Effect Modification of Sex and Age for the Hospital Volume-Outcome Relationship in Abdominal Aortic Aneurysm Treatment: Secondary Data Analysis of the Nationwide German Diagnosis Related Groups Statistics From 2005 to 2014

Abstract

Background Trials and registries associated female sex and high age with unfavorable outcomes in abdominal aortic aneurysm treatment. Many studies showed an inverse correlation between annual hospital volume and in-hospital mortality. The volume-outcome relationship has not been investigated separately for women and men or across the age range. The aim was to analyze whether sex and age are effect modifiers or confounders of the volume-outcome association. Methods and Results In a nationwide setting, all in-hospital cases from 2005 to 2014 with a diagnosis of intact abdominal aortic aneurysm and procedure codes for endovascular or open aortic repair were included. Primary outcome was in-hospital mortality. Using a multilevel multivariable regression model, hospital volume was modeled as a continuous variable. Separate analyses were performed for women and men and for predefined age groups. A total of 94 966 cases were included (12% women; median age, 72 years). Mortality was 4.9% in women and 3.0% in men (3.2% overall). Mortality increased with age. Although there was no significant volume-outcome association in women (P=0.57), there was in men (P=0.02). The strongest volume-outcome association was found in younger men. The younger female subpopulation was found to show a trend for an inverse volume-outcome relationship, whereas an opposite association was found for the women aged >79 years. Conclusions Women have a higher mortality risk after elective abdominal aortic aneurysm treatment. Sex and age are modifiers of the volume-outcome relationship. Unlike in male patients, in women there is no consistent effect of hospital volume on outcome.

Keywords: endovascular aortic repair; hospital performance; hospital volume; in‐hospital mortality; secondary data analysis; sex differences; sex specific.

Figure 1

Case flowchart. EVAR indicates endovascular aortic repair; OAR, open aortic repair.

Figure 2

Association between in‐hospital mortality (primary outcome) and sex, age, comorbidity, and type of treatment: Forest plot of the underlying model. Sex was evaluated at median volume (14 cases) and median male age (72 years); age was evaluated at median volume (14 cases). To account for clustering of patients within hospitals, and to account for effect of calendar year, the year of treatment and hospital site code were entered as random effects.

Figure 3

Odds ratios and 95% CIs of the volume‐outcome relationship by sex (women [left] and men [right]) and age (indicated by color). The median annual volume (14 cases; indicated by vertical line) was set as reference. Odds ratios were compared with the median annual hospital volume (14 cases) and corresponding 95% CI evaluated at the median age within each age group.

Figure 4

Odds ratio of volume compared with an increased volume (5 additional cases a year) with 95% CIs by sex (women [left] and men [right]) and age (indicated by color) for all cases (open aortic repair and endovascular aortic repair [EVAR]). Each line behaves like the first derivative of the corresponding line of Figure 3. At each volume point, the regression coefficients were treated as normally distributed random variables. Then, all variables except the volume were set constant and the linear predictor of the 2 time points was calculated and subtracted. Then, the corresponding estimator of the SE was calculated. Finally, the linear predictor difference with 95% CI was retransformed to the odds ratio scale.

Figure 5

Predicted in‐hospital mortality depending on annual hospital volume by sex (women [left] and men [right]) and age (indicated by color) for endovascular aortic repair (A) and open aortic repair (OAR) (B), amended by highlighted extrema (black dots).

Figure 6

Predicted in‐hospital mortality depending on annual hospital volume by sex (women [left] and men [right]) and age (indicated by color) for all cases (open aortic repair and endovascular aortic repair [EVAR]), amended by highlighted extrema (black dots). Averaged on Figure 5A and 5B by EVAR proportion within each stratum. AAA indicates abdominal aortic aneurysm.