When I first started as an EMT in Newark, N.J., I had the opportunity to work with a more experienced partner who had started working on the ambulance in the mid 1960s, pre-EMT training in the state. He said to me, “Kid, blood pressure, tire pressure—it’s all the same.” While this statement gave me a chuckle, I soon realized a cavalier approach to blood pressure and the tools we use to measure it does not serve us well as clinicians. BP is an essential cardiovascular variable. Its monitoring is mandatory in any EMS patient, and it has a significant impact on patient management. BP is essential for us to determine when a patient is decompensating and proceeding into shock or is in extremis due to congestive heart failure or hypertensive crisis.
English clergyman Stephen Hales made the first measurement of blood pressure in 1733. Blood pressure is an indirect measurement of the pressure of the blood against the arterial walls as it passes through them. Blood travels as a pressure wave, and, using a sphygmomanometer, the high point of the wave is recorded as the systolic pressure, and the low point is noted as the diastolic pressure. If we wanted to directly measure the blood pressure, as they do for patients in the ICU or for some surgical procedures, we would insert a catheter into artery (sometimes referred to as an a-line) to directly measure the actual arterial blood pressure. This has not become an ALS skill yet, but in time we may see it.
Several factors influence BP, one being blood volume (or its absence in hypovolemia), the responsiveness of the arteries to contract (hypertension), or if they overreact and dilate (neurogenic shock). The more sinister complex mechanisms of systemic and capillary vasodilation compounded by leakage of plasma at the capillary level occur in distributive shock.
Even the construct of the BP and its parts are critical. The average blood pressure is 120/80. The normal pulse pressure—the difference between the diastolic and systolic pressures in adults—is 40 mmHg. A widening pulse pressure may be related to rising intracranial pressure; a narrowing pulse pressure may indicate cardiac tamponade or, in a trauma patient, be a warning sign of shock.
Many of the medications we administer affect BP, and the minute changes we see can be indicative of a patient’s status in the yin and yang of resuscitation. Administer nitroglycerin to the cardiac patient with a systolic BP of 60, and our next therapy may be chest compressions.
Obtaining a BP
If we do not practice diligence in assessing BP, we can affect its accuracy. First remove anything covering the arm you’re attempting to get a BP on. The bladder inside the cuff should cover 80% of the upper arm/humerus and be 40% of the width. The edge of the cuff should be 2–4 cm above the antecubital fossa, and the gauge should be level with the heart. If the cuff is too narrow, it will artificially raise a patient’s blood pressure. If it is too wide, it will artificially lower it. If we take too many BPs in rapid succession, we will artificially lower the reading. This is why we say to wait two minutes between measurements on the same arm.
There are three ways to measure blood pressure.
Palpation—Palpation is the first method we teach students to use because the essential elements of BP cuff application and finding the brachial pulse are the rudiments of auscultation. Palpation is an excellent method to use in noisy environments where listening for a BP isn’t possible. The downside is that you will only acquire a systolic pressure—you will not obtain the diastolic pressure or determine the pulse pressure.
Apply the BP cuff. Palpate the brachial artery. While maintaining your fingers on the brachial artery, pump up the BP cuff until the pulse disappears, then 10–20 mmHg above that point. Slowly release the air from the cuff, and the point where the pulse returns will be your systolic pressure.
Auscultation—In auscultation we use our stethoscope to listen for a tapping or thumping as blood flow returns inside the brachial artery. These sounds were first identified by Russian surgeon Nikolai Korotkov in 1905 and bear his name to this day (Korotkoff sounds). As a result auscaltation is the gold standard for the measurement of blood pressure.
First apply the BP cuff. Palpate the brachial artery. Place the diaphragm of your stethoscope on the brachial artery. Now palpate the radial pulse. Pump up the BP cuff to a point about 20–30 mmHg above the point where the radial pulse disappears. Slowly release the air from the cuff in 2–3 mmHg increments. The point where you first hear Korotkoff sounds in your stethoscope will be your systolic pressure, the point where the arteries are under maximal pressure. When they disappear or diminish in volume, this will be the diastolic pressure, or the resting pressure of the arteries.
Oscillation—The advent of noninvasive blood pressure (NIBP) measurement was a technological feat. The first devices to use this technology actually had microphones embedded into the cuff and auscultated the blood pressure. These devices were cost-prohibitive and generally not seen outside operating suites or ICUs. Even then they were considered a luxury.
The use of oscillometry was a step forward to bring NIBP not only to clinicians but to the masses. Oscillometry is used in every NIBP measured by cardiac monitors used in the EMS arena. Knowing how cuff-only oscillometric readings are estimated helps in understanding the limitations of this method.
What is oscillometry? You may have seen it and not realized it. Have you ever taken a BP and seen the needle on the gauge begin to “bounce”? Sometimes that bouncing begins exactly at the point we hear the Korotkoff sounds or feel the return of the brachial pulse. Sometimes it begins before we hear or feel anything, sometimes after we hear the Korotkoff sounds or feel a return of the pulse. The bouncing of the needle reflects that the artery wall oscillates when bloodflows return through an artery during cuff deflation. Oscillometry uses a transducer to detect those oscillations in the arterial wall to arrive at the systolic pressure. It then uses a formula based on a variety of metrics—for example, the peak pressure wave and volume changes within the cuff bladder—to indicate the average of the systolic and diastolic BP within the artery. It combines this average with the rate of change of the pressure wave; the microprocessor then uses a variety of algorithms to estimate the systolic and diastolic BP.
These algorithms vary from machine to machine, resulting in slightly different interpretations of the pressures and depending on a host of factors. While the oscillometric BP may be spot-on, it can also be off enough to influence treatment decisions in the field.
Remember when we would observe oscillations on the gauge when deflating the cuff? The automated oscillometric machines’ differences with respect to the algorithms used to determine the diastolic pressure; the transducers used to detect change; inflation and deflation rates; and cuff sizes and materials all may affect the estimation of BP by 10, 20, or even 30 points. These are significant variances compared with auscultatory readings in the same patient. This is why every manufacturer of NIBP devices instructs users to obtain a manual auscultated BP prior to obtaining a BP using a NIBP device.
A 10–20-point difference in BP may mean the difference for a patient who receives volume resuscitation when they didn’t require fluid, popping the clot and increasing bleeding, or when deferring nitroglycerin in a patient with STEMI because of concerns regarding bottoming the patient’s BP out. This is the basis for manufacturers’ recommendation to acquire a manual BP before a NIBP. For this reason auscultation remains the gold standard for BP measurement.
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Daniel R. Gerard, MS, RN, NRP, is EMS coordinator for Alameda, Calif. He is a recognized expert in EMS system delivery and design, EMS/health-service integration, and service delivery models for out-of-hospital care.