DENVER—One of the first studies of its kind has found that intraoperative hypotension is a common occurrence in ambulatory surgery centers (ASCs).

The researchers found the results surprising given the fact that patients who undergo outpatient surgery at ASCs are generally perceived to be at low risk for experiencing the adverse event, but believe that steps can be taken to reduce its incidence in these settings.

“We know that intraoperative hypotension is associated with a host of negative outcomes,” said Wael Saasouh, MD, the regional director of research at NorthStar Anesthesia and an anesthesiologist at the Detroit Medical Center. “Although the overwhelming majority of prior studies have been performed at major academic centers, it is reasonable to believe that the incidence of hypotension would be less in ASCs because those patients tend to be healthier and the procedures simpler than at academic institutions.

“Nevertheless, data from ASCs is scarce and not nearly as well studied,” he continued. “Given the rise in surgical numbers at ASCs, we set forth to obtain definitive data and compare it to existing literature. We wanted to see if intraoperative hypotension exists to the same extent in ASCs, and if so, the influence of clinician, type of surgery and location.”

A Third of Cases

To do so, the investigators turned to two large U.S. anesthesia practices, both of which contributed retrospective data on 16,750 outpatient cases performed between 2020 and 2021 at ASCs that electronically record and store perioperative blood pressure readings. Cases were included if they comprised ASA physical status II to IV adult patients; nonemergent surgery; general, neuraxial or regional anesthesia; and adhered to the study’s definition of intraoperative hypotension.

“There are many definitions for intraoperative hypotension,” Saasouh told Anesthesiology News. “Depending on which definition you choose, the incidence can be anywhere between 5% and 99%, which makes it very difficult to study something like this.”

The researchers used a primary definition of intraoperative hypotension that aligns with the relevant measure in the Centers for Medicare & Medicaid Services Merit-Based Incentive Payment System. According to this definition, intraoperative hypotension occurs when a patient has experienced a mean arterial pressure below 65 mm Hg for an aggregate of least 15 minutes during the course of a case.

Presenting at the 2023 annual meeting of the International Anesthesia Research Society (abstract 9), he noted that the cases included in the analysis seemed fairly typical of an outpatient setting. Indeed, some 75% were younger than 65 and 80% were ASA physical status II. Furthermore, 87% of the cases had between 0 and 5 anesthesia base units, representing lower complexity cases; the median duration of anesthesia was 65 minutes.

It was found that patients experienced intraoperative hypotension in 30.9% of cases. Of note, the incidence of the adverse event was greatest among younger patients (ages 18-39 years), females and ASA II patients (vs. ASA III or IV). And while the incidence of intraoperative hypotension was also found to be highest in a small number of cases with ASA base units greater than 11, it was nevertheless higher in cases with 0 to 5 base units than in those with 6 to 10 base units.

Healthy Patients Get Hypotension

As Saasouh explained, he and his colleagues were surprised to find that the hypotension rate revealed by the study did not differ much from that found in previous research in large academic centers.

“If you look at the published literature, the incidence is around 30% for our definition, meaning we’re finding almost the same amount of hypotension in ASCs, where cases are much less complicated,” he said.

Part of the reason for this, Saasouh explained, may be that clinicians assume that surgical patients presenting to ASCs—who tend to be younger and healthier than their inpatient counterparts in hospitals—can tolerate deeper and more sustained episodes of intraoperative hypotension.

“On the other hand, if you see intraoperative hypotension in a patient who’s an ASA IV with a history of heart failure, you’re going to put in an arterial line and be very focused on hemodynamics because this person is frail and can’t tolerate low blood pressure,” he said.

When the researchers performed multivariate analysis of factors that may be associated with the incidence of intraoperative hypotension, they were surprised to find that the single most important driver was anesthesia provider.

Addressing such disparities, he added, begins with training anesthesia providers on the signs and risk factors for intraoperative hypotension. “I believe we should standardize how hemodynamics are looked at in an evidence-based way,” Saasouh said. “This means you don’t just immediately start giving blood pressure medications. You hydrate first to make sure the organs are getting enough perfusion. Then if you need more, you can give blood pressure medication.”

Future research, the investigators added, should examine the clinical burden of intraoperative hypotension in the ambulatory setting, as well as the effect of preventing or treating that hypotension on clinical outcomes.

“We know from other studies that hypotensive patients are much more likely to have worse outcomes,” Saasouh concluded. “So it’s fair to assume that would be the case in ASCs as well, but we need to support that with data.”

By Michael Vlessides

Saasouh reported no relevant financial disclosures. The manuscript on which the poster presentation was based was recently published (J Clin Anesth 2023;90:111181); the investigators also published similar research (Perioper Med [Lond] 2023;12(1):29) looking at intraoperative hypotension.