What to know about St. Mary's children's heart surgery mortality rates

Story highlights

  • CNN calculated the mortality rate for babies having open heart surgery at St. Mary's Medical Center was three times the national average from 2011 to 2013
  • The hospital, in West Palm Beach, and the state of Florida have criticized CNN's methodology
  • Here's answers to what you should know about children's heart surgery mortality rates
Update July 1st, 2015: St. Mary's has posted a new risk adjusted mortality rate of 4.7% for its Pediatric Congenital Cardiac Surgery program. You can read St. Mary's statement here.

Atlanta (CNN)CNN's investigation of pediatric open heart mortality rates at St. Mary's Medical Center in West Palm Beach, Florida, showed that the hospital had a much higher death rate than the national average. Both the hospital and state of Florida have responded with documents that criticize CNN's methodology.

We wanted to further explain how mortality rates are calculated. We also want to help readers better understand the responses and our reporting and how to find more information about mortality data.
Following are 12 key questions and answers:
    1. St. Mary's Medical Center says it has a 5.3% risk-adjusted mortality rate. How did the hospital arrive at that number?
    The hospital doesn't say.
    Two numbers are necessary to calculate a mortality rate: How many surgeries were performed and how many patients died. St. Mary's doesn't give either in its response.
    Many other hospitals are clear about these two figures.
    For example, from July 2010 through June 2014, Johns Hopkins All Children's Heart Institute performed 883 congenital heart surgeries and had 30 deaths. Divide 30 by 883 and multiply by 100 and you have a mortality rate of 3.4%.
    Duke University Hospital did 828 surgeries during that same period and had 25 deaths. Do the math, and that's a 3% mortality rate.
    While many other hospitals are transparent about the precise number of surgeries and the number of patients who died, St. Mary's has not made that data public.
    2. Does the hospital say anything about the number of surgeries performed?
    Yes. It has a bar graph titled "Volume of STS Congenital Heart Surgery Procedures/Patients at St. Mary's Medical Center." It shows that from July 2011 until June 2014, it did 132 procedures on 90