Echocardiography for the Assessment of Myocardial Viability
John C. Lualdi, MD, and Pamela S. Douglas, MD, FACC, Boston, Massachusetts
The identification of viable myocardium in the setting o f acute myocardial infarction or chronic coronary artery disease with reduced left ventricular function has important prognostic and therapeutic implications. Many noninvasive methods have been used to assess viability, and recently, dobutamine stress echocardiog- raphy has been studied for this purpose. Dobutamine stress echocardiography is a safe, accessible, and rela- tively inexpensive technique. Moreover, its accuracy for detecting viability approaches that o f positron emission
tomography and thallium scintigraphy. In addition to dobutamine stress echocardiography, other echocar- diographic techniques, such as myocardial contrast echocardiography and dipyridamole stress echocardi- ography, are being developed to delineate viability. In the future, echocardiographic methods may identify viability with enough accuracy to allow us to better select patients for revascularization procedures when the indications are otherwise unclear. (J Am Soc Echo- cardiogr 1997;10:772-81.)
Left ventricular systolic function is among the most powerful predictors of morbidity and mortality in patients with acute myocardial infarction or chronic coronary artery disease. Regional wall motion abnor- malities contribute prominently to left ventricular dysfunction and, for many years, were regarded as irreversible, representing previously infarcted, scarred tissue. More recently, it has been recognized that regional left ventricular dysfunction may be reversible and exists in territories of viable myocardium.
Coronary revascularization can improve left ven- tricular function and favorably alter functional ca- pacity and prognosis in patients with coronary artery disease. >9 Thus, a me thod of identifying those patients whose ventricular function is likely to improve after acute ischemic injury or revascu- larization versus those with irreversible myocardial damage would be of clinical value and likely lead to more appropriate and cost-effective use ofrevascu- larization procedures.
MYOCARDIAL VIABILITY: OVERVIEW
Currently, most decisions regarding revascularization are based on clinical status including evidence of
ischemia, coronary anatomy, and left ventricular function, without consideration of viability. The fail- ure to use viability data has been due, in part, to the lack of a readily available, accurate method for its acquisition. The diagnosis of viability after an acute ischemic insult (stunned myocardium TM) has been based on clinical signs of an "uncompleted" myocar- dial infarction such as a small creatine phosphokinase leak or non-Q-wave electrocardiographic changes. With chronic coronary artery disease (hibernating myocardiumn), both a history of angina and pre- served wall thickness within a hypocontractile region have been clues to viability. Such criteria have not been optimal, however, and studies have shown an approximately 40% rate of discordance between positron emission tomography evidence of viability and independent clinical decisions regarding revascu- larization. 6a2,13 Also striking has been the poor out- come (48% average event rate over 12- to 36-month follow-up periods) noted in paticnts with viable myo- cardium when these regions were not revascularized (Table 1). 6"8,12 This high rate of cardiac events is particularly concerning given that the average event rates were considerably lower (11% to 16%) in pa- tients with viable segments that were revascularized, as well as in patients without viability regardless of revascularization (Table 1).
Although prospective, randomized studies are needed to confirm these observations, a retrospec- tive analysis has shown that viability by F-18 de- oxyglucose uptake can influence clinical decision making and alter therapy for chronic ischemic heart disease. 14 Hal f o f patients referred for t reatment o f chronic heart failure and over one third o f those
Journal of the American Society of Echocardiography Volume 10 Number 7 Lualdi and Doug las 7 7 a
Table 1 Influence of viability by positron emission tomography and coronary revascularization on cardiac event rates in patients with coronary artery disease and left ventricular systolic dysfunction
Study n F/U (too) PET+R+ PET+R- PET-R+ PET-R--
Eitzman et al. 6 82 12 12% 50% 7% 13% Yoshidah and Gould i;* 35 36 10% 0% 50% 50% Dicarli et al. 7. 93 13 12% 41% 6% 9% Lee et al. s 129 17 16% 62% 11% 18%
Total 339 13% 48% 11% 16%
F/U~ Follow-up period; too, months; n~ number of patients in'study; PET+, viability by positron emission tomography; PET-, no viability by positron emission tomography; R+, revascularization; R- , no revascularization. Adapted from Lee et al. 8 *Cardiac deaths only.
Table 2 Noninvasive methods of assessing myocardial viability
referred for transplantation undcrwent revascular- ization on the basis of the demonstration of viabil- ity by F-18 deoxyglucose uptake. Thus, the identi- fication of viable myocardium is likely to significantly affect the management of congestive heart failure in the setting of chronic coronary artery disease.~S
Multiple noninvasive imaging techniques have been studied for the assessment of myocardial via- bility (Table 2). Although F-18 deoxyglucose positron emission tomography is regarded as a gold-standard, 16 it may not be the most practical method because of high cost, inconsistent reim- bursement, and limited accessibility. Thallium-201 scintigraphy with the use of multiple different im- aging protocols has also been used to determine viability. ~7 Recently, echocardiographic techniques including dobutamine stress echocardiography have been proposed for the identification of viabil- ity. Dobutamine stress echocardiography appears to have a similar overall accuracy to F- 18 deoxyglu- cose positron emission tomography and thallium scanning. 18 Importantly, this method is also inex- pcnsive, accessible, and safe, and its use for detect- ing viability is increasing rapidly. Additional echo- cardiographic techniques under investigation for the assessment of viability include myocardial con- trast echocardiography, dipyridamole stress echo- cardiography, and ultrasonic backscatter.
DOBUTAMINE STRESS ECHOCARDIOGRAPHY
Dobutamine stress echocardiography involves compar- ison of regional left ventricular wall motion during basal conditions and with incremental doses of intravenous dobutamine. The most common protocol for perform- ing dobutamine stress echocardiography is to start with 5 ixg/kg/min and to subsequently increase the dose to 10, 20, 30, and ultimately 40 txg/kg/min as tolerated in 3-minute stages. Other protocols have also been studied with low- and intermediate-dose stages at 4 and 12 txg/kg/min 19 or the addition of 2.5 and 7.5 Ixg/ kg/min stages to the standard protocol, z°,21 Although one study found that 7.5 Ixg/kg/min was the low dose at which viability was most often identified, zl all studies regardless of protocol have produced similar results for the detection ofviabifity.
The response of regional left ventricular function to the beta adrenergic receptor agonist dobutamine is useful to characterize myocardium (Figure 1). Nor- mal resting wall motion and the development of hyperdynamic function with increasing doses of do- butamine are hallmarks of normally perfused myocar- dium. The development of new wall motion abnor- malities or the worsening of baseline systolic dysfunction with escalating doses of dobutamine in- dicate myocardial ischemia. Contractile reserve, on the other hand, is consistent with viability and char-
Journal of the American Societ?, of Ecl~ocardiography 774 Lualdi and Douglas September 1997
Normal
Ischemic
Viable
Viable/Ischemic
(Biphasic)
Scarred
F i g u r e 1
D O B U T A M I N E D O S E
Baseline Low-dose Peak-dose
_ . h . _ __A_
Responses of myocardial segments to escalating doses of dobutaminc.
acterized by baseline regional wall motion abnormal- ities that improve with low-dose dobutamine. When such low-dose augmentation of function is followed by progressive systolic dysfunction with higher doses (a biphasic response = with viability and superim- posed ischemia), the accuracy of predicting future changes in regional systolic function after acute myo- cardial infarction 23 or with revascularization in chronic coronary artery disease 21 may be even greater. Left ventricular segments that remain aki- netic or dyskinetic despite dobutamine infusion are nonviable and likely reflect scar.
The ability to detect resting wall motion abnormal- ities, in addition to viability and inducible ischemia with the use of a single diagnostic test are important advantages of dobutamine stress echocardiography when compared with resting thallium scintigraphy or positron emission tomography. Other important ad- vantages of dobutamine stress echocardiography are its relatively wide availability, low cost, and safety. Current reimbursement by Medicare in 1996 for the combined technical and professional components of dobutamine stress echocardiography is about $311.00, compared with $688.00 for exercise myocardial per- fusion scintigraphy with single photon emission com- puted tomography. At the present time, positron emission tomography is not reimbursed by Medicare. In two reviews of over 4000 total dobutamine-atro- pine stress echocardiographic studies, there were no deaths related to the test. 24,25 Moreover, in the larger of these two studies, the rate of life-threatening events or events requiring treatment or admission to the hospital was less than 0.5%. 24 A significant limi-
ration of the test, however, is inadequate acoustic windows which are seen in approximately 10% of patients.
DOBUTAMINE STRESS ECHOCARDIOGRAPHY FOR THE ASSESSMENT OF STUNNED MYOCARDIUM
Acute myocardial infarction treated with reperfusion therapy by thrombolysis or percutaneous translumi- nal coronary angioplasty is a clinical setting associated with myocardial stunning. Because prognosis after myocardial infarction is related to left ventricular function, the distinction between stunned and ne- crotic myocardium is clinically important and may affect risk stratification, as well as subsequent thera- peutic interventions. Multiple studies have evaluated the use of dobutamine stress echocardiography for the identification of viable myocardium after reperfu- sion therapy for acute myocardial infarction (Table 3) and found overall predictive accuracies of 71% to 88% when comparing evidence of contractile reserve with subsequent improvement of resting wall mo- tion 19"23"26-28 or with evidence of viability by positron emission tomography scanning. 29 Similar results are reported for patients treated with t h r o m b o l y s i s ]9-23-29
and primary angioplasty. 26 Pierard et al. 29 evaluated patients with acute ante-
rior myocardial infarction treated with thrombolysis. Mt patients with viable myocardium and normal per- fusion by positron emission tomography (indicative ofnonischemic segments with dysfunction caused by
Journal of the American Society of Echocardiograph'y Volume 10 Number 7 Lualdi and Doug las 7 7 5
Table 3 Utility of dobutamine stress echocardiography in predicting segmental improvement in regional left ventricutar systolic function after acute myocardial infarction (stunned myocardium)
Study n F/U Sens % Spec (%) PPV (%) NPV (%) OPV (%)
Pierard ct a1.29"~ 17 9 mo 80 93 94 76 85 Smart et al) 9. 51 2 mo 86 90 86 90 88
Previtali et al. 23. 59 2 mo 79 68 89 50 7I Watada et al. 26 21 25 days 83 86 89 80 84 Salustri et aL 27 57 3 mo 66 94 79 88 88
Elhendy et ai. a8 30 3 mo 77 84 69 89 82 Elhendy et al. 28. 30 3 mo 79 75 73 80 77
NPV, negative predictive value; OPV, overall predictive value, PPV, positive predictive value; Sens, sensitMty; Spe 6 specificity. Other abbreviations as in Table 1. *Data analyzed by patient (as opposed to by myocardial segment). ~'Data iu comparison with viability by positron emission tomography.
Table 4 Utility of dobutamine stress echocardiography in predicting segmental improvement in left ventricular systolic function after revascularization in chronic coronary artery disease (hibernating myocardium)
Study n F/U Sens (%) Spec (%) PPV (%) NPV (%) OPV (%)
Marzullo et al. 31 14 1,1 wk 82 92
Cigarro et al. a2* 25 >4 wk 82 86 82 86 84 La Canna et al. sa 33 3 mo 92 75 83 87 84 Charney et al. 34 17 7 days 78 86 78 86 83 Afridi et al. 2~ 20 ->6 wk 74 73 59 86 75 Afridi et al. 21. 20 ->6 wk 90 60 69 86 75 Perrone-Filardi et al. as 18 34 days 88 87 91 82 87 Arncsc et al. "% 38 3 mo 74 95 85 93 91 Vanoverschelde et ai. a7* 40 5.3 rno 84 76 76 84 80 Vanoverschelde et alY* 73 5.5 mo 88 77 84 82 84 Vanoverschelde et a lY 73 5.5 mo 751-, 84:~ 86j , 68:~ 82]', 74:~
Abbreviations as in Tableg 1 and 3. *Data analyzed by patient (as opposed to by myocardial segment). tData for akinetic segments. ~:Data for hypokinetic segments.
stunning) also had contractile reserve on dobutamine stress echocardiography and showed significant func- tional recovery at 9-month follow-up. Smart et al. ~9 demonstrated that contractile reserve by low-dose (4 b~g/kg/min) dobutamine stress echocardiography was an independent predictor of functional recovery after thrombolysis for myocardial infarction and su- perior to clinical criteria such as non-Q-wave infarc- tion and peak creatine phosphokinase <1000 I U / ml. Other clinical criteria, such as patency of the infarct-related artery and revascularization were not significant predictors of recovery.
Viability- within the infarct zone is a common oc- currence in these studies. In a study by Previtali et al., 23 73% of the patients had evidence of viability which was highly predictive of spontaneous func- tional recovery at follow-up. 23 It is noteworthy that patients with superimposed ischemia showed less spontaneous regional recovery on follow-up, and thus viability alone appeared to identify patients with jeopardized myocardium, which may improve in function with early revascularizarion. Consequently,
the assessment of viability and ischemia by dobut- amine stress echocardiography after acute myocardial infarction should assist decision malting concerning revascularization. Other investigators have noted that spontaneous recovery is more frequent in baseline hypokinetic than akinetic segments and that the sen- sitivity of contractile reserve for predicting functional recovery is greater in hypokinetic than aldnetic seg- ments. 27
Although no studies to date have addressed whether the identification and revascularization of hypocontractile, viable myocardium improves patient outcome, Barilla et al. a° reported that patients with viable myocardium treated medically had less recov- ery of systolic function than those who were revascu- larized. This finding suggests that revascularization of viable myocardium identified by dobutamine stress echocardiography post-infarction may lead to im- proved overall left ventricular function.
In summary, low-dose dobutamine stress echocar- diography performed in patients with acute myocar- dial infarction treated with reperfusion therapy accu-
Journal of the American Society of Echocardiography 776 Lualdi and Douglas September 1997
Table 5 Dobutamine stress echocardiography versus myocardial contrast echocardiography for predicting improvement in regional left ventricular systolic function in the setting of acute coronary artery disease after myocardial infarction and chronic coronary artery disease after revascularization
MCE, Myocardial contrast echocardiography; DSE, dobutamine stress echocardiography. Other abbreviations as in Table 3.
rately detects reversibly dysfunctional or stunned myocardium. Further investigation is needed to bet- ter define the utility of the identification of stunned myocardium for prognostication and its impact on the use of additional diagnostic studies and revascu- larization procedures.
DOBUTAMINE STRESS ECHOCARDIOGRAPHY FOR THE ASSESSMENT OF HIBERNATING MYOCARDIUM
Dobutamine stress echocardiography has also been used to evaluate chronically dysfunctional, hibernat- ing myocardium and can predict recovery of function after revascularization with overall accuracies of 74% to 91% (Table 4). 21,31-38 Mthough studies suggest that the accuracy of the test is not significantly af- fected by the degree of overall left ventricular dys- function, recent data indicate that dobutamine stress echocardiography may be more accurate for aldnetic than hypokinetic segments. 37 Viability by dobut- amine stress echocardiography is common in patients with moderate-to-severe systolic dysfunction and, in one study, was present in about 30% of such cases. 39
A biphasic response to dobutamine not only diag- noses ischemia 22 but may be more accurate for pre- dicting improvement of regional wall motion abnor- malities with revascularization. 21 Of four observed responses to dobutamine infusion (biphasic, sus- tained improvement, worsening, and no change), the biphasic response had the highest positive predictive values for both segmental recovery and recovery of function in more than two contiguous segments. 21 Other investigators have shown that dobutamine stress echocardiography accurately predicts changes in systolic function after revascularization of left ven- tricular segments with resting wall motion abnormal- ities and hypoperfusion by thallium scintigraphy2 s Moreover, contractile reserve by dobutamine stress echocardiography has been found to be similarly useful for delineating viable myocardium in the set-
tang of both critically stenotic and chronically oc- cluded vessels associated with wall motion abnormal- ities .40
Like viability by positron emission tomography scanning, 6-8,12,41 viability by dobutamine stress echo- cardiography has prognostic value and implications for patient selection for revascularization which may, in turn, affect outcome. 39 In patients with viable or ischemic myocardium treated medically, the event rate for death, myocardial infarction, and late myo- cardial revascularization as a result of unstable angina was significantly higher (43%) than that in patients with scarred, nonviable myocardium (8%). 39 More- over, the presence of viability predicted events inde- pendently of left ventricular ejection fraction and a g e . 39
In summary, dobutamine stress echocardiography is an accurate means of identifying hibernating myo- cardium in patients with coronary artery disease and reduced ejection fraction. Moreover, identification of hibernating myocardium by this method appears to have important prognostic implications.
DOBUTAMINE STRESS ECHOCARDIOGRAPHY VERSUS POSITRON EMISSION TOMOGRAPHY AND THALLIUM SCINTIGRAPHY
In the literature there are few studies directly com- paring dobutamine stress echocardiography with positron emission tomography for the detection of viable myocardium. One study showed a 79% concor- dance rate between the two techniques for differen- tiating viable from necrotic myocardium after acute anterior myocardial infarction. 29 In patients with chronic ischemia, positron emission tomography and dobutamine stress transesophageal echocardiography were compared and had 86% to 90% agreement with respect to the presence of viable myocardium. 42,43 Although the sensitivities for dobutamine stress transesophageal echocardiography and positron emission tomography in predicting left ventricular
Journal of the American Society of Echocardiograph'y Volume 10 Number 7 Lualdi and Douglas 7 7 7
recovery were both high (92% and 96%, respectively), the specificity ofdobutamine stress echocardiography (88%) was greater than that of positron emission tomography (69%). 4a The use of a transesophageal approach to imaging may improve the accuracy of dobutamine stress echocardiography over that of the transthoracic method because of better endocardial definition. Baer et al.44 have also shown that dobut- amine stress transesophageal echocardiography and dobutamine magnetic resonance imaging have com- parable concordances with positron emission tomog- raphy, as well as similar sensitivities and specificities for detecting viability. 44
Dobutamine stress echocardiography has been more extensively compared with thallium scintigra- phy for the identification of viable myocardium, and, although the two techniques appear comparable, do- butamine stress echocardiography is often reported to be more specific than thallium scintigraphy. EI- hendy et al. 28 compared dobutamine stress echocar- diography with rest-redistribution thallium scintigra- phy performed within 7 + 3 days of acute myocardial infarction treated with thrombolysis or no specific reperfusion therapy. 28 Both methods had similar sen- sitivities, but dobutamine stress echocardiography was significantly more specific than thallium scintig- raphy at predicting recovery of regional function after the acute ischemic event.
In patients with resting ventricular dysfunction from chronic coronary artery disease, Marzullo et al.a~ showed comparable sensitivity and specificity of rest-redistribution thallium scans and dobutamine stress echocardiography. A second study also per- formed on patients with chronic coronary artery dis- ease found the sensitivity of dobutamine stress echo- cardiography (78% at 5 txg/kg/min and 71% at 10 ixg/kg/min) to be less than that of thallium (95%), whereas its specificity (86% at 5 Ixg/kg/min and 93% at 10 jxg/kg/min) was comparable with that of thal- lium (85%)? 4 Moreover, the optimal positive (92% at l0 Ixg/kg/min) and negative (86% at 5 Ixg/kg/min) predictive values of dobutamine stress echocardiog-
0 0 raphy were similar to those of thallium (90 ~ and 92 ~, respectively). 34
Further studies, however, suggest that dobutamine stress echocardiography may be superior to exercise- redistribution-reinjection thallium for identifying vi- ability. In one such study, although univariate analy- sis showed that contractile reserve by dobutamine stress echocardiography and thallium defect scores were both predictive of recovery in left ventricular function after revascularization, only improvement in wall motion with dobutamine and left ventricular end-diastolic volume were independent predictors of
functional recovery? 7 One possible explanation for this finding is that a higher level of myocyte func- tional integrity is necessary for the demonstration of contractile reserve by dobutamine stress echocardi- ography than for the membrane function necessary for thallium uptal<e, 4s malting dobutamine stress echocardiography more specific and thallium scintig- raphy more sensitive. This hypothesis is supported by not only the data of Elhendy et alY presented previ- ously, but also by Panza et al. 2° who noted that only 56% of segments with resting systolic dysfunction in patients with chronic coronary artery disease had contractile reserve with dobutamine administration, whereas 84% were considered viable by thallium? °
Two additional studies appear to confirm these findings and their importance. Amese et al. 36 noted a higher specificity and positive predictive value for dobutamine stress echocardiography (specificity 95%, positive predictive value 85%) compared with thal- lium (specificity 48%, positive predictive value 33%) in predicting left ventricular improvement after revas- cularization. The sensitivity of thallium scintigraphy (89%), however, was greater than that of dobutamine stress echocardiography (74%). Vanoverschelde et al. as found that, although exercise-redistribution-re- injection thallium scintigraphy was comparable with dobutamine stress echocardiography for predicting improvement in overall left ventricular function, do- butamine stress echocardiography had significantly greater specificity than thallium scintigraphy for the prediction of segmental recovery. 38
In summary, dobutamine stress echocardiography compares favorably with positron emission tomogra- phy and thallium scintigraphy for the assessment of viability, and, although dobutamine stress echocardi- ography may be less sensitive than these methods, it appears to be more specific.
OTHER ECHOCARDIOGRAPHIC METHODS OF ASSESSING VIABILITY
Myocardial contrast echocardi0graphy is a relatively new technique used to assess the integrity of the coronary microcirculation by the performance of two-dimensional echocardiography during the intra- vascular injection of a sonicated contrast medium. 46 The visualization of microbubbles by this technique assesses flow through coronary vessels less than 10 ~m in diameter, and evidence ofmicrovascular integ- rity may be a marker for viable myocardium. In patients with acute myocardial infarction and occlu- sion of the infarct-related artery, the presence of collateral flow by myocardial contrast echocardiogra-
Journal of the American Society of Echocardiography 778 Lualdi and Douglas September 1997
phy correlated with improvement in regional wall motion 1 month after successful coronary angio- plasty. 47 A subsequent study showed that in patients with documented patency of the infarct-related artery after recent myocardial infarction, there was a strong correlation between evidence of an intact microcircu- lation and subsequent improvement in regional wall motion. 48 During acute myocardial infarction, pa- tients with evidence ofreflow by myocardial contrast echocardiography in the myocardial area at risk after reperfusion therapy had greater improvement in global and regional left ventricular function on fol- low-up than patients with no reflow. 49 With respect to chronic ischemia, perfusion by myocardial contrast echocardiography correlated with improvement in regional wall motion and global left ventricular func- tion after revascularization in a population of patients with previous myocardial infarction and reduced left ventricular ejection fraction, s°
Thus, myocardial contrast echocardiography is a promising technique for assessing myocardial viabil- ity in the setting of acute and chronic ischemia.
Two recent studies have compared myocardial contrast echocardiography with dobutamine stress echocardiography for the identification of viable myocardium (Table 5). In the setting of an acute myocardial infarction, myocardial contrast echocardi- ography was more sensitive than dobutamine stress echocardiography, whereas dobutamine stress echo- cardiography was markedly more specific in the iden- tification of viable myocardium. 5~ Hence, the two tests provide different, but complementary informa- tion: myocardial contrast echocardiography will de- tect the microcirculation which is essential for viabil- ity of even a small number ofmyocytcs admixed with scar, whereas dobutamine stress echocardiography investigates the ability of a given region to increase contractile performance. In the setting of chronic coronary artery disease and left ventricular dysfunc- tion, dobutamine stress echocardiography and myo- cardial contrast echocardiography were not signifi- candy different in predicting functional improvement in baseline hypoldnetic segments, although dobut- amine stress echocardiography was significantly more specific and had a higher positive predictive value in baseline akinetic segments. 52
Though far less studied than dobutamine stress echo- cardiography, dipyridamole stress echocardiography has been used to delineate viability. Dipyridamole is a potcnt vasodilator which may elicit contractile reserve in segments of viable myocardium through increasing cor- onary flow and metabolic changes in the myocardium.53 Analogous to the use ofdobutamine to assess viability, the optimal dose ofdipyridamole for examining revers- ible ventricular dysfunction may be "infta-low" (0.28 mg/kg over 4 minutes) because it fails to elicit con-
founding ischemic responses, s3 In a comparison of di- pyridamole stress echocardiography and rest thallium scintigraphy for thc identification of viable myocardium in patients with known coronalw artery disease and baseline regional wall motion abnormalities, the two tests were concordant for 91% of scgments studied, s4 Investigators have also shown that the combination of low-dose dipyridamole stress echocardiography and do- butamine stress echocardiography significantly in- creases the overall accuracy of idcntifying viable myocardium above that of cither test alone, s5 In a head-to-head study comparing "infi:a-low" dose dipy- ridamole with dobutamine stress echocardiography, the two tests had similar sensitivities / 78% and 76%, respec- tively) and identical specificities 194%~ 53 Although di- pyridamole stress echocardiography may prove useful for assessing myocardial viability in the future, more research and validation are needed with this technique before it can be put into clinicaJ practice.
Ultrasound myocardial tissue characterization with integratcd back scatter has also been used to identify viable myocardium in animal models of stunning s6 and in patients with acute myocardial infarction, s7 In an open-chested dog model of stunning by means of transient coronary occlusion, cardiac cycle-depen- dent cyclic variation recovered to baseline far faster after reperfusion than did wall thickening. 56 In pa- tients with acute myocardial infarction, those with patent infarct-rclated arteries ~i.e., stunned myocar- dium) showed recovery in wclic variation at 24 hours after the onset of symptoms, whereas those with occluded arteries showed no such recovery, s7 To date, there have been no published studies of ultra- sonic tissue characterization with cyclic variation for the identification of hibernating myocardium or comparisons of this technique to dobutamine stress echocardiography.
CONCLUSIONS
Left ventricular systolic function is a major determi- nant of prognosis in patients with acute and chronic coronary artery disease. It is now recognized that regional contractile dysfunction is commonly revers- ible in the setting of acute ischemia with reperfusion (stunned myocardium) or after revascularization of chronically ischemic tissue (hibernating myocardi- urn). Several methods are used to detect viable myo- cardium including F- 18 deoxyglucose positron emis- sion tomography and thallium scintigraphy. More recently, dobutamine stress echocardiography has been shown to be both safe and accurate for this purpose. Furthermore, other echocardiographic techniques such as myocardial contrast echocardiog- raphy and dipyridamole stress echocardiography are
Journa of the American Society of Echocardiography Volume 10 Number 7 Lualdi and Douglas 779
b e i n g deve loped . T h e relative ut i l i ty a n d cost-ett'ec-
t iveness o f the va ry ing modal i t i es for the assessment
o f viabi l i ty will n e e d to be s t u d i ed prospec t ive ly be-
fore defini t ive r e c o m m e n d a t i o n s a b o u t tes t se lec t ion
can be made . I n add i t i on , m u c h still r ema ins to be l ea rned a b o u t
h o w best clinically to use these t e c h n i q u e s a n d w h a t
the i r impac t o n the rapeu t i c decis ions , use o f r evascu -
l a r iza t ion p rocedures , a n d p a t i en t o u t c o m e s will be.
Areas o f fu tu re research s h o u l d i n c l u d e r a n d o m i z e d
trials to con f i rm the obse rva t ions o f an increased risk
o f cardiac events in pa t i en t s w i th nonrevascu la r i zed ,
ye t viable m y o c a r d i u m , as well as the s y m p t o m a n d
mor ta l i t y benef i ts f rom its revascular iza t ion . A t the
p r e sen t t ime , the assessment o f viabi l i ty by echocar-
d iograph ic t e c h n i q u e s offers the p romise o f improv-
i ng o u t c o m e s in pa t i en t s w i t h co rona ry ar tery disease
a n d left vent l ' icular dys func t ion .
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