Assessment of reperfusion after thrombolytic therapy for myocardial infarction

CURRICULUM

IN CARDIOLOGY

Assessment of reperfusion after thrombolytic therapy for myocardial infarction Anita Zeiler Arnold,

DO, and Eric J. Topol,

MD Cleveland,

Since 1980 when DeWood et a1.l established that 90% of acute myocardial infarctions are associated with int,racoronary thrombosis, the treatment of myocardial infarction has expanded rapidly, especially in the area of thrombolytic therapy. Numerous randomized, placebo-controlled trials have convincingly demonstrated the benefits of thrombolytic therapy in reducing mortality associated with acute myocardial infarction.2-4 Improvement in left ventricular function associated with thrombolytic therapy is presumably on the basis of a patent infarct-related artery (IRA), but there may be other mechanisms involved such as limiting infarct expansion that improve survival, also related to a patent IRA, independent of salvage of myocardium. Coronary artery patency has been shown to be the most important and independent factor to decrease the occurrence of late potentials after myocardial infarction, late potentials being a marker of spontaneous and inducible ventricular tachycardia as well as sudden death.5 It is therefore crucial to document reperfusion of the IRA as soon as possible. Unfortunately, there is a 25 ‘% to 30 % failure rate for thrombolytic therapy that is associated with a significantly worse prognosis; there is at least two to three times the death rate in those patients compared with patients with a patent IRA. If thrombolysis has failed to restore patency, the patient could be considered a candidate for fallback (secondary) mechanical intervention (coronary angioplasty or surgical revascularization). It is therefore highly desirable to find a means of documenting the success of thrombolytic therapy in a timely fashion. Coronary angiography was established as the “gold standard” for assessing IRA patency in the early studies of intracoronary streptokinase.6, 7 However, the standard of care was changed with the develop-

From

the Department

Received Reprint Cleveland 411/38403

for publication requests: Clinic

of Cardiology, Jan.

14, 1992;

The Cleveland accepted

Clinic March

Foundation. 2, 1992.

Eric J. Topol, MD, The Department of Cardiology, The Foundation, 9500 Euclid Ave., Cleveland, OH 44106.

Ohio

ment of intravenous thrombolytic agents with patency rates equivalent to those of intracoronary streptokinase.8, g Thrombolytic therapy could now be given without the necessity of coronary angiography, and indeed routine angiography may be undesirable in the setting of acute myocardial infarction, as demonstrated by the Thrombolysis In Myocardial Infarction (TIMI) study group.” Since angiography is no longer routinely performed, noninvasive markers have been sought to document reperfusion. There are a number of noninvasive methods that have been proposed to verify the patency of the IRA. The purpose of this review is to discuss the methods available, and the advantages and limitations of each method. BEDSIDE/CLINICAL

DETERMINANTS

Several noninvasive markers can indicate reperfusion after thrombolytic therapy with a reasonable degree of accuracy. As long as 50 years ago, noninvasive markers such as resolution of ST segment shifts on electrocardiography or the development of an accelerated idioventricular rhythm, were observed in experimental animals. l1 These markers are observed in humans as well, and are still reported in the current literature as signs of reperfusion. Saran et a1.12 used the percent reduction in ST segment elevation to predict IRA patency. When the ST segment elevation fell by less than 25 % of the baseline value, persistent coronary occlusion was likely, with a predictive accuracy of 86%. Califf et all3 found that complete resolution of ST-T wave changes was associated with a 96% patency rate on go-minute angiograms after thrombolytic therapy, but that this occurred in only 6 % of patients. Recent reports focus on electrocardiographic signs of reperfusion, either by frequently recorded 12-lead-electrocardiograms (ECGs), or by continuous ST segment shift monitoring and analysis (Figs. 1 and 2). The Thrombolysis and Angioplasty in Myocardial Infarction (TAMI) 7 trial14 recently reported that when compared with other clinical variables, ST segment digital trend recovery has provided the vast majority of information concerning IRA patency. Continuous vectorcar441

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Fig. 1. Continuous ST segmentmonitoring in a patient with successfulreperfusion. Resolution of ST segment elevation in the anterior leads is documented over time.

ABBB

B

Fig. 2. Continuous ST segmentmonitoring in a patient with initially successfulreperfusion (A) and intermittent occlusion of the infarct-related artery (B).

diography using a computerized monitoring system (MIDA 1000, Ortivus Medical, Taby, Sweden) for on-line dynamic analysis of QRS complex and ST segment changes has been used successfully by Dellborg et a1.15 to accurately predict reperfusion. Of 21 patients analyzed who had thrombolytic therapy or angioplasty for acute myocardial infarction, 16 had a patent IRA and five had persistent occlusion at the time of emergency angiography. Using vectorcardiography analysis (Fig. 3), the authors correctly identified 15 of 16 patients with a patent IRA and five of six with persistent occlusion (sensitivity, 94%; specificity, 80 5%). Some authors have found reperfusion arrhythmias to be a marker for IRA patency,16 while other investigators have not found it to be as reliable.17q l8 In 386 patients treated with tissue plasminogen activator (TPA) for acute myocardial infarction in the TAMItrial, ventricular tachycardia, ventricular fibrillation,

second- and third-degree heart block, and severe sinus bradycardia were all documented with similar frequencies following thrombolytic administration. None of the arrhythmias recorded, however, were associated with rates of reperfusion any higher than 77% (range: 64% for second-degree heart block to 77% for ventricular fibrillation).13 It therefore appears that bedside or clinical parameters are severely lacking in accuracy to detect patients who have failed reperfusion, or even to identify patients at high risk for recurrent spontaneous ischemia after thrombolysis.ig LABORATORY

DETERMINANTS

Early peaking of the total creatine kinase (CK) level and the CK-MB isoenzyme have identified patients with successful reperfusion after streptokinase therapy,6, 2o but the delay in laboratory determination and the number of patients not accurately iden-

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0.200 mV. 40.

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Fig. 3. Pharmacologic reperfusion. Print-out directly from the computer screenof a trend analysis of a trend curve showingchangesin the ST vector magnitude and in the QRS vector difference over time. This patient has an acute inferior myocardial infarction. Acute coronary angiography after 25 minutes of vectorcardiographic monitoring showedTIMI grade 3 flow in the right coronary artery. Arrow indicates first injection of contrast material. (Reprinted with permissionof the American Heart Association).

tidied by this method make it a less desirable marker of IRA patency. Recent reports have focused on the use of peak serum myoglobin to detect coronary reperfusion. Dillon et al.“’ analyzed plasma myoglobin (Mb) concentration-time curves and the results of immediate angiography in 20 patients with acute myocardial infarction undergoing attempted reperfusion. They concluded that their mathematical model allowed the prediction of successful reperfusion with an overall predictive accuracy of 95% and was reliably estimated from two samples drawn within 60 minutes of therapy.21 OthersZ2 have reported similar results monitoring peak serum Mb concent,ration as a marker for reperfusion. Although the Mb concentration is a useful marker and can be assayed quickly (10 minutes by latex agglutination turbidmetry) in certain laboratories, there are still patients who will not be diagnosed as thrombolytic failures by these methods.2” NUCLEAR IMAGING Thallium-201 imaging. While thallium-201 imaging has been used as a marker for early reperfusion as well asto quantify myocardial salvage,24it has several important limitations. The most significant is the delay in administering thrombolytic therapy to obtain initial images. Also, the hyperemic phase of reperfusion affects the distribution of the tracer,

which tends to concentrate thallium in the infarct area if given too soon after reperfusion. Delayed redistribution also makes it difficult to distinguish persistently ischemic myocardium from scar tissue,2” which has important implications for treatment. Thallium-201 combined with intracoronary technetium 99m (Tc-99m) pyrophosphate can predict myocardial salvage after intracoronary thrombolysis, but this requires an invasive approach and is not preferred over angiography to determine IRA patency.26 Technetium-99m stannous pyrophosphate imaging. Wheelan et a1.27used early and late Tc-99m stannous pyrophosphate (PPi) as a marker of reperfusion after thrombolytic therapy. They reported that strongly positive early Tc-99m-PPi images were as reliable as early peaking CK-MB fractions in predicting reperfusion. In the 78% of their patients with early peaking CK-MB fractions, the majority (91 ?C.) also had strongly positive scans suggesting reperfusion. None of the control patients had early CK-MB peaks, and none had positive PPi scans. These results were confirmed by Hashimoto et a1.,‘8 who accurately detected 89 7;’ of patent IRAs as confirmed by angiograms performed several weeks after acute myocardial infarction. None of their patients with unsuccessful reperfusion by angiography had positive scans. This degree of accuracy was achieved with

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Table

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I. Characteristics

of noninvasive

markers

of reperfusion

Method

after thrombolytic

Advantages

therapy

1992

Heart Journal

_____ Limitations

Electrocardiography

ST segment resolution Reperfusion arrhythmia Continuous ST segment monitoring

Readily available, inexpensive Readily available, inexpensive Highly accurate

Limited accuracy Low specificity Not readily available, special equipment

Easily obtained Estimates reperfusion in 60 minutes

Time delay Needs further validation

Available in most hospitals

Time delay for initial images Reperfusion hyperemia affects distribution Delayed redistribution affects interpretation Inferior wall imaging limited

Chemical

Creatine kinase-MB Peak serum myoglobin Nuclear

imaging

Thallium-201

Technetium Magnetic

99-isonitrile

resonance

imaging

No time delay for images No redistribution (MRZ)

Tissue characterization

Conventional

Gadolinium Cine-MRI

enhanced

Altered T2 images, long imaging times, metallic prostheses, arrhythmia interference with gating Similar to conventional MRI Similar to conventional MRI

Better tissue characterization High spatial resolution, low acquisition time, superior temporal resolution

Echocardiography

Regional wall motion

Readily available, bedside imaging

Transmitral Doppler Sonicated albumin

Readily available, bedside imaging Perfusion study, enhanced images

From Dellborg

M, Top01 EJ, Swedberg

K. AM HEART

Abnormality may persist several days after reperfusion Accurate in 256 of patients Not widely applied

J 1991;122:943-8

emission computed tomography (CT) imaging, which corrects for adjacent osseous activity to obtain better quality images when compared with conventional planar scanning. Technetium-99m hexakis-2-methoxy-2-isobutyl-isonitrile imaging. A radiopharmaceutical recently

used in reperfusion imaging is technetium-99m-hexakis2-methoxy-2-isobutyl-isonitrile (Tc-99-isonitrile), which is distributed in proportion to myocardial blood flow. Advantages include the virtual lack of redistribution, so it can be given immediately before thrombolytic therapy without the delay of obtaining initial images, as with conventional thallium-201 imaging. Scans done several hours later will still reflect the prethrombolytic anatomy. Also, because it has a higher energy peak (140 keV), its image is superior to that of thallium-201 (except when the inferior left ventricular wall overlaps the liver, which limits interpretation). 2g IRA patency can be indirectly as-

sessed several hours after thrombolytic therapy with the administration of a second dose of isotope. Several studies30-32 have documented the effectiveness of this radiopharmaceutical to assess the extent of jeopardized myocardium and subsequent myocardial salvage after thrombolytic therapy. NUCLEAR

MAGNETIC

RESONANCE

IMAGING

Magnetic resonance imaging (MRI), with its unique ability to characterize tissue, has the potential to delineate normal from ischemic myocardium after reperfusion. The edema associated with myocardial infarction and necrosis, prolongs relaxation times Tl and T2, therefore altering the intensity of the myocardial signal. With the prolonged relaxation times, the T2 weighted images result in a loss of signal with decreased delineation of normal myocardium. This loss results in longer imaging times and lower specificity for conventional MRI.

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Gadolinium-DTPA (diethylene triamine pentaacetic acid) enhances differences between normal and infarcted tissue and makes MRI a more useful technique. Although this enhanced difference with gadolinium was more evident ex vivo, gadolinium-DTPA images may play a role in post-thrombolytic assessment of IRA patency.33 Van Rossum et a1.34studied 19 patients with gated MRI using gadolinium-DTPA 47 to 72 hours after the onset of myocardial infarction. The results were compared with coronary angiograms taken just before the MRI scan. Contrast ratios of the signal intensity of infarcted tissue to normal tissue were constructed. Although the maximum contrast ratios (infarct tissue/normal) were not different between patients with a patent IRA compared with those with persistent occlusion, the contrast ratio as a function of time did identify patent IRAs. The authors admit, however, that the number of patients studied was small and that clinical use was limited by overlap between contrast ratios. In contrast, Van der Wall et a1.35reported that gadolinium-DTPA-enhanced MRI could not accurately identify patients in whom thrombolytic therapy was successful. While this technique may prove useful in the future, MRI imaging is less accurate than other available methods and poses logistic problems that prohibit its widespread application. Cine-MRI has been studied to detect left ventricular functional characteristics after successful reperfusion. The technique has a high spatial resolution, a shorter acquisition t,ime, and superior temporal resolution when compared with spin-echo MRI. As of yet, however, it has not been widely used to predict IRA patency. Several limitations of tine-MRI, such as metallic hardware in the patient that precludes examination, the inaccuracy of imaging with cardiac dysrhythmias, and the length of time required for examination in critically ill patients36 currently make this technique less than optimal. ECHOCARDIOGRAPHY

Echocardiography is useful in evaluating regional wall motion abnormalities after myocardial infarct,ion, and in evaluating myocardial salvage after t.hrombolytic treatment. Broderick et a1.37noted wall motion abnormalities in all patients before the administration of thrombolytic therapy, and found that 80”;. showed improvement in the IRA subserved segments at the time of late study (3 days later). In the patients whose regional wall motion scores did not improve, most required additional intervention with angioplasty or coronary artery bypass surgery.

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Because of the postischemic improvement in left ventricular contractile function that occurs over several days, conventional echocardiography may not be an adequate method to assess the initial success of reperfusion with thrombolytic agents. Touchstone et a1.38 found that 75% of their patients treated with intravenous streptokinase had a patent IRA several hours later, but only 45 % had echocardiographic evidence of improvement in regional wall motion. In the remainder, improvement occurred up to 10 days later. Oh et a1.3g at the Mayo Clinic reported echocardiographic improvement in wall motion abnormalities in 29% of patients who had successful reperfusion, but also in 11% of patients who did not reperfuse after lytic therapy. While in some patients who reperfuse improvement in wall motion can be detected as early as 24 hours after thrombolytic therapy,40 this does not appear to be a sufficiently sensitive marker of reperfusion. Feigenbaum41 has advocated the use of digital recording of the echocardiogram, with a storage system in the intensive care unit that allows for easy retrieval and the creation of a continuous-loop display of several images. These can be compared with baseline studies to observe subtle changes after thrombolytic therapy.41 Myocardial perfusion imaging with the use of sonicated albumin may increase the diagnostic accuracy of establishing reperfusion with echocardiography, but this technique is not widely applied.42, 43 Left ventricular diastolic filling, as measured by transmitral Doppler flows, has also been studied as a method to assess reperfusion. If reperfusion occurs, there should be improvement in left ventricular diastolic filling, detected by echo-Doppler measurements of transmitral flows. Masuyama et a1.,44 however, were able to show improvement in left ventricular diastolic filling via pulsed Doppler echocardiography in only 25% of patients in whom the IRA was patent. Ultrasonic tissue characterization with conventional two-dimensional echocardiography can define ventricular wall motion abnormalities. The hypothesis is that pathologic changes of the myocardium, such as those produced by acute myocardial infarction, alter the physical properties of the tissue. These alterations can then be quantified by frequency-dependent ultrasonic attenuation and backscatter. Milunski et a1.45 used this technique to determine if ultrasonic tissue characterization could identify acute myocardial infarction, and if the early effects of reperfusion were reflected accordingly. They found that patients with documented IRA patency had a characteristic increase in the cyclic variation in sig-

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nals over time, whereas there was no such recovery in patients with an occluded IRA. While this technique requires some modification of the conventional echocardiographic equipment, it may be a useful modality in detecting reperfusion. CONCLUSIONS

Thrombolytic therapy conclusively decreases the mortality associated with acute myocardial infarction. Still, between 25 ‘% and 30 6%of patients are not successfully reperfused. Interventions that can restore IRA patency can be implemented if patients can be identified who did not reperfuse with thrombolytic treatment. Several noninvasive techniques can assist in this determination (Table I), but as of now the “gold standard” remains coronary angiography. We need to continue to evaluate and refine noninvasive modalities to identify patients who are candidates for further intervention. SUMMARY

The benefits of thrombolytic therapy in reducing the mortality associated with acute myocardial infarction are well documented. Presumably, this is on the basis of a patent IRA, although other mechanisms may be involved. Because there is a 25 % to 30 ‘% failure rate for thrombolytic therapy that is associated with a significantly worse prognosis, it is crucial to document reperfusion in a timely fashion. In cases of failure to reperfuse, the patient could be considered a candidate for secondary mechanical intervention. While coronary arteriography is presently the “gold standard” to document reperfusion, this is an invasive procedure associated with small but defined risks for the patient. A noninvasive marker that is readily available and highly accurate is most desirable. There are a number of methods currently used to document coronary reperfusion noninvasively. This review discusses the advantages and disadvantages of each method, and the need for continued evaluation and refinement in noninvasive modalities to identify patients who are candidates for further intervention.

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