The Kaiser Permanente patient registry provides a unique opportunity to study the likelihood of rupture
By Sue rochman
An abdominal aortic aneurysm is often referred to as a "time bomb." That's because if the aneurysm bursts, it is likely to cause a deadly torrent of internal bleeding.
These aneurysms, bulges that develop in the abdomen in the lower part of the aorta, the largest artery in the body, usually do not cause symptoms. Most people do not know they have one until it is identified on an orderly CT scan due to another health problem. But they are not uncommon. In the US, about 200,000 people are diagnosed with an abdominal aortic aneurysm each year and about 10,000 people die after an aneurysm ruptures, making it the 15thth leading cause of death.
Robert Chang, MD, Vascular Surgeon, The Permanente Medical Group and Medical Investigator, Research Division.
Doctors generally do not recommend that a patient undergo surgery until the aneurysm has grown large enough to potentially rupture. For decades, the size threshold for surgery was 5.5 cm (2.2 inches) for men and 5 cm (2 inches) for women. But how likely is it that such a large abdominal aortic aneurysm will actually rupture? Robert Chang, MD, a vascular surgeon in The Permanent Medical Group and Research Physician at Kaiser Permanente Northern California Investigations Division, is the lead author of the largest study ever conducted to answer that question. The findings, published in July in the Journal of Vascular Surgery, is expected to influence the way doctors help patients decide whether to consider surgery to remove the blockage.
Chang discussed why it was important to study the natural history of large abdominal aortic aneurysms and how the Kaiser Permanente Northern California (KPNC) health care system made it possible.
Why did you decide to do this study?
Chang: Abdominal aortic aneurysms remain one of the leading causes of rupture death in this country. The way we treat them is by repairing them when they reach a certain size, but the size threshold we have used has been based on information from studies that are quite old. We believed that it was important to use contemporary data to identify a patient's risk of a ruptured aneurysm. Before our investigation, no one had done this.
How did you use KPNC's electronic medical records in your research?
Chang: We have an electronic registry of 15,000 patients with abdominal aortic aneurysms that we have used for surveillance for 18 years. We use that registry to monitor our patients, but it has never been used for research purposes. It took us many steps to figure out how to use the registry to study the outcomes of these patients. We also had to develop a natural language processing tool that could examine medical records and find out which imaging studies showed an abdominal aortic aneurysm and also tell us the size of the aneurysm when it was first diagnosed and how much it grew over time. It would have taken us years to go through all those records. The software could do it in a few hours.
You learned?
Chang: The obvious question was: Can we find out if the size threshold we use based on previous studies to advise patients about surgery still makes sense? This type of study has never been done before because no one has had access to this type of data set. So we looked to see if there were patients in the registry who had large aneurysms that, for whatever reason, weren't immediately repaired. And then we look to see what happened after the aneurysm reached the threshold where we would talk to a patient about surgery. Did the aneurysm rupture? Were they operated on later? They died
It turned out that we had over 3,000 patients who had a large aneurysm. Approximately 40% had one at the threshold size (5.5 cm in men, 5 cm in women) at the first imaging study, and the other 60% had small aneurysms that grew over time. What we found was that, essentially, the incidence of rupture is lower than we think across the board for each starting size. We also learned that the risk of breakdown is not cumulative. The curve is much flatter, so there isn't a huge increase in breakout risk if you wait longer.
What we found was that, essentially, the incidence of rupture is lower than we think across the board for each starting size.
Is this good news for patients?
Chang: It is a realistic and more accurate news. If I have a male patient with a 5.5cm aneurysm, I need to be able to explain the risk of it rupturing today, the risk of it rupturing in the next 5 years, and the risks associated with surgery. We had thought that if you have a 5.5 cm aneurysm, you have a 5% risk of rupture per year, and that the risk is cumulative, that is, in 5 years it would be 25%. Now, I use the data from our study that shows that the 1-year breakup risk is only 1.3%, which is one-fifth of what I would have told someone in the past. And then they can weigh that risk against the risks associated with the surgery.
Did you find differences between women and men?
Chang: We show that women have a different risk of ruptured abdominal aortic aneurysm, and that it may actually be higher than men. Right now, we don't really know why. But this is something we can explore because we have so many women in our cohort. Most studies have used databases that do not have as many women as we do.
Whats Next?
Chang: The next question we ask ourselves will focus on patients with small aneurysms. We want to see if we can predict who will need surgery. This information would help us conduct surveillance and potentially intervene at the right time to prevent a patient from experiencing a catastrophic rupture. The goal of aneurysm treatment is to prevent rupture, but unnecessary surgery should not be done either.
