Relationship of hospital size, case volume, and cost for coronary artery bypass surgery: Analysis of 12,774 patients operated on in Massachusetts during fiscal years 1995 and 1996

Objective: This study investigates the relationship between the cost of cor onary artery bypass graft surgery and both hospital size and case volume. Methods: Retrospective administrative and cost data were obtained for all 1 2,774 patients who underwent isolated coronary bypass surgery at 12 Massach usetts hospitals during 1995 and 1996. Hospitals were stratified by number of operating beds into 3 groups (group I, < 250 beds; group LI, 250-450 bed s; group III, > 450 beds). Total (diagnosis-related groups 106 + 107) annua l coronary bypass cases per hospital varied from 271 to 913 (mean 532). Uni variate and multivariable analyses were used to study the relationship betw een the direct and total cost and a number of patient (age, sex, acuity cla ss, payer) and hospital (bed capacity, annual case volume per diagnosis-rel ated group, cardiothoracic residency) predictor variables. For each hospita l, we also studied the relationship between changes in coronary bypass case volume and the corresponding changes in average cost from 1995 to 1996. Results: Scatterplots revealed a broad range of mean direct cost of coronar y bypass surgery among hospitals with comparable case volumes. When annual cases were analyzed as continuous variables, there was no linear relationsh ip of case volume with direct or total cost of coronary bypass (r = -0.05 t o +0.08) for any diagnosis-related group or year. When hospital bed capacit y and case volume were grouped into strata and studied by analysis of varia nce, there was no evidence of an inverse relationship between these variabl es and cost. In multivariable analysis, patient acuity class and diagnosis- related group were the most important predictors of cost. Beds and case vol ume met inclusion criteria for most models but added little to the "explana tion" of variability R-2, Often less than 1%. Finally, substantial interhos pital differences were noted in the magnitude and direction (direct vs inve rse) of their 1995 to 1996 change in volume versus change in cost. Conclusions: Within the range of hospital size and case volume represented in this study, there is no evidence that either variable is related to the cost of performing coronary bypass surgery. Massachusetts hospitals appear to function on different segments of different average cost curves. It is n ot possible to predict the relative cost of coronary bypass grafting at a g iven hospital based primarily on volume.