1. Mechanistic Overview of Antiemetic Classes

Effective control of nausea and vomiting hinges on targeting key receptors in the central and peripheral emetic pathways. Understanding these mechanisms allows for rational combination therapy.

5-HT3 Receptor Antagonists (e.g., Ondansetron, Granisetron)

• Mechanism: Block serotonin (5-HT3) receptors on vagal afferent nerves in the gut and centrally in the chemoreceptor trigger zone (CTZ) and nucleus tractus solitarius (NTS).
• Pharmacokinetic Highlights: Ondansetron has a half-life of 3–4 hours and is metabolized by CYP3A4, 2D6, and 1A2. Granisetron has a longer half-life of approximately 9 hours, allowing for once-daily dosing.
• Adverse Effects: Headache and constipation are common. Dose-dependent QT interval prolongation is a key safety concern.

Dopamine D2 Antagonists (e.g., Metoclopramide, Domperidone)

• Mechanism: Block dopamine D2 receptors in the CTZ. Metoclopramide also acts as a 5-HT4 agonist, which enhances gastric motility (prokinetic effect).
• Pharmacokinetic Highlights: Metoclopramide is renally cleared (half-life 5–6 hours). Domperidone has minimal penetration across the blood-brain barrier, reducing central side effects.
• Adverse Effects: Metoclopramide carries a risk of extrapyramidal symptoms (EPS) and a black box warning for tardive dyskinesia. Domperidone is associated with QT prolongation.

NK1 Receptor Antagonists (e.g., Aprepitant, Fosaprepitant)

• Mechanism: Block substance P at neurokinin-1 (NK1) receptors in the brainstem, which is particularly effective for preventing delayed nausea and vomiting.
• Pharmacokinetic Highlights: Aprepitant (oral) is a CYP3A4 substrate. Fosaprepitant is the IV prodrug.
• Adverse Effects: Fatigue and hiccups are common. They have significant potential for drug-drug interactions via CYP3A4 inhibition.

Corticosteroids (e.g., Dexamethasone)

• Mechanism: Not fully elucidated, but thought to involve central inhibition of prostaglandin synthesis and modulation of neurotransmitter receptors in the NTS.
• Dosing: Typically 4–8 mg IV once daily. Should be tapered after 3–5 days to minimize side effects.
• Adverse Effects: Hyperglycemia, fluid retention, immunosuppression, and myopathy with prolonged use.

2. First-Line Pharmacotherapy

For patients with moderate-to-high emetic risk, a combination of agents targeting at least two different emetic pathways is recommended to maximize efficacy.

Initiation Dosing & Titration

• Ondansetron: 4 mg IV every 8 hours (maximum 32 mg/day IV).
• Granisetron: 1 mg IV every 24 hours or one 3.1 mg/24h patch.
• Metoclopramide: 10 mg IV every 6–8 hours (reduce to 5 mg if CrCl < 40 mL/min).
• Dexamethasone: 8 mg IV daily.
• Aprepitant: 125 mg PO on day 1, followed by 80 mg PO on days 2 and 3.

Advantages vs. Disadvantages of First-Line Classes

3. Second-Line & Adjunctive Agents

For refractory symptoms or specific etiologies like vestibular-mediated nausea, agents targeting alternative pathways should be considered.

• Antihistamines (e.g., Diphenhydramine, Meclizine): Block H1 receptors in the vestibular nuclei. Effective for motion sickness and vertigo. Main side effects are sedation and anticholinergic effects.
  • Dosing: Diphenhydramine 25–50 mg IV/PO q6h; Meclizine 25–50 mg PO daily.
• Anticholinergics (e.g., Scopolamine): Block muscarinic receptors in vestibular pathways. Highly effective for motion sickness.
  • Dosing: Scopolamine patch 1.5 mg applied every 72 hours.
• Benzodiazepines (e.g., Lorazepam): Reduce anticipatory nausea through anxiolysis and amnesia. Not a primary antiemetic but a useful adjunct.
  • Dosing: Lorazepam 0.5–1 mg IV/PO as needed.

4. PK/PD & Dosing Adjustments in Critical Illness

Critical illness significantly alters pharmacokinetics (PK) and pharmacodynamics (PD), necessitating careful dose adjustments.

• Increased Volume of Distribution (Vd): In fluid-overloaded states, hydrophilic drugs like metoclopramide may have an increased Vd, potentially requiring higher loading doses to achieve therapeutic concentrations.
• Hypoalbuminemia: For highly protein-bound drugs, low albumin levels increase the free (active) fraction, raising the risk of toxicity. Monitor closely.
• Hepatic Impairment: The dose of ondansetron should be reduced by 50% in patients with severe hepatic impairment (Child-Pugh Class B–C).
• Renal Dysfunction: Metoclopramide requires a 50% dose reduction if CrCl falls below 40 mL/min.
• Renal Replacement Therapy: Metoclopramide is significantly removed by continuous renal replacement therapy (CRRT), requiring supplemental dosing. NK1 antagonists are minimally removed.

5. Route Selection & Drug Delivery

The route of administration must be chosen to ensure bioavailability and efficacy, especially when oral intake is compromised or IV access is problematic.

• IV Infusion Pumps: Continuous infusion of ondansetron (e.g., 0.1 mg/kg/h) can maintain steady-state concentrations and may limit the peak concentration-dependent risk of QT prolongation compared to bolus dosing.
• Subcutaneous Pumps: Elastomeric pumps can deliver a continuous subcutaneous infusion of metoclopramide (e.g., 20 mg over 24 hours), useful when IV access is poor. Monitor for site reactions, especially in patients with poor peripheral perfusion.
• Transdermal Systems: Granisetron and scopolamine patches provide convenient, multi-day therapy. Avoid application on inflamed skin or in hypotensive patients where absorption may be erratic.

6. Monitoring & Pharmacoeconomics

A successful antiemetic strategy requires a balance of efficacy, safety, and cost-effectiveness through ongoing monitoring.

Efficacy Monitoring

Use a simple 0–10 Visual Analog Scale (VAS) for nausea and patient-reported diaries of vomiting episodes. The therapeutic goal is a VAS decrease of ≥20 mm or 2 points within 24 hours.

Safety Monitoring

• ECG: Obtain a baseline and daily ECG to monitor the QTc interval when using 5-HT3 antagonists and/or dopamine antagonists, especially in combination or in patients with electrolyte abnormalities.
• Neurologic Exam: Regularly assess for extrapyramidal symptoms (akathisia, acute dystonia) in patients receiving metoclopramide.
• Glucose Monitoring: Check blood glucose levels frequently in patients receiving dexamethasone, particularly those with or at risk for diabetes.

Pharmacoeconomics

Ondansetron and metoclopramide are generally cost-effective first-line options. While NK1 antagonists have a higher acquisition cost, they can be cost-effective in preventing refractory chemotherapy-induced nausea and vomiting, thereby reducing rescue therapy needs and hospitalizations. Standardized protocols and electronic order sets can improve dosing accuracy and optimize resource utilization.