Let’s put some pressure on the situation . . . Pump it up!
As the heart pumps it pushes blood through the vasculature to be able to bring vital oxygen and nutrients to each cell in our body. We’ve discussed in previous weeks how hypotension, particularly associated with shock can lead towards the body’s demise. But how do we measure the pressure exerted by the heart and the underlying resistance in the vasculature? We take their blood pressure.
Near the beginning of our initial medical classes we were taught to take blood pressures. We were taught to place a sphygmomanometer (Blood pressure cuff). We were taught to pump it up and then listen for the beginning and ending of hearing the blood pulse through the arteries. We are actually not checking blood pressure but the flow. The two are closely related by not the same. In most normal patients the readings are very similar, but what occurs to our patients in hypovolemic or cardiogenic shock? The flow measurement is no longer accurate.
In fact there are a few common mistakes that we make when taking someone’s blood pressure. In 2005, the American Heart Association released 21 pages of steps and controls needed to be able to determine an accurate blood pressure from flow. If anyone is having a hard time sleeping, I suggest reading this . . . The American Society of Anesthesiology is considering using this as the new surgical anesthesia.
The most common mistake is the use of an incorrect sized BP cuff. When we place the cuff on a patient’s upper arm the bladder should be placed to cover the anterio-medial aspect of the arm and when wrapped around the end should fall within a marked range on the cuff. If too big or too small the readings will be affected. Too many practitioners feel that it takes too long to find the proper sized cuff or perhaps the service doesn’t have them available. There are multiple sized cuffs that must be considered. Sizes range from infant on up to “I never miss a meal” sizes and must be used to ensure accuracy. A quick reference is to cover 2/3 of the distance between the shoulder and elbow with the cuff. Manufactures make their cuffs proportional and the larger the width, the longer the length. Carrying three sizes (Pediatric, Adult Regular, Adult Large) will suffice for the majority of our adult patients. If we treat many children, smaller sizes will be needed.
As we listen thorough the stethoscope we hear the blood flowing through the arteries; known as Korotkoff sounds, these occur in 5 distinct stages during cuff deflation.
- first detectable sounds, corresponding to appearance of a palpable pulse
- sounds become softer, longer and may occasionally transiently disappear
- change in sounds to a thumping quality (loudest)
- pitch intensity changes and sounds become muffled
- sounds disappear
According to the most current AHA guidelines, we would record our systolic number at the very beginning of state one and diastolic at the beginning of stage 5. It would be interesting to try to identify these stages during your next practice.
As we take someone’s blood pressure, it is important to properly prepare the body. A patient should be given at least 5 minutes to rest comfortably before taking their blood pressure. As we move around our heart adjusts its rate and contractility to supply the increased cellular need. Allowing the patient to relax for a few minutes allow these effects to normalize.
In an effort to normalize the effect of gravity on the patient, we need to position the patient in a similar position every time. It’s best to use a seated position with their arm supported along the side of their body. Allowing the arm to hang may show a false high reading. The upper arm should be positioned at the same level of the heart. If an arm is elevated, a falsely low reading could occur (2 mm Hg for each inch elevation). If the patient cannot be placed in a seated position, we must adjust the arm to be at approximately the same position as the heart; for most patients along the mid-axillary line. Because we are trying to reduce gravity, any measurements above the level of the heart will reduce the reading while any done below the heart will elevate. Similar to the cuff, an aneroid transducer must be positioned at the same level to avoid errors.
Crossing of the legs can also have a dramatic elevating effect on blood pressure. Studies have shown an average increase of 8.1 mm Hg in patients with their legs crossed because of decreased blood flow beyond the cross, causing the remainder to be higher. The patient should sit with his feet flat on, and supported by the floor. Dangling legs cause similar increased readings. This false high may affect your treatment plan.
Another common problem is placement. The cuff should be placed on the upper arm against bare skin between the antecubital fascia (inside of elbow) and armpit. The reference line on the cuff should align with the brachial artery. Care should also be taken that the clothing does not constrict the arm above the cuff. While some people try to assess vital signs from forearms, wrists or lower extremities, we must consider the same relation to the heart as previously described. There have also been studies the show while the Mean Arterial Pressure (MAP) remains consistent throughout the body, the systolic is elevated and diastolic is reduced the more distal we assess. This widening pulse pressure may lead to a false negative when assessing for tamponade following blunt force trauma.
To demonstrate the differences in body position, consider orthostatic blood pressures. Take a reading for a laying patient. Wait 1 minute and take it on the same person, now sitting. Wait another minute and take the reading on the now standing patient. The readings will be different; slightly in a healthy adult, significantly in someone with circulatory or cardiac problems.
Perhaps the biggest error comes from our own biases. We expect to hear around 120 and lose around 80. Our mind can play tricks and override our ears. In loud environments, we get lazy and may measure based on a bouncing needle or random ranges within normal. Also, because we expect the “normal” range we may not fill our cuff properly. Underinflation is easy to understand regarding inaccurate readings, but overinflation can lead to falsely high readings.
As we properly assess a patient’s blood pressure, we should place the cuff properly and assess a radial or brachial pulse, inflating until the pulse goes away + 10 mm Hg. At this time a stethoscope should be placed with the appropriate listening side to the patient along the medial 1/3 of their antecubital fascia. As we lower the pressure in the cuff, we should listen for the 5 stages listed above. When we can first hear the pulse flow, we would note the number as the systolic. Continue listening as we lower the pressure until the flow disappears. An irregular rate may affect your expected time for a pulse and consequently an accurate reading. We must deflate slower in patients with irregular rates.
Lastly, we need to consider our equipment, when was the last time that the cuff was calibrated? Has the unit been dropped? Shock damage can change how the meter will read. Are you familiar with the errors in an automatic cuff? How does it compare to the gold standard, a mercury sphygmomanometer?
We were taught in Emergency Medicine that the exact blood pressure is not as big of a deal as its trending during our care. But why take a known inaccurate reading? Also failure to standardize variables may lead to trend changes that are not real. We may not need to assess the Blood Pressure on all of our patients, but when we do they deserve accuracy.