Less than 5 percent of patients hospitalized for acute asthma exacerbation will require mechanical ventilatory support. However, ventilator management in status asthmaticus is unique and the impact of increased airway resistance on ventilation strategies must be understood to avoid life-threatening complications like severe acidosis, barotrauma and hypotension. 

Dynamic Hyperinflation:

Dynamic hyperinflation during mechanical ventilation, also known as auto-PEEP or intrinsic PEEP, is caused by collapse of unstable airways early in expiration or insufficient expiratory time, preventing exhalation of the entire tidal volume. Ventilator waveforms can provide the first clue of worsening airway resistance, dynamic hyperinflation and patient-ventilator asynchrony, enabling clinicians to both diagnose and manage this ventilator emergency. Dynamic hyperinflation is commonly seen in patients with obstructive lung disease and is most readily observed when the expiratory flow waveform fails to return to the zero baseline (i.e. no flow) prior to delivery of the next breath. 

Auto-PEEP has many dangerous consequences resulting from an increased resting lung volume and plateau pressure. 

Consequences of Auto-PEEP:

• Increased intra-thoracic pressure which ultimately leads to decreased venous return, hypotension, and possible cardiovascular collapse
• Increased inspiratory workload from suboptimal lung function at a higher resting lung volume
• VQ mismatch from regional hyperinflation
• Increased risk of barotrauma, e.g. pneumothorax
• Ventilator asynchrony
• Increased respiratory effort required to trigger ventilator (i.e. trigger asynchrony)

The ventilator is alarming for “high peak inspiratory pressure” in your asthmatic patient and you determine that the patient is experiencing dynamic hyperinflation. Now what?

Mechanical ventilation strategies to prevent and reverse auto-PEEP:

1. Decrease the respiratory rate (e.g. 10-12 breaths per minute) to increase the total time for each respiratory cycle, thereby lengthening expiratory time
2. Decrease the tidal volume to allow more time for exhalation
3. Permissive hypercapnia: reducing minute ventilation with the above strategies will likely cause respiratory acidosis, but this is acceptable. Avoid the urge to correct acidemia or normalize the pH. 
4. Increase the I:E ratio to lengthen the expiratory time for each respiratory cycle (e.g. 1:2 to 1:3)
5. If auto-PEEP is present on an end-expiratory hold maneuver or the patient develops hemodynamic instability, disconnect the patient from the ventilator and apply pressure to the chest wall to augment expiratory flow. If no ventilator changes are made, dynamic hyperinflation is likely to recur.
6. Treat the underlying obstructive process: bronchodilators, steroids, magnesium sulfate, frequent suctioning

References:

Ahmed SM, Athar M. Mechanical ventilation in patients with chronic obstructive pulmonary disease and bronchial asthma. Indian J Anaesth. 2015 Sep; 59(9): 589–598.

Leatherman J. Mechanical ventilation for severe asthma. Chest. 2015 Jun;147(6):1671-1680. doi: 10.1378/chest.14-1733. PMID: 26033128.

Smith TC, Marini JJ. Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction. J Appl Physiol (1985). 1988 Oct;65(4):1488-99. doi: 10.1152/jappl.1988.65.4.1488. PMID: 3053583.

The Society of Critical Care Medicine, 2017. Fundamental Critical Care Support. 6th ed. Mount Prospect: Third Printing, pp.81-82.