Ventilator waveforms: air-trapping and ineffective triggering

A CICM cross-table data viva on ventilator waveform interpretation. The candidate reads the flow and pressure scalars, recognises incomplete expiration and ineffective triggering, identifies intrinsic (auto-)PEEP on the expiratory hold, and explains the time-constant physiology and the management.

Prompt

A ventilator scalar screen (flow–time and pressure–time) with an inspiratory-hold measurement and a simultaneous arterial gas, from an intubated patient with COPD — interpret.

Stimulus

A 70-year-old man with severe COPD is intubated for a hypercapnic exacerbation. He is on volume-controlled ventilation. You are shown the ventilator scalar screen, an expiratory-hold manoeuvre, and a simultaneous arterial gas.^[1]

The flow–time scalar: inspiratory flow is delivered as a constant upward deflection; the expiratory flow descends but does not return to the zero baseline before the next breath begins — the expiratory limb is truncated, with residual expiratory flow still present at the onset of the next inspiration.^[1]

The pressure–time scalar: several small negative deflections appear below the baseline at irregular intervals that do not trigger a ventilator breath (ineffective efforts).^[1]

The expiratory hold: a two-second end-expiratory occlusion records an airway pressure of 8 cmH2O above the set PEEP of 5 cmH2O (set PEEP 5; total PEEP 13).^[1]

Variable	Value
Mode	Volume control
Tidal volume	420 mL
Set respiratory rate	16 /min
FiO2	0.4
Set PEEP	5 cmH2O
Measured intrinsic PEEP	8 cmH2O
Peak inspiratory pressure	28 cmH2O
pH	7.32
PaCO2	58 mmHg
PaO2	78 mmHg
Bicarbonate	30 mmol/L

Take ninety seconds, then present a structured interpretation.^[1]

References2Show

[1]Bersten AD, Soni N (eds). Oh's Intensive Care Manual. 8th edition, Elsevier, 2018.
[2]Marino PL. The ICU Book. 4th edition, Lippincott Williams & Wilkins, 2014.