How to Get the Measurement Right

By George Thomas, MD and Marc Pohl, MD

The diagnosis and management of hypertension depends on accurate blood pressure measurement and interpretation.

Blood pressure screening typically is done in the clinician’s office, a sub-optimal setting for this activity because a variety of factors – notably patient anxiety or “white-coat effect” — may affect measurement accuracy and contribute to misdiagnosis. Between 20 and 30 percent of patients who exhibit hypertension in a clinician’s office are normotensive in non-medical settings.

At Cleveland Clinic’s Glickman Urological & Kidney Institute, we advocate ambulatory or home blood pressure monitoring to diagnose and confirm hypertension. If neither option is feasible, automated oscillometric monitoring in the physician’s office is preferable to standard manual measurement.

Both ambulatory and home monitoring produce readings 5 to 10 mm Hg lower than those typically obtained in a medical office and are more representative of a patient’s usual blood pressure. Office-obtained measurements using automated oscillometric equipment more closely match ambulatory and home readings than those obtained using office-based manual (auscultatory) means.

Here’s more about our blood pressure monitoring rationale and guidelines.

Ambulatory monitoring

Ambulatory blood pressure monitoring (ABPM) is the reference standard for hypertension diagnosis and is a better predictor of cardiovascular problems than is conventional office-based screening.

ABPM utilizes a wearable, automated device that collects measurements during a 24- to 48-hour period, with a frequency of every 15 to 20 minutes during daytime and every 30 to 60 minutes while the patient sleeps. A computer averages the day and night readings.

ABPM-defined hypertension is a 24-hour average BP ≥ 125/75 mm Hg, average daytime BP ≥ 130/80 mm Hg, or average nighttime BP ≥ 110/65 mm Hg. A patient with elevated office-obtained BP but normal daytime ABPM is considered to have transient hypertension produced by the stress of the medical office visit and/or direct interaction with medical professionals (i.e., white coats).

Due to cost, minimal insurance reimbursement and many clinicians’ unfamiliarity with its efficacy, ABPM is not routinely available in medical offices.

Home monitoring

Semi-automatic home monitoring units’ relatively low cost ($40-$60) and their ability to eliminate the influence of office-induced hypertension or the white-coat effect make them a viable option for hypertension diagnosis and management.

The clinician should check the patient’s home monitor for accuracy initially and at least yearly thereafter. Patients or their caregivers should be taught correct BP measurement techniques.

Devices with manually inflated cuffs may require significant exertion that can temporarily elevate BP in patients who are self-monitoring by as much as 12/9 mm Hg. Patients using automatically inflated units may experience brief rises in systolic pressure of as much as 7 mm Hg, probably because they tense in anticipation of inflation.

With manual devices, inflating the cuff to at least 30 mm Hg above systolic rather than the usual 20 mm Hg and deflating no more than 2–3 mm Hg per second will allow time for transient exertion effects to dissipate and increase accuracy. With either modality, taking multiple measurements and discarding the first should eliminate any effect from anticipatory anxiety.

Office-based measurement

This is likely to remain the primary means for diagnosing and managing hypertension, due to the cost/availability issues previously mentioned for ambulatory and home monitoring.

Automated or manually operated oscillometric devices that use an electronic pressure sensor to record arterial pressure oscillations and calculate BP are gradually replacing auscultatory devices, whose accuracy varies by the operator’s training and experience. The presence of a medical professional during measurement with either an oscillometric or auscultatory device, as noted, can produce anxiety-elevated readings.

Other factors that can influence the accuracy of office-based BP measurement include:

• Time of day: BP is variable throughout the day due to activity level, stress, food and caffeine intake, environmental temperature, smoking, and timing of anti-hypertensive ingestion. For optimal accuracy, sequential readings should be taken at the same time of day and prior to medication intake.
• Cuff size and placement: A properly sized cuff is critical; if it is too small, inflated cuff pressure may substantially exceed intra-arterial pressure, which could cause overestimation of systolic pressure by 10–50 mm Hg in obese patients. The cuff should be placed with its bladder midline over brachial artery pulsation and with the lower end of the cuff 2–3 cm above the antecubital fossa to minimize noise from the stethoscope touching the cuff. If using the auscultatory method, the cuff should be inflated to 30 mm Hg more than systolic pressure to avoid an improperly interpreted auscultatory gap. The stethoscope should be placed lightly over the brachial artery so as not to delay the disappearance of Korotkoff sounds.
• Patient position: The patient should be seated, with legs uncrossed and back supported to avoid artificially elevated BP readings. The arm should be supported at heart-level, not hanging downward, which can raise BP by 10–12 mm Hg due to gravity-enhanced hydrostatic pressure. When using the auscultatory method, the patient should sit quietly for five minutes before measurement is taken to allow for a normative reading.
• Clinician influence: Talking to the patient during BP measurement can elevate the reading by 8–15 mm Hg. Neither the patient nor the clinician should talk during measurement. Unattended automated oscillometric measurement is ideal.
• Stress and anxiety associated with office visits.

Number of measurements and diagnostic confirmation

If an automated device is not used, BP should be measured at least twice during an office visit. If the readings vary by more than 5 mm Hg, repeat measurements should be taken until values stabilize. The BP recorded in the patient’s chart should be the average of the last two measurements.

Many patients who appear to be hypertensive at the initial clinical visit actually are normotensive. BP readings from at least three visits, each separated by at least a week, are necessary in order to diagnose or rule out hypertension in the absence of other clinical indicators.

Diagnostic confirmation ideally should be done with ABPM. If home monitoring is the method of choice to establish a hypertension diagnosis, 12–14 measurements, morning and evening, taken during one week, are needed.

Dr. Thomas is a specialist in clinical hypertension and Director of Cleveland Clinic Glickman Urological & Kidney Institute’s Center for Blood Pressure Disorders.

Dr. Pohl is an emeritus staff member of the Glickman Urological & Kidney Institute’s Department of Nephrology and Hypertension.