Study: Ambulatory BP Monitors Deemed Erratic

NEW ORLEANS — Bringing ambulatory blood pressure monitors (ABPMs) into the clinic remains a challenge for many centers, and it doesn’t help that available devices may be providing inaccurate readings.

The 2017 hypertension guidelines from the American College of Cardiology and American Heart Association gave ABPM Class IIa recommendations to screen for white-coat or masked hypertension in certain groups.

But when two popular oscillometric ABPMs (the Oscar 2 from SunTech Medical and Spacelabs 90207/90217) were tested in the lab, both came up short against mercury column readings taken manually as reference: the Oscar overestimated systolic blood pressure (BP) by at least 5 mm Hg in 90% of cases, and the Spacelabs devices in 70%, according to V. Patteson Lombardi, PhD, of the University of Oregon in Eugene.

Lombardi and colleagues presented a poster at the American Heart Association annual Hypertension meeting. This group had previously associated auscultatory Accutracker II ABPMs with excessive inter-device variability and increase in patient misclassifications.

Currently Available ABPMs: A Trust Issue

“Oscar and Spacelabs oscillometric ABPMs are erratic in estimating individual BPs, have a low level of agreement and can differ clinically and statistically from observers using a Hg column in a controlled laboratory setting,” the authors said.

“Our results … do not provide confidence in 24-hour measurements during a time when patients are active and assume multiple postures. At the very least, U.S. and international protocols for validating ABPMs must develop more stringent standards and require postural testing. Until then, we have difficulty putting our trust in ambulatory BP monitors,” they concluded.

In the new study, 10 people (70% normotensive) got multiple BP readings in various positions.

Each time, one arm underwent ABPM assessment while the other was subject to a mercury column by a trained observer. Arms were subsequently switched and each reading done in triplicate. Observers were blinded to each other’s results.

The average overestimation of systolic BP was 8.8 mm Hg for Oscar and 5.4 mm Hg for the Spacelabs. As for diastolic BP, these figures were 1.2 mm Hg and 1.4 mm Hg, respectively.

Moreover, the proportion of readings that would have misclassified patients’ BP status (based on 2017 American guidelines) was 20% with the Oscar 2 device and 10% with Spacelabs.

“These inaccuracies become increasingly important with new U.S. guidelines shifting the definition of hypertension to lower levels,” Lombardi and colleagues said.

They also discovered that deviances from reference BP varied with posture: Oscar overestimated systolic BP by 3.5 mm Hg (and diastolic BP by 6.5 mm Hg) in the supine position (P<0.05) and 4.7 mm Hg seated (P<0.01); Spacelabs systolic BP overestimations by posture did not reach statistical significance, but diastolic BP was underestimated by 5.8 mm Hg standing (P<0.05) and 6.4 mm Hg overestimated in the supine position (P<0.05).

“Note the variability for both ABPMs based on posture and the stepwise progression of diastolic BP differences from supine to seated to standing, indicating static, nomogram equations that do not adjust for posture,” according to the authors.

ABPM Implementation Woes

But even if an ABPM is found to be reliable, there are many things that can go wrong when implementing a new ambulatory monitoring program, some said during another session at the Hypertension meeting.

Even though the U.K. has recommended ABPMs since 2011, they still haven’t implemented it well. “But of course, we’re even farther behind them,” according to Anthony Viera, MD, MPH, of Duke University in Durham, North Carolina. In fact, he said, some clinicians even confuse ambulatory with home BP monitoring.

Viera told meeting attendees that it’s hard to get an ABPM at his institution.

One barrier to implementation is the cost of each monitor, which can run $2,000-3,000 on average, he said. A study showed that Medicare paid a median of $52 for ABPM claims submitted from 2007-2010; an informal reimbursement log from 2016 between Viera and his colleagues showed payments ranging from as low as $39 from Medicare to $155 from private insurers.

Despite the cost, however, “you probably need more ABPMs than you think you need,” according to session panelist Joshua Samuels, MD, MPH, of McGovern Medical School at UTHealth in Houston.

Another issue is the difficulty in finding trained staff who can work with patients on the ABPM process.

“ABPM requires work, actually. You don’t just slap it on,” said fellow panel member Gbenga Ogedegbe, MD, MPH, of NYU School of Medicine in New York City. “Don’t underestimate the staff you need for ABPM.”

For one, the staff must explain the practicalities of the ABPM to the patient and prepare him or her to have “a cord dangling behind them” and a machine that buzzes regularly around the clock. Staff must also schedule patients to come in 2 days in a row, which may be problematic for people who do shift work for instance, according to Viera.

Then, there are the challenges in incorporating ABPM readings into the electronic health records (EHR) system: It’s “not such a slam dunk right now, it’s not so simple,” the speaker said.