A 68-year-old woman with severe osteoarthritis of the knee is undergoing epidural anesthesia for total knee replacement. Immediately after epidural catheter placement and test dose injection of 3 mL of 1.5% lidocaine with 1:200,000 epinephrine, the patient develops sudden tachycardia (HR 120/min), hypertension (BP 165/95 mmHg), and tremor. Oxygen saturation and end-tidal CO₂ are normal. What is the most likely cause of this acute reaction?

Explanation

## Clinical Scenario Analysis The patient develops acute sympathomimetic signs (tachycardia, hypertension, tremor) immediately after epidural test dose injection. The presence of **epinephrine in the injectate** and the **immediate onset** of cardiovascular and adrenergic symptoms are the key diagnostic clues. ## Mechanism of Epinephrine-Induced Response ```mermaid flowchart TD A[Epidural test dose injected]:::action --> B{Needle/catheter in vessel?}:::decision B -->|Yes - Intravascular| C[Epinephrine enters circulation]:::urgent B -->|No - Epidural space| D[Normal epidural effect]:::outcome C --> E[α and β-adrenergic stimulation]:::action E --> F[Tachycardia, hypertension, tremor]:::outcome E --> G[Anxiety, restlessness]:::outcome ``` ## High-Yield: The Epinephrine Test Dose **Purpose:** The 1:200,000 epinephrine (5 µg/mL) in the test dose serves as a **marker of intravascular injection**. **Expected response if intravascular:** - Heart rate increase ≥20 bpm - Systolic BP increase ≥15 mmHg - Tremor, anxiety, restlessness - Onset: **within 15–45 seconds** This patient's immediate tachycardia and hypertension fit this profile perfectly. ## Key Point: The **epinephrine test dose is NOT a test for local anesthetic toxicity**—it is a test for **inadvertent intravascular placement**. A positive epinephrine response indicates the needle or catheter tip is in a blood vessel, and the epidural injectate should NOT be administered. ## Clinical Pearl: If a positive epinephrine response occurs: 1. **Stop the injection immediately** 2. **Withdraw the needle/catheter** 3. **Reposition** and attempt re-insertion at a different level 4. **Reassess** with a new test dose The sympathomimetic response to epinephrine is **self-limited** and resolves within minutes without treatment (though beta-blockers can be used if severe). ## Mnemonic: POSITIVE EPINEPHRINE TEST DOSE SIGNS **PACED** = Palpitations, Anxiety, Cardiac acceleration (tachycardia), Elevation of BP, Diaphoresis/tremor [cite:Miller's Anesthesia 8e Ch 45]