Myocardial transport of hexakis(2-methoxyisobutylisonitrile) and thallium before and after coronary reperfusion

Chemotherapy-induced changes in cardiac capillary permeability measured by fluorescent multiple indicator dilution

Anthracyclines cause severe irreversible cardiac toxicity. The study of changes in cardiac permeability with chemotherapy could enhance the understanding of mechanisms behind cardiac damage, and provide useful information to evaluate anthracycline cardiotoxicity. Thirty-six rats (12 Sprague-Dawley, 12 Wistar, 12 Fischer-344) were randomly assigned to control (n = 21) or doxorubicin (n = 15), and injected i.p. with a cumulative dose of 18 mg/kg doxorubicin in saline (vehicle) or vehicle alone over 12 days. Echocardiography was performed at baseline and on day 11. An isolated heart experiment was done on day 12 to obtain perfused heart pressure values, and to measure cardiac capillary permeability using a Texas Red/sodium fluorescein multiple indicator dilution method. Control animals had significantly lower average permeability-surface-area-products (0.035 ± 0.013 cm(3)/s) than doxorubicin animals (0.066 ± 0.023 cm(3)/s), PSP ± SD, p < 0.001. These permeability changes correlated with significant functional changes. There was a significant decline in cardiac function with a deleterious effect of chemotherapy on fractional shortening (p < 0.001), left ventricular developed pressure (p < 0.001), contractility (p < 0.001), and relaxation (p = 0.02). Based on our results, cardiac capillary permeability changes can be detected after in vivo chemotherapy treatment using our fluorescent multiple indicator dilution technique, and may provide valuable information in evaluating cardiotoxicity of novel drugs.

Postreperfusion myocardial technetium-99m-sestamibi defect corresponds to area at risk: experimental results from an ischemia-reperfusion porcine model

INTRODUCTION

Technetium-99m-sestamibi (MIBI) is the most frequently used myocardial perfusion tracer in patients with ischemic heart disease. In patients with acute ST-elevation myocardial infarction, we previously found that the defect in myocardial MIBI uptake was the same in patients injected with MIBI before primary angioplasty and in patients injected immediately after successful treatment. Thus, reperfusion may not be followed by increased uptake of MIBI. Instead, the MIBI defect after reperfusion may reflect the area at risk (AAR) defined by MIBI injected before treatment. We intended to investigate whether myocardial imaging with MIBI administered after reperfusion reflects myocardial perfusion or rather the ischemic AAR.

METHODS

In 12 pigs, left anterior descending coronary artery was totally occluded for 45 min with an angioplasty balloon. After a 2-h reperfusion, MIBI was injected intravenously, and (153)Gd-microspheres were injected in left atrium. AAR and infarct size (IS) were determined by histochemical staining. MIBI and microsphere distribution were evaluated by counting the sliced left ventricle on a gamma camera. Defects were defined as uptake less than 45% of maximum uptake.

RESULTS

The mean±S.D. defect size as a fraction of left ventricle was for MIBI 21%±5.5%, AAR 25%±6.3%, IS 13%±3.9% and microspheres defect size 7.3%±5.5%. MIBI defect size overestimated IS (P=.0005) and microspheres defect size (P=.0001), but it was not significantly different from AAR (P=.30).

CONCLUSION

In a porcine model of myocardial infarction after 45 min of ischemia, MIBI administered 120 min after reperfusion delineates AAR.

Kinetic analysis of 99mTc-sestamibi evaluates the protective effects by ischaemic preconditioning on ischaemic myocardium in an isolated rabbit heart

OBJECTIVE

To analyse the kinetic changes of uptake, washout and retention of Tc-sestamibi in order to evaluate the protective effects and possible mechanism of ischaemic preconditioning and adenosine preconditioning on myocardium injured by ischaemia/reperfusion.

METHODS

Isolated ischaemia/reperfusion rabbit heart models, as established by Langendorff, were used. Eighteen rabbit hearts perfused in Krebs-Henseleit (KH) buffer were randomly assigned to three groups: ischaemia/reperfusion (I/R, n=6), adenosine preconditioning (AD, n=6), and ischaemic preconditioning (IPC, n=6). Tc-sestamibi (55.5 MBq) in KH was perfused for 40 min and washed out for 40 min. The kinetic changes of Tc-sestamibi within myocardial tissue was monitored during the uptake and washout phases. Cardiac haemodynamic parameters, creatine kinase and lactate dehydrogenase leakage in coronary effluent, and myocardial infarct size were measured to assess myocardial injuries in rabbit hearts.

RESULTS

In the early phases of uptake, there were no significantly different uptake rates of Tc-sestamibi between AD (before 20 min), IPC (before 15 min) and I/R myocardium (all P>0.05). Uptake rates of Tc-sestamibi in myocardium of the three groups all tended to increase, with the uptake time increasing. In the late phases of uptake, AD and IPC were significantly higher than I/R (all P<0.05). In the washout phases, the retention fractions of Tc-sestamibi in myocardium of the three groups all showed a descending tendency with washout time increasing. The retention fractions in AD and IPC were all higher than I/R (all P<0.05). There were no statistical differences in uptake rates and retention fractions of Tc-sestamibi between AD and IPC (all P>0.05). Cardiac haemodynamic parameters, creatine kinase and lactate dehydrogenase leakage, and myocardial infarct size demonstrated there is lighter injury in AD and IPC myocardium than in I/R (all P<0.05). The retention of Tc-sestamibi and myocardial infarction weight were significantly negatively correlated (r=-0.8384, P<0.001).

CONCLUSION

Adenosine preconditioning has similar myocardial protective effects on ischaemia/reperfusion myocardium as does ischaemic preconditioning. Tc-sestamibi may be a sensitive and reliable measure for evaluating the importance and mechanism of ischaemic preconditioning and adenosine preconditioning.

Low-flow ischaemia has no deleterious effect on the steady-state kinetics of 201Tl and 99mTc sestamibi within myocardial tissue

OBJECTIVES

This study aimed to specifically analyse the impact of low-flow ischaemia on the ability of myocytes to trap and accumulate Tl and sestamibi (MIBI) within myocardial tissue.

METHODS

In order to reach steady-state conditions for the interstitial/cellular concentration ratios (Ci/Cc) of the tracers and thereby simulate the conditions of cell cultures studies, Tl and MIBI were injected continuously during an 80 min period within the coronary circulation of isolated hearts submitted to normoxia (n=7) or low-flow ischaemia (n=7; >50% reduction in coronary flow). Ci was determined by using interstitial microdialysis and Cc was determined from Ci and myocardial retention values of the tracers.

RESULTS

At the end of the experiments, under steady-state conditions, Ci/Cc was equivalent between low-flow ischaemia and normoxia for both Tl (ischaemia, 0.60 +/- 0.25% vs normoxia, 0.63 +/-0.34%; NS) and MIBI (ischaemia, 1.00 +/- 0.68% vs normoxia, 0.76 +/- 0.32%, NS), whereas tissue concentrations of ATP were more than 4-fold lower in ischaemia than in normoxia (5.1 +/- 3.5 nmol.g vs 22.5 +/- 4.8 nmol.g; P< 0.001).

CONCLUSIONS

In contrast to the published results concerning the effects of anoxia on cell cultures, low-flow ischaemia within myocardial tissue has no deleterious effects on the ability of the cells to accumulate Tl and MIBI under steady-state conditions. This gives definitive evidence of the negligible impact of cellular metabolic disorders in the decrease in Tl or MIBI uptake, which is documented by stress-SPECT within low-flow ischaemic myocardium.

Mechanism of reversible (99m)Tc-sestamibi perfusion defects during pharmacologically induced vasodilatation

Reversible perfusion defects on (99m)Tc-sestamibi imaging during hyperemia are thought to occur due to myocardial blood flow (MBF) "mismatch" between regions with and without stenosis. We have recently shown that myocardial blood volume (MBV) distal to a stenosis decreases during hyperemia, resulting in a reversible perfusion defect on myocardial contrast echocardiography (MCE). In this study, we hypothesized that a reversible perfusion defect on (99m)Tc-sestamibi imaging during hyperemia results from the same mechanism. We tested our hypothesis under the following conditions: 1) increases in MBF in the absence of changes in MBV by using direct intracoronary infusion of adenosine (group I, n = 10 dogs); 2) decrease in MBV despite an increase in MBF by left main infusion of adenosine proximal to a noncritical coronary stenosis placed on either coronary artery (group II, n = 13 dogs); and 3) reduction in both resting MBF and MBV by placement of a severe stenosis (group III, n = 7 dogs). In group I dogs, no difference in MBV or (99m)Tc-sestamibi uptake was found between the two coronary beds despite an up to fourfold increase in MBF in one bed with adenosine. In group II dogs, MBV distal to the stenosis decreased during hyperemia despite a twofold increase in mean MBF. A good correlation was found between (99m)Tc-sestamibi uptake and MBV ratios from the stenosed versus normal bed (r = 0.91, P < 0.001). In group III dogs, both MBF and MBV were decreased in the stenosed bed at rest with a good correlation noted between (99m)Tc-sestamibi uptake and MBV ratios from the stenosed versus normal bed (r = 0.92, P = 0.004). We conclude that reversible defects on (99m)Tc-sestamibi during vasodilator stress imaging are related to decreases in MBV distal to a stenosis and not to "flow mismatch" between beds. The decrease in MBV results in reduced (99m)Tc-sestamibi uptake during hyperemia.

The value of Tc-99m MIBI SPECT during isosorbide dinitrate infusion in assessment of viable myocardium in patients with myocardial infarction

Tc-99m MIBI myocardial SPECT has shown promise for evaluation of coronary artery disease. But its role in predicting myocardial viability is still under investigation. The purpose of this study was to evaluate the value of Tc-99m MIBI myocardial SPECT during isosorbide dinitrate (ISDN) infusion in the assessment of myocardial viability. Thirty-seven patients with previous myocardial infarction (the infarct age ranged from < or = 30 days to 900 days) were studied, of them 13 patients had Tc-99m MIBI studies before and after coronary artery bypass grafting (CABG). The results showed that out of 134 segments with hypoperfusion at resting SPECT, 56 segments (41.8%) had an increase in Tc-99m MIBI uptake during ISDN infusion. Among them, 17 segments (30.4%) were normalized, 6 segments (10.7%) were significantly improved and 33 segments (58.9%) were improved. The degree of improvement in perfusion was related to the age of the myocardial infarction. In 13 patients with CABG, of 31 segments with improvement in perfusion post CABG, 25 segments (80.6%) showed perfusion improvement during ISDN infusion, and of 28 segments with improved wall motion post CABG, 23 segments (82.1%) showed improvement in perfusion during ISDN infusion. Tc-99m MIBI SPECT during ISDN infusion may therefore be a useful approach for assessing myocardial viability.

Rest technetium 99m-sestamibi tomoscintigraphy in hibernating myocardium

The myocardial uptake of rest-injected technetium 99m sestamibi on single-photon-emission computed tomographic images was assessed in 25 patients. All had an area of myocardial dysfunction that could be related to a coronary artery stenosis. None of the patients had clinical evidence of a myocardial infarction. Three months after revascularization, viability was demonstrated by contrast angiography and center-line analysis in 21 (78%) of the 27 formerly hibernating territories. Among these, none had a transmural defect, and 38% had a normal technetium 99m-sestamibi uptake. The four transmural preoperative defects were located in territories without viability. Eight of the 9 territories that were normal at scintigraphy proved to be viable postoperatively. It is concluded that as long as some residual technetium 99m-sestamibi uptake is present, viable myocardium is also present.

Myocardial technetium 99m-labeled sestamibi single-photon emission computed tomographic imaging in the detection of coronary artery disease: comparison between early (15 minutes) and delayed (60 minutes) imaging

BACKGROUND

Previous studies have demonstrated that there is a "partial" myocardial redistribution of 99mTc-labeled sestamibi (MIBI) between 1 and 3 hours after intravenous injection at stress. The purpose of this prospective study was to compare MIBI single-photon emission computed tomographic (SPECT) imaging performed 15 and 60 minutes after the injection at stress in the detection of coronary artery disease.

METHODS AND RESULTS

Thirty-five patients with coronary artery disease (26 underwent coronary angiography and 23 had a positive 201Tl study result) were included in this study. SPECT imaging started 15 minutes after the injection of 25 to 30 mCi MIBI at peak stress (180-degree arc, 32 angles, 25 sec/view, and high-resolution collimator). Patients underwent reimaging at 60 minutes according to the same protocol and with the same gamma camera. A rest study was obtained 75 minutes after the injection of MIBI (25 to 30 mCi) at rest, 48 hours later. Images (divided for a total of 19 segments per patient) were interpreted by two blinded observers for patient diagnosis and segmental comparison. The patient diagnosis was the same for the two protocols: normal = 3, ischemia = 27, and scar = 5. The segmental agreement (kappa = 0.90) was 632/665 (95.0%). The imaging performed at 15 minutes detected normal, ischemia, and scar in 413, 189, and 63 segments, respectively, whereas the imaging performed at 60 minutes detected 422, 180, and 63 segments, respectively (difference not significant). The early and delayed images were placed side by side for subjective comparison of the extent of the defect. Early imaging showed slightly larger defects in six patients, equal defects in 24 patients, and slightly smaller defects in five patients. Ischemic/normal wall ratios were 0.67 +/- 0.16 at 15 minutes and 0.68 +/- 0.15 at 60 minutes.

CONCLUSIONS

There is no clinically significant difference between SPECT imaging performed at 15 minutes or 60 minutes after the injection of MIBI at stress. Furthermore, this study showed that it is feasible to obtain good-quality MIBI images even 15 minutes after the injection at stress.

Transcapillary exchange of indium 111-labeled anticardiac myosin Fab and thallium 201 in isolated reperfused rabbit hearts

BACKGROUND

The physiologic mechanisms of 111In-labeled anticardiac myosin antibody [111In]-AM) imaging are fairly well established. However, to better understand the transcapillary exchange characteristics of normal and reperfused myocardium, a standard first-pass, indicator-dilution analysis was undertaken in hearts subjected to global no-flow and low-flow ischemia.

METHODS AND RESULTS

The first-pass myocardial transport of 201Tl and [111In]-AM was evaluated in an in vitro rabbit model of no-flow ischemia/reperfusion with indicator-dilution analysis during normal and ischemic flows and whole-blood perfusate. The maximum extraction fraction (Emax) of 201Tl was dominated by flow rate as expected and averaged 0.75 (+/- 0.009) and 0.57 (+/- 0.008) during ischemic and normal flows, respectively (p < 0.01). Emax values for [111In]-AM, which were 0.02 or less in all hearts at control, increased to 0.06 or greater at moderate to longer perfusion times after 50 or more minutes of no-flow ischemia. Permeability surface area (in milliliters per minute per gram) tended to decline for 201Tl with longer reperfusion periods in both ischemic and normal flow groups and paralleled the changes observed for Emax for [111In]-AM.

CONCLUSIONS

These data demonstrate that the first-pass extraction of [111In]-AM is quite low in this model of acute coronary occlusion and reflow and enhanced only in severe ischemia-reperfusion treatment. Therefore in this model there appears to be no significant [111In]-AM uptake in either normal or mildly ischemic myocardium. Consequently, [111In]-AM uptake into myocardium must depend on tracer recirculation, as well as sarcolemmal cell wall disruption, to achieve specific and sufficient [111In]-AM uptake for localization of clinical imaging.

Assessment of myocardial perfusion with single photon emission tomography

In order to compare Tc-99m-Sestamibi to T1-201 in the assessment of myocardial perfusion in the presence of coronary artery disease, 100 patients were studied with single photon emission computerized tomography (SPECT). Segmental analysis was carried out on 77 patients who underwent cardiac angiogram. The overall sensitivity and specificity rates in patients with myocardial infarction were respectively 92.5% and 98% for MIBI, 86.6% and 95% for T1; whereas in patients with chest pain, they were respectively 76.7% and 80% for MIBI, 69.8% and 70.5% for T1. Tc-MIBI correctly detected 11 lesions that were normal on T1-201, whereas the latter detected one lesion that was missed by Tc-MIBI. Concluding, we have the impression that Tc-MIBI not only correlated well with T1-201 but also showed marginal superiority over T1-201. Thus, Tc-MIBI is a valid alternative to T1 whenever the latter is not available.

Myocardial kinetics of radiolabeled perfusion agents: basis for perfusion imaging

The myocardial deposition of radiolabeled perfusion agents permits the noninvasive assessment of regional coronary blood flow. The design of imaging protocols and the optimal interpretation of clinical perfusion studies are based on an understanding of the kinetics of blood-tissue exchange for these compounds. Thallium 201 and the technetium 99m-labeled compounds sestamibi, teboroxime, and tetrofosmin show differing myocardial extraction and retention. This review focuses on studies that used cell culture, isolated heart, and intact animal models that form the basis of our current understanding of the myocardial kinetics of these imaging agents.

Myocardial technetium 99m sestamibi kinetics after reperfusion in a canine model

Clinical sestamibi imaging protocols after reperfusion therapy are based on the premise that redistribution does not occur. However, animal studies that use punch biopsies or imaging have variably reported either some or no redistribution. This study was designed to (1) utilize implantable radiation detectors to determine whether sestamibi is redistributed after reperfusion, (2) accurately determine the time course, extent, and kinetics of the redistribution, and (3) determine whether sestamibi kinetics can be used to document reperfusion and salvage after a single dose of sestamibi. Twenty-five dogs were injected with 5.0 mCi of technetium 99m sestamibi and microspheres during circumflex occlusion, and reperfusion occurred within 5 minutes in group 1 (15-minute occlusion) and group 2 (1-hour occlusion). Group 3 was not reperfused. Sestamibi activities in the normal and occluded zones were monitored with radiation detectors for 2 hours, and serial gamma camera imaging and arterial blood sampling was begun. No dogs in group 1 and all dogs in groups 2 and 3 had infarcts as shown by triphenyltetrazolium chloride stain. The final occluded/normal zone technetium 99m activity ratio was significantly higher than the flow ratio at the time of sestamibi injection only in the group 1 dogs (0.51 +/- 0.05 vs 0.38 +/- 0.06, p = < 0.0001). In addition, the mean 2-hour fractional sestamibi clearance from the occluded/reperfused zone (0.03 +/- 0.02) was significantly slower in the group 1 dogs compared with normal zone clearance (0.09 +/- 0.01, p = 0.03). Gamma camera images demonstrated large posterior wall perfusion defects initially, which persisted 2 hours later with no visual evidence of redistribution in any of the reperfused dogs in groups 1 and 2. Thus in an experimental animal model under ideal conditions, sestambi is redistributed into reperfused viable myocardium; however, the amount of this redistribution is small and could not be perceived by visual image analysis. Sestamibi is not redistributed into reperfused nonviable myocardium or into nonreperfused myocardium. Therefore sestamibi kinetics after a single dose of tracer in an experimental animal model can be used to document reperfusion of viable myocardium but cannot differentiate reperfusion of the infarcted territory from nonreperfused infarcted myocardium.

Experimental studies of the physiologic properties of technetium-99m agents: myocardial transport of perfusion imaging agents

The physiologic properties of new technetium-99m-labeled myocardial imaging agents (Tc-99m sestamibi, an isonitrile; and Tc-99m teboroxime, a boronic acid adduct of technetium dioxime) are discussed and compared to thallium-201 (Tl-201). Studies with isolated hearts, subcellular fractions and cell cultures indicate that Tc-99m sestamibi, Tc-99m teboroxime and Tl-201 do not share common transport or sequestration mechanisms. Although peak Tc-99m sestamibi myocardial extraction over time is about half that of Tl-201 at equivalent coronary blood flows, the amount of Tc-99m sestamibi that remains in the heart is similar to that of Tl-201 because of its higher retention efficiency. The high retention efficiency for Tc-99m sestamibi also results in minimal redistribution. In contrast, Tc-99m teboroxime myocardial extraction is higher than that of Tl-201, but its retention is less efficient, resulting in relatively rapid washout characteristics which may quickly result in tracer redistribution. During reperfusion after a no-flow period, Tc-99m sestamibi extraction and retention increase, but for Tc-99m teboroxime and Tl-201 these values tend to decrease. All tracers show adequate transport characteristics for perfusion imaging, and differences in transport and retention should lead to the development of new clinical protocols.