At the preoperative clinics, nurses document histories using a structured electronic questionnaire (Clinical Anesthesia Information System PreOp Clinic, Adjuvant Informatics, Flamborough, Ontario, Canada) that captures age, sex, comorbidities, and medications in a linkable data set.24 (link) Each record includes an ASA-PS score assigned by the anaesthesiologist in the clinic (Table 1). Case records from the clinic database were linked to the Enterprise Electronic Data Warehouse (EDW), which captures all information recorded by the hospital electronic charting system (MISYS EPR; Quadramed Corporation, Reston, VA, USA). The EDW includes information on surgeries, laboratory tests, in-hospital medications, hospital length-of-stay, in-hospital mortality, and International Classification of Diseases 10th Revision (ICD-10) diagnostic codes. Documented surgical information includes an ASA-PS score assigned by the anaesthesiologist in the operating theatre.

Description of ASA-PS classes

ASA-PS classDescription
Class IA normal healthy patient
Class IIA patient with mild systemic disease
Class IIIA patient with severe systemic disease
Class IVA patient with severe systemic disease that is a constant threat to life
Class VA moribund patient who is not expected to survive without operation
Class VIA declared brain-dead patient whose organs are being removed for donation
The primary variables of interest were ASA-PS scores assigned in the preoperative clinics and operating theatres. Patients' age, sex, surgery, preoperative creatinine concentration, hospital length of stay, in-hospital 30 day mortality, and postoperative myocardial injury (troponin I concentration exceeding 0.30 μg litre−1) were captured from the EDW. We ascertained specific comorbidities using the clinic data set (hypertension, coronary artery disease, heart failure, diabetes, cerebrovascular disease, asthma, chronic obstructive pulmonary disease) and EDW (Charlson comorbidity index).23 (link)25 (link) We calculated the RCRI score using information from the EDW (surgical procedure and preoperative creatinine concentration) and clinic data set (other comorbidities).11 (link)