The American Journal of Managed Care October 2006
Outpatient Medication Use and Health Outcomes in Post-Acute Coronary Syndrome Patients
Our study is a prospective observational study, with surveys at 3 months and 8 months that captured information about the period preceding the interview. Without experimental assignment of medications to respondents, the use of postdischarge cardiac medications may correlate with health outcomes. For example, healthier patients or patients with more sufficient discharge medication are more likely to tolerate ACEIs and b-blockers, and they are also the ones who will have better outcomes, regardless of their medication use.22-24 Therefore, we used the propensity score matching method to first predict the propensity to use ACEIs/ARBs or ß-blockers at 3 months or 8 months conditional on patients' demographic features, health status (ejection fraction, DASI, CCI), and discharge medication. Then we compared the outcomes among patients who had the same propensity to receive postdischarge cardiac medications conditional on whether they actually used them or not using Epanechnikov kernel-based matching with the default bandwidth at 0.06 in Stata to calculate the average treatment effect of postdischarge medication use on hospital readmission. We also bootstrapped the standard errors of the average treatment effect with 1000 repetitions to test the significance of the average treatment effect.
Table 1 summarizes characteristics of the 433 patients who participated in both the discharge survey and the 3-month survey. The study sample included older adults with a mean age of more than 60 years (SD = 11.51 years); the majority (64%) were male. Approximately 15% of the sample had poor heart function, with ejection fractions less than or equal to 35%. The average DASI score at discharge was 32.48, which represented higher functional status than that found in the Bypass Angioplasty Revascularization Investigation study (mean DASI = 21).25 The study sample showed a moderate average level of comorbidity, with an average CCI score of 1.65 and standard deviation of 1.31.
Postdischarge Healthcare Utilization
Table 2 depicts the dynamic nature of the postdischarge healthcare, including cardiac medications, in greater detail. At discharge, 273 patients, approximately 65% of the sample, were on a ß-blocker, and 230 were on an ACEI/ARB. At 3 months after discharge, these numbers increased, with 323 on a ß-blocker and 268 on an ACEI/ARB. The number of people on a lipid lowering-medication increased from 283 at discharge to 310 at the 3-month interview. There was a minor increase in the number of patients on aspirin from 351 at discharge to 374 at 3 months.
For the 381 patients who completed the 8-month survey, we found the number of people on prescribed cardiac medications did not change appreciably from 3 months to 8 months, with 257 patients on a ß-blocker at 3 months and 279 at 8 months. Similarly, the number of patients on an ACEI increased unsubstantially from 200 to 228. There were also minor changes in number of people on aspirin from the 3-month to the 8-month survey.
Most of the changes in medication use occurred between the index hospital discharge and the 3-month survey. For example, among the 323 patients who were on a ß-blocker at 3 months after discharge, 65% of them had been discharged with a ß-blocker, and 35% had added a ß-blocker during this time interval. Among the 268 patients who were on an ACEI/ARB at the 3-month survey, 60% were discharged with an ACEI/ARB prescription, and 40% added this medication after discharge. Similarly, among the 208 people who were taking a lipid-lowering medication at the 3-month survey, 57% were discharged with a prescription, and 43% added it. The only type of medication with consistently high use was aspirin. For those who were on aspirin at 3 months, 94% were discharged with an aspirin recommendation.
A high percentage of patients reported taking cardiac medications at both the 3-month to the 8-month surveys. For example, 92% of the patients on a ß-blocker at 8 months also reporting used it at the 3-month survey, and 83% of those on an ACEI/ARB used it at the time of the 3-month survey. Concerning inpatient care, 124 patients were readmitted to a hospital between discharge and 3 months, and 76 patients were readmitted between 3 months and 8 months. It is interesting to note that among the 76 patients who were hospitalized between 3 months and 8 months, 30 (40%) of these patients had been previously readmitted between discharge and the 3-month survey.
Effect of Medication Use on Hospital Readmission
The results of multivariable logit regression of outpatient prescription drug use on hospital readmission are shown in Table 3, demonstrating that patients who were taking a ß-blocker or an ACEI/ARB at 3 months were significantly less likely to be readmitted to a hospital. Taking a ß-blocker only (coefficient = 0.68, P < .05), an ACEI/ARB only (coefficient = 1.09, P < .05), or both (coefficient = -0.69, P < .1) all were associated with a lower probability of hospital readmission by 3 months after discharge. Use of ß-blockers, ACEIs/ARBs, lipid-lowering medications, or aspirin did not have a significant effect on readmission rates between 3 months and 8 months. However, readmission to the hospital between discharge and 3 months did significantly predict hospitalization again at a later time (coefficient = 0.94, P < .05). As expected, health status/severity of illness was important. People with a lower ejection fraction (ejection fraction =35%, coefficient = 0.68, P < .1) or higher DASI score (coefficient = -0.02, P < .05) were more likely to have had a hospital readmission by the time of the 8-month survey.
Propensity Score Matching Results
The propensity score matching results are depicted in Table 4, together with the calculated analytical treatment effect from the logit regression. The average treatment effect of taking ACEIs/ARBs or ß-blockers from propensity score matching is -0.158, with the bootstrapped standard error at 0.083 (P < .1). This means that, on average, the predicted probability of hospital readmission for those who take ACEIs/ARBs or ß-blockers is 0.158 lower than the probability for those who don't take these medications. This result is very similar to the calculated average effect of taking ß-blockers or ACEIs/ARBs on hospital readmission at 3 months obtained from the logit regression at -0.154, with an analytical standard error of 0.03 (P < .05). Therefore, the propensity score matching confirmed that after controlling for possible selection bias in postdischarge medication use due to either better health conditions or more sufficient discharge prescriptions, the use of ACEIs/ARBs or b-blockers at 3 months after discharge was still demonstrated to significantly reduce the probability of hospital readmission.
From these results, we suggest that in addition to hospital discharge, the period within 3 months after discharge appears to be critical for initiating and/or adjusting medication therapy for ACS patients. Based on this study, most of the postdischarge adjustments in medications occurred during this time interval. Taking at least 1 type of ß-blocker or ACEI/ARB within this period helped reduce significantly the probability of hospital readmission for ACS patients. Once patients were readmitted during this critical period, they were more likely to be admitted to the hospital a second time or more. Although we did not assess the relationship between mortality and outpatient medication use in this study, Mukherjee et al26 found that the use of combination evidence-based medical therapies was independently and strongly associated with lower 6-month mortality in patients with ACS. Although we are not sure as to the exact mechanism, this study suggests that patients in this study were more likely to use their cardiac medications up to 8 months after hospital discharge and maintain better health conditions if they used them up to 3 months after discharge.
Such results also suggest that outpatient physicians could be as important for ACS patients as the inpatient physicians who write the discharge prescriptions. The outpatient physicians are responsible for the continuous care of ACS patients after their discharge from the hospital. Reviewing patients' health status, setting up reasonable recovery goals, and motivating patients to adhere to their discharge prescription or adjust their prescriptions to fit their health needs are critical tasks of outpatient physicians. Although other studies have shown the importance of hospital discharge medications, our results demonstrate that a focus on the months shortly after discharge may be equally important for attention to medication therapy.
We did not find that taking lipid-lowering medications had a significant effect on hospital readmission rates for up to 8 months after discharge. This may be because lipid-lowering medications need a longer time frame to show benefit. In general, hospital readmissions within several months after ACS are due to congestive heart failure, for which ACEIs/ARBs and ß-blockers have more benefit. We did find that aspirin is efficacious in preventing hospital readmission with negative point estimates, but the results were not statistically significant. We suspect this is due to ceiling effects, as more than 80% of the patients were on aspirin constantly in this study.
This study does have some limitations. First, we experienced expected attrition in a sample of patients hospitalized for ACS (27% from baseline to the 8-month survey). During the follow-up period, better functional status but the presence of more depression and smoking were significant predictors of attrition. It is not possible to know whether our results held true for those who dropped out. Second, we did not know the reasons for rehospitalization from the self-report data. However, based on the statistically significant relationship between cardiac medication use, heart function measurement (ejection fraction, DASI), and the probability of hospital readmission, but the nonstatistically significant relationship between comorbidity measurement (CCI) and hospital readmission, it is possible that the majority of the hospital readmission in this study were due to cardiac-related conditions. Third, the propensity score matching confirmed the consistency of our estimation of the efficacy of cardiac medication use after discharge. However, we cannot determine the reason for the adjustment of prescription medication therapy between discharge and 3 months. Possible reasons include seeing a specialist versus an internist, the quality of care delivered by outpatient physicians, or the presence or absence of contraindications to any of the medications. Future research to specifically address these issues may provide further insight.