Abstract
OBJECTIVES
A weekend effect with increased mortality has previously been reported in surgical patients and those with acute myocardial infarction (MI). We hypothesized that a similar phenomenon may exist in coronary artery bypass grafting (CABG).
METHODS
Patients undergoing non-elective isolated CABG (2011–2017) were included from a multicentre regional Society of Thoracic Surgeons database. Patients were stratified by weekend versus weekday operations and further analysed by specific day of the week.
RESULTS
A total of 14 374 patients underwent urgent or emergency isolated CABG with 410 (2.9%) operated on over the weekend. Weekend operations were more often emergency (36.1% vs 5.0%, P < 0.001) and more likely to be in the setting of MI (70.0% vs 51.2%, P < 0.001). Cardiopulmonary bypass times were similar [91 min (71-114) vs 94 min (74-117), P = 0.0749] and the frequency of complete revascularization equivalent (83.4% vs 85.3%, P = 0.284) between weekend and weekday operations. In risk-adjusted analyses, there was no increased odds for mortality in patients operated on over the weekend [odds ratio (OR) 1.07, P = 0.811]; however, there was an increased odds of major morbidity (OR 1.37, P = 0.034). Furthermore, compared with Monday, morbidity increased as the operative day approached the weekend (Tuesday 0.98, P = 0.828; Wednesday 1.07, P = 0.469; Thursday 1.12, P = 0.229; Friday 1.19, P = 0.041; weekend 1.47, P = 0.014).
CONCLUSIONS
While patients requiring surgery on the weekend are higher risk, there is no independent effect of weekend surgery on mortality. However, these patients are at increased risk for major morbidity, the causes of which require further investigation.
Keywords: Coronary artery disease, Perioperative care, Quality care, Weekend effect
INTRODUCTION
Coronary artery bypass grafting (CABG) remains the most common cardiac surgery operation performed in the USA with >150 000 procedures performed annually [1]. Although CABG is often scheduled, the proportion of urgent or emergency operations is growing with >60% of patients requiring non-elective operations [1]. Many of these non-elective operations are due to acute coronary syndromes, and it has previously been reported that patients who have acute coronary syndromes over the weekend have higher in-hospital mortality than patients treated on weekdays [2–4]. The observation that there is increased morbidity or mortality for patients who are treated on the weekend when compared with similar patients treated during the week has become known as the ‘weekend effect.’ Similar to patients with acute coronary syndromes, a ‘weekend effect’ has also been described in many surgical fields. However, the literature is conflicting on the relative impact of the contributing factors to these observations such as an increased patient acuity and variations in care or specialty service availability [5–7].
With the rising non-elective CABG volume, it is unclear if there is additional risk for patients operated on over the weekend outside of normal operative times. Fortunately, CABG is a common operation with well-defined clinical pathways and validated risk adjustment tools and is therefore uniquely suited to study this phenomenon. Therefore, the purpose of this study was to evaluate the impact of the ‘weekend effect’ on outcomes across a regional multicentre quality consortium of cardiac surgery centres.
PATIENTS AND METHODS
Ethical statement
The Institutional Review Board has determined that this project met criteria for exempt review (IRB # 20321, expiration 28 November 2019).
Patient data
The Virginia Cardiac Services Quality Initiative includes 19 hospitals and surgical groups in Virginia and North Carolina. Virginia Cardiac Services Quality Initiative data include 99% of all adult cardiac surgery in the region. Clinical data and cost methodology have been described previously [8]. Standard Society of Thoracic Surgeons (STS) definitions were used for all variables [9]. STS data from individual centres are compiled in a central registry. This study was exempt from review by the University of Virginia’s Institutional Review Board due to the de-identified nature of the quality database.
All patients who underwent isolated CABG between July 2011 and June 2017 were extracted from the Virginia Cardiac Services Quality Initiative database. Patients were excluded if they underwent elective operations. Patients were stratified by whether they underwent surgery on the weekend (Saturday or Sunday) versus the weekday (Monday–Friday). The primary outcome of interest was risk-adjusted odds of operative mortality. Secondary outcomes included risk-adjusted odds of composite major morbidity and individual major morbidities. A sensitivity analysis evaluated the impact of the day of the week on outcomes, with Monday treated as the reference. Additional outcomes of interest included intraoperative variables such as cardiopulmonary bypass time, number of distal anastomoses completed and complete revascularization (determined from the ratio of distal anastomoses to the number of diseased vessels).
Statistical analysis
Continuous variables are presented as median (interquartile range) due to skewed distributions. Wilcoxon rank sum test was used for non-normal distributed continuous variables and the χ2 test for all categorical variables. Hierarchical logistic regression with a generalized linear regression model was used to analyse risk-adjusted outcomes. Separate models were created for each outcome, adjusting for the logit of the appropriate STS risk score (log transformed), time from admission to surgery, year of operation, preoperative antiplatelet use and cardiopulmonary bypass time while accounting for centre level clustering with hospital treated as a random effect. These variables were selected a priori as variables felt to impact outcomes. SAS version 9.4 (SAS Institute, Cary, NC, USA) statistical software was used for analysis with a statistical threshold of 0.05 set for significance.
RESULTS
Study population
A total of 21 623 patients underwent isolated CABG during the study period. Of these, 7249 patients were excluded from the analysis due to elective (n = 7244) or unknown status (n = 5) leaving 14 374 patients in the final analysis. This included 410 (2.9%) patients operated on during the weekend. Throughout the week, there was a clustering of non-elective operations at both the beginning and end of the week, with the greatest number of operations performed Friday followed closely by Monday and the fewest on Wednesday (Fig. 1). The majority of urgent/emergency operations were performed for ischaemia- or angina-related indications (64.8%) and approximately one third (30.1%) were considered urgent/emergency due to anatomic concerns or inability to complete percutaneous intervention. A minority of indications included cardiac failure (4.4%) and traumatic of mechanical reasons were rare (0.8%).
Figure 1:
Non-elective operations throughout the week broken down by presenting symptoms. The total number of operations decreases during the first half of the week and then increases with the most urgent or emergency operations occurring on Friday. NSTEMI: non ST-elevation myocardial infarction; STEMI: ST-elevation myocardial infarction.
Weekend operations
Patients undergoing weekend operations had similar baseline characteristics and comorbidities to those with weekday operations, with the exception of a lower prevalence of hypertension (83.7% vs 87.3%, P = 0.030) and diabetes (40.1% vs 48.9%, P < 0.001) in weekend patients (Table 1). However, despite similar demographics, the weekend operations were performed in patients with higher acuity. Patients more often presented with ST segment elevation myocardial infarction (MI) (26.6% vs 8.0%, P < 0.0001), required preoperative intra-aortic balloon pump (41.5% vs 9.6%, P < 0.001) and had an MI within 24 h (34.9% vs 7.5%, P < 0.001, Table 2). In addition, the time from admission to surgery was shorter for weekend patients [1 (0–3) vs 3 (2–5) days, P < 0.001]. The shorter preoperative hospital time was not only seen in comparison of the weekend versus weekday but also progressively noted throughout the week. Patients operated on Mondays had the longest average preoperative hospitalization, which decreased on each subsequent day throughout the week. This trend persisted despite stratification by the type of presentation (Fig. 2). The presenting acuity translated into significantly higher predicted risk of mortality for weekend operations compared with weekday operations [2.1% (1.0–5.2) vs 1.2% (0.7–2.6), P < 0.001].
Table 1:
Baseline characteristics of patients operated on over the weekday versus weekend
| Weekday surgery | Weekend surgery | P-value | |
|---|---|---|---|
| n | 13 964 | 410 | |
| Age (years), median (IQR) | 65 (57–72) | 66 (58–73) | 0.565 |
| Female gender, n (%) | 3811 (27.3) | 128 (31.2) | 0.079 |
| Heart failure, n (%) | 3307 (23.7) | 111 (27.1) | 0.112 |
| Hypertension, n (%) | 12 179 (87.3) | 343 (83.7) | 0.030 |
| Diabetes, n (%) | 6823 (48.9) | 164 (40.1) | <0.001 |
| Peripheral artery disease, n (%) | 1982 (14.2) | 58 (14.2) | 0.978 |
| Prior stroke, n (%) | 1214 (8.7) | 31 (7.7) | 0.444 |
| Dialysis, n (%) | 426 (3.1) | 13 (3.2) | 0.889 |
| Previous cardiac intervention, n (%) | 4317 (30.9) | 128 (31.2) | 0.901 |
| Reoperation, n (%) | 319 (2.3) | 5 (1.2) | 0.152 |
Table 2:
Presentation of patients operated on over the weekday versus weekend
| Weekday surgery | Weekend surgery | P-value | |
|---|---|---|---|
| N | 13 964 | 410 | |
| Prior MI, n (%) | 9248 (66.2) | 323 (78.8) | <0.001 |
| Timing of MI,an (%) | <0.001 | ||
| <6 h | 172 (2.4) | 45 (15.7) | |
| 6–24 h | 372 (5.1) | 55 (19.2) | |
| 1–7 days | 5747 (78.8) | 168 (58.5) | |
| 8–21 days | 890 (12.2) | 12 (4.2) | |
| >21 days | 75 (1.0) | 4 (1.4) | |
| Presenting symptoms, n (%) | <0.001 | ||
| Asymptomatic | 840 (6.0) | 9 (2.2) | |
| Angina | 5976 (42.8) | 114 (27.8) | |
| NSTEMI | 6033 (43.2) | 178 (43.4) | |
| STEMI | 1110 (8.0) | 109 (26.6) | |
| Preoperative IABP, n (%) | 1344 (9.6) | 170 (41.5) | <0.001 |
| Urgency, n (%) | <0.001 | ||
| Urgent | 13 242 (94.8) | 258 (62.9) | |
| Emergency | 692 (5.0) | 148 (36.1) | |
| Salvage | 30 (0.2) | 4 (1.0) | |
| Admit to surgery time (days), median (IQR) | 3 (2–5) | 1 (0–3) | <0.001 |
| PROM, (%), median (IQR) | 1.2 (0.7–2.6) | 2.1 (1.0–5.2) | <0.001 |
| PROMM, (%), median (IQR) | 17.8 (8.9–21.9) | 26.6 (11.2–38.5) | <0.001 |
In patients with prior MI.
IABP: intra-aortic balloon pump; IQR: interquartile range; MI: myocardial infarction; NSTEMI: non ST-elevation myocardial infarction; PROM: predicted risk of mortality; PROMM: predicted risk of morbidity or mortality; STEMI: ST-elevation myocardial infarction.
Figure 2:
Time from admission to surgery for each day of the week. There is decreasing preoperative admission time throughout the week and into the weekend. This trend persists for all types of presenting syndromes. + represents mean, box represents 25th–75th quartile and whiskers represent 10th and 90th percentiles. CABG: coronary artery bypass grafting.
Although there were incrementally and likely clinically insignificantly, albeit, statistically significantly fewer distal anastomoses performed on the weekend [3 (2–3) vs 3 (2–4) distal anastomoses, P < 0.001], the rate of complete revascularization was equivalent (83.4% vs 85.3%, P = 0.284) and cardiopulmonary bypass times were similar [91 (71–114) vs 94 (74–117), P = 0.075]. Patients operated on over the weekend were initially intubated longer (8.8 vs 5.9 h, P < 0.001) and ultimately required additional time in the intensive care unit (ICU) (53.5 vs 47.4 h, P < 0.001). Consistent with preoperative risk scores, unadjusted major morbidity (21.7% vs 12.3%, P < 0.001) and unadjusted operative mortality (3.9% vs 2.0%, P = 0.008) were significantly higher for patients requiring operations on the weekend compared with those operated on during the week (Table 3).
Table 3:
Unadjusted outcomes of patients operated on over the weekday versus weekend
| Weekday | Weekend | P-value | |
|---|---|---|---|
| n | 13 964 | 410 | |
| Cardiopulmonary bypass time (min), median (IQR) | 94 (74–117) | 91 (71–114) | 0.075 |
| Cross-clamp time (mins), median (IQR) | 68 (52–86) | 64.5 (48–80) | <0.001 |
| Complete revascularization, n (%) | 11 914 (85.3) | 342 (83.4) | 0.284 |
| Median number of distal anastomoses, median (IQR) | 3 (2–4) | 3 (2–3) | <0.001 |
| Transfusion required, n (%) | 3819 (27.4) | 152 (37.1) | <0.001 |
| Initial ventilation time (h), median (IQR) | 5.85 (3.95–11.12) | 8.82 (4.54–19.49) | <0.001 |
| Total intensive care unit time (h), median (IQR) | 47.4 (25.5–79.5) | 53.5 (33.8–109) | <0.001 |
| Surgical reoperation, n (%) | 301 (2.2) | 20 (4.9) | <0.001 |
| Renal failure, n (%) | 320 (2.3) | 18 (4.4) | 0.006 |
| Prolonged ventilation, n (%) | 1312 (9.4) | 79 (19.3) | <0.001 |
| Permanent stroke, n (%) | 184 (1.3) | 3 (0.7) | 0.302 |
| Deep sternal wound infection, n (%) | 35 (0.3) | 1 (0.2) | 0.980 |
| Major morbidity, n (%) | 1713 (12.3) | 89 (21.7) | <0.001 |
| Operative mortality, n (%) | 282 (2.0) | 16 (3.9) | 0.008 |
IQR: interquartile range.
Risk-adjusted analyses
Regression analysis demonstrated no significant increase in the odds of mortality for patients operated on over the weekend [odds ratio (OR) 1.07, 95% confidence interval (CI) 0.61–1.90; P = 0.811] when compared with all weekdays, after adjusting for STS predicted risk of mortality, time from admission to surgery, year of operation, cardiopulmonary bypass time and treating hospital. However, adjusting for similar factors, there was an increased risk for morbidity (OR 1.37, 95% CI 1.02–1.82; P = 0.034) including significant increases in the odds of prolonged ventilation and reoperation (Table 4). In subgroup analysis where the same risk adjustment is performed but stratified by urgent and emergency cases, the increased morbidity is no longer statistically significant, but a trend towards increased morbidity in both groups remains (urgent cases—mortality OR 1.15, 95% CI 0.45–2.90; P = 0.771; emergency cases—mortality OR 0.91, 95% CI 0.40–2.11; P = 0.831; urgent cases—morbidity OR 1.12, 95% CI 0.74–1.72; P = 0.59; emergency cases—morbidity OR 1.29, 95% CI 0.82–2.01; P = 0.268). When each day of the week was considered individually and compared with Monday as a reference, the CIs for mortality were wide and all crossed 1 (Fig. 3A). However, there was a general trend towards increasing odds for morbidity in the days leading up to the weekend and over the weekend, with statistically significantly increases on Friday (OR 1.19, CI 1.01-1.41; P = 0.041) and Saturday/Sunday (OR 1.47, CI 1.08-2.01; P = 0.014, Fig. 3B) when compared to Monday as a reference.
Table 4:
Results of hierarchical logistic regression for operative mortality and major morbidity by weekend status
| Weekend surgery |
||||
|---|---|---|---|---|
| Risk-adjusted odds ratioa | 95% confidence interval | P-value | C-statistic | |
| Mortality | 1.07 | 0.61–1.90 | 0.811 | 0.794 |
| Major morbidity | 1.37 | 1.02–1.82 | 0.034 | 0.770 |
| Surgical reoperation | 2.04 | 1.25–3.31 | 0.004 | 0.675 |
| Renal failure | 1.68 | 0.97–2.93 | 0.065 | 0.837 |
| Prolonged ventilation | 1.41 | 1.04–1.92 | 0.028 | 0.800 |
| Permanent stroke | 0.50 | 0.156–1.57 | 0.233 | 0.711 |
Risk adjustment was performed with: appropriate STS risk score, time from admission to surgery, year of operation, preoperative antiplatelet use, and cardiopulmonary bypass time while accounting for center level clustering with hospital treated as a random effect.
Figure 3:
Risk-adjusted odds for (A) mortality and (B) major morbidity for each day referenced to Monday. Error bars represent 95% confidence intervals and point estimates for odds ratio are depicted below each point. *Significance with P < 0.05. In (A), the confidence intervals all cross 1 and no time point reaches statistical significance. In (B), there is a trend towards increased odds of morbidity throughout the week with significant increases on Friday (OR 1.19, P = 0.04) and weekends (OR 1.47, P = 0.01).
DISCUSSION
In this regional analysis of nearly 15 000 non-elective, isolated CABG operations, there was no independent effect of weekend surgery on mortality. However, we observed an increase in risk-adjusted morbidity in patients who required operations over the weekend. Moreover, this association appears to gradually increase throughout the week as the operative day approaches the weekend. While intraoperative care appears to be consistent and there is no increased mortality for those operated on over the weekend, our study demonstrates a ‘weekend effect’ and patients may be at risk for increased adverse events when operated on or immediately prior to the weekend. This study provides important insight into the ongoing controversy of the true nature of the ‘weekend effect’ and is strengthened by the study population, which assessed a highly standardized operation with reproducible and validated risk scores.
The purported causes of the weekend effect are based on 2 main arguments. First, patients who require medical care on weekends have a greater severity of illness and increased acuity. This argument is intuitive and has been proven rather robustly in previous studies. Unfortunately, the majority of the early work in this field has relied on administrative databases, with the inability to accurately account for patient profiles and details of presentations [2, 10–16]. Two recent national studies from Europe investigating this phenomenon within cardiac surgery utilized clinical databases and found that with adequate ability to adjust for patient risk profiles, there was no evidence of a weekend effect for operative or long-term mortality [17, 18]. In addition, a 2016 study utilizing clinical data evaluating mortality in stroke admissions contradicted previous findings in patients with stroke that had only utilized administrative data to adjust for patient factors [16, 19]. This contradiction highlights the importance of using clinical risk adjustment when analysing populations who present at different times and, as a result, may be inherently different. Our results are consistent with the findings of increased acuity in weekend surgical patients and the absence of an association between weekend surgery and increased mortality when clinical markers are used to risk adjust. However, even when accounting for the differences in presentation and acuity, the increased odds for adverse events remain in our population.
The ‘weekend effect’ has also been attributed to alterations in staffing and specialty care availability on weekends [20]. In a study of the amount of consultant time dedicated to emergency weekend admissions, there was a notable difference in the number of hours of specialty care provided with fewer hours for patients arriving over the weekend. However, no association between adjusted mortality rates and the relative involvement of specialty consultant services was found [5]. Although this finding argues against consultation time impacting weekend outcomes in emergency department admissions, the phases of care are somewhat different in cardiac surgery and observations from 1 specialty may not be generalizable to another [21]. Patients requiring cardiac surgery generally follow predefined care pathways and move from the operating room to the ICU for at least the first postoperative day, with high levels of monitoring and treatment concentrated in the early postoperative period. Alterations in ICU staffing coupled with reduced access to consultant services could support this observation, especially as the major morbidity with the greatest increase is the ICU-related metric of prolonged intubation. Alterations in intensivist and respiratory care staffing on weekends may lead to the delayed time to extubation seen in the present analysis and represent an area for potential improvement. Our finding of progressively increased morbidity as the operative day approaches the weekend supports this hypothesis that altered staffing over the weekend could be one of the contributing factors to the ‘weekend effect’. As the operative day approaches the weekend, it is more likely that the postoperative ICU course will involve a weekend day and therefore susceptible to postoperative adverse events.
The causal link to explain the higher likelihood of a reoperation cannot be directly ascribed to variations in postoperative staffing. There is, however, a clear trend for decreased preoperative time in patients who undergo weekend surgery. This is true for the study population in general, and for specific presentations such as those with MI or angina. Patients operated on soon after MIs demonstrate worse outcomes than those who undergo a longer period of preoperative optimization [22, 23]. In addition, for patients who receive preoperative P2Y12 receptor inhibitors, a shorter waiting period results in an increased bleeding risk, which could explain the significantly higher rate of reoperation seen in patients operated on over the weekend with shorter preoperative hospitalizations [24]. Surgeons often do not have the option of delaying an operation due to ongoing symptoms or high-risk lesions, and many patients operated on Fridays or over the weekend may have been those who were felt to not be stable enough to wait until the following week for revascularization. Although the option for delaying revascularization is not always available, the shorter preoperative time for weekend patients may explain some of the excess morbidity and higher rate of reoperation and this was present even when accounting for differences in preoperative antiplatelet administration. In situations where waiting is possible, it may be advisable to delay surgery until platelet function returns to normal.
Limitations
This study has several limitations including its retrospective nature, which exposes the analysis to an element of selection bias. To account for this, conclusions were drawn from adjusted analyses, though the potential for residual confounding does exist. Second, we were unable to correlate the observations seen in this analysis with provider availability. Third, the frequency of non-elective operations performed throughout the week, with a peak on Monday and Friday, may represent decisions based not purely on clinical factors with operating room availability, desire to avoid deterioration over the weekend for patients awaiting surgery and surgeon preference to operate on Monday for borderline cases potentially playing a role. Fourth, holidays that occur on weekdays were not treated differently in the analysis; however, we expected there to be few holidays treated consistently among different centres without significant impact on the conclusions of the study. In addition, practice patterns between institutions across the multicentre network vary including weekend staffing of ICUs and availability of cardiac anaesthesiologists. Finally, only patients who underwent surgical revascularization were captured by this data set. It does not include patients who underwent alternative forms of revascularization, those who were treated with medical therapy or those who acutely decompensated without intervention.
CONCLUSION
In conclusion, patients requiring urgent or emergency surgery on the weekend are at higher risk than those operated on during the week, yet there was no independent effect of weekend operations on mortality. Nevertheless, a possible ‘weekend effect’ following CABG is apparent with an increased risk of postoperative morbidity. While patients should not be denied needed operations on the weekend, processes to mitigate some of the increased morbidity risk should be sought.
Funding
This work was supported by the National Heart, Lung, and Blood Institute [T32 HL007849, UM1 HL088925].
Conflict of interest: none declared.
Author contributions
Jared P. Beller: Conceptualization; Data curation; Formal analysis; Methodology; Writing—original draft; Writing—review & editing. William Z. Chancellor: Conceptualization; Data curation; Methodology; Writing—review & editing. J. Hunter Mehaffey: Conceptualization; Formal analysis; Methodology; Writing—review & editing. Robert B. Hawkins: Conceptualization; Formal analysis; Methodology; Writing—review & editing. Elizabeth D. Krebs: Conceptualization; Formal analysis; Methodology; Writing—review & editing. Alan M. Speir: Conceptualization; Methodology; Writing—review & editing. Mohammed A. Quader: Conceptualization; Methodology; Writing—review & editing. Leora T. Yarboro: Conceptualization; Methodology; Writing—review & editing. Gorav Ailawadi: Conceptualization; Methodology; Supervision; Writing—review & editing. Nicholas R. Teman: Conceptualization; Methodology; Supervision; Writing—review & editing.
ABBREVIATIONS
- CABG
Coronary artery bypass grafting
- CI
Confidence interval
- ICU
Intensive care unit
- MI
Myocardial infarction
- OR
Odds ratio
Presented at the 55th Annual Meeting of the Society of Thoracic Surgeons, San Diego, CA, 27 January 2019.
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