By Mike McEvoy, Ph.D., REMT-P, RN, CCRN
EMS providers often measure blood pressure (BP) in difficult situations that pose tremendous chances for error. Let’s review the most likely sources of error in blood pressure measurement and discuss some tips for obtaining blood pressures in tough conditions.
Actual measurement of blood pressure requires placement of a monitor directly into an artery. Short of this, the best that can be done is to measure blood flow. Typically, pressure and flow are related, except in shock states when flow is reduced and pressure rises from increased vascular tone. In the prehospital environment, what we call blood pressure measurement is actually measuring blood flow.
The most common error measuring blood pressure is using a wrong size 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 most practical way to 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 most of the adult population. Multiple smaller sizes will be needed if you frequently treat pediatric patients.
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. To do this, the arm (or leg) where the cuff is applied must be at mid-heart level. An extremity above heart level will provide a falsely low BP whereas false high readings will be obtained from limbs located below heart level. Seated upright and supine patients pose little difficulty as long as the extremity in which the pressure is taken remains at the patient’s side. Side-lying or other oddly positioned patients 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.
There are circumstances when BP measurement is simply not possible. We can roughly estimate systolic BP (SBP) by assessing pulses in a patient. Presence of a radial pulse typically requires an SBP of at least 80 mmHg. A femoral pulse requires an SBP of at least 70, and a palpable carotid pulse needs an SBP over 60.
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. Obtaining SBP using palpation of a distal pulse while deflating the blood pressure cuff is generally accurate to within 10 to 20 mmHg of an auscultated reading. Oximeter waveforms are also useful in measuring return of blood flow during BP cuff deflation and are as accurate as pressures obtained by palpation.
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. 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 comparable to traditional measurement in the upper arm.
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 ensure accuracy from electronic units, it is important to verify the displayed pulse with an actual patient pulse. Differences of more than 10% will seriously alter the unit’s calculations and produce incorrect systolic and diastolic values on the display screen.
Mike McEvoy, Ph.D., RN, CCRN, REMT-P, is the EMS coordinator for Saratoga County, New York. A former forensic psychologist, he now works in the Cardiac Surgical ICU at Albany Medical Center and teaches at Albany Medical College in NY. He is a paramedic for Clifton Park-Halfmoon Ambulance Corps and medical advisor for West Crescent Fire Department. He presently serves as a member of the New York State EMS Council and the State Emergency Medical Advisory Council and chairs the EMS Section of the New York State Association of Fire Chiefs.
