Gas trapping & breath stacking - What is your asthmatic patient telling you?

Approximately 97,000 Australians presented to the emergency department with asthma in 2022-23. In the same period, over 31,000 required hospitalisation for asthma.¹ 

A significant proportion of ICU admissions in general are based on the need for mechanical ventilation.² However, invasive mechanical ventilation in patients with acute status asthmaticus can be challenging.³ Effective use of this ventilation technique in severe asthma exacerbations involves the ability to recognise and manage high airway pressures and dynamic hyperinflation. 

The problem with dynamic hyperinflation

Dynamic hyperinflation, or dynamic hyperventilation, refers to the failure to fully exhale any inhaled air at the end of the respiratory cycle, resulting in impaired alveolar emptying and  gas trapping. This in turn increases the risk of:²⁴

• increased pulmonary pressure

• barotrauma and pneumothoraces

• decreased cardiac output

• systemic hypotension

• haemodynamic instability

The cardiac complications of dynamic hyperinflation arise from reduced blood flow in the vessels of the thoracic cavity, leading to reduced left ventricular preload and afterload. The subsequent compromise to cardiac function causes increased pulmonary vascular resistance, and, in severe cases, may result in pulsus paradoxus once the patient has been extubated and returns to unassisted breathing.³

It should be noted that patients with asthma are at high risk of gas trapping due to inflammation of the airways, bronchospasm, and obstruction from mucus plugs.² The incidence of pneumothorax is typically reported as 3-6% among these patients, with a higher risk among those for whom the end-inspiratory lung volume goes beyond 20 mL/kg, approaching total lung capacity.³

Managing dynamic hyperventilation during mechanical ventilation

As dynamic hyperventilation occurs due to the patient’s inability to fully exhale air, avoiding this scenario requires adjusting the inspiratory-to-expiratory ratio. Understanding the interplay of lung pressures and lung compliance, and how acute asthma exacerbations can influence these factors is integral to modifying the approach during mechanical ventilation.²

Dynamic hyperventilation may be monitored with:³

• total positive end-expiratory pressure with an end-inspiratory hold manoeuvre

• pulse pressure monitoring as an indication of cardiac output at risk

Strategies to achieve optimal inspiratory-to-expiratory ratio include:²

• decreasing respiratory rate

• increasing inspiratory flow

• administering inspiratory flow with a square waveform

• optimise sedation to avoid the patient hyperventilating

• reduce airway obstruction with pharmacological measures, such as bronchodilators and corticosteroids

If the patient is experiencing systemic hypotension or persistent breath stacking, it may be reasonable to disconnect the ventilator to enable manual decompression and full exhalation.²⁴

Dynamic hyperinflation (A) Apnoea ventilation to eliminate trapped volume (B)⁵

Though mechanical ventilation is a life-saving intervention, it does come with some risk of complications. Acknowledging that patients experiencing acute asthma exacerbations are prone to dynamic hyperinflation while on this treatment will enable the critical care nurse to take steps to avoid it, while remaining vigilant for signs of air trapping. 

Related course

Critical Bytes - Status Asthmaticus

References

1. Asthma Australia. Statistics. 2024. Available at: https://asthma.org.au/what-is-asthma/statistics/. (last accessed March 2025).

2. Mora Carpio AL, Mora JI. Ventilator Management. [Updated 2023 Mar 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK448186/

3. Gayen S, Dachert S, Lashari BH, Gordon M, Desai P, Criner GJ, Cardet JC, Shenoy K. Critical Care Management of Severe Asthma Exacerbations. J Clin Med. 2024 Feb 1;13(3):859. doi: 10.3390/jcm13030859. PMID: 38337552; PMCID: PMC10856115.

4. Talbot T, Roe T, Dushianthan A. Management of Acute Life-Threatening Asthma Exacerbations in the Intensive Care Unit. Applied Sciences. 2024; 14(2):693

5. Kostakou E., Kaniaris E., Filiou E., et al Acute severe asthma in adolescent and adult patients: current perspectives on assessment and management Journal of Clinical Medicine 2019 (8) 1283 http://dx.doi.org/10.3390/jcm8091283