(Circulation. 1995;91:990-998.)
© 1995 American Heart Association, Inc.
Articles |
Relation Between Thallium Uptake and Contractile Response to Dobutamine
Implications Regarding Myocardial Viability in Patients With Chronic Coronary Artery Disease and Left Ventricular Dysfunction
From the Echocardiography and Nuclear Cardiology (V.D., P.T.K.) Laboratories, Cardiology Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md.
Correspondence to Dr Julio A. Panza, Director of Echocardiography, National Institutes of Health, Bldg 10, Room 7B-15, Bethesda, MD 20892.
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Abstract |
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Background Both thallium scintigraphy and dobutamine echocardiography have been used to assess myocardial viability. However, thallium uptake and the inotropic response to dobutamine are expressions of different cellular phenomena. The present study was undertaken to investigate the relation between the two methods in patients with chronic coronary artery disease and left ventricular dysfunction to derive insights into the mechanisms related to myocyte viability.
Methods and Results Thirty patients (28 men and 2 women;
age, 59±10 years) with chronic coronary artery disease and impaired
left ventricular systolic function at rest (mean ejection fraction,
32±9%) were included in the study. Patients underwent transesophageal
echocardiography during incremental doses of dobutamine from 2.5 to a
maximum of 40
µg · kg-1 · min-1 and
single photon emission computed tomographic thallium scintigraphy using
a stress-redistribution-reinjection protocol. The left ventricle was
divided into 16 segments for analysis of echocardiographic and
thallium images. Segmental myocardial contractile function was graded
as normal, hypokinesis, akinesis, or dyskinesis at each incremental
dose of dobutamine. Thallium uptake in each myocardial segment was
graded on a 5-point scale from 0 (absent) to 2 (normal) for each of the
stress, redistribution, and reinjection images. A segment was
considered viable if the assigned thallium score was 1 or higher
(normal uptake or only mild to moderate defect) in any of the stress,
redistribution, or reinjection images. Among 472 myocardial segments
available for analysis, 311 had resting wall motion abnormalities,
of which 56% (173/311) showed contractile improvement with dobutamine
(usually first observed at 
10
µg · kg-1 · min-1) and 84%
(262/311) were considered viable by thallium scintigraphy
(P<.0001). Of the 262 segments considered viable by
thallium, 167 (64%) had a contractile improvement with dobutamine; in
contrast, only 6 of the 49 segments (12%) considered nonviable by
thallium had a positive dobutamine response (P<.0001).
Furthermore, a positive inotropic response to dobutamine was
significantly related to the magnitude of thallium uptake: the
proportion of segments with a positive dobutamine response rose with
increasing magnitude of thallium uptake (P<.001). The
disagreement between the two tests was related primarily to segments
considered viable by thallium that did not show contractile improvement
with dobutamine.
Conclusions These findings demonstrate the existence of a relation between thallium uptake and the inotropic response to dobutamine in patients with chronic coronary artery disease and left ventricular dysfunction. However, the proportion of segments showing a positive response to dobutamine is significantly lower than those with thallium uptake, suggesting that the cellular mechanisms responsible for a positive inotropic response to adrenergic stimulation require a higher degree of myocyte functional integrity than those responsible for thallium uptake.
Key Words: coronary disease • myocardial contraction • echocardiography • scintigraphy
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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Identifying areas of myocardium with depressed but potentially reversible contractile function may bear important significance in the therapeutic strategy and perhaps the prognosis of patients with coronary artery disease and left ventricular dysfunction.1 2 3 4 Consequently, the noninvasive assessment of dyssynergic but viable myocardium, which may contribute to the overall improvement in left ventricular function after revascularization, has generated great interest among cardiologists and cardiac surgeons.5 6 7 8 9 10 11 12 13 14 Among the several techniques used for this purpose, thallium scintigraphy (using rest-redistribution or stress-redistribution-reinjection protocols) has been shown to be a useful method, largely based on the observations that the presence of thallium uptake in a myocardial region with depressed systolic function correlates with contractile improvement after revascularization.15 16 17 18 19 20 21 22 23 24
Infusion of dobutamine (a synthetic catecholamine with a predominant ß1 agonist effect) has been combined with echocardiography for detection of myocardial ischemia in patients with known or suspected coronary artery disease.25 26 27 28 29 30 Recently, dobutamine echocardiography has also been proposed as a tool to assess myocardial viability within poorly contractile segments of the left ventricle in patients with coronary artery disease. Thus, areas of dyssynergic myocardium that display an improvement in contractile function with dobutamine appear to have a greater probability of recovery after revascularization compared with areas in which dobutamine does not induce a significant change in contractile function.31 32 33
Although both thallium scintigraphy and dobutamine echocardiography have been used to assess left ventricular dysfunction in patients with chronic coronary artery disease, the mechanisms by which these two methods identify viable myocardium are different. Thallium scintigraphy demonstrates the ability of the myocardium to take up a cation by an active process that takes place at the level of the cell membrane.14 Dobutamine echocardiography, on the other hand, assesses the ability of the myocardium to increase its contraction in response to an adrenergic stimulus.34 Thus, although the uptake of thallium and a positive inotropic response to dobutamine both indicate the presence of viable myocardium, each reflects different cellular processes characteristic of viable myocytes. The purpose of the present investigation was to determine whether there is a differential effect of chronic coronary stenoses on these two processes by studying the relation between myocardial thallium uptake and the contractile response to dobutamine in patients with coronary artery disease and left ventricular dysfunction.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Study Population
The study included 30 consecutive patients with coronary artery disease and depressed left ventricular systolic function at rest (ejection fraction at rest <45%) admitted to the Clinical Center of the National Institutes of Health for coronary angiography who underwent both thallium scintigraphy and transesophageal dobutamine echocardiography. The 28 men and 2 women had a mean age of 59±10 years. All patients underwent cardiac catheterization as part of their evaluation for suspected coronary artery disease, systolic dysfunction, chest pain syndrome, or previous myocardial infarction. In each patient, coronary angiography demonstrated
70% narrowing in the ID
of at least one major coronary artery. Sixteen patients had undergone
previous revascularization (coronary artery bypass surgery in 10 and
angioplasty in 6); in these patients, the status of the coronary
vascular tree at the time of the most recent cardiac catheterization
(performed after revascularization in all patients) was considered for
allocating the patients into the one-, two-, or three-vessel disease
category. Of the 30 patients, 12 (40%) had single-vessel disease, 7
(23%) had two-vessel disease, and the remaining 11 (37%) had
involvement of all three major coronary arteries. Sixteen of the study patients (53%) had suffered a previous myocardial infarction, but not in the 3 months before the study. Eleven patients were asymptomatic; the remaining 19 had stable symptoms, including 17 patients with chest pain. Ten patients were in New York Heart Association functional class III or IV. Patients with evidence of cardiomyopathy, congenital or valvular heart disease, recent myocardial infarction, unstable angina, or uncontrolled systemic hypertension were excluded from the study. All patients were in sinus rhythm. Antianginal medications were withdrawn at least 48 hours before the studies. In 28 of the 30 patients (93%), both thallium scintigraphy and transesophageal dobutamine echocardiography were performed within the same week; in the remaining 2 patients, both studies were performed within 1 month. No patient suffered any intervening cardiac events. In 16 patients, thallium scintigraphy was performed before transesophageal dobutamine echocardiography; in the remaining 14 patients, the order was reversed. The study was approved by the National Heart, Lung, and Blood Institute Investigational Review Board, and each patient gave informed written consent for all tests.
Radionuclide Angiography
Gated cardiac blood pool studies
were performed to assess left
ventricular ejection fraction at rest with red blood cells labeled in
vivo with 20 to 25 mCi of 99mTc. Imaging was done with a
conventional Anger camera equipped with a high-sensitivity
parallel-hole collimator, as previously described.35 The
left ventricular ejection fraction was derived by computer analysis
of the scintigraphic data. The lower limit of normal for resting
ejection fraction by our technique is 45%. In each study patient, rest
radionuclide ventriculography showed a left ventricular ejection
fraction <45% (mean±SD, 32±9).
Transesophageal Dobutamine Stress Echocardiography
Transesophageal echocardiograms were performed with a
Hewlett-Packard biplane (12 patients) or omniplane (18 patients) 5-MHz
probe. The studies were performed with patients in the left lateral
decubitus position. The oral pharynx was anesthetized with aerosol
benzocaine spray. Intravenous sedation with midazolam hydrochloride was
administered before introduction of the probe and during the study as
needed (total dose of midazolam per patient was 6±3 mg). Heart rate
and oxygen saturation were monitored continuously during the study. A
12-lead ECG and measurements of systemic blood pressure were recorded
at the end of each stage of the dobutamine protocol while the
echocardiographic images were obtained.
Dobutamine was infused starting at 2.5 µg · kg-1 · min-1. After 5 minutes, the dose was increased to 5 µg · kg-1 · min-1. Subsequently, the dose was increased by 5-µg · kg-1 · min-1 increments every 5 minutes to a maximum of 40 µg · kg-1 · min-1. Two-dimensional (2D) views were acquired during the last 2 minutes of each stage. The stress test was stopped when the maximum dose of dobutamine was reached or if severe chest pain, a severe increase in systolic blood pressure (>250 mm Hg), clinically significant arrhythmias, extensive wall motion abnormalities, or intolerance to the probe developed. A fall in blood pressure not accompanied by the development of extensive wall motion abnormalities was not considered a criterion for termination of the test.36 37 Sublingual nitroglycerin, nifedipine, and intravenous propranolol were available.
At baseline and at each stage of the dobutamine infusion, transgastric short-axis and transesophageal long-axis, four- and two-chamber views were obtained following the same sequence in all patients. First, the transgastric short-axis view was obtained at both the papillary muscle and the mitral valve levels. Subsequently, the probe was pulled out to the esophagus, and the long-axis, four-chamber, and two-chamber views were recorded. Immediately after completion of the image acquisition, the dose of dobutamine was increased to the next level, and the probe was gently advanced again across the cardias and into the stomach to start with the transgastric short-axis view during the next stage. The long-axis view was obtained by lateral flexion of the longitudinal probe when the biplane probe was used38 and by rotating the transducer 135° from the horizontal position when the omniplane probe was used.39 During acquisition of the transesophageal views, particular attention was paid to avoid foreshortening of the left ventricular cavity that would result in poor visualization of the apical segments. This was achieved by using the maximum degree of retroflexion of the scope without losing contact with the esophageal wall. In addition, visualization of the apex was ascertained by ensuring that the length of the left ventricular cavity (from the plane of the mitral valve to the apical endocardium) was the same in all transesophageal views (ie, four-chamber, two-chamber, and long axis).
Two-dimensional views were continuously recorded on videotape and digitized on-line with a Nova Microsonics Image Vue system (Nova Microsonics). The system permits creation of a continuous loop of a single cardiac cycle by displaying still frames acquired at regular intervals in a continuous fashion. Twelve sequential frames of each 2D view were captured at 33-millisecond intervals with an ECG R wave–triggered mechanism at baseline and at each dose of dobutamine.
Transesophageal echocardiographic images digitized on-line during the dobutamine stress study were transferred to optical disks for permanent storage and review. Continuous videotape recordings were not routinely used for analysis but were available during the review process. Images were analyzed by two independent observers blinded to the results of the thallium scintigraphy study. In cases of disagreement, the studies were reviewed and a consensus was reached.
At each stage of the dobutamine protocol, the left ventricle was divided into 16 segments following the recommendations of the American Society of Echocardiography.40 Regional myocardial contractile function was graded as normal, hypokinesis, akinesis, or dyskinesis in each myocardial segment; particular attention was paid to systolic wall thickening rather than to endocardial motion.
For the purpose of this study, only segments with wall motion abnormalities during basal conditions were analyzed. For each segment, a positive response to dobutamine was considered an improvement in contractile function of at least one grade, regardless of whether worsening in wall motion developed during later stages of the test. For each segment, the lowest dose of dobutamine that produced an unequivocal improvement in contractile function was determined. In addition, for each myocardial segment with abnormal resting wall motion, the behavior during the increasing doses of dobutamine was analyzed and assigned to one of the following categories: sustained improvement—progressive increase in contractile function throughout the test; biphasic response—initial improvement followed by worsening in contractile function with higher doses of dobutamine; worsening only—basal hypokinesis deteriorating to akinesis with dobutamine without initial improvement (a change from basal akinesis to dyskinesis was not considered in this category41 ); hypokinesis without change—basal hypokinesis without noticeable improvement or worsening during the test; or akinesis without change—basal akinesis (or dyskinesis) without improvement (or with development of dyskinesis) during dobutamine infusion.
Thallium Scintigraphy
After an overnight fast, all patients
underwent exercise
201Tl single photon emission computed tomographic study
according to a standardized multistage, symptom-limited treadmill
exercise test. At peak exercise, 2 to 3 mCi thallium was administered
intravenously, and the patient continued to exercise for an additional
45 to 60 seconds. Conventional stress and 3- to 4-hour redistribution
images were acquired. Immediately after redistribution, an additional
1-mCi dose of thallium was administered at rest, and reinjection images
were acquired 10 to 15 minutes thereafter.17
Thallium studies were analyzed by two independent reviewers who were unaware of the dobutamine echocardiography results. From the raw scintigraphic data, horizontal long-axis, vertical long-axis, and short-axis tomograms were reconstructed. The reconstructed stress, 3- to 4-hour redistribution, and reinjection images were then analyzed semiquantitatively in the basal, mid, and apical short-axis views. The left ventricle was divided into 16 segments matching the segmentation used with echocardiography. The uptake in each myocardial segment for each of the stress, redistribution, and reinjection images was graded on a 5-point scale as follows: 2, normal; 1.5, mildly reduced; 1, moderately reduced; 0.5, severely reduced; and 0, absent.
For the purpose of analysis, a segment was considered to have a reversible defect if the thallium uptake in the stress images was reduced but was increased by at least one grade in the redistribution or reinjection image. If the thallium uptake in the stress images was reduced and did not increase during redistribution or reinjection, the segment was considered to have an irreversible defect. These irreversible defects were then subgrouped on the basis of severity of reduction in tracer activity: mild to moderate (thallium score of 1 or greater) and severe (thallium score of <1) defects. We showed previously that when irreversible thallium defects on redistribution images are subgrouped on the basis of the severity of reduction in thallium activity, mild to moderate irreversible defects were found by positron emission tomography to be metabolically active and viable, whereas regions with severe irreversible thallium defects were shown to be metabolically inactive and hence scarred by positron emission tomography.42 43 Thus, for the purpose of the present study, myocardial regions with normal or mild to moderate reduction in thallium activity were considered viable.
Statistical Analysis
Proportions were compared by
2 or
Fisher's exact test as appropriate. A probability value of <.05 was
considered significant. All data are expressed as mean±SD.
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Results |
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Transesophageal Dobutamine Stress Echocardiography
Transesophageal dobutamine stress echocardiograms were completed in all patients without complications. The study was terminated for development of extensive wall motion abnormalities in 12 patients, after completion of the protocol in 11 patients, and because of severe chest pain in the remaining 7 patients. A significant fall in blood pressure associated with extensive wall motion abnormalities was observed in only 1 patient. In no patient was the test stopped because of intolerance to the probe or side effects. The median maximum dose of dobutamine used in each patient was 30 µg · kg-1 · min-1 (range, 10 to 40 µg · kg-1 · min-1).
Of 472 segments available for analysis (average, 15.7 segments per
patient), 311 segments (66%) had wall motion abnormalities during
basal conditions (134 were hypokinetic; 177 were akinetic or
dyskinetic). Fig 1
shows the distribution of the
different types of responses to dobutamine in these 311 segments. Only
a minority of segments that were hypokinetic during basal conditions
had either no change or worsening only with dobutamine infusion (5%
and 7% of all dyssynergic segments at rest, respectively).
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A total of 173 of the 311 segments with resting wall motion abnormalities (56%) showed a contractile improvement with dobutamine (sustained improvement in 63; biphasic response in 110). A positive response to dobutamine was observed more frequently among hypokinetic segments (97 of 134, 72%) than among akinetic (76 of 170, 45%) or dyskinetic (0 of 7) segments (P<.0001).
In the vast
majority of segments, the improvement was first detected
with the initial three doses of dobutamine (in 148 of the 173 segments
with a positive response to dobutamine [86%], the improvement was
observed at a dose of 10
µg · kg-1 · min-1 or
less). In only
4 segments (1%) was the improvement first detected with a dose higher
than 20 µg · kg-1 · min-1
(Fig 2).
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Thallium Scintigraphy
Of the 311 myocardial segments with
resting wall motion
abnormalities, 262 (84%) had normal or only mild to moderate reduction
in thallium activity (score 1) in at least one stage of the
stress-redistribution-reinjection protocol, and 49 (16%) had severely
reduced or absent thallium activity (score <1). The range of segmental
thallium score was as follows: 28 segments received a score of 0, 21 a
score of 0.5, 75 a score of 1, 39 a score of 1.5, and 148 a score of
2.
A total of 110 of the 311 segments (35%) with resting wall motion abnormalities had normal thallium uptake on the stress images. Of the remaining 201 segments with reduced uptake on the stress images, 124 had increased uptake on the redistribution or reinjection images and were therefore considered reversible defects; the other 77 did not have increased thallium uptake on corresponding redistribution or reinjection images and were considered to show irreversible defects.
Relation Between Dobutamine Echocardiography and Thallium
Scintigraphy
The number of segments with resting wall motion
abnormalities that were considered viable by thallium was significantly
greater than the number of segments showing a contractile improvement
in response to dobutamine (84% versus 56%, respectively;
P<.0001) (Fig 3). A significant relation was
observed between thallium uptake and response to dobutamine. Among 262
segments considered viable by thallium, 167 (64%) showed contractile
improvement with dobutamine; in contrast, only 6 of the 49
segments (12%) considered nonviable by thallium had a positive
dobutamine response (Fig 4). The rate of agreement
between thallium uptake and a positive response to dobutamine was 68%
(Fig 5); the disagreement between the two tests was
related primarily to the finding of segments considered viable by
thallium that did not show contractile improvement with dobutamine
(95 segments, or 30% of the total number of segments with resting wall
motion abnormalities) (Fig 6). The latter observation is
emphasized by the following analysis: of the 138 myocardial
segments without contractile improvement with dobutamine, 95 (69%) had
normal or mildly to moderately reduced thallium uptake and were
therefore considered viable by this method.
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The finding of a positive
inotropic response to dobutamine was
significantly related to the magnitude of thallium uptake: the
proportion of segments with a positive dobutamine response rose with
increasing magnitude of thallium uptake (Fig 7). Even
among the 77 segments with irreversible thallium defects, the
proportion of segments with contractile improvement with dobutamine was
related to the magnitude of thallium uptake: 42% (17 of 42) in
segments with mildly to moderately reduced thallium uptake compared
with 14% (5 of 35) in segments with severely reduced or absent
thallium uptake (P<.02).
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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Relation Between Thallium Scintigraphy and Dobutamine Echocardiography
The results of the present investigation indicate that a relation exists between thallium scintigraphy and dobutamine echocardiography in their ability to detect viable areas of dyssynergic myocardium. Thus, myocardial regions with positive thallium uptake have a significantly greater likelihood of showing a positive inotropic response to dobutamine compared with segments with absent or severely reduced thallium uptake. Importantly, the contractile response to dobutamine is significantly related to the magnitude of thallium uptake, such that the proportion of myocardial segments with contractile improvement in response to dobutamine progressively rises with increasing magnitude of thallium uptake.
At the same time, our findings demonstrate that among regions of the myocardium with reduced contraction at rest, the number of segments with thallium uptake is significantly greater than the number of those capable of increasing their contractile force in response to dobutamine. In our study, 84% of resting dyssynergic segments demonstrated normal or only mild to moderate reduction in thallium activity on the stress-redistribution-reinjection protocol and were therefore considered to be viable by this technique. This conclusion is based on the results of previous studies showing that myocardial segments with normal or mildly to moderately reduced thallium uptake have preserved metabolic activity by positron emission tomography42 43 and a higher likelihood of contractile recovery after revascularization compared with segments demonstrating severe reduction in thallium activity.20 21 In contrast, only 56% of the dyssynergic segments in our study patients showed a positive inotropic response to dobutamine. Such a response to dobutamine has also been shown to correlate with improvement in contractile function after revascularization in patients with chronic left ventricular dysfunction.33 The rate of agreement between thallium scintigraphy and dobutamine echocardiography in our study was 68%. It is important to emphasize that the major discordance between the two techniques (32%) was found primarily in the subgroup of segments with normal or mild to moderate segmental thallium uptake but without inotropic response to dobutamine (30%). In only 6 of the 311 (2%) segments analyzed was the discordance related to the finding of a positive response to dobutamine in segments with severely reduced or absent thallium uptake.
Implications Regarding Myocardial Viability
Taken as a whole,
our findings indicate that cell membrane pump
integrity has a different sensitivity to the mechanisms that determine
myocardial dysfunction compared with the process subserving the
contractile response to adrenergic stimulation. Thus, in segments with
reduced resting contractile function, the myocardial processes
necessary for thallium uptake appear to be less sensitive to the
inhibitory effects of chronic coronary stenoses than those needed for a
positive response to dobutamine. This is not surprising because
transmembrane pump activity must be more critical for the preservation
of myocyte viability than the maintenance of contractile function.
From the results of the present investigation, it appears that a spectrum of myocardial dysfunction exists in patients with chronic coronary artery disease. Mild myocardial dysfunction can be characterized as a stage in which basal contractile function is reduced but cellular membrane integrity and the capacity to respond to an inotropic stimulus are both preserved. This stage, observed in 167 of the 311 myocardial segments (54%) with reduced systolic function at baseline, is identified by both thallium uptake and positive inotropic response to dobutamine. A more severe form of systolic dysfunction would be that in which there is reduced basal contractile function with absent response to inotropic stimulation but with preserved membrane integrity. This stage, observed in 95 of 311 dyssynergic myocardial segments (30%) in our study patients, is identified by positive thallium uptake but negative response to dobutamine. The mechanisms responsible for producing these discordant effects on thallium uptake and contractile response to dobutamine cannot be determined from the findings of the present investigation. Possible explanations include a reduction in blood flow insufficient to maintain a positive response to dobutamine but sufficient to maintain transmembrane pump function, a greater extent of myocardial necrosis in these regions compared with those with a positive response to dobutamine, or more frequent episodes of ischemia leading to a more severe form of chronic myocardial "stunning"44 in these regions. Finally, a state of irreversible dysfunction, found in 43 of the 311 dyssynergic segments (14%) in our study, would be that in which both the ability to respond to an inotropic stimulus and the membrane integrity are lost. This is identified by negative thallium uptake and a negative response to dobutamine. The remaining 6 segments (2%) that displayed a positive response to dobutamine in the absence of thallium uptake cannot be explained with this concept and could probably be attributed to the error inherent in the comparison of the two techniques, including poor anatomic correspondence of left ventricular segmentation in some patients.
Limitations
It must be emphasized that in the present
investigation, we
did not have the opportunity to assess the potential recovery in
contractile function after revascularization. Therefore, we cannot
ascertain whether the greater proportion of segments with positive
thallium uptake compared with those with a positive response to
dobutamine truly indicates a higher sensitivity of thallium
scintigraphy for detection of viable myocardium than dobutamine
echocardiography. However, it was not the purpose of our study to
assess the relative merits of thallium scintigraphy and dobutamine
echocardiography for the prediction of functional recovery after
restoration of blood flow. Instead, we used these techniques to
investigate the relation between transmembrane pump integrity and
contractile reserve in myocardial regions with chronically depressed
systolic function.
It must also be noted that in the present study, we considered only the presence or absence of a positive inotropic response to dobutamine. However, a minority of myocardial segments were hypokinetic (and therefore displayed some degree of systolic thickening) during basal conditions and did not improve their systolic function in response to dobutamine (hypokinesis without change). In addition, other hypokinetic segments showed a worsening-only type of response to dobutamine, compatible with myocardial ischemia. Although these patterns of contraction do not reflect increased inotropism, they probably represent the existence of viable myocardium45 46 and should be considered in assessments of myocardial viability with dobutamine echocardiography. It must be emphasized, however, that these types of responses were observed in only 12% of the dyssynergic segments in our study patients. Such a low prevalence may be explained by the slow increases in dobutamine dose used in our study. It is possible that if higher doses of dobutamine are used initially (eg, 10 µg · kg-1 · min-1), myocardial ischemia may develop before any contractile improvement is detected, thereby increasing the prevalence of segments with hypokinesis without change or with worsening only. Furthermore, the importance of starting with very low doses of dobutamine for detection of the response of dyssynergic segments is emphasized by the observation that in the vast majority of segments, the improvement in response to dobutamine was first detected at a dose of 10 µg · kg-1 · min-1 or lower.
An important aspect of our study is the fact that the response to inotropic stimulation with dobutamine was assessed by transesophageal echocardiography. Although this technique may not be amenable for routine evaluation of all patients with coronary artery disease, it is a highly reliable tool for investigation of myocardial response to dobutamine and other interventions in which an accurate assessment of myocardial function is critical. In a previous study,47 we reported on the safety, feasibility, and diagnostic accuracy of transesophageal dobutamine stress echocardiography in the overall population of patients with known or suspected coronary artery disease. In agreement with the findings of that study and those of other investigations related to the use of transesophageal echocardiography during different forms of stress,48 49 50 51 the technique was well tolerated by all patients, and in no case did the study have to be discontinued because of intolerance to the probe or the occurrence of serious side effects.
Finally, it must be emphasized that the assessment of myocardial viability by both thallium scintigraphy and transesophageal dobutamine echocardiography in our study was based on qualitative criteria. Because both techniques are potentially amenable to quantification, it is important to determine whether a quantitative assessment of wall thickening and thallium uptake might provide a better understanding of the relation between membrane integrity and functional contractile reserve in myocardial regions with resting wall motion abnormalities.
Conclusions
The findings of the present investigation
demonstrate that in
patients with chronic coronary artery disease and left ventricular
dysfunction, a relation exists between thallium uptake and the
inotropic response to dobutamine; such a relation is determined by
the magnitude of thallium uptake. However, the proportion of segments
with preserved thallium uptake is significantly greater than those
showing a positive response to dobutamine. These findings emphasize the
difference in the mechanisms involved in the identification of
myocardial viability by the two techniques and suggest that the
cellular processes responsible for a positive inotropic response to
adrenergic stimulation require a higher degree of myocyte functional
integrity than those responsible for thallium uptake.
Received June 9, 1994; revision received August 12, 1994; accepted September 23, 1994.
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