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The stepwise improvement in patient outcomes with higher institutional volume is a testament to the learning curve that stills exist in transcatheter aortic valve replacement (TAVR), according to a study spanning more than a decade.

By the time centers reached their most experienced phase with more than 300 TAVR cases, those institutions had a significantly lower all-cause mortality rate at 30 days for their patients (3.3%), with the results as follows:

• Initial era (cases 1-75): 9.6% (OR 3.83, 95% CI 1.93-7.60)
• Early era (cases 76-150): 7.9% (OR 2.41, 95% CI 1.51-5.03)
• Intermediate era (cases 151-225): 5.8% (OR 2.53, 95% CI 1.19-5.40)

Once the hospital reached 226-300 TAVRs, the odds of patient survival did not improve substantially with additional experience (4.8% versus 3.3%, P=0.525), reported Asim Cheema, MD, PhD, of St. Michael's Hospital in Toronto, Ontario, and colleagues , writing online in .

However, the most experienced centers still had better outcomes in producing the fewest deaths, strokes, major bleeding, vascular complications, surgical conversions, and renal failures after TAVR. Moreover, low-volume TAVR institutions (annual TAVR case volume <50) had worse 30-day all-cause mortality rates and worse early safety compared with higher-volume groups, whereas intermediate (50-100) and high annual volume centers (>100) shared similar patient outcomes.

"There are many low-volume centers performing <50 procedures a year both in the United States and internationally," Cheema and co-authors wrote. "This trend is likely to continue with potential utilization of this technology in lower-risk patients. The findings from the present study suggest a minimum annual volume threshold to provide the best clinical outcomes for patients undergoing TAVR procedures and can serve as a guide for optimal distribution of resources and technology."

Included in the study were 3,403 patients who had TAVR in 2005-2016. Sixteen centers across North America, South America, and Europe participated in the registry.

Low-volume centers with annual TAVR case volume <50 were the most likely to use the transfemoral approach and self-expanding valves.

Writing in an , John Carroll, MD, of the University of Colorado School of Medicine in Aurora, said: "A question that is being asked around the world is whether the procedure has evolved to a state of maturity with advanced technologies and technique with low complication rates that there is no or a trivial learning curve and that subsequently there is no clinically meaningful relationship of outcomes to the on-going volume of TAVR procedures performed at individual centers."

The answer, he said, is no.

Additionally, Carroll urged, low-volume centers should consider referral of the highest-risk patients to a high-volume center that has excellent outcomes; he acknowledged, though, that this may be a thorny issue in the U.S., where private and public hospitals compete with each other for market share.

He said that in light of the reconsideration by the Centers for Medicare & Medicaid Services of the National Coverage Determination for TAVR -- a panel having convened in July for this purpose -- there is a "danger of lowering TAVR quality of care by doing away or relaxing volume requirements."

"This would create many new centers starting a new learning curve, result in more low-volume centers, and potentially diluting the case volume and advanced skills of established and high-volume centers. Therefore, despite the efforts of many parties to use training and proctoring to eliminate the TAVR learning curve, it continues to exist and needs to be acknowledged as new centers start TAVR programs."

Missing from the study by Cheema and colleagues, the editorial stated, are details regarding the impact that policies and facility distribution in different healthcare systems have on access to TAVR.

The investigators also acknowledged the baseline differences that existed across case volume groups, which they said they tried to account for with multivariable adjustment. Other limitations to the study were the relatively few participating centers and the lack of stratification by individual operator experience.

"Further research is required to determine whether newer TAVR technology, focused training, and proctoring can abbreviate the TAVR learning curve," the team concluded.