Eur J Cardiothorac Surg 2002;21:32-35
© 2002 Elsevier Science NL
Myocardial perfusion correlates with improvement of systolic function of the left ventricle after CABG. Dobutamine echocardiography and Tc-99m-MIBI SPECT study
L. Paluszkiewicza*,
P. Kwineckia,
M. Jemielitya,
A. Szyszkab,
W. Dyszkiewicza,
A. Cie
li
skib
a Department of Cardiac Surgery, Institute of Cardiology, 61-848 Pozna
ul. D
uga 1/2, Poland
b Department of Cardiology, Institute of Cardiology, 61-848 Pozna ul. Duga 1/2, Poland
Received 10 October 2000;
received in revised form 3 October 2001;
accepted 5 November 2001.
* Corresponding author. Tel.: +48-61-852-1021; fax: +48-61-826-6338
e-mail: lpalusz{at}poczta.onet.pl
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Abstract
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Objective: The aim of the study was to assess the effect of surgical revascularization [coronary artery bypass grafting (CABG)] on systolic function and perfusion of the left ventricle using dobutamine echocardiography (DE) and Tc-99m-MIBI SPECT (SPECT). Methods: There were 32 patients mean age 52.2±7.2 years in whom DE and SPECT were performed before and 3–4 months after CABG using standard protocols. Wall motion score index (WMSI) and perfusion index (PI) were calculated. Results: Significant improvement of WMSI at rest (1.44±0.46 vs 1.33±0.41; P=0.03) as well as after maximal dose of dobutamine (1.49±0.42 vs 1.39±0.44; P=0.02) was observed after CABG as compared to preoperative examination. Similar relation was observed during SPECT study. Perfusion index diminished significantly after revascularization during rest acquisition (2.19±0.71 vs 1.93±0.70; P=0.0008) and after Dipirydamole administration (2.73±0.73 vs 2.20±0.69; P=0.0001) as compared to preoperative examination. We found correlation between PI and WMSI at rest before CABG (R=0.46; P=0.01), PI after Dipirydamole and WMSI after maximal dose of Dobutamine before CABG (R=0.37; P=0.04), PI and WMSI at rest after CABG (R=0.39; P=0.03), PI after Dipirydamole and WMSI after dobutamine after CABG (R=0.38; P=0.03). Conclusions: Surgical revascularization significantly improves both perfusion and contractility. Increased perfusion after CABG correlates with improvement of systolic function of the left ventricle.
Key Words: Dobutamine • Technetium-99m-sestamibi • Echocardiography • Revascularization • Systolic function
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1. Introduction
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As the aim of surgical procedure is now not only restoration of blood flow in coronary arteries but also improving function of myocardium supplied by this arteries, identification of dysfunctional but viable myocardium suggesting potential reversibility of myocardial function is of clinical importance in patients undergoing coronary surgery. Unfortunately restitution of coronary blood flow does not simply mean improvement in myocardial function. Some imaging techniques such as myocardial uptake of F-18 fluorodeoxyglucose with positron emission tomography (PET) [1,2] or thallium-201 reinjection scintigraphy [3] has been proved clinically useful for predicting which asynergic myocardial segments will demonstrate improvement after revascularization [4]. Tc-99m-MIBI SPECT with very good physical characteristics and passive diffusion driven by mitochondrial transmembrane potential closely reflects blood flow distribution [5]. Dobutamine echocardiography became widely used in clinical practice in assessment of myocardial function because of relatively low costs and high efficiency in predicting recovery of myocardial function after revascularization [6,7]. Simultaneous use of both techniques seems to be a good choice in estimation of the results of coronary surgery.
The aim of the study was to assess effect of coronary revascularization on myocardial perfusion using Tc-99m-MIBI SPECT and on myocardial function of the left ventricle using dobutamine echocardiography.
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2. Methods
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We examined 32 consecutive patients 1 week before and 3–4 months after surgical revascularization. Beta blocking drugs were stopped 24 h before examination.
Dobutamine echocardiography: resting 2-dimensional echocardiogram was recorded and infusion of 5 µg/kg per min of dobutamine was started. The dose increased of 5 µg/kg per min in 3 min stages to maximal of 40 µg/kg per min. Two dimensional echocardiogram was recorded every 3 and 10 min after the maximal dose. End point for the study were: (1) completion of peak dobutamine dose; (2) attaining of 85% of age–predicted heart rate; (3) development of a new wall motion abnormality in two or more left ventricular segments; (4) typical angina; (5) symptomatic hypotension with decrease in systolic blood pressure >15 mmHg; (6) increase of blood pressure >220/110 mmHg; and (7) sustained arrhythmia. Left ventricle was divided into 16 segments. Systolic function was assessed using four point scale for contractility: one-normokinesis, two-hypokinesis, three-akinesis and four-dyskinesis. Wall motion score index (WMSI) was calculated as the summation of the individual score of the 16 segments divided by the number of segments.
Tc-99m-MIBI SPECT: examination was done over 2-day protocol. On day 1, a pharmacological exercise test with Dipirydamole was performed. On day 2, giving the same radiopharmaceutical agent, the protocol was conducted at rest. Patients were given 0.56 or 0.74 mg/kg body weight of Dipirydamole over a period of 4 or 7 min. During infusion, ECG parameters and arterial blood pressure were monitored. In case of side effects (tachycardia, angina, arterial hypertension) the action of Dipirydamole was neutralized by administering aminophylline. A total of 2 min after infusion, Tc-99m-MIBI was given intravenously, at a dose of 740 MBq (20 mCi). The left ventricle was divided into nine segments and perfusion was assessed on a five-point scale: one-normal perfusion; two-slightly impaired; three-moderately impaired; four-markedly impaired; five-no perfusion. Perfusion index was calculated as the summation of the individual score of the nine segments divided by the number of segments.
Statistical analysis: all data were expressed as mean±SD. The Wilcoxon's signed rank test was used for analysis of continuous data. Correlation between continuous variables was assessed by the Spearman correlation coefficient. Differences were considered significant at P<0.05.
The study was approved by the local ethics committee.
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3. Results
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The clinical characteristic of patients is presented in Table 1. Age of examined patients ranged from 38 to 68 years (mean 52.2±7.2). Twenty-four of them had previous history of myocardial infarction. Three vessel disease was detected in 19 patients, two vessel in 12 and one stenosed artery was found in one patient. From 82 stenosed arteries 93% were revascularized. Neither electrocardiographic changes suggesting myocardial infarction nor increased CK-MB levels were observed in perioperative period.
Dobutamine test was classified as positive in 28 patients before operation (88%) and in 20 (62%) after CABG. Mean time of dobutamine test increased significantly after revascularization from 992.18±310.39 to 1173.11±312.73 s (P=0.003). Mean dobutamine dose increased from 27.67±8.70 µg/kg per min before operation to 32.67±8.37 µg/kg per min after surgery (P=0.003). Maximal dobutamine dose was achieved in three patients before and in 17 after operation. Wall motion abnormalities were the reason for termination of examination in 20 patients before surgery and in eight after CABG. Chest pain during dobutamine test was observed in three patients after CABG as compared to ten patients before operation.
Total number of 512 segments were analyzed during dobutamine echocardiography. Before operation impaired contractility at rest was observed in 121 segments. Among them were 76 hypokinetic, 36 akinetic and nine dyskinetic segments. After operation improved contractility was noted in 63 (52%) of analyzed segments. Improvement was noted in 40 hypokinetic, 21 akinetic and two dyskinetic segments.
Wall motion score index before and after revascularization is shown in Table 2. Significant improvement of WMSI after CABG at rest and after maximal dose of dobutamine was noted.
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Table 2. Wall motion score index (WMSI) assessed at rest and after maximal dobutamine dose before and after coronary artery bypass graft (CABG)
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Perfusion index before and after CABG is shown in Table 3. Significant decrease of perfusion index was observed after CABG at rest as well as after Dipirydamole.
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Table 3. Perfusion index (PI) assessed at rest and after Dipirydamole before and after coronary artery bypass graft (CABG)
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We found correlation between PI and WMSI at rest before CABG (R=0.46; P=0.01), PI after Dipirydamole and WMSI after maximal dose of Dobutamine before CABG (R=0.37; P=0.04), PI and WMSI at rest after CABG (R=0.39; P=0.03), PI after Dipirydamole and WMSI after dobutamine after CABG (R=0.38; P=0.03).
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4. Discussion
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Accumulation of Tc-99m-MIBI in viable cells mostly depends on coronary flow [8]. The Tc-99m-MIBI SPECT uptake is present even in myocytes with impaired metabolism when the cell membrane integrity is preserved. It can bind to the mitochondrial membrane as long as they are not irreversibly damaged and allows to identify stunned or hibernating myocardium with comparable sensitivity and specificity to 201Tl [9]. Homogenous uptake of Tc-99m-MIBI SPECT means normal blood flow throughout myocardium. In patients with coronary artery stenosis decreased uptake of radiopharmaceutical agent is present depending on the extent of artery stenosis and myocardial viability. Decreased uptake after Dipirydamole infusion and improvement during acquisition at rest (reversible defect) indicates ischemia. Complete revascularization in such areas will result in perfusion improvement. Irreversible perfusion defect mostly indicates scar tissue. In our patients perfusion index assessed by Tc-99m-MIBI SPECT was significantly lower after revascularization as compared with preoperative examination that means better perfusion at rest and smaller perfusion defect after Dipirydamole. Similar results were reported [10,11]. Perfusion index however improved, did not returned to normal. The possible reason could be incomplete revascularization, diffuse changes in coronary arteries or myocardium could be irreversibly damaged in this patients. The diameter of anastomosis seems to influence perfusion after operation. It is important to notice that even fixed perfusion defect does not exclude improvement of contractility after revascularization. Observed correlation between perfusion expressed as perfusion index and contractility expressed as wall motion score index suggest importance of myocardial blood flow in myocardial function. Perfusion correlated with contractility as well before as after surgery.
We observed significant decrease of wall motion score index after revascularization what is related to improvement of myocardial contractility in those segments. Improvement was noted at rest as well as after maximal dose of dobutamine. Similar results were published by many others [12–14]. However 3 months after operation in almost 48% of segments with previously impaired contractility improvement of myocardial function was not observed in our patients. Possible explanations of lack of improvement of myocardial contractility are: incomplete revascularization, early graft occlusion, perioperative myocardial infarction and progression or new stenosis in non-grafted arteries. We did not performed control coronary angiography so we can not exclude graft occlusion or progression of stenosis in non-operated arteries. Perioperative myocardial infarction is less probable in our patients as no significant electrocardiographic changes or increased CK-MB levels suggesting myocardial infarction were observed in perioperative period.
To achieve improvement of myocardial function after revascularization some other important conditions must be fulfilled. The specific amount of viable myocardium must be present in order to produce visible changes in contractility [15,16], so lack of visible, macroscopic improvement of contractility during echocardiographic examination does not exclude improved myocardial function. Dobutamine echocardiography was found to be more specific in predicting functional recovery as compare with positron emission tomography [17].
Time from operation to examination also plays a very important role. Improvement of myocardial function was reported as soon as after 7 days to even months after surgery [14]. Rahimtoola proposed different forms of hibernation depending on time and extent of structural changes in myocardium [18,19]. Time from surgery to assessment of myocardial function in our study seems to be sufficient to allow restoration of function in case of acute and subacute hibernation, though we can not exclude segments with chronic hibernation. In such situation because of structural changes of myocytes recovery of myocardial function can be delayed even 12 months [19,20].
Increased dobutamine dose during test, longer test time, less patients presenting chest pain during test, less wall motion abnormalities after operation confirm positive clinical effect of surgical revascularization. Surgical revascularization significantly improves both perfusion and contractility. Increased perfusion after coronary artery surgery correlates with improvement of systolic function of the left ventricle.
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Footnotes
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Presented at the 14th Annual Meeting of the European Association for Cardio-thoracic Surgery, Frankfurt, Germany, October 7–11, 2000.
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Appendix A. Conference discussion
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Mr S. Large (Cambridge, UK): This is a very interesting area, particularly if you are interested in working on poor left ventricular function. Hunting out hibernating myocardium and really targeting that group of patients may be very profitable. My question to you here is that you haven't really introduced the subject of anti-anginal therapy. So much of it is based on agents that affect diastolic and, for that matter, systolic function. I wonder what role they played in the assessed ventricular function before and after your surgery?
Dr Paluszkiewicz: The normal procedure before dobutamine test and before scintigraphy is to stop beta blockers at least for 24 h before the examination. Nitrates were used only in the case of anginal pain. As I remember there were only two patients who really needed to use nitrates during 24 h prior to examination. All patients were hospitalized and we tried to avoid any unnecessary stress or physical effort.
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Abstract
1. Introduction
