A 28-year-old trauma patient with severe head injury and suspected elevated intracranial pressure is brought to the operating room for emergency craniotomy. The anesthesiologist must choose between ketamine and thiopentone for induction. Which feature best distinguishes ketamine from thiopentone in this neurosurgical emergency?

Explanation

## Distinguishing Ketamine from Thiopentone: Cerebral Hemodynamics in Head Injury ### The Critical Discriminator: Cerebral Perfusion Pressure Preservation **Key Point:** Ketamine uniquely **maintains or increases blood pressure and cerebral perfusion pressure (CPP)** while reducing intracranial pressure (ICP), making it the preferred agent in head injury. Thiopentone causes profound hypotension and reduced CPP, worsening cerebral ischemia. ### Comparative Pharmacology in Neurotrauma | Feature | Ketamine | Thiopentone | |---------|----------|-------------| | **BP & SVR** | Maintained or ↑ (sympathomimetic) | ↓↓ (profound depression) | | **Heart Rate** | ↑ (mild) | ↓ (bradycardia risk) | | **Cerebral Blood Flow** | Maintained | ↓ | | **Cerebral Metabolic Rate (CMRO₂)** | ↑ slightly | ↓↓ | | **ICP** | ↓ (via maintained CPP & reduced CMRO₂ balance) | ↓↓ (via ↓ CMRO₂) | | **Airway Reflexes** | **Preserved** | Abolished | | **CPP = MAP − ICP** | **Maintained (MAP ↑)** | **Compromised (MAP ↓)** | | **Emergence Delirium** | Yes (20–30%) | No | ### Why Ketamine Is Superior in Head Injury 1. **Sympathomimetic effect:** Ketamine increases catecholamine release → maintains or increases MAP. 2. **Preserved CPP:** Even if ICP rises slightly from increased CMRO₂, the rise in MAP more than compensates, maintaining CPP = MAP − ICP. 3. **Airway protection:** Ketamine preserves airway reflexes better than thiopentone — critical in emergency intubation. 4. **Reduced secondary brain injury:** Thiopentone's hypotension causes cerebral ischemia, worsening outcome in head injury. **Clinical Pearl:** In head injury, the **worst mistake is hypotension** — it causes secondary ischemic brain injury. Ketamine's hemodynamic stability is life-saving; thiopentone's hypotension is dangerous. **High-Yield:** Ketamine is the **induction agent of choice in head injury, trauma, and hemodynamically unstable patients**. Thiopentone is **contraindicated in head injury** due to hypotension and worsening CPP. **Mnemonic:** **KETAMINE = Keep Everything Intact** — keeps BP, keeps airway reflexes, keeps CPP. **THIOPENTONE = Terrible in Trauma** — causes hypotension and secondary brain injury. ### Why Other Options Are Incorrect - **Airway reflexes:** Ketamine preserves them *relatively* better, but this is secondary to CPP preservation. - **Onset time:** Both have rapid onset (10–30 sec); not a discriminator. - **Emergence delirium:** Ketamine causes it; thiopentone does not. However, in an emergency head injury, emergence effects are irrelevant — immediate brain protection is paramount. [cite:KD Tripathi 8e Ch 12]