8.4 Blockade, Comfort, and Gastrointestinal Adjunct Agents
Key Takeaways
- Lidocaine is an amide local anesthetic injected intradermally or subcutaneously for painful access; EMLA® cream (lidocaine 2.5% plus prilocaine 2.5%) is applied under an occlusive dressing about 60 minutes before venipuncture and is a pediatric mainstay.
- H2 receptor antagonists—cimetidine, famotidine, or ranitidine—are given before Meckel diverticulum scintigraphy to inhibit parietal-cell secretion so pertechnetate is retained in ectopic gastric mucosa instead of washing into the lumen.
- Stable iodine blockade with Lugol's solution, SSKI, or potassium perchlorate protects the thyroid before radioiodinated agents such as I-123 MIBG and I-123 ioflupane; blockade starts before the dose and continues per the package insert.
- Atropine is an anticholinergic given in small incremental IV doses during dobutamine stress when the heart-rate target is not reached, and for symptomatic bradycardia—not as a vasodilator reversal agent.
- A fatty oral supplement such as Boost® or Ensure® stimulates endogenous cholecystokinin to contract the gallbladder, but standardized sincalide infusion remains the reference method when a numeric gallbladder ejection fraction is required.
8.4 Blockade, Comfort, and Gastrointestinal Adjunct Agents
Quick Answer: Lidocaine / EMLA® = comfort at the needle (EMLA needs ~60 minutes under occlusion). H2 blockers = retain pertechnetate in ectopic gastric mucosa for Meckel's. Lugol's / SSKI / perchlorate = thyroid blockade before radioiodinated agents. Atropine = rescue chronotropy during dobutamine stress. Boost®/Ensure® = fatty-meal gallbladder contraction. Contrast media = hybrid CT, with allergy, renal, and iodine-load consequences.
Section 8.2 covered the stress, renal, biliary, brain, and therapy-support adjuncts. The blueprint also names a second group that shows up in real workflow constantly and on the exam through prep and timing items.
Local Anesthesia: Lidocaine and EMLA® Cream
| Agent | Form and use | Timing | Cautions |
|---|---|---|---|
| Lidocaine | Amide local anesthetic, commonly 1%–2% injected intradermally/subcutaneously before difficult IV access, arterial lines for Y-90 mapping support, or physician-performed intrathecal punctures | Onset within minutes; duration roughly 1–2 hours | Confirm allergy history (amide vs ester agents); use preservative-free product when the route requires it; aspirate to avoid intravascular injection; systemic toxicity presents as perioral tingling, tinnitus, then arrhythmia or seizure |
| EMLA® cream | Eutectic mixture of lidocaine 2.5% + prilocaine 2.5%, applied topically under an occlusive dressing | Apply about 60 minutes before venipuncture (longer for deeper procedures) | The prilocaine component carries a methemoglobinemia risk in infants and in patients on other oxidizing drugs; respect maximum application area and dwell time by weight |
Practical scheduling matters: an EMLA site must be prepped an hour ahead, so pediatric bone scans and renograms are ordered with cream applied on the floor before transport. Technologists apply and remove topical anesthetics under institutional policy; injected lidocaine follows the site's credentialing rules.
Acid Blockade for Meckel Diverticulum Scintigraphy
Tc-99m pertechnetate is secreted by gastric mucin-producing cells, so ectopic gastric mucosa in a Meckel diverticulum concentrates it. Secretion into the bowel lumen then washes activity away and blurs the focus.
| Premedication | Effect |
|---|---|
| Cimetidine, famotidine, or ranitidine (H2 antagonists) | Inhibit parietal-cell acid secretion, which reduces washout of pertechnetate from the mucosa and raises target-to-background — the standard, well-tolerated choice |
| Pentagastrin | Stimulates uptake but is less favored because of side effects |
| Glucagon | Slows peristalsis (about 50 µg/kg IV, max 1 mg, in sterile water—not saline); avoided in diabetic patients and used selectively |
H2 blockade is typically started the day before or the morning of the study per protocol. Trap: giving a perchlorate blocking agent instead—perchlorate blocks the trapping mechanism and would defeat the study.
Stable Iodine Thyroid Blockade
Free radioiodide from radioiodinated agents concentrates in the thyroid. Blockade saturates or blocks the sodium-iodide symporter (NIS).
| Agent | Notes |
|---|---|
| Lugol's solution | Strong iodine solution; dose in drops per protocol |
| SSKI (saturated solution of potassium iodide) | Equivalent blockade; often easier to dose |
| Potassium perchlorate | Competitive NIS blocker; the alternative when iodine allergy or sensitivity is a concern |
When it applies: I-123 MIBG (AdreView®), I-123 ioflupane (DaTscan®), and other radioiodinated products per their inserts. Blockade begins before the radiopharmaceutical (commonly about an hour ahead) and may continue for a defined period afterward. Never give stable iodine to a patient scheduled for a thyroid uptake, radioiodine survey, or I-131 therapy—it floods the iodine pool and ruins the study.
Atropine
Atropine is an anticholinergic that blocks vagal tone and increases heart rate.
| Use | Practice |
|---|---|
| Dobutamine stress | Small incremental IV doses when the target heart rate is not achieved at the top infusion stage |
| Symptomatic bradycardia | Emergency use per ACLS and standing orders |
| Cautions | Narrow-angle glaucoma, significant urinary retention/prostatism, tachyarrhythmias; expect dry mouth, blurred vision, and urinary hesitancy |
Trap: atropine supports dobutamine; it does not reverse it. Excess dobutamine effect is managed by stopping the infusion and, if needed, a short-acting beta blocker. Aminophylline reverses vasodilator stress, not either of these.
Oral Fat Challenge: Boost® and Ensure®
A fatty oral supplement triggers endogenous cholecystokinin release and gallbladder contraction.
| Scenario | Role |
|---|---|
| Prolonged fasting or TPN before hepatobiliary imaging | Contract a distended, sludge-filled gallbladder so tracer can enter |
| Gallbladder emptying assessment when standardized sincalide is unavailable | Acceptable alternative, but reference values differ |
| Quantitative gallbladder ejection fraction | Standardized sincalide infusion is the reference method—do not substitute a milkshake and report a numeric GBEF against sincalide norms |
Also remember the subdiaphragmatic activity use case: a fatty snack or milk can accelerate hepatobiliary clearance before repeat myocardial perfusion imaging when liver and bowel activity crowd the inferior wall.
Contrast Media (Oral and IV)
Hybrid imaging brings CT contrast into the nuclear medicine department.
| Consideration | Technologist action |
|---|---|
| Allergy history | Screen for prior contrast reaction; premedication with corticosteroid plus antihistamine is a physician order with a defined lead time |
| Renal function | Check creatinine/eGFR per policy before iodinated IV contrast |
| Metformin | Follow institutional policy on holding and restarting after IV contrast |
| Iodine load | Iodinated contrast blocks thyroid radioiodine uptake for weeks to months—never give it shortly before a thyroid uptake, radioiodine survey, or planned I-131 therapy |
| Extravasation | Recognize swelling, pain, and firmness; stop the injection, elevate, and follow the reaction protocol |
| Oral contrast | Bowel opacification for diagnostic CT; may cause attenuation-correction artifacts on PET/CT |
Matching Table
| Prompt | Adjunct |
|---|---|
| Child terrified of the needle for a bone scan | EMLA® applied about an hour ahead |
| Meckel scan ordered for painless rectal bleeding | H2 blocker premedication |
| I-123 MIBG for suspected pheochromocytoma | Lugol's/SSKI or perchlorate blockade |
| Dobutamine at 40 µg/kg/min, heart rate still 62 | Atropine increments per protocol |
| Hepatobiliary study in a patient on TPN for two weeks | Sincalide pretreatment or a fatty meal per protocol |
| PET/CT with diagnostic-quality CT ordered | IV/oral contrast with allergy and renal screening |
Bottom line: these agents change comfort, secretion, uptake, or heart rate—never the radionuclide. Learn the timing relative to the tracer and the one screening question that stops the study.
Why is an H2 receptor antagonist such as cimetidine or famotidine given before Meckel diverticulum scintigraphy?
EMLA® cream is ordered for a pediatric patient before a renogram. Which statement reflects correct practice?
A patient scheduled for I-123 MIBG imaging asks why she was told to take an iodine solution before the injection. The best explanation is that stable iodine: