We examined the relationship between volume of CHF and clinical processes, outcomes, and costs. We found that patients discharged from hospitals with a higher volume of CHF patients received higher quality of care, on average, and had better outcomes, but at modestly higher cost; these relationships were independent of other key hospital characteristics including teaching status and hospital size. The strongest impact of volume on outcomes was seen in the group of hospitals with the lowest volume (those with a case volume less than 200 over our 23 month study period). The effects of volume on outcomes appeared to diminish beyond 200 to 400 discharges over the study period. Our findings suggest that the volume-outcome relationship, seen previously in procedure-based conditions, also exists for CHF and may extend more broadly to chronic medical conditions.
We have known that higher volume is associated with better outcomes for major surgeries(4
) and cardiovascular procedures(8
) for nearly three decades.(22
) However, the relationship between volume and outcomes for medical care is less well-understood. Our findings suggest that, at least for CHF, the effects can be substantial: patients discharged from hospitals with 200 CHF discharges had, on average, 18% lower odds of death compared to those discharged from hospitals with 20 CHF discharges. Given the prevalence of this disease, these differences have important clinical and public health implications. The average hospital with 200 discharges would have an HQA score that was 13.5% higher compared to the hospital with 20 CHF discharges, with the average patient having 18% lower odds of readmission. However, the typical hospital with 200 CHF discharges would cost approximately $400 more per hospitalization. The gains in quality and outcomes appear to be worth the extra cost – but one would need formal and long-term cost-effectiveness analyses to be sure.
How might volume lead to better outcomes? One possible explanation is that greater knowledge of and adherence to process measures account for the differences. However, while high-volume hospitals did have higher HQA scores, these measures are, unto themselves, unlikely to lead to lower short-term mortality or reduced readmissions.(23
) It is also possible that high-volume hospitals have both higher HQA scores and better clinical outcomes as a common result of their greater experience in caring for CHF patients. Hospitals with more CHF patients might have more incentive to retain familiarity with professional society care guidelines, or be more likely to have invested in systems to monitor for adherence to quality metrics. Standardized admission or discharge forms, increased familiarity with CHF-specific patient education, or involvement of discharge planners in CHF patients’ care, might also be more prevalent at high-volume institutions, although there are no data currently available to assess this directly. Nurses at high-volume centers might have more familiarity with CHF-specific patient care and education needs, although we do not have data to support or refute this notion. Additionally, high-volume centers might also have specialty inpatient and/or outpatient CHF services, concentrating similar patients under a team of providers; this could improve quality and coordination of care. Further work is needed to explore whether these types of services are more often present in high volume centers and whether they account for some or all of the differences we found.
Finally, it could be that better outcomes lead to higher volume, rather than the other way around. Hospitals that spend more to provide higher-quality services may attract more patients through referral and self-referral, thus increasing case volume. Luft et al examined this possibility using data from 736 hospitals across the country, and using analyses of referrals-in and transfers-out, as well as patterns of mortality for different conditions, concluded that selective referral was important in explaining the volume-outcome relationship.(24
) Selective referral requires either that data on hospital quality is available and easy for patients and referring physicians to understand, or that high performing hospitals have a reputation for quality that attracts patients and referring physicians. Prior studies have found that publicly available quality data are rarely used by patients for selecting a hospital or clinician(25
) and publishing provider performance has little effect on their market share.(26
) Therefore, if selective referral is a key part of the mechanism by which high performing hospitals have higher volume, it is likely through the reputational effects of being a high quality hospital.
Our finding that higher volume was associated with greater inpatient costs is novel, and has important implications for the health policy debate. We unfortunately could not determine how the extra money was spent, or whether extra spending during the index hospitalization led directly to the improved outcomes. Our analysis suggests that the efficiency gains often seen when institutions perform a high volume of surgical procedures may not bear out for conditions like CHF, at least in the short term, and challenge the assumption that hospitals with more experience caring for a particular condition should be able to do so at lower cost. Multiple studies from the Dartmouth Atlas have demonstrated that higher Medicare spending is not associated with better health outcomes at the hospital referral region level or at the individual hospital level.(28
), although their models of costs examine data over a longer period of time (typically the last 6 months or 2 years of life) and are “looking backwards” after a death. Our costs are episode based and our results are not in direct contradiction to the Dartmouth work. It is possible that intensive care of CHF, while more expensive in the short run, may lead to lower longer-term costs.
We found a threshold beyond which additional volume was associated with little additional benefit. The incremental benefit of volume decreased once volume reached approximately 200 Medicare CHF discharges. The fact that roughly one-third of all U.S. hospitals, caring for nearly half of all patients, achieve this level of experience has important implications for quality improvement and regionalization of care. First, our findings suggest that we can get the largest benefit by targeting quality improvement efforts at hospitals with lower volume. Second, policy makers have advocated that for high-risk surgeries,(4
) patients should be sent to the few hospitals that achieve very high volumes. In CHF, this degree of centralization may not be necessary given that a large number of hospitals meet the volume threshold. Interestingly, while improvement in quality of care and clinical outcomes leveled off, there was no obvious threshold effect for costs, suggesting that beyond a certain level of spending, additional resource utilization may not be associated with better outcomes. Whether this is due to unnecessary use of expensive tests or procedures at these very high volume hospitals is unclear, and additional studies are needed to better understand this issue.
Prior studies of the impact of volume on outcomes for medical conditions have demonstrated mixed results. For example, Thiemann et al demonstrated that elderly patients presenting with acute MI had lower rates of 30-day and one-year mortality if they presented to a high-volume hospital.(11
) An in-hospital survival benefit in high-volume centers has been seen for patients with AIDS(33
) and lupus,(36
) although we know less about longer-term outcomes for these conditions. However, Lindenauer et al found no relationship between volume and outcomes for chronic obstructive pulmonary disease (COPD),(12
) and demonstrated that hospitals with a high volume of pneumonia patients actually had both worse performance on process measures and worse clinical outcomes than hospitals with a lower volume of pneumonia patients.(13
) A recent analysis by Ross et al found that hospitals with higher volume had lower 30-day mortality rates for acute myocardial infarction, CHF and pneumonia.(14
) Our findings extend this work by both quantifying the benefits of volume on mortality, and by demonstrating that the benefits include better processes and lower readmissions.
We are unaware of prior studies that have directly examined the relationship between volume and costs for medical conditions. Hospitals with higher volumes for surgical procedures usually have lower costs, presumably by reducing costly complications and decreasing length of stay.(37
) However, CHF care may fit a different paradigm than a surgical procedure. As policymakers increasingly focus on value (balancing the importance of both outcomes and costs), our findings suggest that there may be no easy solution for improving CHF outcomes and that better care might require greater spending. Further, because CHF is a chronic, relapsing disease, rather than a single episode of care, up-front investment in quality may lead to downstream, rather than immediate, savings. Policy efforts aimed at building greater coordination and accountability among providers may have the potential to encourage hospitals to invest in better clinical CHF care if they create a mechanism through which they could reap the financial benefits of downstream cost savings.
Our study has several limitations. We lack data regarding the clinicians providing care for these patients, and were unable to assess the impact of clinician volume or clinician specialty on outcomes. Similarly, because we used administrative rather than clinical data, while we were able to calculate a nurse-to-patient ratio for each hospital, we did not have data regarding the intensity of nursing care delivered to each patient individually. Additionally, administrative data presents difficulties in terms of our ability to fully account for variations in severity of illness across hospitals. However, although administrative data are imperfect, they are standardized, validated, and increasingly used, even for public reporting. Furthermore, high-volume hospitals generally had sicker patients, and inadequate risk-adjustment may have led to underestimating differences in outcomes between these hospitals and their counterparts with a less complex population. We focused on Medicare patients; while these patients make up more than 80% of CHF admissions,(39
) whether our findings apply to non-Medicare patients is unclear. Finally, as with any non-experimental study design, we could not assess whether the relationships we found were causal, or rather markers of other factors associated both with volume and outcomes.