Ventilator dyssynchrony from inadequate inspiratory flow — FFICM MCQ

Correct answer: D — Flow starvation causing patient-ventilator dyssynchrony

The correct answer is D, flow starvation causing patient-ventilator dyssynchrony. In volume control ventilation the inspiratory flow rate is fixed regardless of patient effort, so when respiratory drive is high (as in septic shock with ARDS) the patient's demand outstrips the delivered flow. This produces the classic scooped-out concavity on the pressure-time curve as the patient generates negative pressure trying to pull more flow, with vigorous accessory muscle use but only modest peak pressure because the ventilator is flow-limited rather than pressure-limited. The plateau pressure of 24 cmH2O confirms compliance is not acutely deteriorating, and regular capnography excludes a circuit or airway catastrophe, leaving mismatched flow delivery as the explanation for both the waveform and the worsening respiratory alkalosis from increased effective minute ventilation. Why the other options are wrong: B. Auto-triggering from circuit water: this produces spurious triggering without true patient effort or accessory muscle recruitment, and typically shows a normal, non-scooped inspiratory pressure profile rather than a concave effort pattern. C. Endotracheal tube obstruction: this raises peak pressure disproportionately to plateau pressure due to increased airway resistance, and would usually alter the capnography waveform, neither of which is seen here. E. Tension pneumothorax: this causes acutely rising peak and plateau pressures with haemodynamic compromise and often absent breath sounds, not a stable plateau of 24 cmH2O with regular capnography. A. Severe bronchospasm: this produces a high peak to plateau pressure gradient, a rising expiratory limb, and prolonged expiratory time on the flow-time curve, none of which are described. Key point: A scooped inspiratory pressure-time curve with normal plateau pressure and preserved capnography in volume control ventilation signals flow starvation, not a mechanical or circuit problem.

Reference: Patient-Ventilator Dyssynchrony in Critically Ill Patients, Journal of Clinical Medicine 2021 (PMC8509510), applied per FICM/ICS UK critical care ventilation practice: https://pmc.ncbi.nlm.nih.gov/articles/PMC8509510/