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Original Articles: Adult Cardiac

Changing Operative Characteristics of Patients Undergoing Operations for Coronary Artery Disease: Impact on Early Outcomes

Cardiothoracic Centre, Castle Hill Hospital, Kingston-Upon-Hull, East Yorkshire, United Kingdom

Accepted for publication July 17, 2008.

^_* Address correspondence to Mr Ngaage, Department of Cardiothoracic Surgery, Castle Hill Hospital, Kingston-Upon-Hull, East Yorkshire, HU16 5JQ, United Kingdom (Email: dngaage{at}yahoo.com).

	Abstract

Top Abstract Introduction Material and Methods Results Comment References

 
Background: Aggressive nonsurgical revascularization results in high-risk patients presenting for operation at a later stage of coronary artery disease (CAD). This study investigated the effect of temporal changes in operative characteristics on outcomes of surgical revascularization.

Methods: We compared preoperative, intraoperative, and postoperative variables of 5633 patients who underwent surgical revascularization for CAD between April 1998 and January 2007, divided into early (1998 to 2002, n = 2746) and late (2004 to 2007, n = 2887) eras. End points were major adverse outcomes (postoperative myocardial infarction, stroke, new dialysis) and operative mortality.

Results: Median age (66 vs 68 years, p < 0.0001), prevalence of left ventricular systolic dysfunction, left main stem disease, prior angioplasty, diabetes mellitus, concomitant valve operation, and aprotinin use increased steadily over time. Severe symptoms, nonelective operations, mean number of grafts, postoperative bleeding, reopening for bleeding, and blood transfusion declined. Major complications were evenly distributed between the eras. Operative mortality for isolated coronary artery bypass grafting did not change (2.0% vs 1.8% p = 0.62) despite increasing operative risk (p < 0.0001); there was a 100% reduction in the absolute risk (110% to 210%) over time. The markers for operative difficulties, such as longer bypass times, were determinants of operative mortality and, in addition to other predictors like age and left ventricular systolic dysfunction, were more prevalent in the late era.

Conclusions: Coronary operations are increasingly performed in higher-risk patients; however, surgical revascularization is nearly twice as safe in current practice compared with a decade ago.

	Introduction

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Advances in the management of coronary artery disease have expanded the role of nonsurgical therapies. Medical treatment with aspirin, clopidogrel, statins, β-blockers, and angiotensin-converting enzyme inhibitors have led to improved survival [1–4]. Better diagnosis, treatment devices, and catheter skills, and a continuously evolving drug-eluting stent technology, boosts the widespread use of percutaneous intervention for coronary artery disease [5–9]. Consequently, patients undergoing interventions for coronary artery disease are becoming increasingly highly selected and, with advancing age [10, 11] prove technically challenging.

To meet these challenges, new surgical management strategies are constantly being developed, increasing the safety of operations and thereby also allowing patients previously considered too high risk for an operation to receive treatment [12–14]. Patients undergoing surgical treatment for coronary artery disease in the current era therefore are higher risk and more technically demanding. Changes in demographic and clinical profiles of cardiac surgical patients and the effect on outcome have been reported [10, 11]. We undertook this study to investigate changes in operative characteristics of patients undergoing first-time surgical procedures for coronary artery disease and to determine the effect of these changes on operative outcome.

	Material and Methods

Top Abstract Introduction Material and Methods Results Comment References

 
We identified and retrieved preoperative, intraoperative, and postoperative variables for 5633 consecutive patients who underwent a first-time surgical procedure for coronary artery disease from April 1998 through January 2007. We obtained approval and a waiver for individual patient consent from the Medical and Ethics Committee of our institution. All the patients were referred for and scheduled for coronary artery bypass grafting (CABG) with or without valve repair.

Data retrieved included demographic profile, clinical presentation, reports of coronary angiography, electrocardiography and echocardiography, operative details, and postoperative variables. Medical records were also reviewed where necessary to obtain missing data. To elucidate temporal trends and identify the effect of time-related changes in clinical profile and operative characteristics on outcomes, we separated the study population into two groups according to the dates of operation: from 1998 to 2002 (early era) and 2003 to 2007 (late era). During the study period, procedures were performed by the same surgeons, and waiting times dropped from more than 12 months to within 3 months.

Perioperative Care
A full median sternotomy and cardiopulmonary bypass (CPB) with hypothermia (temperature between 32° and 34°C) was used in most cases. A few cases, however, were performed without CPB. Except in the early years of the first era, when crystalloid cardioplegia was a common method of myocardial preservation, blood cardioplegia and intermittent cross-clamp fibrillatory arrest were more often used for the rest of the study period.

Standard St. Thomas’ cardioplegia solution (potassium chloride, magnesium chloride, and procaine hydrochloride) was added to Ringer's lactate solution for crystalloid cardioplegia: 50 mL added to 450 mL of Ringer's solution to provide 75 mmol of potassium for high-dose administration, and 25 mL in 475 mL of Ringer's solution to provide 35 mmol of potassium for low dose top-ups. Ringer's lactate solution was replaced with blood in a ratio of 4:1 of cardioplegia solution for blood cardioplegia. Routinely, 1000 mL of cardioplegia is given to initiate cardiac arrest, and this is followed by 200 to 300 mL top-ups at 20- to 30-minute intervals. Antegrade cardioplegia administration was used in most patients.

Postoperative care changed in the course of time. Protocols, regularly adapted to reflect prevailing evidence-based practice, facilitated care by rotational medical staff. Standard practice in the late era evolved over time to include:

• Strict screening for methicillin-resistant Staphylococcus aureus and treatment of patients with isolates preoperatively using nasal and topical Bactroban (GlaxoSmithKline, Middlesex, UK);

• regular use of Swan-Ganz catheter for hemodynamic management;

• training and routine use of transesophageal echocardiography intraoperatively and postoperatively;

• tight glucose control, maintaining blood glucose levels between 4 and 8 mmol/L postoperatively;

• high threshold for blood product transfusions, from hemoglobin level of 10 g/dL to 8 g/dL, and

• very early postoperative administration of aspirin (300 mg rectally after 6 hours), β-blockers, and angiotensin-converting enzyme inhibitors (within 24 hours).

Anesthetic management philosophy also changed to enable immediate to early postoperative extubation during this period.

Statistical Analysis
The study primary end point was operative mortality (death in the hospital or within 30 days postoperatively). Secondary end points were major cardiac, cerebral, and renal postoperative morbidity, namely, myocardial infarction (new and persistent S-T changes, new Q waves on electrocardiography), stroke (transient and permanent neurologic deficit), and new dialysis (in a preoperatively nondialysis patient).

Dichotomous variables are reported as percentages and compared between groups using the ² test. Continuous variables with symmetric distribution are reported as mean ± standard deviation (SD) and compared between groups using the t test and, for continuous variables with asymmetric distribution, the medians are reported with the 25th and 75th percentiles as interquartile range (IQR) and compared between the groups using the Mann-Whitney U test. The two-sided p values are reported for the comparisons, and significance was set at p < 0.05. To identify the predictors for the primary and secondary end points, a backward, stepwise, multifactorial logistic regression model was constructed using all the variables listed in Tables 1 and 2 (only reopening for bleeding or tamponade under postoperative morbidity was included). The odds ratio (OR), 95% confidence interval (CI), and p values are reported for the predictors. Statistical analysis was performed using SPSS 15.0 software (SPSS Inc, Chicago, IL).

	Results

Top Abstract Introduction Material and Methods Results Comment References

Operative Characteristics
Table 2 outlines the operative characteristics of the two groups. Most patients in the two eras underwent isolated CABG. Although the frequency of isolated CABG declined over time, CABG combined with aortic or mitral valve replacement/repair was on the ascendancy. The enthusiasm for off-pump CABG also declined, and its use became more selective, at 296 (10.8%) vs 104 (3.6%, p 0.0001). The average number of bypass grafts performed (2.6 ± 0.9 vs 2.5 ± 0.8, p = 0.001) decreased from the early to late era, even though the mean numbers of occluded coronary arteries remained constant (2.6 ± 0.6 vs 2.6 ± 0.6, p = 0.31).

Incomplete revascularization was considered to be present when the discrepancy between the number of occluded coronaries or ischemic territories on one hand and the number of grafts or revascularized territories on the other, was in favor of the former. Among patients undergoing isolated CABG, there was no difference in the incidence of incomplete revascularization (14.6% vs 14.9%, p = 0.74) during the study period. Incomplete myocardial revascularization was more likely in the setting of coronary artery disease combined with valvular disease than with isolated coronary artery disease (30% vs 14%, p < 0.0001).

Urgent/emergency operations were performed with decreasing frequency from the early to the late era. More patients had cardiopulmonary bypass reinstituted for hemodynamic deterioration in the late era than in the early era. Recommencement of CPB was more common in patients who had preoperative ejection fraction of 0.50 or less (46% vs 25%, p < 0.0001), intravenous nitrates for acute deterioration (18% vs 8%, p < 0.0001), treated heart failure (29% vs 20%, p = 0.05), and combined CABG and valve procedures (34% vs 13%, p < 0.0001).

The use of aprotinin increased from the early to the late era and median blood loss, rates of reopening for bleeding or tamponade, and transfusions of blood and blood products decreased remarkably.

Postoperative Morbidity and Mortality
The frequencies of major postoperative complications in both eras were comparable (Table 2). Postoperative myocardial infarction was not common, at 27 (1.0%) in the early era vs 40 (1.4%) in the late era (p = 0.16), and by multivariate analysis, had strong associations with operative factors such as reinstitution of CPB (OR, 9.50; 95% CI, 3.92 to 22.98; p < 0.0001), incomplete revascularization (OR, 4.86; 95% CI, 1.68 to 14.05; p = 0.003), blood/blood product transfusion (OR, 2.16; 95% CI, 1.12 to 4.17; p = 0.02), and reopening for bleeding or tamponade (OR, 4.00; 95% CI, 1.77 to 9.07, p = 0.001), as well as patient factors such as age (OR, 1.06; 95% CI, 1.02 to 1.10) and diabetes mellitus (OR, 0.15; 95% CI, 0.04 to 0.64; p = 0.01).

Postoperative stroke was observed in 42 patients (1.5%) in the early era and in 48 (1.7%) in the late era. The risk factors were previous stroke (OR 2.93; 95% CI, 1.70 to 5.04; p < 0.0001), age (OR, 1.04; 95% CI, 1.01 to 1.08; p = 0.008), combined mitral and coronary surgery (OR, 5.03; 95% CI, 1.30 to 19.44; p = 0.02), diabetes mellitus (OR, 1.85; 95% CI, 1.09 to 3.14; p = 0.02), and blood/blood product transfusion (OR, 1.74; 95% CI, 1.05 to 2.88; p = 0.03).

Twenty nondialysis patients (0.7%) in the early era and 29 (1.0%; p = 0.26) in the late era were treated with renal replacement therapy postoperatively. The determinants of this outcome included preoperative renal insufficiency (OR, 16.35; 95% CI, 5.84 to 40.37; p < 0.0001), male gender (OR, 5.36; 95% CI, 1.39 to 20.63; p = 0.02), reopening for bleeding or tamponade (OR, 4.61; 95% CI, 1.54 to 13.79; p = 0.006), left ventricular systolic ejection fraction of less than 0.30 (OR, 4.56; 95% CI, 1.53 to 13.52; p = 0.006), preoperative stroke (OR, 3.17; 95% CI, 1.29 to 7.79; p = 0.01), and peripheral vascular disease (OR, 2.45; 95% CI, 1.03 to 5.81; p = 0.04).

In the late era, more patients were treated with inotropes, and this corresponded with the trend for preoperative left ventricular systolic dysfunction and intravenous nitrate therapy for acute symptom deterioration. However, mechanical hemodynamic support with intraaortic balloon pump did not change over time. A progressive approach toward early extubation is reflected by a substantial drop in the median duration of mechanical ventilation; however, this did not translate to shorter median length of postoperative hospitalization because the patients were older and at higher risk.

There was no difference in the crude rates for overall (2.4% vs 2.4%; p = 0.96) and procedure-specific operative mortality between the early and late eras (Table 2), but the rates were constantly lower than the predicted rate at every point in time. For isolated CABG, the operative mortality was 2.0% for the early era vs 3.1 ± 2.4 mean predicted European System for Cardiac Operative Risk Evaluation (EuroSCORE), and 1.8% for the late era vs 3.6 ± 2.4 mean predicted EuroSCORE (Fig 1). Significantly though, the absolute risk of operative mortality was reduced by 100% with time (110% vs 210%, Fig 1).

View larger version (14K): [in this window] [in a new window]   Fig 1. Trends in operative mortality from the early to the late era are illustrated. (A) Shows the rising mean predicted mortality (dashed line) by the European System for Cardiac Operative Risk Evaluation (EuroSCORE) against the observed operative mortality (solid line). (B) Displays a steady increase in the magnitude of absolute reduction in operative mortality rate.

	Comment

Top Abstract Introduction Material and Methods Results Comment References

 
Our study shows that patients undergoing surgical intervention for coronary artery disease in the current era, compared with previous eras, are often older, diabetic, and with left main stem disease or advanced coronary artery disease complicated by left ventricular systolic dysfunction, but usually present with less severe symptoms probably due to better medical therapy and enthusiastic coronary angioplasty with insertion of drug-eluting stents. These patients are usually at a higher risk and surgical intervention in them is more technically demanding because the coronary arteries are less suitable for grafting; in addition, concomitant valve replacement/repair is often required. Despite this, the operative mortality rate has not increased; in fact, there was a 100% reduction in the absolute risk of operative mortality from the early to late era, indicative of improvement in surgical outcome despite greater predictive risk and more complex operations.

Even with a significant reduction in waiting times for surgical revascularization, increasing numbers of patients undergoing procedures in the late era had a higher predicted risk and evidence of advanced coronary artery disease, contrary to expectation. This is probably due to prolonged nonsurgical management before referral. Perhaps, shorter waiting times only affects surgical revascularization by reducing nonelective operations, because patients undergo the procedure soon after referral, before severe or unstable symptoms develop. This inadvertent selection process chooses high-risk patients for operation at a later stage of the disease. In addition, compelling evidence in support of the prognostic benefit of CABG compared with angioplasty with stents [15, 16] favors the selection of diabetic patients for CABG, and they can typically have problematic coronary artery targets. These nuances present a new set of operative challenges. Not uncommonly, therefore, coronary targets are found to have diffuse calcific disease, distal stenoses, and to be small, and occasionally, to be replaced with a cast of multiple stents. These can preclude placement of bypass grafts beyond every occluded major coronary artery and possibly contributed to the reduction in number of bypass grafts performed. Furthermore, the imposing operative difficulties not infrequently result in a change in the operative strategy [17], such as conversion from off-pump to on-pump CABG, which can jeopardize the operative result.

The influence of temporal changes in patient demography and clinical profile on operative outcome has been well reported [10, 11, 18]; however, changes in operative characteristics and the effect on outcome are not clearly known. Historical data showed that unlike patient characteristics, which did not significantly affect mortality, changes in patient clinical profile and operative factors stalled improvement in surgical results for two decades [19]. Preexisting risk factors allow preoperative planning and institution of preemptive measures in a controlled manner, which is not the case for operative factors. Often unexpected operative difficulties demand an impromptu and, sometimes, ingenious response that usually involves an alteration of the surgical strategy. Our findings indicate that the consequences of these intraoperative exigencies adversely influence outcome. For example, operative markers of technical difficulties such as the duration of CPB, reinstitution of CPB, and reopening for bleeding or tamponade were predictors of operative death. Unlike patient and preoperative factors, these operative characteristics are not captured by conventional risk scoring systems [20–22]; therefore, the predicted risk does not place these complex cases with challenging coronary artery pathologies in the correct perspective.

As the complexity and difficulty of surgical intervention for coronary artery disease has increased with time, operative risk factors have become more prevalent in the surgical population. Age, for example, has steadily increased, such that in the current era a significant proportion of patients were older than 70 years. Similarly, a greater proportion of patients in the current era had left ventricular systolic dysfunction and peripheral vascular disease and required concomitant valve operations. The adverse effects of these risk factors are mitigated by advances in operative technique and perioperative care. The progressive increase in the use of aprotinin during the period explains the significant reduction in blood loss, blood/blood product transfusions, and reopening for bleeding or tamponade, and as we have reported previously, the use of aprotinin was not associated with adverse outcomes in our patients [23]. Also contributing to the improvement in outcomes were the routine use of perioperative strategies that have been shown to improve clinical results, such as tight glucose control [24, 25], early administration of aspirin [26], and early extubation [27, 28].

This study has some limitations. Complete data were not available for all patients, as indicated in Tables 1 and 2, but because of the large sample size, missing data affecting few patients and a small number of variables did not significantly affect the findings of this study. Second, although we demonstrated a relative improvement in operative outcome, we have not established direct associations between specific time-related practice changes and outcomes. Study eras as defined did not correspond to definite changes in practice because these evolved sequentially and in some cases over overlapping periods of time; hence, their influence on operative results is collective. Our list of improvements in operative and perioperative care does not represent the exhaustive record of changes that occurred during the study period, because changes in other supportive treatment modalities that facilitate recovery are not included. Notwithstanding, in this study we established the temporal trends in outcome of surgical intervention for coronary artery disease over a decade and defined the impact of changing operative characteristics on these trends.

In conclusion, the expanding and enthusiastic practice of angioplasty with drug-eluting stents for coronary artery disease leaves for surgical intervention a highly select group of higher-risk patients with a higher prevalence of diabetes mellitus, extensive coronary artery disease, left ventricular systolic dysfunction, and concomitant valvular disease. Although increasing operative difficulties and complexities lead to longer CPB time and more reinstitution of CPB, which threaten improved outcomes, the risk of operative mortality has dropped by 100% in one decade, making surgical revascularization nearly twice as safe.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Dumbor L. Ngaage Steven Griffin Levent Guvendik Michael E. Cowen Permission Requests Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ngaage, D. L. Articles by Cale, A. R.J. Search for Related Content PubMed PubMed Citation Articles by Ngaage, D. L. Articles by Cale, A. R.J. Related Collections Coronary disease