The rate of hospitalization with a primary diagnosis of acute coronary syndrome (ACS) — which includes acute myocardial infarction, with or without ST-segment elevation, and angina pectoris — changed very little between 2003 and 2011, with a rate of roughly five admissions per 100 patient years during the first year of dialysis among incident patients, and of slightly more than four among prevalent patients. By far, the most common primary discharge diagnosis has been subendocardial infarction (i.e., NSTEMI).
The rate of hospitalization with either a primary or leading secondary diagnosis of ACS, in contrast, changed meaningfully during the same era, with a decline during the interval preceding the advent of MS-DRGs and a subtle increase during the interval afterward. That latter feature should be assessed closely in subsequent reports, as it may be a harbinger of further increases due to widening use of more sensitive troponin assays, rather than the leading edge of increased incidence. Regarding the broader definition including primary and leading secondary diagnoses, most of the decline during the interval preceding the advent of MS-DRGs can be attributed to sharp declines in coding of unstable angina, specifically intermediate coronary syndrome (ICD-9-CM diagnosis code 411.1), which was declared to be only a complicating condition under MS-DRG. This suggests that the incidence of unstable angina, in contrast to the incidence of myocardial infarction, may be more sensitive to reimbursement rules and thus more difficult to discern from Medicare claims.
There is some geographic variation in the incidence of coronary syndrome among both incident and prevalent dialysis patients, with a tendency toward higher rates in the northeastern quadrant of the country.
Among incident dialysis patients, there was a clear downward shift between 2003 and 2010 in the rate of hospitalization for ACS during the first three months of dialysis, and less of a shift thereafter. The reasons for this decrease in early risk are unclear. It may reflect better medication management before initiating dialysis, improved delivery of dialysis (specifically, better management of arterial blood pressure), or both.
Interestingly, there is strong evidence of seasonality in the incidence of ACS, with periodic peaks during the winter months and an especially high peak during the winter of 2011. This latter case is interesting in itself, as the Centers for Disease Control and Prevention has characterized the severity of influenza during the winter of 2010–2011 as less pronounced than during the preceding winter. Of course, the final months of 2010, during which the rate of hospitalization for ACS spiked, also coincided with sharp changes in anemia management in anticipation of the ESRD Prospective Payment System, complicating attribution of the spike in risk to specific mechanisms. In any case, the recurrent wintertime peaks in hospitalization for ACS point toward a potentially fruitful target for quality improvement.