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Prediction of Cardiovascular Death in Men Undergoing Noninvasive Evaluation for Coronary Artery Disease
13 auth. Kiernan Morrow, C. Morris, V. Froelicher, Alisa Hideg, Dodie Hunter, E. Johnson, T. Kawaguchi, K. Lehmann, P. Ribisl, R. Thomas, ...
Clinical evaluation, exercise testing, and coronary angiography are used routinely by physicians to decide whether interventions are needed in patients with coronary artery disease [1, 2]. Various conflicting clinical prediction rules have been prop…
Clinical evaluation, exercise testing, and coronary angiography are used routinely by physicians to decide whether interventions are needed in patients with coronary artery disease [1, 2]. Various conflicting clinical prediction rules have been proposed [3]. In a first report, we described our method of outcome assessment in patients who had undergone exercise testing and coronary angiography within a 3-month period and compared our prediction rules with those from other samples [4]. Our two main findings were that the results of coronary angiography and exercise-induced ST depression were not independently associated with cardiovascular death or infarct-free survival. The purpose of this investigation was to predict cardiovascular death using variables available from a standard noninvasive work-up of patients with known or suspected coronary artery disease. The use of this larger cohort, uninfluenced by selection for cardiac catheterization, allowed assessment of work-up bias. Methods Patients Patients were selected from a consecutive series of 3609 persons who underwent routine clinical exercise testing between 1984 and 1990; 30% of this group had coronary angiography within 3 months of testing and were excluded from the analysis. Also excluded were women (who constituted less than 2% of the sample), patients with significant valvular disease, and those who had previous coronary artery bypass surgery. Most of the remaining 2456 (84%) patients had been referred for testing because of chest pain or for the evaluation of exercise capacity. Clinical Definitions Myocardial infarction was defined by the presence of two or more of the following factors: 1) serial electrocardiographic changes; 2) typical chest pain; and 3) myocardial enzyme increase. Congestive heart failure was defined by typical symptoms and signs, plus echocardiographic or radiographic confirmation of cardiomegaly and pulmonary edema. Before treadmill testing, angina pectoris was classified as typical if the patient described substernal pressure, tightness, or pain that was brought on by exertion or emotions, lasted several minutes, and was relieved by nitroglycerin or rest. Angina was considered atypical in the absence of one or more of these features if the pain was thought to be cardiac in origin. Exercise Testing The exercise test was done using a standard progressive treadmill protocol [5]. Except for patients undergoing testing before discharge after myocardial infarction, each test was sign or symptom limited using standard recommended criteria for termination [2]; fatigue or chest pain was the reason for termination in most patients. In addition to the maximal systolic blood pressure achieved, the blood pressure response during exercise was coded as a score reflecting exercise-associated changes in systolic blood pressure (0 points = increase > 40 mm Hg; 1 point = 31 to 40 mm Hg; 2 points = 21 to 30 mm Hg; 3 points = 11 to 20 mm Hg; 4 points = 0 to 11 mm Hg; and 5 points = decrease below standing systolic blood pressure taken before testing) [6]. The treadmill was stopped abruptly at the completion of exercise, and the patient was placed in the supine position within 1 minute [7]. Exercise capacity was estimated in multiples of resting oxygen consumption (METs) and was also analyzed as a percentage of normal for age according to an equation derived from a normal subset of our referral group [8]. Electrocardiographic Measurements Left ventricular hypertrophy was coded according to Romhilt and Estes criteria [9]. Patients lacking left ventricular hypertrophy with more than 0.5 mm ST depression in any lead were coded as having resting ST depression. The exercise electrocardiogram was interpreted as previously described [7]. Measurement of Outcome Variable Since 1984, the Department of Veterans Affairs Health Care System has developed a series of programs to support Veterans Affairs Medical Center clinical functions as part of the Decentralized Hospital Computer Project (DHCP). Death certificates are routinely completed by Veterans Affairs Medical Center physicians for inpatient and outpatient deaths. Information on care received elsewhere is routinely requested for clinical purposes, and all patients were scheduled for routine appointments at 6-month intervals after testing. Data on hospitalizations and deaths are entered, and retrieval programs are available to obtain dates and information regarding the most recent clinical visit and prescription received as well as those regarding hospitalization or death. To avoid bias, the coding of death certificates and other outcome variables was blinded to the predictor (exposure) variables. Although not designed for research purposes, this administrative and clinical database helped us obtain complete follow-up information. Data Analysis All data were entered into R:Base (Microrim, Redmond, Washington) and were analyzed using R:Base, Statgraphics (Statistical Graphics Corporation, Rockville, Maryland), True Epistat (Epistat Services, Richardson, Texas), Confidence Interval Analysis (American College of Physicians, Philadelphia, Pennsylvania), and EGRET (SERC, Seattle, Washington) on a standard 80386-SX-based personal computer (Vectra RS/20C, Hewlett Packard, Palo Alto, California). Survival time in person-days was measured from the time of the exercise test and was censored at the time of noncardiac death, coronary artery bypass surgery, or percutaneous transluminal coronary angioplasty. Survival Analysis Analysis was done to predict cardiovascular deaths and infarct-free survival (that is, cardiovascular death and nonfatal myocardial infarction). Kaplan-Meier survival curves were evaluated stratifying one or more variables to explore the data for interactions. The Cox proportional-hazard model was then applied to clinical and resting electrocardiographic variables, hemodynamic variables from treadmill testing, and electrocardiographic changes and angina during the treadmill test. Each variable grouping was also analyzed independently and by combining the strongest or most logical variables. Analysis was also done on the total group, including those who underwent catheterization (588 patients) because they were seen before the decision to catheterize. Results Follow-up Computed clinical information was available for all 2546 patients, and follow-up was initiated in February 1991. Of these, 85% were confirmed to be alive by a clinic visit or prescription filled at a minimum of 1 year after their treadmill date, and 187 (7.5%) had died after a mean follow-up period of 45 17 months. Contact either by telephone or letter led to follow-up and verification of vital status in 99%. After review of autopsy, death certificate, or hospital charts, 119 of the deaths (63%) were classified as cardiovascular. Forty-four patients had nonfatal myocardial infarctions, 34 developed congestive heart failure, 46 underwent coronary bypass surgery, and 18 received one or more angioplasties. The average annual cardiac mortality rate was 1.5%. Clinical Characteristics Table 1 shows the clinical characteristics of the study cohort grouped by end point. The mean age (SD) was 59 10 years. One fifth of the patients had typical angina pectoris, and one fifth had a history of previous myocardial infarction or electrocardiograms with diagnostic Q waves. Medications were not changed or withheld before exercise testing; 22% were taking -blockers, and 8% were taking digoxin. Statistically significant differences between the no cardiovascular event and cardiovascular death groups were observed for age, congestive heart failure, myocardial infarction, digoxin use, and most resting electrocardiographic abnormalities (P < 0.01). Table 1. Clinical Features of the Total Study Population and Number and Percentage with a Given End Point Hemodynamic and Electrocardiographic Responses Group averages for pre-exercise standing heart rate, systolic blood pressure, and double product were 76 beats per minute, 130 mm Hg, and 9800 (heart rate times systolic blood pressure), respectively. Table 2 shows the hemodynamic and electrocardiographic responses during the exercise test. No significant differences were found among end point groups for perceived exertion and occurrence of premature ventricular contractions. Table 2. Hemodynamic and Exercise Electrocardiographic Features of the Total Study Population* Cox Proportional Hazards Model The univariate scores and P values for the variables are listed in Appendix Table. No significant interactions were discovered, and thus none are included. Similar results were obtained both when infarct-free survival was considered as an end point (variable order, coefficients, and level of significance) and when the entire cohort was analyzed. The score test statistic listed is the relative weight or importance assigned the variables in the Cox model. Using stepwise selection, the Cox model was allowed to build on each variable group (clinical variables alone entered first with subsequent addition of other variables) to arrive at the final model that chose history of congestive heart failure or digoxin use, the change in systolic blood pressure score, exercise capacity (METs), and exercise-induced ST depression. A score was then formed using the coefficients from the Cox model with only these four variables entered as follows: 5 x (congestive heart failure or digoxin use [yes = 1; no = 0]) + exercise-induced ST depression in millimeters + change in systolic blood pressure score METs. Three groups were formed using a scoring system in which 2 indicated low risk, 2 to 2 indicated moderate risk, and greater than 2 indicated high risk. The hazard ratios, confidence intervals (CIs), and P values for these groups are shown in Table 3, and the Kaplan-Meier survival curves are shown in Figure 1. This score enabled identification of a low-risk group (77% of the cohort) with an annual cardiovascular mortality rate of less th
Published in
Annals of Internal Medicine
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8
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7 | 1993 |
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