Sunday, November 22, 2015

Vasodilator Stress Agents


Mechanism of action
T 1/2
Administration
Side effects
Contraindications
Adenosine
A2A receptor à direct coronary vasodilation à 3.5- 4 x increased blood flow à stenotic epicardial coronaries will have attenuated hyperemic response.
True ischemia may be induced by coronary steal phenomenon.
Secs
140 mcg/kg/min over a 6 min period
Minimum time to tracer injection: 2 mins
Continue infusion for 2 mins after tracer injection
Nonspecific chest pain: not an indicator of CAD
ST depression >1mm: indicative of significant CAD
A1 receptor: AV block à does not require termination of infusion
A2b receptor: Hypotension
A2b and A3 receptors: Bronchospasm
Resolve in few seconds (due to extremely short half-life of adenosine)
Asthma with ongoing wheezing
2/3 AV block without a pacemaker
SBP <90 mmHg
Use of dipyridamole in last 48 hours; aminophylline in 24 hours or caffeine in last 12 hours (pentoxifylline OK)
ACS
Regadenoson
A2A receptor à direct coronary vasodilation (lower affinity than adenosine)
2-4 mins
0.4mg/10 ml given as rapid injection followed by saline flush
Tracer injection 10-20 secs after
SOB, flushing, headache
Most resolve within 15 mins, headaches resolve in 30 mins
(Use aminophylline 50-250 mcg infused slowly)
Hypotensionѱ
Asthma with ongoing wheezing
2/3 AV block without a pacemaker
SBP <90 mmHg
Use of dipyridamole in last 48 hours; aminophylline in 24 hours or caffeine in last 12 hours (pentoxifylline OK)
ACS
Dipyridamole
Prevents reuptake and deamination of adenosine à indirect coronary vasodilator
Dipyridamole induced hyperemia lasts more than 15mins
30-45 mins
(liver)
0.56mg/kg IV over 4 mins (142mcg/kg/min)
Tracer injection after 3-5 mins
Flushing, chest pain, headaches, dizziness.
Resolve in 15-25 mins
(Use aminophylline 125-250 mcg infused slowly- should also be used in the presence of ischemic changes after dipyridamole)
Same as adenosine.
CAN be used in patients taking oral dipyridamole.
Dobutamine
Direct B1 and B2 stimulation
Increase in heart rate, blood pressure and myocardial contractility.
Increases regional myocardial blood flow (similar to exercise, less than adenosine).
Does not increase venous return (no increase in wall stress)
2 mins
5mcg/kg/min increased at 3 minute intervals to 40mcg/kg/min. Tracer injection at peak dose and continue infusion for 2 mins after
Palpitations, chest pain, headache, flushing, dyspnea
(Use esmolol 0.5mg/kg over 1 min)
ACS
Hemodynamically significant LVOT obstruction
Severe AS
Prior h/o VT
Aortic dissection/large aortic aneurysm
Uncontrolled hypertension
B blockers
Ѱ Risk of hypotension higher in patients with autonomic dysfunction, hypovolemia, left main coronary artery stenosis, stenotic valvular heart disease, pericarditis, pericardial effusions, stenotic carotid artery disease with cerebrovascular insufficiency

Other Myocardial Tracers

Rb-82
Ultra short half life
Myocardial perfusion
PET scanning
O-15 Water
Free diffusible, uptake directly proportional to blood flow
Best tracer for myocardial blood flow detection
PET Scanning
N-13 ammonia

Myocardial perfusion
PET Scanning
C-11 acetate
Oxidative substrate
Can be used for both perfusion and metabolism
PET Scanning
C-11 palmitate
Fatty acid substrate
Metabolism
PET Scanning
FDG
Glucose metabolism
Metabolism
PET Scanning
BMIPP (I-123)
Fatty acid
Myocardial ischemic memory imaging
(normal myocardium metabolizes fatty acids, during ischemia switches to glucose; if ischemia occurred dark spots on BMIPP imaging, used for detection of ischemia that occurred hours prior. Also, provides viability assessment)
MIBG
Similar to norepinephrine
Sympathetic neuronal imaging for heart failure

PET Perfusion Tracers


Rb-82
N13 ammonia
T 1/2
75 secs
10 mins
Dose/Injection Rate
20mCi
Bolus <30secs
BGO/LSO/GSO scanners
10-20mCi
Bolus <30secs
Extraction Mechanism
Na/K ATPase pump
Extraction decreases with increasing blood flow
Can be decreased by hypoxia, acidosis and ischemia
Na/K transporter
Passive diffusion
Type of stress used
Pharm stress
(short t1/2 of Rb, patient needs to remain still in camera)
Pharm stress
Imaging delay after injection
EF>50%: 70-90secs
EF <50%: 90-130 secs
1.5-3mins
Imaging duration
3-6 mins
10-15 mins
Reconstructed pixel size
4 mm
2-3 mm
Organ receiving maximum dose
Kidney
Urinary bladder

Radiotracers: Thallium and Technetium


Thallium 201
Technetium 99m
(Sestamibi/ Cardiolite, Tetrofosmin/Myoview)

Potassium analog; monovalent cation
Lipid soluble, cationic
Preparation
Cyclotron
Generator (eluted from Mo 99m)
Half life
73.1 hours

6 hours (therefore larger doses can be given resulting in higher counts, less scatter and less tissue attenuation)
Decay
Electron capture to Hg-201/ X rays
Isomeric transition/ Gamma ray emission
Energy release
68-80 keV
140 keV
First pass extraction
High (85%)
Lower (55%)
Pharmacodynamics
Active membrane transport in to the myocyte through Na/K pumps
Rapid clearance from intravascular space
Diffuses in to cells- no active uptake
Uptake depends on blood flow
Enters mitochondria due to transmembrane energy potentials (mitochondria à negatively charged membrane)
Myocardial extraction coefficient
Highest: 85% (peak concentration 10 mins after injection)

65% (S)
54% (T)
Redistribution
Mono-exponential  (depends on initial tracer concentration in the myocyte and myocardial blood flow) washout that starts 10-15 mins after injection
Negligible washout therefore no redistribution
Clearance
Kidneys
Hepatobiliary system (tetrofosmin clears liver earlier than sestamibi)
Dose administered
2.5-4 mCi
10-20 mCi: sestamibi
5-33 mCi: tetrofosmin
Whole body radiation dose
0.68 rad
0.5 rad
Whole body effective dose
6.3mSv/mCi of Tl-201 injected
0.3mSv/mCi of Tc 99m injected
Collimator
GAP/ LEAP
High Resolution (LEHR)
Imaging protocol
Exercise/Pharm Stress àTh-201 injection à 10-15mins àImaging (stress) à 2-4 hours à Imaging (rest/redistribution)
1 day protocol: Patient at rest à  Tc injection à 45 mins à Imaging à
-Exercise Stress à Tc injection (2-3 times higher dose) à15-20 mins àImaging
-Pharm stress à Tc injection (2-3 times higher dose) à 45mins à Imaging

2 day protocol*: Stress imaging on day 1; rest imaging on day 2 (if needed)
Time to imaging
Rest/ resdistribution:
Pharmacologic stress:
Exercise:

2.5- 4 hours after stress
10-15 mins
10-15 mins

45-60 mins (S)/ 30-45 mins (T)
60 mins (S)/ 45 mins (T)
15-20 (S)/ 10-15 mins (T)
Limitations
Low energy emission, therefore:
-More image attenuation (especially obese patients)
-Longer imaging times
Short shelf life: must be ordered everyday/ twice a day

Clinical uses
CAD diagnosis
Viability assessment
CAD diagnosis: sharper images
LV function
Minimum number of counts
100 counts/ pixel
200 counts/pixel
Minimum number of projections
32
60-64
*2-day protocol used in BMI >30; female patients where significant breast attenuation in expected