A 58-year-old male with a history of myasthenia gravis (MG) is scheduled for elective cholecystectomy. During induction, propofol and remifentanil are used. For muscle relaxation, a non-depolarising agent is chosen. Thirty minutes into surgery, the neuromuscular monitoring shows a train-of-four (TOF) ratio of 0.35 (moderate blockade). The surgeon requests deeper relaxation. The patient's last dose of pyridostigmine was 8 hours ago. What is the most appropriate next step?

Explanation

## Clinical Context: Non-depolarising Muscle Relaxant in Myasthenia Gravis Myasthenia gravis is an autoimmune disorder causing antibodies against the nicotinic acetylcholine receptor (nAChR) at the neuromuscular junction. Patients with MG have profound sensitivity to both depolarising and non-depolarising muscle relaxants, requiring special anesthetic considerations. ## Pathophysiology in MG **Key Point:** In MG, the number of functional nAChRs is reduced by 70–80% due to autoimmune destruction. This leads to: - Exaggerated response to non-depolarising agents (prolonged blockade at lower doses) - Unpredictable response to depolarising agents (risk of prolonged apnoea if pseudocholinesterase deficiency coexists) - Difficulty reversing blockade with standard reversal agents ## Why Cisatracurium Is the Optimal Choice ```mermaid flowchart TD A[Patient with MG requires muscle relaxant]:::outcome A --> B{Which agent?}:::decision B -->|Rocuronium/Vecuronium| C[Organ-dependent elimination<br/>Prolonged blockade in MG<br/>Difficult reversal]:::urgent B -->|Succinylcholine| D[Contraindicated in MG<br/>Risk of prolonged apnoea<br/>Hyperkalemia]:::urgent B -->|Cisatracurium| E[Hofmann elimination + Ester hydrolysis<br/>Independent of organ function<br/>Predictable duration]:::action E --> F[Administer 0.1 mg/kg bolus<br/>Maintain infusion 1-2 μg/kg/min]:::action F --> G[Neuromuscular monitoring with TOF]:::action G --> H[Easier reversal with sugammadex<br/>or spontaneous recovery]:::outcome ``` ## Comparison of Muscle Relaxants in MG | Agent | Elimination | Duration in MG | Reversibility | Use in MG | | --- | --- | --- | --- | --- | | **Cisatracurium** | Hofmann + Ester hydrolysis | Predictable (60–90 min) | Sugammadex or spontaneous | **PREFERRED** | | **Rocuronium** | Hepatic (60–70%) + Renal | Prolonged (>2 hrs) | Sugammadex effective | Acceptable if sugammadex available | | **Vecuronium** | Hepatic (80–90%) + Renal | Prolonged (>2 hrs) | Difficult; neostigmine unreliable | Avoid | | **Succinylcholine** | Pseudocholinesterase | Unpredictable; prolonged apnoea risk | — | **CONTRAINDICATED** | | **Atracurium** | Hofmann + Ester hydrolysis | Predictable | Spontaneous recovery | Alternative to cisatracurium | **High-Yield:** Cisatracurium undergoes Hofmann elimination (spontaneous degradation at physiological pH and temperature) and ester hydrolysis, making its duration independent of organ function. This is ideal in MG, where hepatic and renal function may be variable. ## Dosing in MG **Clinical Pearl:** Patients with MG require **reduced initial doses** and **longer intervals** between doses: - Cisatracurium: 0.05–0.1 mg/kg (vs. 0.15–0.2 mg/kg in non-MG patients) - Maintenance infusion: 1–2 μg/kg/min (titrate to TOF) - Avoid bolus redosing; use continuous infusion instead ## Neuromuscular Monitoring **Key Point:** TOF monitoring is mandatory in MG patients: - TOF ratio 0.35 = moderate blockade (acceptable for most surgery) - Target TOF 0.1–0.2 for adequate surgical relaxation - Do NOT exceed 0.2 to avoid prolonged recovery - Baseline TOF may be reduced in untreated MG; use patient's own baseline as reference ## Reversal Strategy - **Sugammadex** (4–16 mg/kg) is the preferred reversal agent; it encapsulates rocuronium and cisatracurium metabolites - **Neostigmine + Glycopyrrolate** is unreliable in MG and may worsen myasthenic crisis; avoid - **Spontaneous recovery** is acceptable with cisatracurium if TOF >0.9 **Warning:** Do NOT use succinylcholine in MG patients. Risk of: 1. Prolonged depolarisation blockade (dual blockade) 2. Hyperkalemia (especially if concurrent rhabdomyolysis or renal disease) 3. Myasthenic crisis from acetylcholine depletion