The expiratory time constant (RCexp) is measured breath-by-breath on all Hamilton Medical ventilators. As RCexp is the product of compliance and resistance, this single variable gives us an overview of the overall respiratory mechanics.
It is very useful for diagnosing the lung condition and its severity in order to optimize the ventilator settings (1). In pediatric patients, compliance (C) and resistance (R) must always be checked in parallel to differentiate clearly between restrictive versus obstructive and mixed disease. Mixed disease is a common lung condition in pediatric patients with acute respiratory failure.
This bedside tip provides examples of adjusting settings for pressure, tidal volume (Vt), inspiratory and expiratory time (Ti, Te), and frequency according to the measured airway mechanics (RCexp, C, R) for different lung conditions in children. Note that the flow curve should also be checked in all cases to avoid end-inspiratory and end-expiratory flow interruption. This promotes the best gas distribution during Ti and helps avoid AutoPEEP/air-trapping during Te.
Original monitoring data from mechanically ventilated children
Severe restriction: 2-year old boy with severe diffuse / interstitial pneumonia under veno-venous ECMO (bodyweight 12 kg). After recovery from self-induced lung injury, sedation was adjusted to limit the respiratory drive. Due to the very low RCexp of 0.10 (C = 2.5 ml/cmH2O; Rinsp = 10 cmH2O/l/s), a protective Pressure support of 9 cmH2O with a resulting Vt of 3.2 ml/kg of bodyweight was chosen. Expiratory trigger sensitivity (ETS) for flow cycling was set to 5% to prevent inspiratory flow interruption occurring too early. Note that the patient compensates the low Vt with a high breathing frequency of 60 bpm, which does not lead to end-expiratory flow interruption.
Obstructive lung condition: 4-year old girl with pneumonia with obstruction (bodyweight 20 kg / length 112 cm) under controlled ventilation. High resistance causes a prolonged RCexp, and the frequency (20 bpm) is lowered with a Ti of 1.1s and Te of 1.9s in order to avoid any flow interruption (see flow curve).
Mixed disease; obstruction predominant: 1.5-year old girl with a BPD history and acute human metapneumovirus pneumonia (bodyweight 9.1 kg) under controlled ventilation. Low frequency (28 bpm) is set together with a protective Vt of 6.6 ml/kg, which is only possible with sedation and permissive hypercapnia (see PetCO2). End-inspiratory and end-expiratory flow interruption is avoided by setting a prolonged Ti and Te.
Normal lung mechanics: 9-year old girl with neuromuscular disease breathing spontaneously and ready for extubation. Lung mechanics after recovery from pneumonia are normal, with the exception of augmented Cstat due to higher chest wall compliance with reduced muscular tonus (Vt ~7 ml/kg requiring only 3 cm pressure support).
Applying an individualized ventilation strategy in pediatric patients
- Find the right diagnosis of the lung condition for each individual child.
- Verify the diagnosis by evaluating the airway mechanics based on RCexp, R and C.
- Adjust the ventilator settings according to airway mechanics:
- Predominantly restriction: apply lung-protective driving pressure to achieve the lowest possible Vt/C, compensate with higher frequencies.
- Predominantly obstruction: reduce frequencies to avoid any flow interruption, compensate with higher Vt up to 10 ml/kg.
- Check the flow curve for end-inspiratory and end-expiratory flow interruption.
- Kneyber MCJ et al. Recommendations for mechanical ventilation of critically ill children from the Paediatric Mechanical Ventilation Consensus Conference (PEMVECC). Intensive Care Med (2017) 43:1764–1780.
- Recommendations for mechanical ventilation of critically ill children
- Bedside tip: How to use the expiratory time constant
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