To make the best use of your blood pressure monitoring equipment, it helps to have an idea of how the equipment works and the possible sources of error that can affect the readings.
5 Mistakes That Are Giving You Incorrect Blood Pressure Readings
The most common errors in blood pressure reading are:
Using the wrong size cuff
The most common mistake when using indirect blood pressure measurement equipment is using the wrong size cuff. A BP cuff that is too large will give falsely low readings, while a cuff that is too small will give falsely high readings.
The American Heart Association publishes guidelines for blood pressure measurement. It recommends that the length and width of the bladder (the inflatable part of the cuff) be 80% and 40%, respectively, of the circumference of the arm. Most doctors find measuring the circumference of the bladder and arm too time-consuming, so they don’t do it.
Incorrect patient positioning
The second most common error in BP measurement is the incorrect position of the extremities. To accurately assess blood flow in an extremity, the influences of gravity must be eliminated.
The standard reference level for blood pressure measurement by any technique, direct or indirect, is at the level of the heart. When you wear a cuff, the arm (or leg) where the cuff is applied should be at mid-heart level.
Measurement of BP in a limb placed above the level of the heart will give a falsely low BP. Whereas falsely high readings will be obtained whenever a limb is placed below the level of the heart. The errors can be significant, typically 2 mmHg for every inch the limb is above or below the level of the heart.
An upright sitting position provides the most accurate blood pressure, as long as the arm in which the pressure is taken remains at the patient’s side. Patients lying on their side, or in other positions, may present problems with accurate pressure measurement
Incorrect cuff placement
The standard for blood pressure cuff placement is the upper arm using a cuff on bare skin with a stethoscope placed in the elbow crease over the brachial artery.
The patient must be seated, with the arm supported at the level of the heart, the legs uncrossed and without speaking. Measurements can be made at other locations, such as the wrist, fingers, feet, and calves. But they will produce varied readings depending on the distance from the heart.
The mean pressure, interestingly, varies little between the aorta and the peripheral arteries, while the systolic pressure increases and the diastolic pressure decreases in the more distal vessels.
Crossing the legs increases systolic blood pressure by 2 to 8 mm Hg. About 20 percent of the population have pressure differences of more than 10 mmHg between the right and left arms. In cases where significant differences are observed, treatment decisions should be based on the higher of the two pressures.
Normal reading bias
The bias of normal readings contributes significantly to inaccuracies in blood pressure measurement. I would certainly be suspicious if a fellow EMT reported blood pressures of 120/80 in three patients in a row. As creatures of habit, humans expect to hear sounds at certain times, and when extraneous interference makes it difficult to get a blood pressure, there is a considerable tendency to “hear” a normal blood pressure.
Orthostatic hypotension is defined as a drop in systolic blood pressure of 20 mm Hg or more, or a drop in diastolic blood pressure of 10 mm Hg or more, measured after three minutes of standing quietly.
Not factoring electronic units correctly
Electronic blood pressure units, also called noninvasive blood pressure (NIBP) machines. They detect air pressure changes in the cuff caused by blood flowing through the PA cuff tip. The sensors calculate the patient’s mean arterial pressure (MAP) and pulse rate. The machine’s software uses these two values to calculate the systolic and diastolic BP.
To ensure the accuracy of the electronic units, it is important to verify the displayed pulse rate with the patient’s actual pulse rate. Differences of more than 10 percent will seriously upset the unit’s calculations and produce incorrect systolic and diastolic values on the display.
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