Differences in complication rates among the centres in the SPACE study

Despite the high grade of standardisation of study protocols, there is still room for variability among the centres in specific treatment aspects. We evaluated the treatment risk in stent-protected angioplasty of the carotid versus endarterectomy (SPACE) associated with the specific patient enrollment rates of the centres.

The analysed endpoints were ipsilateral stroke or death [primary outcome event (pOE)] and any stroke or death [secondary outcome event (sOE)] until 30 days after treatment. A binary logistic regression analysis with random effects was performed separately for each treatment arm. The centres were secondarily categorised in three classes: I) > or = 25 patients enrolled, II) ten to 24 patients and III) <10 patients and a hierarchic log linear model was fitted to test the three-way interaction of treatment, number of patients per class and outcome.

The random effects logistic regression analysis in the carotid artery stenting (CAS) arm proved a significant increase in pOE with decreasing number of patients enrolled (-0.0190 +/- 0.0085, p = 0.025, deviance 35.7 with 32 df), whereas no such effect was found in the carotid endartectomy (CEA) arm (-0.010 +/- 0.008, p = 0.24, deviance 39.78 with 32 df). In the log linear model, there was a significant interaction between treatment, number of patients per centre and sOE (p = 0.023). The odds ratios for sOE in the enrollment classes (CAS vs. CEA) were 0.98 (95% CI 0.50-1.94, p = 0.95) for class I, 1.13 (95% CI 0.47-2.77, p = 0.77) for class II and 11.56 (95% CI 1.40-253.45, p = 0.01) for class III centres.

Despite rigorous standardisation and quality requirements for operator qualification, there seemed to be a decrease in complication rate with increasing patient enrollment numbers in the CAS arm while this signal could not be detected in the CEA arm of SPACE.