Appropriate BP Site

Question#: 683

Question:

Regarding obtaining a BP, is the standard the upper arm or can alternate sites be acceptable when considering a treatment plan?

Answer:

Taken on many patients, blood pressures are in important diagnostic tool. As per the BLS PCS and ALS PCS, there is no predefined site where you must take a blood pressure. Keep in mind however that there are many variables that will impact the accuracy of said blood pressure, which can ultimately impact your downstream care and treatment plan. As with anything, the trend of the blood pressures is important, and one value shouldn’t be taken in isolation.

The NIBP monitor uses the oscillometric measurement technique. The oscillometric technique does not use Korotkoff sounds to determine blood pressure; rather, it monitors the changes in pressure pulses that are caused by the flow of blood through the artery. The NIBP monitor measures the pulse rate by tracking the number of pulses over time. The NIBP monitor uses artifact rejection techniques to provide accurate results under most operating conditions. When a patient is experiencing arrhythmias during a measurement, the accuracy of the pulse determination may be affected or the time needed to complete a measurement may be extended. In shock conditions, the low amplitude of blood pressure waveforms makes it difficult for the monitor to accurately determine the systolic and diastolic pressures.

Before we consider the site, it’s important to optimize the controllable factors to obtaining accurate blood pressures. Cuff size is an important variable. Choose the appropriately sized BP cuff as that can skew the accuracy of your results - a BP cuff that is too large will give falsely low readings, while an overly small cuff will provide readings that are falsely high. If too small a cuff is used, the pressure generated by inflating the cuff may not be fully transmitted to the brachial artery; in this setting, the pressure in the cuff may be considerably higher than the intraarterial pressure, which can lead to overestimation of the systolic pressure by as much as 10 to 50 mmHg in patients with obesity. To be technical, the AHA recommends that the bladder length and width (the inflatable portion of the cuff) should be 80 percent and 40 percent respectively, of arm circumference. Obviously, this isn’t practical on a 911 call, so a quick and easy way is to choose a cuff that covers two-thirds of the distance between your patient’s elbow and shoulder. Commercial cuffs also have markings on them to allow for accurate sizing and avoiding miscuffing. Position the ‘Artery Index Marker’ over the patient’s brachial artery and ensure that the marking falls within the printed range markings on the cuff. This should ensure the correct size. Moreover, the cuff should be placed on bare skin, or a thin sleeve, and shirt sleeves shouldn’t be rolled up as it may create a tourniquet effect, nor should the measurement be taken over a thick sleeve, as this may lead to an overestimate of the patient's BP.

Other controllable factors include patient positioning, and correct cuff location. With respect to patient positioning, blood pressures should be conducted with the patient in a seated or supine position, and the BP cuff should be at the level of the patient’s right atrium. If in the supine position, if the arm is resting on the bed, it will be below the level of the hear, so the cuffed arm should be supported with a pillow. If the arm is allowed to hang down unsupported, the measured BP will be elevated by 10 to 12 mmHg due to the added hydrostatic pressure induced by gravity.

Research has shown that SBP has been reported to be 3-10 mmHg higher in the supine vs seated position. In a seated position, the right atrium level is the midpoint of the sternum or 4th intercostal space. If the patient does not have their back supported while seated, the SBP and may be increased by 5-15 mmHg. Other factors that may artificially raise the SBP include having crossed legs (5-8 mmHg increase), having to urinate (>10 mmHg increase), having the upper arm below the right atrium (i.e. the arm dangling while supine), or the patient holding their arm up themselves. In an ideal world, patients should have an empty bladder, sit quietly with a supported back, keep both feet flat on the floor, have uncrossed legs, and the BP cuff should be placed directly on the bare arm (not over clothes) at the level directly above the AC fossa; though patients in our world are often not in ideal situations.

With respect to cuff location, the standard location is the patient’s bicep, but it is important to realize that the systolic and diastolic pressures vary substantially in different parts of the arterial tree. In general, the systolic pressure increases in more distal arteries, whereas the diastolic pressure decreases. Having said that, there are scenarios when placing a BP cuff on the patient’s arm may not be possible (i.e. injury, surgery, indwelling catheters, fistulas, grafts, body habitus, etc.).

In this case, the leg can be considered an alternative site. The principles of BP measurement in the leg are similar to those described above in the arm, with use of an appropriately sized cuff.

Ankle BPs are recommended rather than calf or thigh measurements because they generally cause less discomfort and the cuff is easier to fit, particularly in obese patients. It’s important to note that in normal subjects, the systolic pressure in the lower extremity is usually higher than that in the brachial artery. In a systematic review of 44 studies comparing arm and leg BP readings performed in the supine position, mean systolic BP was higher in both the calf (by 10 mmHg, 95% CI 4 to 16 mmHg) and the ankle (17 mmHg, 95% CI 15.4 to 21.3 mmHg) than in the arm, however there is a lot of variability in these studies making accuracy hard to reliably determine. In addition, no oscillometric BP monitors (such as the LP15 or Zoll X-Series) have been validated for lower-extremity BP measurements.

Due to accuracy concerns, taking a BP at the wrist should not be used if you can obtain a brachial BP. Technical considerations may make accurate measurement of BP at the wrist more challenging than measurements taken at the brachial artery. At the wrist, the hydrostatic pressure related to the lower position of the wrist relative to the heart can result in a further false elevation of BP. This can be minimized by taking the BP with the wrist kept at the level of the heart. In addition, an automatic device's sensor must remain directly over the radial artery for an accurate reading, and wrist flexion may interfere with appropriate sensor positioning.

References:
https://www.ahajournals.org/doi/full/10.1161/01.hyp.0000150859.47929.8e
https://www.ahajournals.org/doi/10.1161/HYP.0000000000000087#d1e363
https://www.uptodate.com/contents/blood-pressure-measurement-in-the-diagnosis-and-management-of-hypertension-in-adults?sectionName=Leg%20blood%20pressure&search=blood%20pressure%20in%20mastectomy&topicRef=104714&anchor=H10&source=see_link#H10
https://www.nature.com/articles/s41371-020-0325-5
https://journals.lww.com/jhypertension/Abstract/2019/04000/Defining_the_relationship_between_arm_and_leg.2.aspx
https://www.stryker.com/content/dam/stryker/ems/resources/operating-instructions/international/3314911-030_int-eng_lifepak_15_operating_instructions.pdf