Discussion
This large dataset of the majority patients undergoing CABG in NSW hospitals from 2000 to 2013 shows a 40% decline in medium-term mortality during this time period. This has occurred despite an increase in the clinical risk profile of this cohort. In-hospital mortality fell from 2.5% to 1.6%, and after adjusting for patient risk, the hazard of dying in hospital fell by 50%. There were additional mortality gains post-hospital discharge with a 27% reduced hazard from 2000–2001 to 2012–2013, a finding which was only slightly attenuated (21% reduced hazard) when medium- term survival was evaluated among patients surviving 30 days following their operation.
Australian studies examining temporal trends in outcomes following CABG have reported conflicting data on short-term outcomes. An early report of 8910 patients from a Western Australian cardiothoracic surgical database reported worsening 30-day mortality from 1980 to 1993 attributable to inclusion of higher risk patients.12 However, a more recent report from the Australian and New Zealand Society of Cardiac and Thoracic Surgeons database including 27 hospitals nationally reported a decline in 30-day mortality from 2009 to 2013.13 These findings are consistent with ours; it was in later years in our study that the improvement in hospital outcomes became apparent. Our data are also comparable to those from a recent Danish study which reported a reduction in 30-day mortality from 4.1% 1999–2000 to 2.4% 2011–2012,6 and a report from the US Society of Thoracic Surgeons database which reported a fall in 30-day mortality from 2.4% in 2000 to 1.9% in 2009.5
There have been a number of developments in anaesthetic and surgical technique that may have contributed to the improvement in perioperative outcomes. These include bleeding and transfusion avoidance strategies, accelerated extubation and optimising perioperative analgesia.14 The performance of procedures without cardiopulmonary bypass (off pump) may improve short- term outcomes in high-risk patients, although there is no evidence that the uptake of this procedure has increased over this time period.9 13 15
Studies that have investigated changes in longer-term mortality have also yielded conflicting results.6 7 A Danish cohort study found no improvement in 1-year mortality from 1999 to 2012 which they attributed to the increasing age of their cohort despite adjustment. This study was limited to only three contributing hospitals, which might explain the disparate findings.6 In contrast, a population-based study from Sweden reported an improvement in 4-year survival among most CABG patients from 1987 to 2006.7 The finding of a decline in long-term mortality is plausible and may be attributable to a combination of better grafting strategies and better secondary prevention. Improved vein graft handling has been shown to improve graft patency.16 17 All arterial grafting has been associated with improved outcomes in some series18and is increasing in selected hospitals around Australia.13 19 Continued statin and antiplatelet therapies improve longer-term outcomes following revascularisation.20 Increased uptake of these medications has been documented to have occurred over this period in Australian studies.21
In additional analyses restricted to patients in whom there was complete follow-up, the improvement in mortality was attenuated in the years following CABG. Patients undergoing surgery in 2000–2001 had a 25% greater likelihood of death by 2 years than those having surgery in 2012–2013; in contrast, there was no significant improvement in 5-year survival among the patients for whom there was complete follow-up. There are several explanations for this. First, these patients underwent surgery earlier in the study period, at a time when relative improvements in mortality in our cohort were less apparent. Second, the benefits of bypass surgery relative to medical therapy attenuate over time.2 22 23 This is thought to be primarily attributable to bypass graft attrition24 25 and may also be contributed to by competing risk from other causes of death in older populations.23 As discussed above, improved vein preservation techniques and increasing use of arterial conduits may improve long-term graft patency and survival.9 16 26
Several limitations of our study deserve mention. First, the data were derived from administrative and procedural codes which are subject to error27 28 In addition, we were limited by a lack of procedural details, such as vein graft preservation techniques used, relative changes in the prevalence of arterial grafting and perioperative medications. This meant we were unable to document changes in practice that may have contributed to the observed improvement in perioperative outcomes. Furthermore, we did not have data on long-term medication use or attendance at cardiac rehabilitation in our cohort. This information would allow a determination of the contribution of these factors to improving outcomes. As this cohort was extracted from a broader group of patients undergoing a range of cardiac procedures, it includes only about 90% of CABG procedures performed over this time period. However, we do not believe the method of identification introduces any bias to the population and so should not influence our findings.
In conclusion, we report a consistent reduction in medium-term mortality among a large unselected cohort of NSW patients undergoing CABG between 2000 and 2013. This fall is attributable both to an improvement in outcomes in hospital and in the postdischarge period. The age-standardised death rate for coronary heart disease in Australia has fallen by 4.5% per year between 1980 and 2015. This has been attributed to improvements in disease prevention through risk factor control, better emergency medical care for heart attack and an increase in specialised coronary care units.29 In this paper, we identify an additional important contemporary contributor to this improvement in mortality, better survival among patients undergoing CABG.