## Analysis of IV Induction Agent Properties ### Onset of Action Comparison **Key Point:** Propofol has a FASTER onset of action than thiopentone, not slower. Both are lipophilic, but propofol's rapid onset (30–60 seconds) is superior to thiopentone's (10–20 seconds to peak effect, but slower equilibration). The onset depends on multiple factors: lipid solubility, pKa, protein binding, and blood-brain barrier penetration — not lipid solubility alone. ### Correct Statements Verified | Agent | Respiratory Effect | Cardiovascular Effect | Airway Reflex | Hemodynamic Use | |-------|-------------------|----------------------|---------------|------------------| | Propofol | Marked depression, apnea common | Hypotension, ↓SVR | Preserved initially | Avoid in shock | | Thiopentone | Moderate depression | Hypotension, ↓CO | Preserved | Avoid in shock | | Etomidate | Minimal depression | Excellent preservation | Excellent | Stable patients | | Ketamine | Preserved/maintained | Sympathomimetic ↑HR, ↑BP | Preserved | **Hemodynamic instability** | **High-Yield:** Thiopentone onset is actually 10–20 seconds, while propofol onset is 30–60 seconds — but propofol's effect is more pronounced and predictable. The statement reverses this relationship. **Clinical Pearl:** In a hypotensive trauma patient, ketamine is preferred because it maintains or increases sympathetic tone, whereas propofol and thiopentone cause profound hypotension. ### Why the Distractor is Wrong The statement claims thiopentone has faster onset due to higher lipid solubility. In reality: - Propofol is MORE lipid-soluble (octanol-water partition coefficient ~4000) than thiopentone (~300). - Propofol's rapid onset is partly due to its higher lipophilicity and rapid blood-brain equilibration. - The statement is factually inverted. [cite:Gupta & Rana Clinical Anesthesia 6e Ch 8]
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