What is the most appropriate ventilation strategy for a mechanically ventilated child with status asthmaticus and dynamic hyperinflation?

Ventilation Strategy for Mechanically Ventilated Children with Status Asthmaticus and Dynamic Hyperinflation

In a mechanically ventilated child with status asthmaticus and dynamic hyperinflation, use a controlled hypoventilation strategy with low respiratory rates (≤12 breaths/min), prolonged expiratory time, plateau pressure ≤30 cmH2O, tidal volume ≤10 mL/kg ideal body weight, and accept permissive hypercapnia (PaCO2 45-60 mmHg) as long as pH remains >7.20. 1, 2

Core Ventilator Settings

Pressure Limits:

• Keep plateau pressure ≤30 cmH2O in obstructive airway disease to prevent barotrauma and worsening dynamic hyperinflation 1
• Maintain peak inspiratory pressure ≤45 cmH2O, as deliberately limiting airway pressure below this threshold reduces pressure-related complications 2
• Tidal volume should be ≤10 mL/kg ideal body weight 1, 3

Respiratory Rate and Timing:

• Use low respiratory rates (≤12 breaths/min) to allow adequate expiratory time and prevent air-trapping 2, 4
• Set inspiratory time based on respiratory system mechanics using the time constant (compliance × resistance), and monitor flow-time scalars to ensure complete exhalation 1, 3
• Prolonged expiratory time is mandatory to minimize dynamic hyperinflation 4

PEEP Strategy:

• Add external PEEP when air-trapping is present to facilitate triggering and reduce work of breathing 1
• Set external PEEP below measured intrinsic PEEP (PEEPi) after performing an expiratory hold maneuver 1, 5
• Typical PEEP range is 5-8 cmH2O, but titrate carefully based on intrinsic PEEP measurements 1, 3

Permissive Hypercapnia Approach

Accept elevated PaCO2 (45-60 mmHg) as long as pH >7.20 to avoid the need for high minute ventilation that worsens dynamic hyperinflation 1, 2, 4. The Paediatric Mechanical Ventilation Consensus Conference explicitly recommends accepting higher PCO2 in acute pulmonary patients unless specific diseases dictate otherwise, with a target pH >7.20 1. This controlled hypoventilation strategy has been shown to successfully ventilate children with severe status asthmaticus with reduced pressure-related complications 2.

Critical Monitoring Parameters

Measure the following to guide ventilator adjustments:

• Peak inspiratory pressure, plateau pressure, and intrinsic PEEP (using expiratory hold maneuver) 1, 5
• Monitor flow-time scalars continuously to detect incomplete exhalation and air-trapping 1, 3
• Arterial or capillary blood gases for PCO2 and pH 1, 3
• SpO2 continuously (target ≤97% to avoid excessive oxygen) 1, 3
• End-tidal CO2 monitoring 1, 3

Patient-Ventilator Synchrony

Target optimal patient-ventilator synchrony to reduce work of breathing and prevent patient-ventilator dyssynchrony that can worsen dynamic hyperinflation 1, 3. If pressure support ventilation is used with restored respiratory drive, adjust flow cycling sensitivity and rise time to obtain appropriate inspiratory time 1.

Modes to Avoid

Do NOT use high-frequency jet ventilation (HFJV) in obstructive airway disease due to the risk of worsening dynamic hyperinflation 1. High-frequency oscillatory ventilation (HFOV) has insufficient evidence and should only be considered if conventional ventilation fails, though it can be judiciously performed in obstructive airway disease with extreme caution 1.

Common Pitfalls to Avoid

• Rapid correction of hypercapnia and respiratory acidosis should be avoided as this requires high minute ventilation that worsens dynamic hyperinflation and increases risk of barotrauma 5, 4
• Excessively high respiratory rates lead to incomplete exhalation and progressive air-trapping 3, 4
• Failing to measure intrinsic PEEP can result in inappropriate external PEEP settings 1, 5
• Hand ventilation should be avoided unless specific conditions dictate otherwise, as it often delivers excessive tidal volumes 1

Rescue Therapies

If conventional mechanical ventilation with the above strategy fails:

• Consider ECMO, particularly useful in cases of severe air-leak syndrome (pneumothorax, pneumomediastinum) 1, 5
• Inhaled anesthetics are direct bronchodilators but require close cooperation with Anesthesiology as conventional ICU ventilators cannot deliver them 5
• Heliox may be beneficial but cannot be delivered by every ventilator 5

Expected Outcomes

With appropriate ventilator management using controlled hypoventilation, most children can be weaned and extubated within 48 hours, with median duration of mechanical ventilation around 29-42 hours 6, 2. Pressure-controlled ventilation using these principles has been shown to improve pH from median 7.21 to 7.31 and decrease PaCO2 from 65 to 41 torr within 4 hours 6.