May 29, 2008 - A study of all U.S. Medicare hospital patients who had abdominal aortic aneurysm (AAA) repair between 2001 and 2003 shows that high volume hospitals that used endovascular, rather than open surgical approaches, had improved mortality rates.
Details of the study were revealed in the June 2008 issue of the Journal of Vascular Surgery. Statistics were collected from the national Medicare database by researchers who reviewed the impact of total hospital volume on operative mortality for all AAA repairs, and then compared both endovascular and open surgical procedures. Patients with ruptured AAAs were excluded from the study. The mortality rate was defined as death before discharge or up to 30 days after surgery. All analyses were adjusted for patient risk using logistic regression.
During the three-year study, 80,953 Medicare patients underwent AAA repair and an endovascular approach was used in 26,730 of the cases. Researchers found that hospital volume was significantly related to operative mortality in all comparisons. Mortality rates were 80 percent higher at hospitals in the lowest vs. the highest quartile of total volume when considering all types of repair together, and a similar relationship between total hospital volume and mortality was found when examining each approach separately.
"We found that the proportion of AAAs repaired by an endovascular approach increased from 27 percent to 39 percent during the study," said Justin B. Dimick, M.D., MPH, assistant professor at the University of Michigan and staff member of the Michigan Surgical Collaborative for Outcomes Research and Evaluation, University of Michigan Health Systems, both in Ann Arbor. "The highest volume hospitals used the endovascular approach 44 percent of the time compared to 18 percent at the lowest volume hospitals," he said. "This greater use of endovascular procedures accounted for 37 percent of the difference in mortality between high- and low- volume hospitals."
According to Dr. Dimick, endovascular repair also was associated with a much lower in-hospital mortality rate compared to open repair (6.6 percent vs. 2.5 percent). Mortality rates also were lower for endovascular repair in both elective (2.1 vs. 5.0 percent) and urgent/emergency (5.1 percent vs. 11.2 percent) operations.
"To our knowledge we are the first study to perform a systematic analysis since the introduction of the endovascular approach," said Dr. Dimick. "Our results show that hospital volume is still a valid quality indicator for AAA surgery. Some researchers used to think endovascular technology would change referral patterns, altering case-mix distribution and potentially diminish the apparent volume effect; our study proves that referral patterns have not altered the impact of volume on outcome. Also, our data shows that high volume hospitals are doing fewer open repairs, but still have very low mortality rates for open repair. These data clearly show that high volume hospitals are performing well despite any changes in referral patterns."
Dr. Dimick added that it is important to continue to investigate the relationship between volume and outcome as use of endovascular technology increases. "By investigating outcomes in the next five to 10 years it will become evident whether endovascular technology contributes to the existing relationship between volume and outcome for AAA repair, or if changes are due to a position on the learning curve or are true steady-state volume/mortality relationships," he said.
Dr. Dimick added that although the present study demonstrates that the volume effect is still present, it does little to resolve the ongoing debate about using volume as a proxy for quality. However, information on hospital volume can be used to improve the quality of care of AAA surgery by 1) steering patients to high volume hospital by using public reporting or various financial incentives and 2) having quality of care at all hospitals through can be achieved through collaborative quality improvement.
"Hospital volume in and of itself does not produce better outcomes," said Dr. Dimick. "However, underlying processes of care that differ between high and low-volume settings are responsible for the difference in outcomes." He noted that it is not known which approach, selective referral or quality improvement, will yield the greater results.
For more information: www.VascularWeb.org