Data from the British Columbia Cardiac Registries (BCCR) were used to identify the study participants and their demographic, clinical and treatment characteristics. This population-based patient registry prospectively captures the date of booking request for operating room time, and the date of and reason for removal from the wait list, for all adult patients accepted for CABG in any of the four cardiac centers in the province . To identify cardiac catheterization dates and coexisting medical conditions, we used each patient’s provincial health number to deterministically link BCCR records to the Canadian Institute for Health Information (CIHI) Discharge Abstract Database (DAD) . To identify coexisting conditions, we used diagnoses reported in the DAD within one year prior to the booking request. Census data on the decile of median income in enumeration area were based on the postal code of the patient’s residence.
We studied patients who had a record of registration on a wait list for first-time isolated CABG surgery from January 1, 1992 to December 31, 2005, and who had a record of catheterization procedure in the DAD. The inception cohort had 14,049 records of registration for CABG from January 1, 1991 to December 31, 2005. We excluded 567 records of patients for various reasons: procedure at registration was not isolated CABG (312), procedure at registration or at surgery was not first-time CABG (62), emergency cases at the time of registration (34), missing operating room reports (4), removed on the registration date (101), registration was on a weekend and admission was day after (14), or the patient had multiple episodes (40). We also excluded 1,452 records of patients who were registered in 1991 (797) or did not have a catheterization date (655). The remaining 12,030 records had either the surgery date or the date and reason of removal from the list without surgery.
Primary study variable
The study variable was urgency group at registration categorized as urgent, semiurgent, and nonurgent. When placing patients on wait lists in British Columbia, Canada, all cardiac surgeons indicate the urgency of CABG according to angiographic findings, symptom severity, and left ventricular dysfunction (ejection fraction less than 50%) to ensure timing of revascularization according to the provincial guidelines: within one week for urgent procedures, within six weeks for semiurgent procedures, and within 26 weeks for nonurgent procedures .
The primary outcomes were (1) preoperative death from all causes and (2) unplanned emergency surgery while awaiting a planned CABG. Surgeons on call made the decision to operate on patients who presented to the emergency or admitting department. All admissions from the emergency department and admissions from other locations bearing an emergency code were classified as unplanned emergency surgery. The date at which a surgeon’s office submits the operating room booking request for surgery serves as the date of registration on the list. Because scheduling is done weekly, wait-list time for each patient was computed as the number of calendar weeks from registration to removal from wait lists or end of study period. We restricted the analysis to the first 52 weeks following registration because of the lack of information to identify periods when patients were not ready for surgery, which might have contributed to extended waits.
The existing literature suggests that elderly patients are more likely to undergo revascularization as an urgent procedure , that smaller diameter of the coronary vessels may account for the higher risk of adverse cardiovascular events among women , that co-existing conditions may delay open heart surgery , that institutional constraints and individual care providers may affect clinical outcomes , that patients with a lower socioeconomic status may wait longer for cardiac surgery , and that changes in practice or the availability of supplementary funds may reduce the waiting time until surgery . To identify comorbidities at the time of registration, we used diagnoses reported in the DAD within one year prior to registration. The reference category was defined as no coexisting conditions. The first comparison category was defined as patients with any of the following conditions at presentation: congestive heart failure, diabetes mellitus, chronic obstructive pulmonary disease, cancer, or rheumatoid arthritis . The second comparison category was defined as patients presenting with other coexisting chronic conditions, as defined elsewhere .
Other confounders include hospital booking catheterization to address variation in standards and calendar year of surgery decision as a proxy of changes in practice and available funding. We also included the time between catheterization and surgery, the mode of admission for catheterization, urgency at admission for catheterization, which may differ substantially among hospitals affecting estimates of the total of delays in undergoing the operation . The time between catheterization and registration was computed as the number of calendar weeks. The catheterization dates were obtained from the CIHI DAD and defined as the most recent diagnostic (Canadian Classification of Procedure (CCP) codes 4892–4898, 4996, 4997) or therapeutic (CCP codes 4802, 4803, 4809) catheterization performed within one year preceding and including the date of booking. We used the date of most recent catheterization procedures (diagnostic or therapeutic) because the results of this procedure are most likely linked to decision to operate .
Probability of remaining on the list and weekly event rates
The probability of remaining on the list within a certain time of registration was estimated using the product-limit method . Time to removal from the lists was compared across urgency groups using the log-rank test . Average weekly event rates were calculated as the number of events divided by the sum of observed waiting times measured in weeks.
Cumulative incidence of event
The cumulative incidence function (CIF) of an event is the proportion of CABG candidates experiencing the event of interest (e.g. death) instead of competing events (e.g. planned surgery) by a certain time on the wait list [22, 23]. Both the event rate and the probability of remaining on the list influence the CIF. Therefore, if the CIF of an event differs between two groups when the event rates are the same, then it is the probabilities of remaining on the list that contribute to this difference. Using Gray’s test, the CIF was compared across urgency groups . Further details on the cumulative incidence of event may be found in Additional file 1.
The effect size of urgency group on weekly rates of death and unplanned emergency surgery were estimated using discrete-time survival regression models, which naturally gives rise to the odds ratio (OR) . To estimate the effect of urgency group on the cumulative incidence of death and unplanned emergency surgery, regression methods for CIF were used . Further details on regression of CIF may be found in Additional file 1.
In these regression models, we adjusted for potential confounders allowing for at least 10 events per variable . In the regression models for preoperative death, we adjusted for sex, age decade, comorbidities at registration, calendar period of registration, and time between catheterization and registration. In the regression models for unplanned emergency surgery, we adjusted for sex, age group, coronary anatomy at registration as a proxy for severity of coronary disease, comorbidities at registration, calendar period at registration, institution at registration, institution at catheterization, mode of admission at catheterization, urgency at admission for catheterization, and time between catheterization and registration. We performed additional analyses, in which we adjusted for socioeconomic decile in these models.
The Behavioural Research Ethics Board of the University of British Columbia approved the study protocol, Certificate of Approval H06-80651.