Sumaira MacDonald says that the EVA 3-S trial has demonstrated the world’s first reverse learning curve, and that carotid artery stenting can only get better
Evidence indicates a linear relationship between throughput and outcome for carotid endarterectomy.(1-3) This relationship is less clear for carotid artery stenting, however, it is likely to exist, as carotid stenting is technically complex.
The results of carotid stenting within the French national randomised trial (EVA 3-S) were significantly worse than the results for endarterectomy (4), and this trial has swayed public feeling, moving the (largely French) audience at the recent CACVS congress in Paris to vote in favour of the motion that “Carotid artery stenting is dead”!
Many argue that those performing the procedure within EVA 3-S were inexperienced. Indeed, 85% of all operators performing carotid stenting within this trial had performed ≤50 cases in total, but, despite this, the authors, concluded that “operator experience for carotid stenting was not a significant factor in the poor results”.
The EVA 3-S trial may well have demonstrated the world’s first reverse learning curve, with a nonsignificant increase in adverse event rate as operators “progressed” from their first to their fiftieth case.
The authors of the CAVATAS and SPACE trials, however, documented a more conventional relationship between volume or experience and outcome.(5, 6)
Our group performed a systematic review of the relationship between volume/experience and outcome for carotid stenting. Of over 700 publications yielded by the search strategy, only three randomised trials, two post-marketing surveillance studies, two registries, and four large case series met the inclusion criteria.
The trials were clearly not designed to assess the effect of experience, and the two post-marketing surveillance studies (CASES-PMS and CAPTURE) seemed to suggest that prior experience was not relevant to outcome for carotid stenting.(7, 8) However, they both employed a structured training programme for operators at three different levels of experience and supported them with didactic and hands-on teaching, case reviews and simulator sessions.
Four case series notable for their size (totaling almost 3,000 patients) met the criteria for inclusion in our systematic review; Roubin et al, Boltuch (a series updated from Ahmadi et al), Verzini et al, and Setacci et al.(9-12) These series spanned more than a decade (1994-2006) and this particular period of time heralded conspicuous advances, such as improved pharmacology, dedicated stents, and cerebral protection.
There is evidence highlighting the influence of each of these innovations on outcome. A cynic might suggest that technical advances alone explain the improved results over time. However, meta-analysis of data from our systematic review showed statistically significant improvements with time (p=.0015 and p=.0001 respectively) both before and after the “tipping point” in technology (around 2000-2001).
Furthermore, registries like ProCAS (containing over 5,000 patients) clearly reveal that experience is an independent predictor of outcome, with a significant difference in performance for those who had performed 50 cases compared to 150 cases, and between those who had performed ≤150 and those who had performed ≥150 cases.(13)
While any large registry like this one collects cases over time, and is therefore subject to the major confounding variable of the influence of technical advances on outcomes, ProCAS also demonstrated that the positive temporal trend disappeared when adjustment was made for institutional experience, stressing the importance of experience rather than technical improvements.
To return to the question I was asked to address at the CACVS congress in January, the literature suggests that 50, 80, or 197 cases are required before the stroke and death rates for carotid stenting fall to a level acceptable to those performing endarterectomy.(11, 14, 15) Meta-analysis of these data reveal that it takes, on average, 1.82 years in centres with reasonable volume to get below an (arbitrary) event rate of 5%.
Is this the great Achilles’ heel of carotid stenting? The answer, I think, has many facets.
Endarterectomy is an “index procedure”, i.e. on the curriculum for vascular surgical trainees and is taught and examined formally. carotid stenting is currently not.
Institutional experience is, arguably, as important as operator experience; the scrub nurses, radiographers, ward nurses, and possibly anaesthetists, all have a role to play. Institutional learning extends to decision making, i.e. the whole multidisciplinary team learns appropriate case selection, and this is as vital as pure technical skill for any procedure, not least carotid stenting.
The importance of case selection will be highlighted during the CX Symposium, at which I will present a scoring system developed to aid novices (i.e. those with experience of ≤50 cases) in selecting patients for carotid stenting according to agreed anatomic criteria.
Finally, should we worry about that vote in Paris? Probably not: EVA 3-S did indeed demonstrate that carotid stenting performed by novices in unselected patients is less safe than endarterectomy performed by experts.
An important message for us all, I think.