Show the model answerAttempt it first — that is what makes it stick
What earns the marks10 marks
| (a) Three headings | 4 marks. Physicochemical, pharmacokinetic, pharmacodynamic. Points are overlooked without them |
|---|---|
| (a) Non-organ-dependent metabolism | Specifically credited to good candidates — rapid metabolism by multiple pathways including extrahepatic and extrarenal |
| (a) Muscle relaxation | Specifically recorded as omitted by most |
| (a) Not bioavailability | Explicitly not relevant for an intravenous drug |
| (b) Compare, do not describe | 6 marks. Marks are lost when there is no comparison |
| (b) State the magnitude | "Markedly reduced", "slightly reduced", "no change" — good answers quantified the direction |
| (b) Correlate to the clinical effect | Link SVR and CO to what is observed; the same for respiratory parameters and minute ventilation |
| (b) Not mechanism, not kinetics | No marks were given for either. Respiratory rate was most often written incorrectly |
4 marks
The ideal properties of an intravenous anaesthetic drug
A list, grouped under three headings. The grouping is not presentation — it is what stops points being missed.
| Category | Property |
|---|---|
| Physicochemical | |
| Formulation | Water soluble, so no lipid emulsion or organic solvent is needed |
| Stability | Long shelf life at room temperature; stable in light; ready to use without reconstitution |
| Compatibility | Stable and non-reactive with plastic, glass and metal; compatible with other drugs in the line |
| Ionisation | Mainly unionised at physiological pH, so it crosses the blood–brain barrier readily |
| Painless | No pain on injection; safe if given intra-arterially or extravasated |
| Cost and environment | Inexpensive; minimal environmental impact |
| Pharmacokinetic | |
| Onset | Loss of consciousness within one arm–brain circulation time |
| High lipid solubility | So that it reaches the brain on the first pass |
| Rapid recovery | Short context-sensitive half-time; no accumulation during prolonged infusion |
| Non-organ-dependent metabolism | Rapid metabolism by multiple pathways, including extrahepatic and extrarenal routes, so that hepatic or renal failure does not prolong it |
| Inactive metabolites | No active or toxic metabolite |
| Predictability | Kinetics predictable enough to be modelled, so a target-controlled infusion is possible |
| No interaction | No pharmacokinetic interaction with other anaesthetic agents |
| Pharmacodynamic | |
| Analgesia | Analgesic at sub-anaesthetic concentrations |
| Muscle relaxation | Provides some, so that less neuromuscular blocking drug is required |
| Cardiovascular | Minimal depression of blood pressure, cardiac output and systemic vascular resistance |
| Respiratory | Minimal respiratory depression; no apnoea; no airway irritation or bronchospasm |
| Cerebral | Reduces CMRO₂ and intracranial pressure while preserving cerebral perfusion pressure; anticonvulsant |
| Emesis | Antiemetic, or at least not emetic |
| Emergence | No excitatory movements, no emergence delirium, no hallucinations |
| Safety | No histamine release, no hypersensitivity reaction, no toxicity, no endocrine effect; safe in porphyria and not a malignant hyperthermia trigger |
Commonly lost: Bioavailability is not relevant for an intravenous drug — it is 100% by definition, and listing it is a category error. Muscle relaxation was the property most often omitted. And “list” meant list: lengthy elaboration on each point earned nothing and cost the time the second part needed.
6 marks
Cardiovascular and respiratory effects of propofol and etomidate compared
A comparison table, with the magnitude stated and each parameter correlated to what is actually observed.
| Parameter | Propofol | Etomidate | Clinical consequence |
|---|---|---|---|
| Systemic vascular resistance | Markedly reduced, 15–25% | Minimally reduced, or unchanged | The dominant difference. Propofol removes the vasoconstriction a shocked patient is surviving on; etomidate does not |
| Mean arterial pressure | Markedly reduced, 10–40% | Largely unchanged | Why etomidate is chosen for the cardiovascularly compromised patient |
| Myocardial contractility | Reduced — direct negative inotropy | Largely unchanged | Compounds the fall in output with propofol; absent with etomidate |
| Cardiac output | Reduced | Largely unchanged | Follows from the two rows above |
| Heart rate | Unchanged or reduced; bradycardia and asystole reported | Unchanged | Propofol blunts the baroreflex, so no compensatory tachycardia defends the pressure |
| Baroreceptor reflex | Blunted and reset | Preserved | The single most important row, and the reason the propofol fall is not self-limiting |
| Pulmonary vascular resistance | Slightly reduced | Reduced | Relevant where right ventricular afterload matters |
| Myocardial oxygen consumption | Reduced, with coronary flow falling in parallel | Supply and demand both largely preserved | Etomidate maintains the myocardial oxygen supply–demand balance better in ischaemic heart disease |
| Parameter | Propofol | Etomidate | Clinical consequence |
|---|---|---|---|
| Apnoea after an induction dose | Common — 25–30%, and longer with a concurrent opioid | Occurs, but less frequent and briefer | The reason a difficult airway argues against a large propofol bolus |
| Tidal volume | Markedly reduced — about 40% at 100 μg/kg/min | Reduced | Both depress ventilation; propofol more so |
| Respiratory rate | Increased — about 20% at that infusion rate | Increased transiently, often after a brief period of hyperventilation | Most often written incorrectly. Rate does not simply fall; it is minute ventilation that is unpredictable |
| Minute ventilation | Unpredictable — the rate rise partly offsets the tidal volume fall | Reduced | State the two components rather than the product |
| Ventilatory response to CO₂ | Depressed | Depressed, but less so | Etomidate's respiratory depression is the milder of the two |
| Ventilatory response to hypoxia | Depressed | Depressed | Both; worth one clause |
| Airway reflexes | Markedly obtunded — a supraglottic airway can be placed without a relaxant | Not obtunded | Propofol's outstanding respiratory advantage, and the one place it beats etomidate outright |
| Airway irritation | None; laryngospasm and cough rare | None, though myoclonus may be mistaken for movement | Neither is an airway irritant, unlike thiopentone |
| Hypoxic pulmonary vasoconstriction | Preserved | Preserved | Both intravenous agents preserve it, unlike the volatiles — a mark for one clause |
Commonly lost: Marks are lost when there is no comparison. Describing propofol in one paragraph and etomidate in the next answers a different question, however accurate each paragraph is. And most candidates wrote about respiratory rate incorrectly — a maintenance infusion of propofol increases respiratory rate while reducing tidal volume, which is why the change in minute ventilation is unpredictable rather than simply a fall.
The teaching behind this section — etomidate’s haemodynamic profile