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PH2.1-5 | Autonomic and Peripheral Pharmacology — Assignment

CLINICAL SCENARIO

This assignment asks you to integrate receptor pharmacology, clinical scenarios, and emergency decision-making across the five core competency areas of Autonomic and Peripheral Pharmacology. You will analyse a real-world clinical vignette, justify drug choices on mechanistic grounds, and critically appraise management decisions.

Instructions

Read the following clinical scenario carefully and answer all four parts. Each part targets a distinct competency domain. Demonstrate mechanistic reasoning — do not merely list drugs. Apply the PH arc framework (pathophysiology → therapeutic rationale → drug classification → PK/PD → clinical decision-making). Total marks: 30.

Clinical Scenario:
A 48-year-old sugarcane farmer is brought to a rural PHC at 10 pm. His wife reports he was spraying pesticides in the fields that afternoon without protective equipment. He is drowsy but rousable. On examination: profuse salivation, lacrimation, miosis (both eyes), HR 48/min, BP 90/60 mmHg, diffuse wheeze bilaterally, and visible muscle fasciculations in both forearms. Acetylcholinesterase activity (RBC) is <20% of normal.

Length: 800–1200 words total across all parts. Parts requiring calculations: show working. Parts requiring clinical justification: one concise paragraph per sub-question is sufficient.

What to Submit

Part A — Pathophysiology and Drug Mechanism (8 marks)

Identify the toxidrome and name the causative agent class (2 marks).
Using a structured framework (muscarinic features / nicotinic features / central features), classify each clinical finding in this patient (3 marks).
Explain at the molecular level — including the specific bond formed and the concept of 'ageing' — why this poisoning is potentially irreversible without timely intervention (3 marks).

Part B — Emergency Drug Management (9 marks)

Atropine is prescribed. State the first-line dose for severe OPC poisoning in an adult, the route of administration, and the correct titration endpoint (not heart rate or pupils). Explain why that endpoint was chosen (4 marks).
Pralidoxime (PAM) is added. State its mechanism of action, the critical time window for its efficacy, and what happens if it is given after 'ageing' has occurred (3 marks).
Diazepam is also ordered. Identify its pharmacological role in OPC poisoning management and the receptor mechanism involved (2 marks).

Part C — Receptor Selectivity and Clinical Choice (8 marks)

After stabilisation, the patient is transferred to a tertiary hospital ICU. He develops hypotension unresponsive to fluid resuscitation (MAP 52 mmHg) due to vasodilatory shock from systemic inflammation.
1. Compare the receptor selectivity profiles of noradrenaline, dopamine, and dobutamine. Using this comparison, justify which agent is MOST appropriate as a first-line vasopressor in this distributive shock state (5 marks).
2. A consultant suggests adding a low-dose β1-selective blocker to protect against tachyarrhythmias. Critique this suggestion on pharmacodynamic grounds, citing the specific receptor interaction that makes it risky in this context (3 marks).

Part D — Local Anaesthesia and Peripheral Nerve Block (5 marks)

On day 3, the patient requires a painful central line insertion. The team plans a local infiltration with bupivacaine 0.5%.
1. The patient weighs 65 kg. Calculate the maximum safe volume of 0.5% bupivacaine that can be injected (plain, without adrenaline). Maximum dose = 2 mg/kg (2 marks).
2. Before the procedure, what preparation specific to bupivacaine (NOT applicable to lignocaine) must be immediately available at the bedside? Name the preparation and state its mechanism of action in the event of toxicity (3 marks).

Grading Rubric — Autonomic and Peripheral Pharmacology Assignment Rubric

Criterion	Points	Full-marks descriptor
Part A — Toxidrome identification, structured muscarinic/nicotinic/central classification, and molecular mechanism (AChE phosphorylation + ageing concept) [PH2.2]	8 pts	Toxidrome and causative class correctly identified. ALL muscarinic (SLUDGE + bronchospasm + bradycardia + miosis), nicotinic (fasciculations + weakness), and central (drowsiness) features correctly classified. Molecular mechanism: covalent phosphorylation of serine esteratic site named; 'ageing' (side-chain hydrolysis → irreversible bond) explained with clinical implication (oxime must be given early).
Part B — Atropine dose/endpoint, pralidoxime mechanism/window, and diazepam role [PH2.2 / PH2.3]	9 pts	Atropine: correct adult dose (2–4 mg IV, repeat every 5–10 min), correct endpoint (drying of secretions + clear chest auscultation), correct mechanistic reason (this targets the lethal muscarinic feature). Pralidoxime: nucleophilic attack releasing AChE from phosphate; time window (<24–48 h before ageing); consequence of late use (no reactivation possible after ageing). Diazepam: control of seizures/anxiety via GABA-A positive allosteric modulation.
Part C — Vasopressor receptor comparison, justified selection, and beta-blocker critique [PH2.1 / PH2.3]	8 pts	Complete and accurate receptor selectivity comparison (noradrenaline: α1 >>> β1, minimal β2; dopamine: D1 at low, β1 at mid, α1 at high doses; dobutamine: β1 > β2, minimal α1). Correct selection (noradrenaline first line in distributive shock) with mechanistic justification (restores SVR via α1 without β2-mediated vasodilation). Beta-blocker critique: β1 blockade reduces cardiac output in a patient with borderline haemodynamics; β2 blockade removes residual bronchodilatory compensation in a patient recovering from bronchospasm.
Part D — Bupivacaine dose calculation and lipid emulsion rescue [PH2.5]	5 pts	Calculation correct: 2 mg/kg × 65 kg = 130 mg; 0.5% = 5 mg/mL; 130 ÷ 5 = 26 mL (with working shown). Lipid emulsion (20% Intralipid) correctly named. Mechanism correctly stated: lipid sink — draws bupivacaine out of cardiac Na+ channels into a lipid aqueous compartment, restoring normal cardiac conduction.