8.2 Interventional and Adjunct Pharmaceuticals
Key Takeaways
Pharmacologic MPI stress uses coronary vasodilators (dipyridamole, adenosine, regadenoson) or the inotrope dobutamine; reverse methylxanthine effects with aminophylline and respect caffeine holds
Captopril/enalaprilat unmask renovascular hypertension patterns; furosemide challenges drainage on diuretic renography
HIDA adjuncts: morphine diverts bile into a patent cystic duct; sincalide (CCK) contracts the gallbladder for ejection fraction
Acetazolamide (Diamox®) tests cerebrovascular reserve; insulin-glucose protocols prepare FDG myocardial viability studies
Thyroid pathway adjuncts include Lugol’s/SSKI (stable iodine blockade) and rhTSH (Thyrogen®); PRRT needs antiemetics and amino acids; heparin/ACD support cell labeling
8.2 Interventional and Adjunct Pharmaceuticals
Quick Answer: Adjuncts are nonradioactive drugs that create the physiology the tracer measures. Memorize what each does, when it is given relative to the RP, key contraindications, and reversal (especially aminophylline after dipyridamole/related stress).
Domain III items often fail candidates on timing and holds, not on isotope half-life.
Pharmacologic Stress for MPI
When patients cannot exercise adequately, pharmacologic coronary hyperemia lets perfusion tracers map flow heterogeneity.
Vasodilators: Dipyridamole, Adenosine, Regadenoson
| Agent | Mechanism (teaching) | Typical administration concept | RP timing concept |
|---|---|---|---|
| Dipyridamole | Blocks adenosine reuptake → ↑ endogenous adenosine | ~0.56 mg/kg IV over 4 min (classic) | Inject MPI tracer near peak hyperemia (~3–5 min after infusion per protocol) |
| Adenosine | Direct A2A (and other) receptor agonist | ~140 µg/kg/min IV for ~4–6 min | Tracer at mid-to-late infusion while hyperemia maintained |
| Regadenoson (Lexiscan®) | Selective A2A agonist | Fixed 0.4 mg IV bolus (~10 s) + flush | Tracer typically 10–20 s to ~1 min after injection per protocol |
Shared effects: flushing, chest pressure, dyspnea, headache, AV block risk, hypotension. Caffeine and methylxanthines antagonize vasodilators—hold ~12–24 h (protocol; often 24 h taught for caffeine).
Contraindications / cautions (vasodilators): active bronchospasm/severe reactive airway disease (important for nonselective agents; regadenoson is more A2A-selective but still use caution per protocol), high-grade AV block without pacemaker, symptomatic hypotension, acute coronary syndromes per stress-lab rules, recent caffeine/theophylline.
Reversal: Aminophylline
Aminophylline (a theophylline compound) is the classic reversal agent for dipyridamole-related (and often persistent vasodilator) side effects by adenosine-receptor antagonism. It is not the first-line “antidote” concept for dobutamine (use beta blocker). Have it available in the stress lab; dose per ACLS/stress protocol (commonly discussed ~50–250 mg IV slow for persistent symptoms—follow current lab standing orders).
Dobutamine Stress
Dobutamine is a β1-predominant agonist that raises heart rate, contractility, and myocardial oxygen demand—used when vasodilators are contraindicated (e.g., severe asthma) or for selected viability/stress echo crossover protocols.
| Item | Teaching point |
|---|---|
| Infusion | Graded increase (e.g., 5 → 10 → 20 → 30 → 40 µg/kg/min) to target HR |
| Tracer timing | At peak heart rate / peak stress stage |
| Side effects | Palpitations, arrhythmias, hypertension/hypotension, chest pain |
| Reversal | Short-acting beta blocker (e.g., esmolol/metoprolol per protocol)—not aminophylline |
Atropine may be added if HR fails to rise. Contraindications include severe outflow obstruction and uncontrolled arrhythmias per stress guidelines.
Renal Adjuncts: Captopril, Enalaprilat, Furosemide
| Drug | Role | Timing concept |
|---|---|---|
| Captopril (oral) / enalaprilat (IV) | ACE inhibition for renovascular hypertension captopril renography—drops GFR in the stenotic kidney via efferent arteriole dilation | Give ACEI per protocol before the diagnostic renogram phase; baseline vs post-ACEI comparison |
| Furosemide (Lasix®) | Loop diuretic challenge to distinguish obstructed vs dilated non-obstructed systems | F−15, F+0, F+20, etc.—know that washout after diuretic is the interpretive target |
Cautions: hypotension after ACEI; hold chronic ACEI only as protocol requires for baseline/post comparison validity; ensure hydration and bladder drainage for renograms. Sulfonamide allergy history may matter for some diuretics—follow site policy.
Hepatobiliary Adjuncts: Morphine and Sincalide (CCK)
| Adjunct | Mechanism | When used |
|---|---|---|
| Morphine | Contracts sphincter of Oddi, raising common-duct pressure to fill GB if cystic duct patent | Nonvisualized GB at ~60 min with adequate hepatic excretion and bowel activity |
| Sincalide (CCK analog) | Stimulates GB contraction | Gallbladder ejection fraction; empty viscous GB after prolonged fasting/TPN before tracer in some protocols |
Typical morphine teaching dose is on the order of 0.04 mg/kg IV (max often ~2–4 mg)—confirm protocol. CCK side effects: abdominal cramping, nausea. Opioids given before the study for pain can alter GB filling—document home meds.
Brain, Cardiac Viability, and Thyroid Adjuncts
| Drug | Study context | Key point |
|---|---|---|
| Acetazolamide (Diamox®) | Brain perfusion reserve (e.g., with Tc-99m HMPAO/ECD) | Carbonic anhydrase inhibitor → cerebral vasodilation; compares baseline vs Diamox challenge |
| Insulin / glucose | FDG myocardial viability | Metabolic preparation (glucose loading ± insulin) shifts myocardium to glucose utilization so viable hibernating tissue takes FDG |
| Lugol’s / SSKI | Stable iodine | Blocks thyroid uptake of free radioiodine/radioiodinated byproducts (e.g., some iodinated RPs, MIBG-related teaching) |
| Thyrogen® (rhTSH) | Thyroid cancer pathway | Raises TSH without thyroid hormone withdrawal to stimulate NIS for I-123/I-131 imaging or ablation prep |
Diamox cautions: sulfa allergy cross-reactivity concerns, electrolyte shifts, and contraindication in some cirrhosis/severe renal failure settings. Insulin protocols require glucose monitoring to avoid hypoglycemia.
Supportive Meds for Therapy and Labeling
| Adjunct | Use |
|---|---|
| Ondansetron (Zofran®), aprepitant (Emend®) | Antiemetics for PRRT, high-dose I-131, AA infusions |
| Amino acid solutions | Renal protection during Lutathera (see §8.1) |
| Beta blockers | Rate control; reverse dobutamine; sometimes hold before certain stress tests if ordered |
| Heparin / ACD-A | Anticoagulation of drawn blood for RBC/WBC labeling to prevent clotting in syringes/kits |
| Iodinated / other contrast | Hybrid CT or correlative angiography—coordinate allergy premed, renal function, and interference with iodine uptake if radioiodine therapy/imaging is imminent |
Know when contrast is part of SPECT/CT or Y-90 mapping days and where emergency meds are kept.
Timing Cheat Sheet (RP Relative)
| Scenario | Adjunct before RP? | Adjunct with/after RP? |
|---|---|---|
| Vasodilator MPI | Start pharma stress first | Inject tracer at peak hyperemia |
| Dobutamine MPI | Escalate infusion first | Tracer at target HR |
| Captopril renogram | ACEI per protocol before diagnostic phase | Tracer after ACEI effect window |
| Diuretic renogram | Often tracer first | Furosemide at protocol minute |
| Morphine HIDA | Tracer first; image ~60 min | Morphine if GB still out |
| CCK EF | Tracer fills GB | CCK after GB visualized |
| Diamox brain | Challenge dose timed to perfusion agent | Protocol-specific dual-day or split |
Exam trap: giving caffeine before adenosine, morphine before a “rule-out acute cholecystitis” without documenting opioids, or aminophylline to reverse dobutamine. Always ask: Does this drug create hyperemia, change GFR, divert bile, block thyroid, protect kidney, or anticoagulate a blood label?
A patient develops persistent severe symptoms after dipyridamole pharmacologic stress. Which reversal agent is most appropriate?
Intravenous sincalide to increase coronary flow further
Aminophylline to antagonize adenosine-mediated effects
High-dose furosemide to clear dipyridamole renally within seconds
Sublingual nitroglycerin as the specific pharmacologic antidote for A2A agonism
Morphine is administered during a Tc-99m mebrofenin study primarily to:
Stimulate gallbladder contraction for ejection fraction measurement
Block hepatocyte uptake so only the spleen is visualized
Contract the sphincter of Oddi and promote gallbladder filling if the cystic duct is patent
Reverse adenosine stress before a same-day rest MPI injection
Which pairing of adjunct drug and nuclear medicine purpose is correct?
Acetazolamide — evaluate cerebrovascular vasodilatory reserve with brain perfusion imaging
Regadenoson — primary reversal agent for dobutamine-induced tachycardia
Lugol’s solution — mandatory co-infusion for every Lu-177 PSMA therapy dose
Heparin — oral blockade of thyroid NIS before I-123 scanning
Sections you finish are checked off in the contents.