Nursing Field Notes / Pharmacology · GI · Antiemetics by Receptor
Antiemetics 🤢
Anti-Nausea & Vomiting — one reflex, four receptors, four drug classes
NG-020PHARM · GIADHD-friendly visual edition
Vomiting is a reflex run by the vomiting center in the medulla. Four different pathways can pull its trigger, and each pathway uses a different receptor. That is the whole logic of this drug class: match the antiemetic to the receptor that is being stimulated. Chemo → serotonin. Motion → histamine & acetylcholine. Drugs & toxins in the blood → dopamine. Pick the wrong receptor and the drug does nothing.
Learn this one picture and you never have to memorize a drug list again — you derive it.
🧠 The vomiting center, the CTZ, and the four roads that reach them
EXAM TIP The vomiting center sits in the medulla and is the final common pathway — it actually fires the reflex. The chemoreceptor trigger zone (CTZ) sits next to it in the area postrema on the floor of the fourth ventricle, in a spot with essentially no blood–brain barrier — so it can "taste" drugs, toxins and metabolic waste circulating in the blood and tell the vomiting center about them.
Gold dashed arrows = the four stimulus roads. The CTZ (teal) senses the blood; the vomiting center (red) pulls the trigger.
🧠 "CTZ = Chemical Tasting Zone." It sits outside the blood–brain barrier so it can taste the blood. That single anatomical fact explains why IV chemo, opioids and kidney failure all cause vomiting without ever touching the stomach.
🔒 Four receptors, four blockers — the membrane close-up
🧠 "Four locks, four keys." 5-HT₃ = -setron · D₂ = -pramide / -perazine · H₁ = -zine (meclizine, dimenhydrinate) · muscarinic = scopolamine. Match the lock to the cause.
📊 The receptor comparison table — the page in one grid
Delayed chemo-induced N/V, with a setron + steroid
Fatigue, hiccups; multiple CYP drug interactions
🧠 "SDHA-N" — Serotonin, Dopamine, Histamine, Acetylcholine, Neurokinin. Say it as "Sad-ha, no" — the five receptors that can make you throw up.
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WATCH FOR
STEP 2 · THE TWO THAT GET TESTED
Ondansetron's danger is on the monitor. Metoclopramide's danger is on the face.
🚨 HESI EXIT priority side effect: ondansetron → Torsades de Pointes
Ondansetron prolongs the QT interval. A long QT can degenerate into Torsades de Pointes — a polymorphic ventricular tachycardia that presents as syncope and can deteriorate into ventricular fibrillation.
🧪 Check potassium and magnesium before and during therapy — hypokalemia and hypomagnesemia lengthen the QT further, and a vomiting patient is already losing both.
📈 Know the baseline QTc; use caution with congenital long QT and with other QT-prolonging drugs (many antipsychotics, some antibiotics, antiarrhythmics).
💉 Give IV doses slowly — rapid administration increases the risk.
🤕 Common, benign effects: headache and constipation.
⚠️ Serotonin syndrome is possible when a setron is combined with other serotonergic drugs — agitation, hyperthermia, clonus/hyperreflexia, diaphoresis, tremor.
🧠 "Zofran stretches the Q-T; low K and low Mg stretch it further." The vomiting itself is what depletes K⁺ and Mg²⁺ — so the very reason you gave the drug is what makes it dangerous. Check the electrolytes.
🚨 Metoclopramide → EPS: lip smacking · puffing of the cheeks · blinking of the eyes
Metoclopramide blocks dopamine — and dopamine blockade anywhere produces extrapyramidal symptoms. The three key words from the source are the classic features of tardive dyskinesia, which may be irreversible. Question the prescription and REPORT TO THE HCP IMMEDIATELY.
🚫 HESI: metoclopramide is contraindicated with a bleeding duodenal ulcer — and with GI obstruction or perforation, because it increases GI motility and would worsen the bleed or rupture.
🕰️ EPS risk climbs with higher doses, longer duration and older age. Keep courses short.
💊 Acute dystonia is treated with diphenhydramine or benztropine — but tardive dyskinesia is not reversed that way.
🧠 "M–M: Metoclopramide = Major lip smacking & puffing cheeks." Two M's, one action — report.
😴 Antihistamines & scopolamine — sedation and dry everything
🩹 Scopolamine patch: apply behind the ear on clean, dry, hairless skin; wash hands before and after — drug on the fingers rubbed into an eye causes a dilated pupil and blurred vision.
⏱️ Apply several hours before the trip; it is prophylaxis, not rescue.
🛏️ Sedation is expected with H1 antiemetics — fall precautions, no driving, no alcohol.
👵 In older adults, anticholinergics cause confusion, urinary retention and constipation — a common cause of new delirium.
💉 Promethazine IV carries a serious risk of severe tissue injury with extravasation; dilute, give slowly into a large, patent vein, and stop for any burning.
🧠 Anticholinergic = "can't see, can't pee, can't spit, can't poop" — plus "can't remember" in the elderly.
🧪 Hypokalemia and hypochloremia from lost gastric contents → muscle weakness, dysrhythmias.
⚖️ Metabolic alkalosis — you are literally vomiting acid out of the body.
🫁 Aspiration risk — position side-lying or upright, especially if the client is sedated or has a decreased LOC.
🩸 Hematemesis or coffee-ground emesis, or vomiting after a head injury → escalate immediately.
🧠 "Every vomit costs acid, potassium and water." Replace all three, and turn them on their side while you do it.
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TEACH & ACT
STEP 3 · THE QUESTIONS THEY ASK
Two exam scenarios show up again and again — one is a priority-action item, one is a report item.
⭐ HESI: child vomits DURING a chemotherapy infusion — priority action?
1
STOP the chemotherapy infusion. 🛑
▼
2
Flush the line. 💧
▼
3
Administer ondansetron. 💊
WHY THIS ORDER You always remove the cause before you treat the symptom. Giving the antiemetic first while the offending drug keeps infusing is treating around the problem. ATI: ondansetron is used to decrease nausea and vomiting caused by chemotherapy.
🧠 "Stop · Flush · Zofran." Three beats. If an option says "administer the antiemetic" but another says "stop the infusion" — stopping wins.
✅ General antiemetic teaching
⏰ Give it BEFORE the trigger — before chemo, before the car ride, before ambulating post-op. Antiemetics prevent far better than they rescue.
🥤 Small sips of clear fluids; advance slowly. Cold, dry, bland foods; avoid strong smells and greasy food.
🪑 Sit upright after eating; rise slowly (many of these drugs drop the BP).
🚗 No driving and no alcohol on any sedating antiemetic.
Q1: What is the priority side effect of ondansetron?
QT prolongation leading to Torsades de Pointes. Check baseline QTc and correct hypokalemia and hypomagnesemia — both of which the vomiting itself causes.
Q2: A child vomits during a chemo infusion. Priority nursing action?
Stop the chemotherapy, flush the line, then administer ondansetron.
Q3: A client on metoclopramide has lip smacking, puffing of the cheeks and blinking. What now?
These are extrapyramidal symptoms / tardive dyskinesia and may be irreversible. Question the prescription and report to the HCP immediately.
Q4: Metoclopramide is ordered for a client with a bleeding duodenal ulcer. Response?
Hold and clarify — it is contraindicated. It increases GI motility, which is dangerous with GI bleeding, obstruction or perforation.
Q5: Which receptors matter for motion sickness, and which drugs block them?
Histamine H1 and muscarinic acetylcholine receptors in the vestibular pathway. Meclizine and dimenhydrinate (H1) and scopolamine transdermal (muscarinic). Expect sedation and anticholinergic effects.
Q6: Why can chemotherapy given IV cause vomiting without ever touching the stomach?
The chemoreceptor trigger zone in the area postrema sits outside the blood–brain barrier, so it detects drugs and toxins circulating in the blood and signals the vomiting center. Chemo also damages intestinal cells, releasing serotonin that stimulates 5-HT3 receptors on vagal afferents.