Life-threatening asthma with dynamic hyperinflation — FFICM MCQ

Correct answer: C — Reduce minute ventilation and prolong expiratory time

The correct answer is C, reduce minute ventilation and prolong expiratory time. This patient has severe airflow obstruction with a very high peak pressure but a much lower plateau pressure (55 vs 23 cmH2O), a large intrinsic PEEP of 12 cmH2O, and inspiratory hypotension, all classic features of dynamic hyperinflation (gas trapping) rather than poor lung compliance. Continuing to ventilate aggressively to normalise PaCO2 worsens air trapping, raises intrathoracic pressure, reduces venous return and cardiac output, and increases barotrauma risk. The recognised strategy is controlled hypoventilation: reduce tidal volume and/or respiratory rate, lengthen expiratory time (low I:E ratio), and accept permissive hypercapnia and respiratory acidosis while bronchodilator therapy takes effect. This approach is supported by current UK asthma management guidance, which prioritises avoidance of hyperinflation-related cardiovascular compromise over rapid CO2 correction. Why the other options are wrong: E. Increase respiratory rate to correct PaCO2: raising rate shortens expiratory time further, worsening breath stacking and dynamic hyperinflation, and would aggravate the hypotension already seen. D. Increase external PEEP above intrinsic PEEP: adding PEEP above the auto-PEEP in an obstructive, flow-limited patient increases end-expiratory lung volume and intrathoracic pressure, worsening hyperinflation and hypotension rather than counteracting it. B. Switch to small tidal volumes with inverse ratio ventilation: inverse ratio ventilation shortens expiratory time, the opposite of what is needed in an obstructive picture with gas trapping. A. Start recruitment manoeuvres every hour: recruitment manoeuvres are indicated for derecruited, poorly compliant lungs (eg ARDS), not for hyperinflated, flow-limited asthmatic lungs, and would dangerously increase intrathoracic pressure and barotrauma risk. Key point: In ventilated life-threatening asthma with high intrinsic PEEP and inspiratory hypotension, treat dynamic hyperinflation by lowering minute ventilation and prolonging expiratory time, accepting permissive hypercapnia rather than chasing a normal PaCO2.

Reference: NICE/BTS/SIGN Guideline NG245: Asthma: diagnosis, monitoring and chronic asthma management, and associated British guideline on the management of severe/acute asthma; principle of permissive hypercapnia and avoidance of dynamic hyperinflation in ventilated life-threatening asthma. https://www.nice.org.uk/guidance/ng245