Trends and outcomes of valve surgery in The Netherlands
Source 3: The Cause of Death Registry Ten out of 16 centers participated in the follow-up of valve surgery performed from 2007 to 2010 (n=12,286 heart valve operations). The linkage between the NVT database and the Cause of Death Registry has previously been described in detail.(12;15) Records were matched by date of birth, sex and postal code. At the time of analysis, survival status, date of death and cause of death up to 31 December 2011 was available. In total 11,996 records (97.6%) of valve surgery could successfully be linked. Source 4: The Hospital Discharge Registry The Hospital Discharge Registry contains information on a majority of the hospital admissions in The Netherlands. The linkage between the NVT database and the Hospital Discharge Registry has previously been described in detail.(12;16) At the time of analysis, information on hospital discharges up to 31 December 2010 was available. A subset of valve surgery performed from 2007 to 2009 was linked to the registry (n = 9,126 heart valve operations), to ensure a minimum follow-up of one year for all records.(12) The primary admission of 7724 (84.6%) heart valve operations could be found in the Hospital Discharge Registry and 7654 (83.9%) records could be followed for one year after surgery. Non-linkage was mainly attributable to the fact that linking was performed on date of birth, sex and postal code. This method has been applied in previous studies.(17) Analyses Volumes The annual number of heart valve operations was determined using the BHN database [source 1]. Due to incomplete registration in the first years, a Poisson regression model was used to impute the number of valve operations from 1995 to 2006. The Poisson regression model used institution, age and gender as categorical covariates to model the procedural volume in time, with the size of the population at risk as an offset term. Time was defined as calendar year and modeled as a continuous covariate. Age was divided in 15 categories of each 5 years. The size of the Dutch adult population in each year between 1995 and 2006 was obtained from the National Statistics Bureau.(15) The estimated procedural volumes are presented with 95% confidence intervals between brackets. Annual changes in age, gender and prosthesis type were assessed in the observed dataset. Overall, age was missing for 0.04% and gender for 0.5%. The prosthesis type was unknown for 1.04% in the aortic valve group and for 0.8% in the mitral valve group. Additional surgery for cardiac arrhythmia is included in all subgroups and additional tricuspid valve surgery is included in all mitral valve subgroups, as both are considered to be part of the treatment for (mitral) valve disease. All volumes were calculated on intervention level, which means that the volumes refer to the number of performed operations. 195