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Ann Thorac Surg 2003;75:1849-1855
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Patient characteristics can predict improvement in functional health after elective coronary artery bypass grafting

^a Division of Cardiothoracic Surgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa, USA
^b Center for the Evaluative Clinical Sciences, Dartmouth Medical School, Hanover, New Hampshire, USA
^c Section of Cardiothoracic Surgery Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, USA
^d Center for Shared Decision Making, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, USA

Accepted for publication December 23, 2002.

^* Address reprint requests to Dr Welke, Division of Cardiothoracic Surgery, University of Iowa Hospitals and Clinics, 200 Hawkins Dr, Iowa City, IA 52242, USA.
e-mail: karl-welke{at}uiowa.edu

	Abstract

Top Abstract Introduction Patients and methods Results Comment References

 
BACKGROUND: Despite many patients undergoing coronary artery bypass grafting (CABG) to improve their functional status, literature in this area is limited. The purpose of this study is to determine the effect of CABG on the functional health of an elective population and to identify preoperative patient characteristics associated with improved functional health after surgery.

METHODS: Physical and mental functional health was assessed before and 6 months after surgery with the Short-Form Health Survey (SF-36) in 1,061 consecutive patients undergoing elective, isolated CABG. Survey data were complete in 529 patients (49.9%). Preoperative information on patient demographics, severity of cardiovascular illness, and disease comorbidities was also prospectively collected.

RESULTS: Six months post-CABG the mean summary score for physical function improved by 31.9% over baseline (45.1 versus 34.2, p < 0.0001). The mean summary score for mental function improved by 7.3% over baseline (51.3 versus 47.8, p < 0.0001). Overall 73.2% of patients showed improvement in physical function and 41.6% showed improvement in mental function. Multivariate logistic regression identified certain preoperative characteristics as negative correlates of a significant improvement in physical functioning: body mass index 35 kg/m² or greater, diabetes with sequelae, chronic obstructive pulmonary disease, peripheral vascular disease, and baseline physical function. Baseline mental function and chronic obstructive pulmonary disease were identified as negative correlates and older age as a positive correlate of significant improvement in mental functioning.

CONCLUSIONS: Patient characteristics exist that impact functional health after elective CABG. Knowledge of these characteristics may be helpful when counseling patients about expected improvement in functional health with CABG.

	Introduction

Top Abstract Introduction Patients and methods Results Comment References

 
Selected clinical criteria—mortality, relief of angina—are used to judge the success of coronary artery bypass grafting (CABG). In elective patients, especially those undergoing CABG for symptom relief in hopes of improving activity and energy levels, a measure of functional health may also be appropriate. Operative mortality rates are low in this group of patients and longer life expectancy as a result of surgery is often not expected. In addition elective patients may be most interested in whether CABG will improve their health-related quality of life. Therefore it is important to study the effect of CABG on patients’ functional health.

Despite the large number of patients undergoing CABG with the goal of improving health-related quality of life, the literature in this area is limited. Specifically there is little information about predictors of post-CABG functional health [1–7].

The purpose of this prospective assessment is to determine the effect of CABG on the functional health of an elective population and to identify preoperative patient characteristics associated with improved functional health after surgery.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment References

 
Study population
These analyses are based on a consecutive cohort of patients undergoing elective isolated CABG at Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, from October 1993 to October 2000. Data were collected for 2,965 patients, 1,061 (36.8%) of whom had elective surgery. Patients whose priority at surgery was classified as emergent or urgent were excluded from the study.

Data collection
Physical and mental functional health was assessed using the Medical Outcomes Study 36-item Short-Form Health Survey (SF-36) before surgery and 6 months after surgery. The SF-36 is a standardized, validated, self-administered tool that measures patient functional health in eight domains: physical functioning, role-physical, bodily pain, general health, vitality, social functioning, role-emotional, and mental health. These eight subscores are summarized by a physical component scale (PCS) and a mental component scale (MCS). Possible raw scores range from 0 to 100 with 100 representing the most unrestricted functionality. We reported norm-based scores, which are based on US population means of 50 with a standard deviation of 10 [8].

Surveys were self-administered using the booklet form before April 1998 and the scanable form after April 1998. The survey was administered either at home or in the clinic up to 45 days before the procedure. Follow-up surveys were administered through the mail 6 months after hospital discharge. A reminder postcard was sent to nonrespondents 14 days after the initial mailing; after another 14 days a second copy of the survey was sent [9].

Patient demographic and comorbidity data were collected through the Northern New England Cardiovascular Disease Study Group [10]. The Group maintains a prospective registry of all patients undergoing CABG in the region. The following data were recorded for all patients: age, sex, body mass index (BMI), diabetes (including patients with or without sequelae), Canadian Cardiovascular Society angina score, chronic obstructive pulmonary disease (COPD), dialysis dependent renal failure, congestive heart failure, hypertension, peripheral vascular disease, prior percutaneous transluminal coronary angioplasty or heart surgery, left ventricular ejection fraction, left ventricular end diastolic pressure, surgical priority (elective, urgent, emergent), time on bypass, aortic cross clamp time, perioperative stroke, and discharge status (dead or alive).

Statistical methods
Change in functional health after CABG was assessed in two ways. First, mean values of the summary PCS and MCS were compared preoperatively and 6 months postoperatively using paired t tests. Mean change scores have the advantage of reflecting the magnitude of change as measured by the scale but mask the proportion of patients with postoperative scores that differ from baseline. The 6-month time frame was selected to allow patients to recover from surgery while avoiding the confounding effects of loss of functional health with increasing age.

Second, patients were allocated to two change groups: patients who improved and patients who remained the same or declined. This statistical strategy functions to translate a continuous variable into a clinically meaningful dichotomous variable and captures the proportion of patients with postoperative scores that differ from baseline. As the goal of elective CABG is to improve functional status rather than to maintain an existing state, patients who remained the same were grouped with those who declined. Criteria for a meaningful change in functional status were those suggested by Ware and associates [8]. For the PCS a patient was categorized as improved if the difference between 6-month and baseline PCS exceeded 5.42. For the MCS a patient was categorized as improved if the difference between 6-month and baseline MCS exceeded 6.33. These threshold values are derived as the equivalent value of two standard errors of measurement (approximately the 95% confidence interval) for a 12-month follow-up. Changes large enough to be labeled as improved would be unlikely due to measurement error or chance. Clinically, patients categorized as improved could be described as believing themselves to have experienced a meaningful physical or mental improvement after CABG. Because follow-up scores were recorded at 6 months in our study, any measurement of likelihood in change would be biased toward the null.

Univariate logistic analyses were performed using the changes in PCS and MCS as the dependent variables dichotomized into clinically significant improvement and lack of improvement or decline as described above. Age and sex as well as independent variables with significant associations (p < 0.05) and associations close to significance (p < 0.15) were entered into multivariate logistic models for PCS and MCS. For categorical variables (BMI and diabetes mellitus) all strata were included as long as one stratum was significant. The area under the receiver operating characteristic curve was used as a measure of discrimination. The Hosmer-Lemeshow goodness-of-fit test was used as a measure of calibration. Analyses were performed using Stata statistical software, version 6.0, 2000 (Stata Corporation, College Station, TX).

	Results

Top Abstract Introduction Patients and methods Results Comment References

 
Patient characteristics
In all 529 patients (49.9%) completed surveys at both time points; 532 patients (50.1%) either completed a survey at only one of the time points or did not complete any survey. Of these patients 139 completed the baseline survey only, 270 completed the 6-month survey only, and 123 completed neither survey. Patient characteristics of these two groups were similar (Table 1), with the following exceptions: a greater percentage of patients who completed surveys at both time points had BMIs in the 30 to 34 kg/m² category. The percentage of patients with BMIs 35 kg/m² or greater was similar. Patients who completed both surveys had less peripheral vascular disease than those who did not complete both surveys. They also had a slightly higher mean left ventricular ejection fraction. The groups were similar with regard to Canadian Cardiovascular Society angina classification.

Patients did not uniformly experience the improvement in physical functioning and mental functioning. More patients with low preoperative physical or mental functional health improved after surgery than patients with higher self-reported preoperative function health. Significant improvement was observed in 43.7% of patients in the highest quartile of preoperative physical scores whereas 78.3% of patients with baseline physical scores in the lowest quartile experienced significant improvement (Fig 1). When considered by quartile of baseline mental score, the mean score of patients in the top 25% of baseline MCS declined from 60.9 (95% confidence interval: 60.4 to 61.5) to 56.5 (95% confidence interval: 55.4 to 57.6). Only 0.8% of these patients experienced significant improvement compared with 63.9% of patients in the lowest quartile of baseline MCS (Fig 2).

View larger version (17K): [in this window] [in a new window]   Fig 1. Change in mean physical component summary scale (PCS) score at baseline and at 6 months by baseline PCS quartiles. (Lower 25% = diamonds; 25% to 75% = squares; top 25% = triangles.)

View larger version (16K): [in this window] [in a new window]   Fig 2. Change in mean mental component summary scale (MCS) score at baseline and 6 months by baseline MCS quartiles. (Lower 25% = diamonds; 25% to 75% = squares; top 25% = triangles.)

	Comment

Top Abstract Introduction Patients and methods Results Comment References

The patients in our cohort scored below the mean PCS and MCS values for similar age groups in the US population at baseline and improved to near mean values after CABG. Patients in the top 25% of baseline PCS and MCS had mean baseline scores greater than the US population and well above mean scores for patients with angina, congestive heart failure, or recent myocardial infarction [8]. These patients were less likely to improve significantly after CABG than patients with lower baseline scores. These findings suggest that the functional health benefits from CABG are more likely to be experienced by elective surgery patients with more severe limitations and greater opportunity to improve. Patients with higher baseline health status are likely to obtain little functional improvement and the indications for CABG in these patients should be driven by a survival advantage.

Earlier studies have demonstrated improvement in quality of life with CABG. Reports of factors influencing change in functional health with CABG are few however [1–7]. Limitations in this literature include the use of small cohorts [1, 2, 11, 12], lack of preoperative assessments [13, 14], and the use of postoperative estimates of preoperative functional status [1, 3, 15]. Furthermore, the metrics used to measure quality of life have varied. Some studies have used angina and dyspnea [14, 16] whereas others have measured employment status [17, 18], sexual functioning [19, 20], or social functioning [2, 21].

The Coronary Artery Surgery Study reported improved quality of life in CABG patients measured as relief of chest pain and subjective assessment of activity limitation [16]. While anginal symptoms are correlated with overall functional health [2, 22, 23] they may not be sufficient as the sole measure of functional health. In a single-institution prospective cohort study, Stewart and colleagues [4] found that although women underwent CABG at a lower functional health level than men, they experienced comparable improvement in functional health from CABG. As we used change in functional health as the dependent variable in our models, our finding that gender is not predictive of a change in functional health is consistent with these reports. While women undergoing CABG at a lower functional health level may be in part due to women undergoing surgery in a more advanced disease state, it may also be affected by the observation that women in general score lower on functional health surveys than men [8].

In a single-institution prospective cohort study, Lindsey and coworkers [7] showed increases in the mean scores for all eight SF-36 domain scores after CABG. A higher social network score and higher preoperative health status as measured by the SF-36 were associated with improved health status postoperatively. Patients with lower preoperative SF-36 scores, diabetes mellitus, current cigarette smoking status, younger age, higher socioeconomic deprivation category, and higher alcohol intake had lower SF-36 scores after CABG.

The present study has several limitations. First, the generalizability of our findings is limited by a low response rate and a patient population restricted to elective patients. The low SF-36 response rate may be due to the functional health data being collected not as a concentrated effort inside a research protocol but as a part of the routine workup and follow-up of CABG patients. The 49.9% capture rate speaks to the difficulty of collecting this information as part of routine clinical practice, especially without knowledge of how to apply the data to benefit the patient. The low response rate may have led to a selection bias. Although patients who did and did not fill out both the preoperative and postoperative surveys were not remarkably different by the demographic and comorbidity variables collected, they may have differed by unmeasured factors.

While restricting our study to patients undergoing elective CABG limits the generalizability of our results to all patients undergoing CABG, the decision allowed us to focus on the population most likely benefit from the results of this work. Some elective patients undergo CABG to increase long-term survival but most are interested in improved functional health. Urgent and emergent priority patients are more likely to undergo CABG to improve survival and are likely to consider functional status as an important, but secondary concern. In addition changes in the functional health of patients of urgent and emergent status may not be comparable with those changes in elective patients. Patients with more severe disease are likely to be hospitalized and may not be able to give an accurate perception of their baseline functional status.

Second, the use of conservative criteria for determining what was a significantly improved physical or mental component score may have biased our findings toward the null. Since we were unable to find published criteria for significant change in SF-36 PCS and MCS scores over a 6-month period, we used criteria derived for determining significant change in SF-36 PCS and MCS scores over a 12-month period. Had we used 6-month criteria as our standard we might have found additional variables predictive of change in functional health or stronger predictive values of the variables we did identify. We chose not to use a simple improvement in score, as the clinical meaning of such findings would be questionable.

Notwithstanding these drawbacks our study included prospective data collection, preoperative data from a regional data source, and adequate power. Functional health data were prospectively collected from a single institution with the remainder of the variables collected as part of a well-established regional database thereby eliminating biases in the collection of demographic and comorbidity data. The study was adequately powered to detect a difference between patients with and without a change in functional health while controlling for confounding variables. Residual confounding from unmeasured variables was possible. We did not have data on social or socioeconomic factors that have been shown to be predictors of functional health. Such factors may have been confounders as patients with lower levels of social support and at lower economic levels have more severe disease and lower functional health than those at higher levels [13, 24–26].

This study confirms the finding of previous work that CABG can improve functional health. In addition it goes a step further by identifying certain patient characteristics that can be used to predict which patients might experience such an improvement. Although the patient of urgent or emergent surgical priority is primarily interested in timely symptom relief and prolongation of life, elective patients may have different priorities. Many patients of elective surgical priority undergo CABG not to increase life expectancy but to improve symptoms and quality of life. In-hospital mortality and duration of life are not appropriate measures of success for this patient population. Even relief of angina and dyspnea are not complete measures of what these patients deem success. Patients who are free of pain after CABG may be affected by other physical limitations such that there is no change in their overall physical health. A broader functional health measure such as the SF-36 is a better measure of the goal of CABG in the elective patient. The present data if corroborated by additional studies may lead to a body of knowledge that would be useful when counseling prospective patients as to what improvement in quality of life they might expect from elective CABG.

	References

Top Abstract Introduction Patients and methods Results Comment References

 

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