The Surgical Apgar Score in hip and knee arthroplasty

Abstract

Background: A 10-point Surgical Apgar Score, based on patients' estimated blood loss, lowest heart rate, and lowest mean arterial pressure during surgery, was developed to rate patients' outcomes in general and vascular surgery but has not been tested for patients having orthopaedic surgery.

Questions/purposes: For patients undergoing hip and knee arthroplasties, we asked (1) whether the score provides accurate risk stratification for major postoperative complications, and (2) whether it captures intraoperative variables contributing to postoperative risk based on the three parameters independent of preoperative risk.

Patients and methods: We retrospectively reviewed the electronic records for all 3511 patients who underwent a hip or knee arthroplasty from March 2003 to August 2006 and extracted data to calculate a Surgical Apgar Score. We evaluated the relationship between scores and likelihood of major postoperative in-hospital complications and assessed its discrimination and calibration.

Results: Complication rates increased monotonically as the score decreased. Even after controlling for preoperative risk, each 1-point decrease in the score was associated with a 34.0% increase (95% confidence interval, 0.66-0.84) in the odds of a complication. The overall discriminatory performance of the score was a c-statistic of 0.61. Seventy-six percent of all major complications occurred in patients classified as low risk with scores of 7 or greater.

Conclusions: For patients undergoing hip and knee arthroplasties, the score captures important intraoperative information regarding risk of complications and contributes additional information to preoperative risk, but on its own is insufficient to provide comprehensive postoperative risk stratification for arthroplasties.

Level of evidence: Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Fig. 1

A calibration plot compares the model’s predicted probabilities based on the original general and vascular surgery cohort and observed proportions in our arthroplasty cohort. Triangles display deciles of the predicted probabilities in the arthroplasty cohort. The diagonal line reflects the ideal situation (predicted probability = observed proportion). The curve represents the relation nonparametrically. The histogram in the lower part of the figure shows the distribution of predicted probabilities in this sample where the height of each line represents the number of patients shown on the y-axis on the right side of the figure. Probabilities greater than 50% were truncated. This plot shows the score constantly underpredicted major complications and as such is insufficiently accurate to serve as a comprehensive risk stratification tool.

Fig. 2

Major complication and death rates are shown according to the 10-point Surgical Apgar Score from the operation. The graph shows a monotonic increase in risk across decreasing values of score of categories, indicating the score provides prognostically meaningful information regarding the risk of complications. Below the graph, the absolute number of patients distributed across the score categories is shown. The majority of patients have high score values and only very few have low score values.