Anesthetic Equipment and Safety Flashcards

During low-flow anesthesia with a circle system, you notice the inspired CO₂ is rising despite a functioning soda lime canister. The capnograph shows an elevated baseline. Which mechanism is MOST likely responsible?

Answer: Channeling through the soda lime due to compaction or bypass around the granules

Channeling occurs when CO₂-laden gas finds paths of least resistance through compacted or unevenly packed soda lime granules, bypassing active absorbent. This can occur even with remaining absorbent capacity and before the color indicator changes uniformly. An incompetent inspiratory valve would allow exhaled gas into the inspiratory limb but would present differently on capnography (elevated baseline with characteristic waveform change). MH causes elevated CO₂ production but the absorbent would compensate at higher flows. True exhaustion is indicated by indicator change, which is specifically noted to be absent here.

A vapour analyser shows isoflurane concentration of 1.8% at the common gas outlet, but the vaporiser dial is set to 1.0%. The vaporiser has been recently filled. What is the MOST dangerous explanation for this discrepancy?

Answer: The vaporiser has been filled with desflurane instead of isoflurane

Filling an isoflurane vaporiser (SVP ~32 mmHg at 20°C) with desflurane (SVP ~669 mmHg at 20°C) creates an extremely dangerous situation. Desflurane's vastly higher saturated vapour pressure means the delivered concentration would be dramatically and unpredictably elevated — potentially causing awareness, cardiovascular collapse, or death. Modern vaporisers have agent-specific Selectatec-type connections to prevent this, but cross-filling can still occur with older equipment or keyed filling devices that have been tampered with. The concentration discrepancy (1.8% vs 1.0%) in this context reflects only a fraction of the potential output if desflurane were present. The other options produce smaller, more predictable deviations.

You are anaesthetising a patient in the prone position. Your capnograph suddenly shows a decrease in ETCO₂ from 5.2 kPa to 2.1 kPa with a normal waveform shape, while SpO₂ remains 98% and airway pressures are unchanged. The MOST likely diagnosis is:

Answer: Sampling line disconnection from the airway

A sampling line disconnection produces a sudden drop to near-zero or very low ETCO₂ with a preserved (normal morphology) waveform because the analyser is now sampling room air, which contains ~0.04% CO₂, producing a flat or minimally elevated trace. Crucially, all other parameters remain normal because ventilation and oxygenation are unaffected. Pulmonary embolism causes reduced ETCO₂ but also produces haemodynamic instability, tachycardia and the trace is typically unchanged in shape. Endobronchial intubation would increase airway pressures and reduce SpO₂. Accidental extubation would cause a loss of the capnograph trace entirely and loss of airway pressure, and would be rapidly apparent in the prone position.

In a total intravenous anaesthesia (TIVA) setup using target-controlled infusion (TCI), the Marsh model for propofol is being used. Compared to the Schnider model, which statement MOST accurately describes an important pharmacokinetic difference that affects clinical practice?

Answer: The Marsh model volume of distribution at steady state (Vdss) scales linearly with weight, potentially overestimating distribution in obese patients

The Marsh model scales the central and peripheral volumes of distribution linearly with total body weight (TBW). In obese patients, this can lead to dramatically overestimated volumes, resulting in underdosing (particularly at induction) or, paradoxically, accumulation errors over time. The Schnider model uses lean body mass (LBM) for volume calculations and age as a covariate, making it potentially better suited for elderly and obese patients. The Marsh model does not use age. By default, Marsh targets plasma concentration (Cp), not effect-site; many implementations allow effect-site targeting as an option. The Schnider model has a faster ke0 (~0.456 min⁻¹) meaning faster effect-site equilibration and thus a more pronounced initial peak at induction.

During mechanical ventilation with a circle breathing system, you switch from volume-controlled ventilation (VCV) to pressure-controlled ventilation (PCV) with the same set inspiratory pressure. The tidal volume drops significantly. Before attributing this to increased airway resistance or reduced compliance, which equipment-related cause should be excluded FIRST?

Answer: The compliance of the circuit tubing and humidifier consuming a greater proportion of the set pressure in PCV mode

In pressure-controlled ventilation, the set pressure drives gas into both the patient's lungs AND the compliant elements of the breathing circuit (corrugated tubing, HME filter, Y-piece, and humidifier chamber if present). In VCV, the set volume is delivered regardless — the circuit compliance acts as a loss but the volume target is met (at the cost of higher peak pressures). When switching to PCV with a numerically equivalent inspiratory pressure, the circuit compliance 'steals' a portion of the driving pressure, resulting in a lower effective transpulmonary pressure and lower tidal volume. This effect is amplified with longer tubing, heated humidifiers (larger compliance), or paediatric circuits. Fresh gas flow decoupling is a valid concern at low flows but does not explain a drop in Vt on switching modes. I:E ratio and trigger changes are possible but mode-specific defaults are typically preserved or displayed.

A patient undergoing cardiac surgery requires cardiopulmonary bypass (CPB). During bypass, the membrane oxygenator's gas inlet is connected to a blender supplying a O₂/air mixture. The anaesthetist sets the FiO₂ to 0.5 and a sweep gas flow of 3 L/min. Forty minutes into bypass, a blood gas reveals a PaO₂ of 680 mmHg and a PaCO₂ of 22 mmHg. Which adjustment is MOST appropriate?

Answer: Add CO₂ to the gas blender mixture to correct hypocapnia and reduce the sweep gas flow

In a membrane oxygenator, oxygen transfer (oxygenation) is primarily determined by FiO₂ and blood-gas contact time, while CO₂ removal is controlled by sweep gas flow rate — higher sweep flow 'blows off' more CO₂. With PaO₂ of 680 mmHg and PaCO₂ of 22 mmHg: oxygenation is excessive (hyperoxia) and the patient is significantly hypocapnic. Simply reducing sweep gas flow will correct hypocapnia but does not address hyperoxia. The ideal correction combines reducing the FiO₂ (to lower PaO₂) AND reducing sweep gas flow (to normalise PaCO₂). However, the option that most directly and completely addresses BOTH abnormalities simultaneously — while maintaining the specific ability to fine-tune CO₂ independently — is adding CO₂ to the blender and reducing sweep flow. This is standard practice in many cardiac centres to maintain normocapnia during CPB and avoid the vasoconstrictive effects of hypocapnia on the cerebral circulation.