Show the model answerAttempt it first — that is what makes it stick
What earns the marks10 marks
| Every change needs a consequence | 8 marks. A pharmacokinetic change with no clinical implication attached is half a point. Write them as pairs |
|---|---|
| The liver, in three parts | Liver mass, hepatic blood flow and intrinsic metabolic activity — named separately, because they fall for different reasons |
| The kidney, in two | Renal blood flow and glomerular filtration rate |
| Volume of distribution and redistribution | Not just that it changes, but how it then affects metabolism and elimination |
| The four implications | Loading dose, dosing interval, monitoring, reversal. This is the list almost nobody wrote |
| Do not reduce the mg/kg dose | The loading dose per kilogram is unchanged. A neighbouring sitting called this error dangerous |
| (b) Onset is about delivery | 2 marks. Lower cardiac output, longer circulation time, slower onset — most candidates did get this |
8 marks
Pharmacokinetic considerations, and what each one means at the bedside
Rocuronium is the aminosteroid most dependent on the liver, so ageing acts on it through nearly every route at once. Write the change and its consequence on the same line.
How ageing reaches a rocuronium block
The healthy elderly patient
No disease is stated in the stem, so every change below is the normal physiology of ageing — which is precisely why it applies to every elderly patient on the list, not only the sick ones.
Three separate routes, and the examination wanted all three named
Body composition
Total body water and lean body mass fall; total body fat rises. Rocuronium is a permanently charged, hydrophilic drug confined largely to extracellular fluid, so its volume of distribution falls.
The liver
Liver mass falls, hepatic blood flow falls, and intrinsic metabolic activity falls. All three reduce hepatic uptake and biliary excretion of the parent drug.
The kidney
Renal blood flow and glomerular filtration rate both fall, removing part of the minority elimination route as well.
A smaller volume of distribution means less redistribution away from the effect site, so plasma concentration is sustained; reduced hepatic and renal clearance then removes what remains more slowly. The measured result is a 27% fall in plasma clearance.
The clinical effect
Duration of action and recovery index are both prolonged — slower onset and a longer block, from the same milligram-per-kilogram dose.
| Pharmacokinetic change | Mechanism | Clinical implication |
|---|---|---|
| Distribution | ||
| Volume of distribution falls | Total body water and lean body mass fall while body fat rises; a hydrophilic, ionised drug has less extracellular water to occupy | The mg/kg loading dose is unchanged. A given dose produces a slightly higher initial plasma concentration and reliable intubating conditions — the dose is not reduced |
| Redistribution is reduced | A smaller peripheral compartment to move into | Plasma and effect-site concentrations are sustained for longer, so the clinically apparent duration lengthens before clearance has done anything |
| Metabolism and elimination | ||
| Hepatic clearance falls | Reduced liver mass, reduced hepatic blood flow and reduced intrinsic metabolic activity, acting on a drug taken up by carrier-mediated hepatic transport and excreted in bile | The principal route of elimination is slowed, and this is the dominant term for rocuronium |
| Renal clearance falls | Reduced renal blood flow and reduced glomerular filtration rate | The minority route is slowed too, so there is no compensating pathway |
| Plasma clearance falls by about 27% | The net of the above | Duration of action and recovery index are both increased. This is the number to quote |
| Decision | What changes | Why |
|---|---|---|
| Loading dose | Unchanged in mg/kg | The number of receptors to be occupied has not changed. Onset will be slower, which is a reason to wait longer before laryngoscopy, not a reason to give less |
| Dosing interval | Lengthened | With the exception of atracurium and cisatracurium, the interval between maintenance doses must be increased in older patients to hold the same depth. Repeating on the old schedule is how a block accumulates |
| Monitoring | Objective, quantitative, and non-negotiable | Recovery of neuromuscular function is generally delayed in older patients, and the choice of drug plus monitoring of depth are described as exceptionally important in this group. Incomplete recovery after a long-acting agent is associated with an increased incidence of perioperative pulmonary complications |
| Reversal | Anticipate a deeper block at the end of the case, and reverse on the measurement rather than the clock | Neostigmine needs at least T2 and has a ceiling; if the train-of-four is still absent, sugammadex is the agent that works at that depth. Rocuronium being an aminosteroid, sugammadex is available — which it would not be had the question named atracurium |
Commonly lost: Most candidates mentioned the pharmacokinetic changes of ageing — volume of distribution, metabolism and clearance — but very few outlined the clinical implications for loading dose, dosing interval, monitoring and reversal. That omission, not any missing physiology, is what produced a 15.2% pass.
Commonly lost: The required detail was specific: the effect of liver mass, hepatic blood flow and intrinsic metabolic activity on metabolism, and of renal blood flow and glomerular filtration rate on elimination. “Reduced hepatic and renal function” in one phrase collapses five marking points into one.
2 marks
Pharmacodynamic factors affecting the onset of rocuronium in the elderly
Two marks, and the examination recorded that most candidates did get the central point. Say it, then extend it.
| Factor | Effect on onset | Mechanism |
|---|---|---|
| Reduced cardiac output | Slower | A longer circulation time delays arrival of the drug at the junction. These are ionised quaternary compounds whose onset is perfusion-limited rather than diffusion-limited, so delivery is the rate-limiting step and cardiac output governs it. This is the point the examination noted most candidates made |
| Reduced muscle blood flow | Slower | The same argument applied regionally. Less well-perfused muscle equilibrates with plasma more slowly |
| Changes at the junction itself | Slower and less predictable | The distance between the junctional axon and the motor end plate increases with age, and the end plate flattens — a structural change on top of the delivery one |
| Potency is unchanged | No effect | Worth stating explicitly. Onset across the class is governed by potency and molar dose, and ageing changes neither. The slowing is entirely a delivery phenomenon, which is why it is a pharmacodynamic and circulatory answer rather than a receptor one |
Writing it in the time you actually have
Eight marks and two, so write in a ratio of four to one
Part (a) is 80% of the question. The tempting mistake is to write everything you know about ageing physiology and never reach the implications, which is exactly what happened.
| Minutes | Part | What to write |
|---|---|---|
| 0–1 | Plan | Two columns down the page: change, and implication. The second column is the one that was left blank in 2023 |
| 1–7 | (a) · the pharmacokinetics | Rocuronium's route of elimination first, then body composition and volume of distribution, then liver mass, hepatic blood flow and intrinsic activity, then renal blood flow and GFR, then the 27% fall in clearance |
| 7–13 | (a) · the implications | Loading dose unchanged, dosing interval lengthened, objective monitoring mandatory, reversal anticipated and guided by the monitor. Four lines, six marks' worth of the difference between a pass and a fail |
| 13–16 | (b) · onset | Cardiac output and circulation time first, then muscle blood flow, then the junctional changes, then the note that potency is unchanged |
| 16–18 | Surplus | The atracurium contrast, which shows the whole argument was about organ clearance rather than about age itself |
If this came up in the viva
Viva point
The question this stem turns into when it is asked aloud.
A rocuronium block lasts longer in an 80-year-old. Is that pharmacokinetic or pharmacodynamic? Which relaxant would behave differently, and why?
Answer
Pharmacokinetic. Sensitivity at the neuromuscular junction is essentially unchanged by age; the prolongation comes from reduced hepatic and renal clearance. Atracurium and cisatracurium behave differently because Hofmann elimination is organ-independent, so their duration changes little.