In both incident and prevalent dialysis patient populations, there has been clear progress across the country in reducing hospital admissions due to cardiovascular disease. The first-year admission rate fell by more than 16 percent between annual cohorts of patients initiating chronic dialysis in 2003 and 2010. Among prevalent patients, the rate fell by more than 14 percent between 2004 and 2011.
Large differences in rates among Census Divisions persist, however. By the end of the study period, relative differences between divisions with the highest and lowest rates ranged from 25 to 30 percent. Some of this variation can certainly be attributed to case mix, as all of the displayed rates are unadjusted. The absence of parallel trends across the divisions, however, circumstantially suggests that regional opportunities for quality improvement exist. Increased prescription of and improved adherence to oral medications for primary and secondary prevention of cardiovascular morbidity may be one such opportunity.
An interesting aspect of admission rates for cardiovascular disease is the apparent seasonality in the prevalent population. In each year between 2008 and 2011, there are clear peaks in January or February. The reasons for this pattern are likely complex. Certainly, the incidence of communicable disease, particularly influenza, follows a seasonal pattern in the general population; epidemiologic studies show that seasonal patterns occur in cold-weather and warm-weather states alike, and some have suggested that the patterns are actually more profound in warm-weather states. In the dialysis patient population, infectious diseases are likely to elicit inflammatory reactions that may engender subsequent cardiovascular events, resulting in the seasonal pattern displayed here. Some portion of the pattern may, however, be due to non-modifiable factors like air temperature, atmospheric pressure, and sunlight hours per day.
Cardiovascular morbidity comprises a diverse set of conditions which are important to consider in their own rights, as the pathophysiology and possible iatrogenic risk factors for each differ. In subsequent pages we explore acute coronary syndrome, comprising myocardial infarction and unstable angina; arrhythmia, including atrial fibrillation; heart failure and the related conditions of cardiomyopathy, fluid overload, and pleural effusion; and stroke. In each case, there are challenges in interpreting Medicare claims submitted by hospitals, as the diagnosis codes used to document either the incidence or mere presence of specific diseases tend to be used more or less frequently as reimbursement rules evolve. For this reason, in the case of each specific cardiovascular condition, we display rates of hospital admissions defined by queries of the principal discharge diagnosis code and alternative queries of both the principal and leading secondary discharge diagnosis codes. In many cases, we find that the advent of Medicare Severity Diagnosis Related Groups (MS-DRGs) on October 1, 2007, resulted in substantial changes in secondary diagnosis coding.