Last reviewed: May 2026 | Written by the MplusX Editorial Team
📌 Key Takeaways
- Withholding Rules: Mastery of preoperative medication withholding (e.g., stopping SGLT2 inhibitors 2 to 3 days pre-op) is critical to prevent complications like perioperative euglycemic DKA.
- Malignant Hyperthermia: Triggered by volatile anesthetics and succinylcholine, this hypermetabolic crisis is managed with immediate trigger cessation, hyperventilation with 100% O2, and IV Dantrolene (2.5 mg/kg).
- Risk Stratification: Differentiate surgical risk categories using the ASA Physical Status Classification and predict post-op emesis using the Apfel Score.
- Primary CTA: Download the Pre-operative Drug Withholding Reference — a high-yield printout summarizing withholding timelines and clinical justifications for all major drug classes.
A patient with type 2 diabetes is scheduled for a major elective cholecystectomy tomorrow morning.
He is currently taking Ramipril, Metformin, and Empagliflozin.
Which of these medications must be withheld, and for how long?
Preoperative safety and perioperative medication withholding are among the most heavily tested safety domains on the AMC MCQ. A single mistake here can precipitate severe intraoperative hypotension or life-threatening euglycemic DKA.
As an entry-level clinician in Australia, you are expected to make rapid, safe decisions regarding surgical risk, anesthesia crises, and emergency airways.
This guide provides the master drug withholding timelines, malignant hyperthermia protocols, and trauma airway algorithms you need to secure these high-yield marks.
1. Master Pre-operative Medication Withholding Matrix
Managing a patient’s chronic medications prior to major surgery requires balancing the risk of perioperative withdrawal or disease exacerbation against the risk of surgical complications (like bleeding, hypotension, or metabolic crises). Reading about MplusX vs AMEDEX might also be helpful.
Memorize this master reference matrix for your exam:
| Medication Class | Example Drugs | Withholding Timeline | Primary Clinical Justification | Restarting Parameters |
|---|---|---|---|---|
| SGLT2 Inhibitors | Empagliflozin, Dapagliflozin | Withhold 2 to 3 days prior to major surgery. | Prevents severe, atypical euglycemic diabetic ketoacidosis (DKA). | Restart only when the patient has stable oral intake and normal renal function. |
| Metformin | Metformin | Withhold the morning of surgery. | Minimizes risk of lactic acidosis during periods of perioperative renal hypoperfusion. | Restart 48 hours post-op, once normal renal function (eGFR) is confirmed. |
| ACE Inhibitors / ARBs | Ramipril, Perindopril, Irbesartan | Withhold 24 hours prior to surgery. | Prevents profound, refractory intraoperative hypotension following anesthetic induction. | Restart post-operatively once the patient is hemodynamically stable and hydrated. |
| Oral Diuretics | Frusemide, Spironolactone | Withhold the morning of surgery. | Prevents intravascular volume depletion and severe electrolyte imbalances. | Restart once oral intake is established. |
| P2Y12 Antiplatelets | Clopidogrel, Prasugrel | Withhold 5 to 7 days prior to high-bleeding-risk surgery. | Minimizes risk of surgical hemorrhage. | Coordinate with cardiologist; continue if coronary stent is recent (<12 months). |
| Warfarin | Warfarin | Withhold 5 days prior to surgery. | Allows INR to drop to <1.5. | Bridge with therapeutic LMWH (enoxaparin) if thromboembolic risk is high (e.g. mechanical valve). |
| DOACs | Apixaban, Rivaroxaban | Withhold 24 to 48 hours depending on renal function. | Prevents surgical hemorrhage; clear renal clearance timeline. | Restart 24–48 hours post-op once surgical hemostasis is secure. |
| Beta-blockers | Metoprolol, Bisoprolol | DO NOT WITHHOLD (Continue morning of surgery). | Abrupt cessation precipitates withdrawal tachycardia, arrhythmias, and myocardial ischemia. | Continue uninterrupted. |
Critical Exam Pearl: Aspirin is typically continued throughout the perioperative period for secondary prevention in patients with established ischemic heart disease or coronary stents, unless the surgery is in a closed space with catastrophic bleeding risks (such as neurosurgery or posterior eye surgery).
2. Differentiating Post-Operative Nausea & Vomiting (PONV): The Apfel Score
Post-operative nausea and vomiting is one of the most common and distressing complications of anaesthesia. The AMC MCQ tests your ability to identify high-risk patients and implement appropriate preventive therapy.
The Apfel Simplified Risk Score is the standard tool used to predict PONV risk. It is based on four independent risk factors: 1. Female gender 2. History of PONV or motion sickness 3. Non-smoker status 4. Use of post-operative opioids
PONV Risk Assessment Table
| Apfel Score (Risk Factors Present) | Predicted PONV Risk (%) | Recommended eTG Prophylaxis Regimen |
|---|---|---|
| 0 or 1 (Low Risk) | 10% – 20% | No routine prophylaxis required, or single agent (e.g., dexamethasone). |
| 2 (Moderate Risk) | 39% – 40% | Dual prophylaxis: Dexamethasone 4mg IV at induction + Ondansetron 4mg IV at emergence. |
| 3 or 4 (High Risk) | 60% – 78% | Triple prophylaxis: Dexamethasone IV + Ondansetron IV + Droperidol (or Propofol TIVA maintenance). |
Clinical Rule: If a patient experiences breakthrough PONV in the recovery bay despite receiving ondansetron prophylaxis, do not administer a repeat dose of ondansetron. Instead, select an antiemetic from a different pharmacological class (e.g., metoclopramide 10mg IV or cyclizine 50mg IV).
3. Malignant Hyperthermia: The Hypermetabolic Crisis
Malignant Hyperthermia (MH) is a rare, life-threatening genetic hypermetabolic crisis of skeletal muscle triggered by exposure to volatile anesthetic agents (e.g., halothane, sevoflurane, desflurane) or the depolarizing muscle relaxant succinylcholine (suxamethonium).
Pathophysiology
Trigger agents cause an uncontrolled release of calcium from the sarcoplasmic reticulum via mutated ryanodine receptors (RYR1). This leads to sustained muscle contraction, massive ATP consumption, lactic acidosis, carbon dioxide production, hyperkalemia, and rhabdomyolysis.Immediate Recognition Signs (tested on exam)
1. Rapid, unexplained rise in End-Tidal CO2 (ETCO2): The earliest and most reliable sign. 2. Masseter muscle rigidity (jaw spasm): Following succinylcholine administration. 3. Unexplained tachycardia and tachypnoea. 4. Hyperthermia: A late, pre-terminal sign where temperature rises rapidly (up to 1°C every 5 minutes).Step-wise Crisis Management Pathway
1. Cease Trigger Agents immediately: Turn off the vaporizers and stop the succinylcholine infusion. 2. Hyperventilate with 100% Oxygen: High flows (>10 L/min) through a clean breathing circuit. 3. Administer IV Dantrolene: The definitive antidote. Inject 2.5 mg/kg IV immediately. Repeat the dose every 5 to 10 minutes until the hypermetabolic state (tachycardia, muscle rigidity, rising CO2) is controlled, up to a maximum dose of 10 mg/kg. 4. Cool the Patient: Active cooling using iced saline IV infusions, surface ice packs, and gastric lavage. Stop cooling once core temperature drops below 38.5°C to avoid hypothermic overshoot. 5. Treat Hyperkalemia: If potassium is elevated, administer IV insulin (10 units) with dextrose (50 mL of D50W) and IV calcium gluconate to stabilize the myocardium.4. Local Anaesthetic Systemic Toxicity (LAST)
Local Anaesthetic Systemic Toxicity occurs due to accidental intravascular injection of local anesthetics (commonly bupivacaine, lignocaine, or ropivacaine) or rapid systemic absorption from high-vascularity injection sites (such as intercostal blocks).
Symptoms of LAST
* Central Nervous System (Early): Metallic taste in mouth, perioral numbness, tinnitus, auditory disturbances, visual blurring, and muscle twitching. * Central Nervous System (Late): Generalized tonic-clonic seizures and coma. * Cardiovascular System: Severe bradycardia, conduction blocks, ventricular arrhythmias (especially with bupivacaine), and cardiac arrest.Metallic taste, perioral numbness, tinnitus, seizures] C –> D[Cardiovascular Toxicity:
Bradycardia, arrhythmias, cardiac arrest] D –> E[Immediate Management:
Stop injection, support airway with 100% O2] E –> F[Definitive Antidote:
IV Intralipid 20% lipid emulsion bolus & infusion] style A fill:#0F2D5C,stroke:#fff,color:#fff style C fill:#D69E2E,stroke:#fff,color:#fff style D fill:#991B1B,stroke:#fff,color:#fff style F fill:#166534,stroke:#fff,color:#fff
Emergency Antidote: Lipid Emulsion (Intralipid)
The immediate treatment sequence for LAST is: 1. Stop the injection immediately. 2. Support the Airway: Administer 100% oxygen and ventilate if necessary. Secure the airway to prevent hypoxia and acidosis, which worsen local anesthetic toxicity. 3. Control Seizures: Administer low-dose benzodiazepines (midazolam). 4. Administer Intralipid 20%: A lipid emulsion that acts as a “lipid sink,” drawing the lipophilic local anesthetic molecules out of myocardial and neural tissue. Give an IV bolus of 1.5 mL/kg over 1 minute, followed by a continuous infusion of 0.25 mL/kg/min.5. Emergency Trauma Airway Triage
In the emergency department or trauma bay, securing the airway is the first priority of the ATLS/EMST algorithm. The AMC MCQ tests your clinical reasoning regarding when to intubate and when to select emergency surgical airways.
Use this triage sequence to guide your exam choices:
Key Exam Rules: * In-Line Stabilization: During orotracheal intubation of a trauma patient, you must maintain manual in-line cervical stabilization (MILS). Do not hyperextend the neck to perform laryngoscopy. * Surgical Cricothyroidotomy: If a patient has severe mid-face trauma, airway bleeding, or a larynx obstructed by foreign bodies where orotracheal intubation has failed twice (“can’t intubate, can’t oxygenate”), the correct option is a surgical cricothyroidotomy. Do not attempt a tracheostomy in the emergency room; tracheostomies are elective procedures performed in the operating theatre. * Age Limits: In children aged under 8 years, perform a needle cricothyroidotomy with jet insufflation rather than a surgical cricothyroidotomy, due to the high risk of damaging the small, pliable cricoid cartilage.
5. Post-Operative Complications: Triage and Management
Managing a patient doesn’t end when the surgical drapes are removed. The post-operative ward is a high-risk clinical environment, and the AMC MCQ frequently tests your ability to manage acute clinical deterioration in the first 24 to 72 hours post-op. Two of the most common tested scenarios involve post-operative fever and post-operative oliguria.
Post-Operative Fever: The “5 W’s” Framework
When a patient develops a fever (temperature >38.0°C) after surgery, you must determine the etiology based on the post-operative day (POD). Use the classic “5 W’s” framework to guide your differential diagnosis and exam selection:1. Wind (POD 1–2): Atelectasis or Pneumonia. Pathophysiology:* Shallow breathing due to incisional pain or anesthetic-induced alveolar collapse (atelectasis). Management:* First-line is chest physiotherapy, aggressive mobilization, and incentive spirometry. Order a chest X-ray if respiratory symptoms (hypoxia, cough) occur to exclude pneumonia. 2. Water (POD 3–5): Urinary Tract Infection (UTI). Pathophysiology:* Frequently associated with indwelling urinary catheters (catheter-associated UTI). Management:* Remove the catheter if no longer indicated. Obtain a urine specimen for microscopy, culture, and sensitivity (MC&S), and initiate antibiotics based on local guidelines. 3. Walking (POD 5+): Deep Vein Thrombosis (DVT) or Pulmonary Embolism (PE). Pathophysiology:* Venous stasis from prolonged perioperative immobility. Management:* Diagnosis via compression ultrasound for DVT or CT Pulmonary Angiogram (CTPA) for suspected PE. Initiate therapeutic anticoagulation (low-molecular-weight heparin bridged to warfarin or DOAC) immediately if confirmed. 4. Wound (POD 5–7): Wound Infection (Surgical Site Infection). Pathophysiology: Introduction of skin flora (e.g., Staphylococcus aureus*) during surgery. Management:* Inspect the wound daily for erythema, purulence, or fluctuance. Collect wound swabs. Small superficial infections may require oral cephalexin; deep or fluctuant collections require surgical debridement and drainage. 5. Wonder Drugs (Any time): Drug Fever or Transfusion Reaction. Pathophysiology:* An immunologic reaction to medications (especially antibiotics or anesthetics) or blood products. Management:* Stop the offending drug or cease the blood transfusion immediately and check for hemolytic signs.
Post-Operative Oliguria: Diagnostic Pathway
Oliguria is defined as a urine output of less than 0.5 mL/kg/hour for 6 consecutive hours (or less than 30 mL/hour for an average adult). In the post-op ward, this represents acute kidney injury (AKI) risk and requires a systematic diagnostic approach:Immediate Triage Sequence for Oliguria: 1. Rule out mechanical obstruction: Check if the urinary catheter is kinked, blocked, or malpositioned. Flush the catheter with sterile saline. If urine flows immediately, the oliguria was post-renal (obstructive). 2. Assess fluid status: Examine the blood pressure, heart rate, jugular venous pressure (JVP), mucous membranes, and lung fields. 3. Trial fluid challenge: If the patient is dry or clinically hypovolemic (e.g., due to intraoperative blood loss or fluid restriction), administer an IV fluid challenge of 250 to 500 mL of 0.9% sodium chloride over 15 to 30 minutes. 4. Review medications: Discontinue all potential nephrotoxic drugs, including non-steroidal anti-inflammatory drugs (NSAIDs like ibuprofen/ketorolac) and aminoglycoside antibiotics (gentamicin), which can precipitate acute tubular necrosis.
Frequently Asked Questions
What is the difference between ASA 1 and ASA 6?
The American Society of Anesthesiologists (ASA) Physical Status Classification system ranges from 1 to 6: * ASA 1: A normal healthy patient. * ASA 2: A patient with mild systemic disease (e.g., well-controlled hypertension or diabetes, social smoker). * ASA 3: A patient with severe systemic disease that limits activity but is not incapacitating (e.g., stable angina, poorly controlled diabetes, COPD). * ASA 4: A patient with severe systemic disease that is a constant threat to life (e.g., recent MI <3 months, unstable angina, symptomatic valve disease). * ASA 5: A moribund patient who is not expected to survive without the operation (e.g., ruptured abdominal aortic aneurysm). Reading about AMC MCQ final revision plan might also be helpful. * ASA 6: A declared brain-dead patient whose organs are being removed for donor purposes.When should we resume SGLT2 inhibitors after surgery?
Under eTG guidelines, do not restart SGLT2 inhibitors (empagliflozin, dapagliflozin) until the patient has fully recovered from the acute surgical phase, has stable oral intake (eating and drinking normally), and has had their renal function (eGFR) checked to confirm no acute kidney injury (AKI) occurred perioperatively.Why do we withhold ACE inhibitors 24 hours prior to surgery?
ACE inhibitors block the renin-angiotensin-aldosterone system (RAAS). General anesthesia suppresses the sympathetic nervous system. When both systems are suppressed concurrently, the patient is at high risk of developing profound, prolonged induction-associated hypotension that is refractory to standard vasopressors (requiring metaraminol or vasopressin). Reading about MplusX review might also be helpful.What is the maximum safe dose of local anesthetics for infiltration?
For lignocaine (lidocaine) without adrenaline, the maximum safe dose is 3 mg/kg. With adrenaline (which causes local vasoconstriction, delaying systemic absorption), the maximum safe dose increases to 7 mg/kg. Exceeding these thresholds significantly increases the risk of Local Anaesthetic Systemic Toxicity (LAST).How do we manage post-operative pain under eTG rules?
First-line postoperative analgesia is built around a multimodal framework: scheduled paracetamol and NSAIDs (if not contraindicated by renal or bleeding risks), with short-acting oral opioids (e.g., immediate-release oxycodone) reserved for breakthrough pain. Long-acting, modified-release opioids should not be initiated for acute postoperative pain.How is succinylcholine-induced muscle pain prevented?
Muscle fasciculations and subsequent myalgia caused by the depolarizing agent succinylcholine can be minimized by administering a small, non-depolarizing neuromuscular blocker dose (such as vecuronium or rocuronium at 10% of the standard intubation dose) 2 to 3 minutes prior to succinylcholine injection.Written by the MplusX Editorial Team — dedicated to providing clinically accurate, structured, and practical resources for international medical graduates pursuing licensing with the Australian Medical Council.
References
- John Murtagh‘s General Practice (8th Edition): Chapter 120: Emergency care, Page 3150; Chapter 123: Common skin wounds and foreign bodies, Page 3233
- Therapeutic Guidelines (eTG): Part 1 Analgesic: Chapter 4: Acute pain: perioperative, Page 26; Part 2 CVS: Chapter 30: Periprocedural management of patients with cardiovascular disease, Page 156
Disclaimer: This article is written for AMC MCQ examination preparation and general informational purposes only. It does not constitute medical advice, diagnosis, or treatment recommendations. Clinical decisions should always be based on individual patient assessment, current Australian Therapeutic Guidelines (eTG), and consultation with qualified healthcare professionals. MplusX is an exam preparation platform and is not a substitute for supervised clinical training.