(the opinions in this article are the opinion of the author and do not express the view of the Farmersville Fire Department)
Nurses and physicians often argue over differences between arterial line and non-invasive blood pressure (NIBP) cuff readings. Revised guidelines for management of high blood pressure increased thresholds for diagnosing and treating hypertension, causing further debate and controversy .
To make the best use of blood pressure monitoring equipment, it is helpful to have an insight into how the equipment works and the likely sources of error that can affect readings.
Here’s what many of us do wrong when taking BP, and how to get it right:
1. YOU’RE USING THE WRONG-SIZED CUFF
The most common error when using indirect blood pressure measuring equipment is using an incorrectly sized cuff. A BP cuff that is too large will give falsely low readings, while an overly small cuff will provide readings that are falsely high. The American Heart Association publishes guidelines for blood pressure measurement . recommending that the bladder length and width (the inflatable portion of the cuff) should be 80 percent and 40 percent respectively, of arm circumference. Most practitioners find measuring bladder and arm circumference to be overly time consuming, so they don’t do it.
The most practical way to quickly and properly size a BP cuff is to pick a cuff that covers two-thirds of the distance between your patient’s elbow and shoulder. Carrying at least three cuff sizes (large adult, regular adult, and pediatric) will fit the majority of the adult population. Multiple smaller sizes are needed if you frequently treat pediatric patients.
Korotkoff sounds are the noises heard through a stethoscope during cuff deflation. They occur in 5 phases:
- I – first detectable sounds, corresponding to appearance of a palpable pulse
- II – sounds become softer, longer and may occasionally transiently disappear
- III – change in sounds to a thumping quality (loudest)
- IV – pitch intensity changes and sounds become muffled
- V – sounds disappear
In their 1967 guidelines, the AHA recommended that clinicians record the systolic BP at the start of phase I and the diastolic BP at start of phase IV Korotkoff sounds. In their 1981 guidelines, the diastolic BP recommendation changed to the start of phase V .
2. YOU’VE INCORRECTLY POSITIONED YOUR PATIENT’S BODY
The second most common error in BP measurement is incorrect limb position. To accurately assess blood flow in an extremity, influences of gravity must be eliminated.
The standard reference level for measurement of blood pressure by any technique — direct or indirect — is at the level of the heart. When using a cuff, the arm (or leg) where the cuff is applied must be at mid-heart level. Measuring BP in an extremity positioned above heart level will provide a falsely low BP whereas falsely high readings will be obtained whenever a limb is positioned below heart level. Errors can be significant — typically 2 mmHg for each inch the extremity is above or below heart level.
A seated upright 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, can pose problems for accurate pressure measurement. To correctly assess BP in a side lying patient, hold the BP cuff extremity at mid heart level while taking the pressure. In seated patients, be certain to leave the arm at the patient’s side.
Arterial pressure transducers are subject to similar inaccuracies when the transducer is not positioned at mid-heart level. This location, referred to as the phlebostatic axis, is located at the intersection of the fourth intercostal space and mid-chest level (halfway between the anterior and posterior chest surfaces.
Note that the mid-axillary line is often not at mid-chest level in patients with kyphosis or COPD, and therefore should not be used as a landmark. Incorrect leveling is the primary source of error in direct pressure measurement with each inch the transducer is misleveled causing a 1.86 mmHg measurement error. When above the phlebostatic axis, reported values will be lower than actual; when below the phlebostatic axis, reported values will be higher than actual.
3. YOU’VE PLACED THE CUFF INCORRECTLY
The standard for blood pressure cuff placement is the upper arm using a cuff on bare skin with a stethoscope placed at the elbow fold over the brachial artery.
The patient should be sitting, with the arm supported at mid heart level, legs uncrossed, and not talking. Measurements can be made at other locations such as the wrist, fingers, feet, and calves but will produce varied readings depending on distance from the heart.
The mean pressure, interestingly, varies little between the aorta and peripheral arteries, while the systolic pressure increases and the diastolic 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 has differences of more than 10 mmHg pressure 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.
4. YOUR READINGS EXHIBIT ‘PREJUDICE’
Prejudice for normal readings significantly contributes to inaccuracies in blood pressure measurement. No doubt, you’d be suspicious if a fellow EMT reported blood pressures of 120/80 on three patients in a row. As creatures of habit, human beings expect to hear sounds at certain times and when extraneous interference makes a blood pressure difficult to obtain, there is considerable tendency to “hear” a normal blood pressure.
Orthostatic hypotension is defined as a decrease in systolic blood pressure of 20 mm Hg or more, or diastolic blood pressure decrease of 10 mm Hg or more measured after three minutes of standing quietly.
There are circumstances when BP measurement is simply not possible. For many years, trauma resuscitation guidelines taught that rough estimates of systolic BP (SBP) could be made by assessing pulses. Presence of a radial pulse was thought to correlate with an SBP of at least 80 mm Hg, a femoral pulse with an SBP of at least 70, and a palpable carotid pulse with an SBP over 60. In recent years, vascular surgery and trauma studies have shown this method to be poorly predictive of actual blood pressure .
Noise is a factor that can also interfere with BP measurement. Many ALS units carry doppler units that measure blood flow with ultrasound waves. Doppler units amplify sound and are useful in high noise environments.
BP by palpation or obtaining the systolic value by palpating a distal pulse while deflating the blood pressure cuff generally comes within 10 – 20 mmHg of an auscultated reading. A pulse oximeter waveform can also be used to measure return of blood flow while deflating a BP cuff, and is as accurate as pressures obtained by palpation.
In patients with circulatory assist devices that produce non-pulsatile flow such as left ventricular assist devices (LVADs), the only indirect means of measuring flow requires use of a doppler.
The return of flow signals over the brachial artery during deflation of a blood pressure cuff in an LVAD patient signifies the mean arterial pressure (MAP). While a normal MAP in adults ranges from 70 to 105 mmHg, LVADs do not function optimally against higher afterload, so mean pressures of less than 90 are often desirable.
Clothing, patient access, and cuff size are obstacles that frequently interfere with conventional BP measurement. Consider using alternate sites such as placing the BP cuff on your patient’s lower arm above the wrist while auscultating or palpating their radial artery. This is particularly useful in bariatric patients when an appropriately sized cuff is not available for the upper arm. The thigh or lower leg can be used in a similar fashion (in conjunction with a pulse point distal to the cuff).
All of these locations are routinely used to monitor BP in hospital settings and generally provide results only slightly different from traditional measurements in the upper arm.
5. YOU’RE NOT FACTORING IN ELECTRONIC UNITS CORRECTLY
Electronic blood pressure units also called Non Invasive Blood Pressure (NIBP) machines, sense air pressure changes in the cuff caused by blood flowing through the BP cuff extremity. Sensors estimate the Mean Arterial Pressure (MAP) and the patient’s pulse rate. Software in the machine uses these two values to calculate the systolic and diastolic BP.
To assure accuracy from electronic units, it is important to verify the displayed pulse with an actual patient pulse. Differences of more than 10 percent will seriously alter the unit’s calculations and produce incorrect systolic and diastolic values on the display screen.
Given that MAP is the only pressure actually measured by an NIBP, and since MAP varies little throughout the body, it makes sense to use this number for treatment decisions.
A normal adult MAP ranges from 70 to 105 mmHg. As the organ most sensitive to pressure, the kidneys typically require an MAP above 60 to stay alive, and sustain irreversible damage beyond 20 minutes below that in most adults. Because individual requirements vary, most clinicians consider a MAP of 70 as a reasonable lower limit for their adult patients.
Increased use of NIBP devices, coupled with recognition that their displayed systolic and diastolic values are calculated while only the mean is actually measured, have led clinicians to pay much more attention to MAPs than in the past. Many progressive hospitals order sets and prehospital BLS and ALS protocols have begun to treat MAPs rather than systolic blood pressures.
Finally, and especially in the critical care transport environment, providers will encounter patients with significant variations between NIBP (indirect) and arterial line (direct) measured blood pressure values.
In the past, depending on patient condition, providers have elected to use one measuring device over another, often without clear rationale besides a belief that the selected device was providing more accurate blood pressure information.
In 2013, a group of ICU researchers published an analysis of 27,022 simultaneous art line and NIBP measurements obtained in 852 patients . When comparing the a-line and NIBP readings, the researchers were able to determine that, in hypotensive states, the NIBP significant overestimated the systolic blood pressure when compared to the arterial line, and this difference increased as patients became more hypotensive.
At the same time, the mean arterial pressures (MAPs) consistently correlated between the a-line and NIBP devices, regardless of pressure. The authors suggested that MAP is the most accurate value to trend and treat, regardless of whether BP is being measured with an arterial line or an NIBP. Additionally, supporting previously believed parameters for acute kidney injury (AKI) and mortality, the authors noted that a MAP below 60 mmHg was consistently associated with both AKI and increased mortality.
Since 1930, blood pressure measurement has been a widely accepted tool for cardiovascular assessment. Even under the often adverse conditions encountered in the prehospital or transport environment, providers can accurately measure blood pressure if they understand the principles of blood flow and common sources that introduce error into the measurement process.
1. James PA, Oparil S, Carter BL, et al. 2014 Evidence-Based Guideline for the Management of High Blood Pressure in Adults: Report From the Panel Members Appointed to the Eighth Joint National Committee (JNC 8). JAMA. 2014;311(5):507-520. (Available at: http://jama.jamanetwork.com/article.aspx?articleid=1791497)
2. Pickering TG, Hall JE, Appel LJ, et al. AHA Scientific Statement: Recommendations for blood pressure measurement in humans and experimental animals, part 1: blood pressure measurement in humans. Hypertension. 2005; 45: 142-161. (Available at: https://hyper.ahajournals.org/content/45/1/142.full)
3. Deakin CD, Low JL. Accuracy of the advanced trauma life support guidelines for predicting systolic blood pressure using carotid, femoral, and radial pulses: observational study. BMJ. 2000; 321(7262): 673–674. (Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC27481/)
4. Lehman LH, Saeed M, Talmor D, Mark R, Malhotra A. Methods of blood pressure measurement in the ICU. Crit Care Med. 2013;41:34-40.