Ischemic Heart Disease: Monitoring

MONITORING

Intraarterial pressure monitoring is reasonable for all patients with severe CAD and major or multiple cardiac risk factors who are undergoing any but the most minor procedures. Central venous (or rarely pulmonary artery) pressure can be monitored during prolonged or complicated procedures involv-ing large fluid shifts or blood loss. Less invasive methods of cardiac output determination and vol-ume assessment have been previously discussed in this text. Transesophageal echocardiography (TEE) and transthoracic echocardiography (TTE) can provide valuable information, both qualitative and quantitative, on contractility and ventricular cham-ber size (preload) perioperatively. Intensive care unit staff increasingly use ultrasound to assist in hemo-dynamic management. Numerous “basic” courses in TEE and TTE are available to assist practitioners in performing “hemodynamic,” as opposed to cardiac diagnostic TEE.Intraoperative detection of ischemia depends on recognition of electrocardiographic changes, hemodynamic manifestations, or regional wall motion abnormalities on TEE. Doppler TEE also allows detection of the onset of mitral regurgitation caused by ischemic papillary muscle dysfunction.

A. Electrocardiography

Early ischemic changes are subtle and can often be overlooked. They involve changes in T-wave morphology, including inversion, tenting, or both (Figure 21–1). More obvious ischemia may be seen in the form of progressive ST-segment depression. Down-sloping and horizontal ST depressions are of greater specificity for ischemia than is up-sloping depression. New ST-segment elevations are rare dur-ing noncardiac surgery and are indicative of severe ischemia, vasospasm, or infarction. However, the increasing number of individuals treated with drug-eluting stents can be problematic perioperatively, especially if surgical concerns necessitate discontinu-ation of antiplatelet therapy (eg, emergency spine surgery). Such patients are at very increased risk of thrombosis and perioperative MI. Anesthesia staff should never for nonsurgical reasons (eg, desire to perform a spinal anesthetic) discontinue antiplatelet or anti thrombotic agents perioperatively without first discussing the risks and benefits of the pro-posed anesthetic requiring suspension of antiplatelet therapy with the patient and his or her cardiologist. ACC/AHA offers recommendations on the approach of bringing patients to surgery following percutane-ous coronary interventions and the type of interven-tions suggested when subsequent surgery is expected (Figures 21–2 and 21–3). It should be noted that an isolated minor ST elevation in the mid-precordial leads (V₃ and V₄) can be a normal variant in young patients. Ischemia may also present as an unexplained intraoperative atrial or ventricular arrhythmia or the onset of a new conduction abnormality. The sensitiv-ity of the ECG in detecting ischemia is related to the number of leads monitored. Studies suggest that the V₅, V₄, II, V₂, and V₃ leads (in decreasing sensitivity) are most useful. Ideally, at least two leads should be monitored simultaneously. Usually, lead II is moni-tored for inferior wall ischemia and arrhythmias, and V₅ is monitored for anterior wall ischemia. When only one channel can be monitored, a modified V₅ lead provides the highest sensitivity.

B. Hemodynamic Monitoring

The most common hemodynamic abnormalities observed during ischemic episodes are hyperten-sion and tachycardia. They are almost always a cause (rather than the result) of ischemia. Hypotension is a late and ominous manifestation of progressive ventricular dysfunction. TEE readily will demon-strate a dysfunctional ventricle and ventricular wall motion changes associated with myocardial ischemia. Ischemia is frequently, but not always, associated with an abrupt increase in pulmonary capillary wedge pres-sure. The sudden appearance of a prominent v wave on the wedge waveform is usually indicative of acute mitral regurgitation from ischemic papillary muscle dysfunction or acute left ventricular dilatation.

C. Transesophageal Echocardiography

TEE can be helpful in detecting global and regional cardiac dysfunction, as well as valvular function in selected patients. Moreover, detection of new regional wall motion abnormalities is a rapid and more sensitive indicator of myocardial ischemia than the ECG. In animal studies in which coro-nary blood flow is gradually reduced, regional wall motion abnormalities develop before the ECG changes. Although the occurrence of new intraop-erative abnormalities correlates with postoperative MIs in some studies, not all such abnormalities are necessarily ischemic. Both regional and global abnormalities can be caused by changes in heart rate, altered conduction, preload, afterload, or drug-induced changes in contractility. Decreased systolic wall thickening may be a more reliable index for ischemia than endocardial wall motion alone.