressure down. The American Heart Association recommends a rate of 2 mm to 3 mm per heartbeat. As the pressure is lowered in the cuff, the provider listens for the arterial pulses, known as Korotkoff sounds, using a stethoscope. The cuff is connected to a mercury column, which must be replaced in 2005 per Environmental Protection Agency directive, or an aneroid gauge. Both gauges are calibrated in mm Hg. As the provider bleeds off the cuff pressure, the first Korotkoff sound (phase I) is often described as a sharp thud, which indicates systolic pressure. Following the thud, the provider hears a blowing or swishing sound (phase II), followed by a softer thud (phase III), followed by a softer blowing sound (phase IV). Finally, in phase V, there is no sound. There is some debate over whether phase IV or phase V corresponds with diastolic pressure. This can create a problem as some providers use the onset of muffling in phase IV as the diastolic pressure, while others use the disappearance of sound (phase V). This can mean the difference between prescribing medications and not.

The cuff also presents the potential for a bad blood pressure reading. Some cuffs are designed for use on the right arm, others on the left. The cuff may not properly occlude the brachial artery if it is on the wrong arm and give a bad reading. To check which arm the cuff is designed for, place it over the upper arm with the tubing coming to the center line of the elbow. Hold the cuff in place with your fingers and put your thumb on the edge of the bladder in the cuff while pushing the cuff against your arm. Your thumb and the bladder edge should reach the backside of the arm or at least start the turn to the backside. If the thumb does not reach the back of the arm, the cuff is either too small for you or on the wrong arm.

The size of the cuff is an area where mistakes are common. If the cuff is too small, the diastolic pressure will be high. A cuff that is too large will give a low systolic pressure reading. An adult cuff is probably limited to people weighing between 120 pounds and 180 pounds. A large adult cuff for those weighing between180 pounds and approximately 280 pounds is probably safe. For anyone weighing more than 280 pounds, the provider may have to use a thigh cuff. There are exceptions, so have enough cuff sizes available.

It is becoming more common for blood pressure to be taken at the ankle area in patients older than 50 or in those with circulation problems. Standard cuffs are used for this measurement.

Combine all of the preceding variables with a $6.00 stethoscope, background noise, and hearing that is not as good as it used to be, and it means that a lot of people who may not need to be are on blood pressure medicine and others who should be are not.

Mercury Column Maintenance
The mercury column is considered by many to be the gold standard for blood pressure measurement. While it is accurate if properly calibrated and maintained, it is also toxic and is being phased out of use. Most hospitals do not have a program for checking mercury columns or, for that matter, aneroid units. If you still have mercury in use, you should do at least minimum maintenance once a year. Check the zero level of the column—the top of the mercury should be at the zero point. Check the kidskin filter on the top of the column under the cap—if it is very dirty, replace it. A dirty filter will affect how the mercury moves in the column, meaning the indicated pressure and real pressure could be different. The column can be very dirty. This obscures the top of the mercury at points along the column. The column’s not being perpendicular will also affect the accuracy of the measurement.

Check the needles on aneroid units, and if they are in the rectangle when no pressure is applied, they probably are accurate to use. Adjusting aneroid units is not difficult, but they need to be checked against a known unit after adjustment and a return to use.

Blood pressure cuffs are often considered as beneath the dignity of a biomedical technician, so we avoid checking them or replacing them. Cuffs have several problems that are generally corrected by throwing them away and putting out new ones. Cuffs get dirty and bloody and have all sorts of body fluids on them. Do not clean: Replace. Cuffs develop leaks. Do not patch: Replace. Tubing cracks. To check this, just pull the tubing, and if it looks cracked, replace it. Lint fills the Velcro on the cuff so that it will not close securely and a steady pressure cannot be maintained. Again: Replace. Hospitals will replace a $90.00 patient cable if there is any question that it is bad, but they will keep $10.00 blood pressure cuffs that have been in use for years, even if they are dirty and leaking, and will not stay closed.

David Harrington is director of staff development and training at Technology in Medicine Inc, in Holliston, Mass.

Bob Freeman, a staff biomedical technician for Technology in Medicine at Quincy Medical Center, contributed to this article.

Direct any questions to dharrington@techmed.com.