Show the model answerAttempt it first — that is what makes it stick
What earns the marks10 marks
| (a) Two classes, spelled correctly | 1 mark. Aminosteroidal and benzylisoquinolinium, one example each. Misspelling the second was not accepted |
|---|---|
| (b) Onset is about potency | 2 marks. Inverse relationship, and the reason: a bigger molar dose builds a steeper gradient |
| (b) Perfusion, not diffusion | The discriminating point. These are ionised quaternary compounds, so onset is perfusion-limited |
| (c) A mechanism per class | 4 marks, and the lowest-scoring part. Most got magnesium and stopped. Every class named needs its mechanism |
| (c) Drugs only | Temperature and pH are accurate and earned nothing — the question asked for drugs |
| (d) Both routes, with figures | 3 marks. Hofmann and non-specific ester hydrolysis, their percentages, and their pH and temperature dependence |
| (d) Name the metabolites | Laudanosine and monoquaternary acrylate, then why organ independence matters in renal failure |
1 mark
Classification by chemical structure
One mark, thirty seconds, and no reason to lose it. Write the two classes with one example each and move on.
| Class | Example | What the structure buys |
|---|---|---|
| Aminosteroidal | Rocuronium (also vecuronium, pancuronium) | A steroid nucleus carrying quaternary ammonium groups. Eliminated by liver and kidney, so organ failure matters. Vecuronium is pancuronium minus a single methyl group |
| Benzylisoquinolinium | Atracurium (also cisatracurium, mivacurium) | Two quaternary nitrogens joined by a di-ester chain. That chain is what makes the class breakable in plasma, independent of liver and kidney, and is why part (d) of this question exists |
Commonly lost: Spelling errors such as “benzylisoquinolone” were not accepted. One mark, lost on a word. Write it out twice now: benzylisoquinolinium.
2 marks
What determines the speed of onset
Two marks, and the whole of it is one idea: onset time is inversely related to potency. Get that stated with its mechanism and the marks are secure.
Why the weakest drug in the class is the fastest
Start from potency
Rocuronium’s ED₉₅ is 0.3 mg/kg; cisatracurium’s is 0.05 mg/kg. Rocuronium is the weaker drug by roughly sixfold.
An equipotent dose of the weaker drug therefore contains far more molecules — potency and molar dose are two sides of the same number.
More molecules in plasma means a steeper concentration gradient between the blood and the junctional cleft, and the receptor reserve is saturated sooner.
The measurable result
At the same multiple of ED₉₅, rocuronium reaches maximum block in 1.7 minutes and cisatracurium in 5.2 minutes. Cisatracurium is three to five times more potent than atracurium and correspondingly slower — and its onset can be bought back by raising the dose.
| Factor | Direction | Why |
|---|---|---|
| The drug | ||
| Potency (ED₉₅) | Lower potency, faster onset | The dominant factor. A weaker drug is given in a larger molar dose, which steepens the plasma-to-junction gradient |
| Dose given, as a multiple of ED₉₅ | Higher dose, faster onset | The same mechanism applied deliberately. Rocuronium 1.2 mg/kg is faster than 0.6 mg/kg, at the cost of a much longer block |
| Priming | Faster by 30–60 seconds | A subparalysing dose — about 20% of the ED₉₅, or 10% of the intubating dose — given 2 to 4 minutes ahead occupies part of the receptor reserve in advance. It costs aspiration risk, difficulty swallowing and visual disturbance in an awake patient |
| The patient | ||
| Cardiac output | Higher output, faster onset | Delivery to the junction is the rate-limiting step, so anything that speeds circulation speeds onset. A low-output state delays it |
| Muscle blood flow | Better perfused muscle blocks first | The diaphragm and the laryngeal adductors are reached before the adductor pollicis — which is why the airway can be secured while the monitored hand still twitches |
| Age and circulation time | Slower in the elderly | A longer arm–brain circulation time delays arrival of the drug at the junction |
Commonly lost: On the closely related October 2024 question, most candidates mistook prolonged duration of action for speed of onset and gave incorrect points as a result; some gave factors relating to the onset of local anaesthetics, such as pKa and lipid solubility. That question passed at 6.03%. Onset and duration are different questions with almost opposite answers.
4 marks — the part that decided the question
Drugs that potentiate a non-depolarising block, and how
Most candidates described magnesium correctly and then thinned out. The marks here are one per class with its mechanism attached, so a named class without a mechanism is half a mark thrown away.
| Class | Example | Mechanism |
|---|---|---|
| Magnesium | Magnesium sulphate | Reduced acetylcholine release by competition with calcium at the presynaptic terminal, plus stabilisation of the postsynaptic membrane. The one class most candidates got right |
| Volatile anaesthetics | Desflurane, sevoflurane | Three mechanisms, and better answers gave all three: central depression of somatic reflexes, enhanced affinity of the antagonist for the receptor, and inhibition of postsynaptic nicotinic receptors. Rank order desflurane > sevoflurane > isoflurane |
| Aminoglycoside antibiotics | Gentamicin | Reduced presynaptic acetylcholine release through competition with calcium. Polymyxins, lincomycin and clindamycin behave similarly; tetracyclines act postjunctionally only |
| Calcium channel blockers | Verapamil | Reduced calcium influx into the nerve terminal, so less transmitter is released |
| Local anaesthetics | Lidocaine | Sodium channel blockade — membrane stabilisation. Small doses enhance an existing block; large doses depress transmission on their own |
| Lithium | Lithium carbonate | Sodium channel effects; lithium substitutes for sodium in cellular transport and impairs transmission presynaptically |
| Antiarrhythmics | Quinidine | Sodium channel blockade and membrane stabilisation |
| Another non-depolariser | Rocuronium with cisatracurium | Two agents of different chemical classes are synergistic; two of the same class are additive. Rarely written, and available for a mark |
Commonly lost: This section recorded the lowest scores of the four parts. Most described magnesium correctly, but mechanisms for the other groups were often incomplete or omitted. The list is the easy half; the mechanism is the mark.
Commonly lost: Physiological factors such as temperature and pH, although accurate, were not relevant to the question asked. The stem says drugs. This is the same trap that runs through every sitting of this material.
3 marks
Atracurium metabolism, and its use in renal failure
Two routes, their percentages, their pH and temperature dependence, the metabolites by name, and then the clinical conclusion. All five elements were credited.
The two elimination routes of atracurium
Atracurium in plasma at 37 °C and pH 7.4. It is stored at pH 4 and 4 °C, conditions under which Hofmann elimination is arrested — which is why it lives in the fridge.
Two routes, neither needing an organ
Ester hydrolysis · ~60%
Non-specific plasma esterases, unrelated to plasma cholinesterase. Products: a quaternary alcohol, a quaternary acid and laudanosine. Acidosis accelerates this route, although pH change in the clinical range probably does not alter it much.
Hofmann elimination · ~40%
A spontaneous, non-enzymatic chemical breakdown requiring normal body temperature and pH. Products: laudanosine and a quaternary monoacrylate. Acidosis and hypothermia slow it — the opposite direction to ester hydrolysis.
The clinical conclusion
Elimination is independent of hepatic and renal function. Only about 10% of the drug is excreted renally, and none in bile.
The metabolite to name
Laudanosine is common to both routes: a tertiary amine, so it crosses the blood–brain barrier, a glycine antagonist with CNS-stimulating properties, no neuromuscular blocking activity, and cleared by the kidney. Seizures occur only at concentrations well above those met clinically.
| Point | Detail |
|---|---|
| Why it suits renal failure | Both elimination routes are organ-independent, so clearance and duration are essentially unchanged. Roughly 90% is metabolised, with only about 10% renally excreted and none in bile |
| The concern, and its size | Laudanosine is cleared by the kidney and will accumulate in renal failure. Plasma concentrations remain very low, and adverse effects are unlikely in either theatre or intensive care — better answers said the accumulation is rarely clinically significant in routine use rather than treating it as a contraindication |
| Cisatracurium compared | One of the ten stereoisomers of atracurium, three to five times more potent. It undergoes no direct ester hydrolysis of the parent molecule; Hofmann elimination is the predominant route, giving laudanosine and a monoquaternary acrylate. Because less drug is needed, roughly five times less laudanosine is produced |
| Cisatracurium in organ failure | No change in kinetic profile in end-stage renal or hepatic impairment, and its metabolites have no neuromuscular blocking activity |
| The trade-off | Cisatracurium buys negligible histamine release and less laudanosine; it costs onset, because greater potency means a smaller molar dose and a shallower gradient |
Writing it in the time you actually have
Spend the time where the marks are
The mark weighting is 1, 2, 4, 3. Most candidates write in the order printed and give the classification more space than it is worth.
| Minutes | Part | What to write |
|---|---|---|
| 0–1 | (a) · 1 mark | Two lines. Aminosteroidal — rocuronium. Benzylisoquinolinium — atracurium. Check the spelling and move on. Do not write a paragraph on duration classes |
| 1–4 | (b) · 2 marks | State the inverse potency relationship and its mechanism, then the perfusion-limited point, then dose, cardiac output and muscle blood flow. One ED₉₅ comparison as evidence |
| 4–12 | (c) · 4 marks | The longest section, because it carries the most marks and scored worst. A two-column table: class and mechanism. Aim for seven or eight classes, each with its mechanism. Write no physiology |
| 12–17 | (d) · 3 marks | The two routes with their percentages, the opposite pH behaviour, laudanosine and monoquaternary acrylate by name, then organ independence and the renal failure conclusion |
| 17–18 | Surplus | Cisatracurium as the contrast, or the synergy between chemically dissimilar relaxants in part (c) |