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Diagnosis

Preoperative cardiac risk assessment with thallium scanning helped identify patients at low risk for cardiac complications

ACP J Club. 1992 Sept-Oct;117:52. doi:10.7326/ACPJC-1992-117-2-052

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• Editorial: Evidence about preoperative risk assessment: Why aren't there better studies?


Source Citation

Coley CM, Field TS, Abraham SA, Boucher CA, Eagle KA. Usefulness of dipyridamole-thallium scanning for preoperative evaluation of cardiac risk for nonvascular surgery. Am J Cardiol. 1992 May 15;69:1280-5.


Abstract

Objective

To evaluate whether dipyridamole-thallium scanning (DTS) can identify patients scheduled for nonvascular surgery who are at risk for cardiac complications.

Design

Blinded comparison of preoperative DTS results with clinical measures as predictors of postoperative outcomes.

Setting

Tertiary care teaching hospital.

Patients

109 consecutive patients referred for DTS and scheduled for nonemergency, nonvascular surgery. None were thought able to complete standard treadmill testing. 9 patients were excluded because surgery was cancelled. 61% were men (mean age, 68 y).

Description of test and diagnostic standard

All patients had intravenous planar DTS. Thallium images were graded by blinded observers as normal, fixed deficits only (no thallium redistribution) or ≥ 1 segment with redistribution. Medical charts were abstracted by blinded observers to collect or construct 13 clinical predictors.

Main outcome measures

Ischemic events were unstable angina ( ≥ 2 episodes of ischemic electrocardiographic changes with cardiac signs or symptoms that resolved after treatment), ischemic pulmonary edema (pulmonary congestion with ST-T electrocardiographic changes), myocardial infarction (postoperative electrocardiographic or clinical evidence with a typical enzyme pattern), and cardiac death (sudden postoperative death or death attributable to postoperative myocardial infarction or congestive heart failure).

Main results

9 patients had ≥ 1 postoperative ischemic event (2 cardiac deaths, 2 nonfatal myocardial infarctions, 4 cases of unstable angina pectoris, and 3 of ischemic pulmonary edema). 8 of the 9 patients (89%) with events had thallium redistribution compared with 28 of 91 patients (31%) without events. The presence of redistribution had a sensitivity of 89%, specificity of 69%, positive predictive value of 22%, negative predictive value of 98%, {positive likelihood ratio of 2.9, and negative likelihood ratio of 0.16}*. The multivariate clinical predictors were > 70 years of age and history of congestive heart failure (both P < 0.01). The positive predictive value of DTS increased to 33% among patients with ≥ 1 clinical predictor.

Conclusions

In patients scheduled for nonvascular, nonemergency surgery, preoperative dipyridamole-thallium scanning may be helpful in identifying those at low risk for cardiac events. Clinical predictors may identify a subgroup of these patients in whom the test will be most useful.

Source of funding: Not stated.

Address for article reprint: Dr. C.M. Coley, Medical Practices Evaluation Center, Massachusetts General Hospital, 50 Staniford Street, 9th Floor, Boston, MA 02114, USA.

*Numbers calculated from data in article.


Commentary

Previous work by Coley and colleagues and by others has shown that DTS may have a role in preoperative evaluation of patients scheduled for vascular surgery. The results for patients scheduled for nonvascular surgery in this study are similar. This finding may not be surprising because 80% of the patients were referred for testing because of known or suspected coronary artery disease.

A recent study by Mangano and colleagues (1) questioned the use of DTS as a screening test. In 60 consecutive patients having elective vascular surgery, 7 of the 13 events occurred in patients without redistribution defects. Sensitivity of redistribution was 46% and specificity was 66%. They concluded that routine DTS in patients having vascular surgery may not be warranted.

Why were their results different from those of Coley and colleagues? First, the patient populations may have differed. Mangano and associates studied all patients scheduled for elective surgery whereas Coley and colleagues studied just those patients referred by a consultant because of suspicion of significant coronary artery disease. This referral bias could have increased the apparent test sensitivity. Second, attending physicians were not blinded to the results of DTS in the study by Coley, but they were in the Mangano study. This nonblinding may have increased postoperative surveillance, especially of patients with positive DTS results and thus may have spuriously increased sensitivity (incorporation bias). Finally, in all studies the number of patients and event rates have been small; thus estimates of test performance may be unstable.

How should clinicians proceed? Evidence suggests that in patients with a moderate to high pretest probability of coronary artery disease, DTS may be useful in assessing the risk for an adverse cardiac event. This may not be the case in patients with a low pretest probability. Clinicians should be alert for new data as additional patients are studied.

David J. Malenka, MD
Dartmouth-Hitchcock Medical CenterLebanon, New Hampshire, USA


Reference

1. Mangano DT, London MJ, Tubau JF, et al. Circulation. 1991;84:493-502.