Hybernating Myocardium

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LOW DOSE DOBUTAMINE DETECTION OF HIBERNATING
MYOCARDIUM BY Tc-99m MIBI GATED SPECT
Dr. Asghar H. Asghar
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Hibernating Myocardium (HM)
Persistent painless hypoperfusion of the
myocardium leading to contractile dysfunction,
which gets improve after successful
revascularization.
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Characteristics Of HM

• LV dysfunction at rest.
• Prolonged reduction in perfusion.
• Mismatch in perfusion and metabolism.
• Match in perfusion and contractile function.
• Revascularization usually reverse it.
• No biochemical evidence of ischaemia

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Techniques for Detection of HM

• Flow/Metabolism assessment
• Cell membrane integrity detection
• Wall motion/Contractile function

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CELLULAR METABOLISM FLOW

• Flow Rb-82
• Metabolism
• Glucose F-18 FDG
• Free Fatty Acids I-123 BMIPP
• C-11 Palmitate
• C-11 Acetate

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Glucose Metabolism

• 18FDG PET quite common
• gold standard
• Increased uptake of 18FDG
• Reduced perfusion
• Flow Metabolism Mismatch

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F-18 FDG PET
Studies 11 (n332) Sensitivity 88 Specifi
city 73
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Limitations of Glucose Metabolism

• Substrate Availability
• Fasting
• Glucose Loading
• Diabetes
• Catecholamines and harmone levels
• Difficult to interpret as a measure of
  viability in 20 cases

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Cell Membrane Integrity

• Imaging of choice, where PET is not available.
• Not dependant upon physiological and metabolic
  variables affecting intracellular metabolism.
• Thallium (Tl -201 ) or Tc-99m MIBI are commonly
  radioisotopes used for this purpose.

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Thallium-201(Tl)

• K analogue
• Simple and convenient method
• Widely available isotope
• 85 to 90 first pass extraction
• Redistribute in 3-4 hours

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Protocol of Tl-201 for HM

• Stress, redistribution, and 24 hours delayed
  imaging.
• Stress, redistribution, and reinjection imaging.
• Stress, immediate reinjection, and redistribution
  imaging.
• Rest and redistribution imaging.

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Tl-201 SPECT

• Stress-Redistribution-Reinjection
• Studies 7 (n209)
• Sensitivity 86
• Specificity 47
• Rest-Redistribution
• Studies 8 (n145)
• Sensitivity 90
• Specificity 54

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Relationship of Tl-201 and FDG PET imaging
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Tc-99m MIBI

• Second most commonly used perfusion agent.
• It enters passively through the cell membrane.
• Concentration in cytosol is 51, whereas it
  increases up to 3001 in mitochondria.
• In HM, its functional need is reduced, as
  mitochondrial functions are compromised.

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Wall Motion Detection

• Stress Echocardiography
• Magnetic Resonance Imaging (MRI)
• Radionuclide Ventriculography (RNVG)
• Gated SPECT

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Stress Echocardiography

• Studies 19 (n448)
• Sensitivity 84
• Specificity 81

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MRI

• One of the non-invasive technique.
• It can be used for detection of viability.
• Helps in calculation of EDV, ESV and EF.
• Significant concordance between Gated MRI and
  post-revascularization findings.

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RNVG

• Can also be used for detection of regional wall
  motion/thickening (WM/WT) and EF.
• Viable myocardium shows WM/WT, while scar
  myocardium do not show it.
• Limitations are of breast and diaphragmatic
  attenuation, and of apical thinning.

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Gated SPECT

• It helps in
• Both perfusion and wall motion detection.
• LV EF and ventricular volumes calculation.
• 3-D display of endocardial, epicardial or of
  both.
• Regional quantitation by its polar map system.
• Differentiation of an artifact from actual
  lesion.

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Diagrammatic Representation of Gated SPECT
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Gating Techniques

• Fixed Mode
• Fixed Temporal Resolution Framing
• Forward Formatting
• Backward Formatting
• Variable Temporal Resolution Framing
• List Mode

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Different Techniques for Framing of RR Interval
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Dobutamine

• Sympathomimatic amine.
• Effect on ?-1 (mainly), ?-2, and ?-1
  adrenoceptors.
• At low doses, it has more inotropic effect.
• Helps in differentiation of viable from scar
  myocardium.

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Aims and Objectives

• To assess the role of low dose dobutamine for
  detection of HM by gated SPECT and its comparison
  with perfusion imaging alone.

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Material

• Patients fulfilling the selection criteria.
• Radioisotope ? Tc-99m.
• Radiopharmaceutical ? MIBI
• Dobutamine
• Gamma Camera (Picker International).

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Patient Selection Criteria

• Known patient of CAD.
• In case of MI, not lt3 months old.
• LV dysfunction with EF ? 45.

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Patients Exclusion Criteria

• EF ? 45 or any previous intervention
• Cardiac failure or BBB
• Severe hypertension
• Fresh MI or superceded MI after old MI
• Unstable angina or arrhythmia on ECG

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Method

• Fasting of 4 hours.
• Tc-99m MIBI (30 mCi) was injected I/V and
  scanning started after one hour.
• First baseline and then dobutamine (7 ?g/kg/min)
  induced gated SPECT scans were done.

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Perfusion Grading

• 4 ? Normal perfusion
• 3 ? Mild reduction in perfusion
• 2 ? Moderate reduction in perfusion
• 1 ? Severe reduction in perfusion
• 0 ? No perfusion

Iskandrian AS et Iskandrian et al., JNC
1998,5(2)206-209
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WM/WT Grading

• 4 ? Normal WM/WT
• 3 ? Mild hypokinesia/reduction in WT
• 2 ? Moderate hypokinesia/reduction in WT
• 1 ? Severe hypokinesia/reduction in WT
• 0 ? Akinesia/dyskinesia

Iskandrian AS et al. Iskandrian et al., JNC
1998,5(2)206-209
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Segmental Division of SPECT Slices
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Demographic Results

• Study period from July, 99 to Jan, 2000.
• 21 patients (17 M 4 F) aged 51?11 Yr.
• 13 patient had Q-wave MI.
• 6 had received streptokinase.
• Average EF of all patient was 36 ? 8.

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Risk Factors

• Risk factors were
• DM
• Hypertension
• High lipid profile
• Family H/O IHD
• Smoking

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Echocardiographic Results
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Angiographic Findings
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Stenotic Finding of Angiography
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Non-Viable Segments (G Vs UG)
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Non-Viable Segments (BG Vs DG)
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Viable Segments (BG Vs DG)
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Effect of Dobutamine
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Limitation of Study

• Duration of study was insufficient.
• Revascularization is a costly procedure.
• QGS software was not available.
• Limitation of exact matching of segments.
• Low dose dobutamine echo. was not done.

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Conclusions

• Routine gated SPECT has no added advantage as
  compared to ungated perfusion SPECT at least for
  detection of viable myocardium (pNS).
• Its better to use gating in conjunction with low
  dose dobutamine, as it significantly enhances the
  detection of viable myocardium (plt0.006).

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THANKS