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Using measured airway mechanics in pediatrics

Article

Author: Süha Demirakca

Date of first publication: 12.06.2018

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.
Using measured airway mechanics in pediatrics

Introduction

It is very useful for diagnosing the lung condition and its severity in order to optimize the ventilator settings (Kneyber MCJ, de Luca D, Calderini E, et al. Recommendations for mechanical ventilation of critically ill children from the Paediatric Mechanical Ventilation Consensus Conference (PEMVECC). Intensive Care Med. 2017;43(12):1764-1780. doi:10.1007/s00134-017-4920-z1​). 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.

The following images show original monitoring data from mechanically ventilated children.

Severe restriction

Figure 1 shows a two-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.

Screen showing monitoring parameters and waveforms
Figure 1: Severe restriction
Screen showing monitoring parameters and waveforms
Figure 1: Severe restriction

Obstructive lung condition

Figure 2 shows a four-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).

Screen showing monitoring parameters and waveforms
Figure 2: Obstructive lung condition
Screen showing monitoring parameters and waveforms
Figure 2: Obstructive lung condition

Mixed disease; obstruction predominant

Figure 3 shows an eighteen-month 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.

Screen showing monitoring parameters and waveforms
Figure 3: Mixed disease with obstruction predominant
Screen showing monitoring parameters and waveforms
Figure 3: Mixed disease with obstruction predominant

Normal lung mechanics

Figure 4 shows a nine-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).

Screen showing monitoring parameters and waveforms
Figure 4: Normal lung mechanics
Screen showing monitoring parameters and waveforms
Figure 4: Normal lung mechanics

Applying an individualized ventilation strategy in pediatric patients

  1. Find the right diagnosis of the lung condition for each individual child.
  2. Verify the diagnosis by evaluating the airway mechanics based on RCexp, R and C.
  3. Adjust the ventilator settings according to airway mechanics:
    1. Predominantly restriction: apply lung-protective driving pressure to achieve the lowest possible Vt/C, compensate with higher frequencies.
    2. Predominantly obstruction: reduce frequencies to avoid any flow interruption, compensate with higher Vt up to 10 ml/kg.
  4. Check the flow curve for end-inspiratory and end-expiratory flow interruption.

Footnotes

References

  1. 1. Kneyber MCJ, de Luca D, Calderini E, et al. Recommendations for mechanical ventilation of critically ill children from the Paediatric Mechanical Ventilation Consensus Conference (PEMVECC). Intensive Care Med. 2017;43(12):1764-1780. doi:10.1007/s00134-017-4920-z

Recommendations for mechanical ventilation of critically ill children from the Paediatric Mechanical Ventilation Consensus Conference (PEMVECC).

Kneyber MCJ, de Luca D, Calderini E, et al. Recommendations for mechanical ventilation of critically ill children from the Paediatric Mechanical Ventilation Consensus Conference (PEMVECC). Intensive Care Med. 2017;43(12):1764-1780. doi:10.1007/s00134-017-4920-z



PURPOSE

Much of the common practice in paediatric mechanical ventilation is based on personal experiences and what paediatric critical care practitioners have adopted from adult and neonatal experience. This presents a barrier to planning and interpretation of clinical trials on the use of specific and targeted interventions. We aim to establish a European consensus guideline on mechanical ventilation of critically children.

METHODS

The European Society for Paediatric and Neonatal Intensive Care initiated a consensus conference of international European experts in paediatric mechanical ventilation to provide recommendations using the Research and Development/University of California, Los Angeles, appropriateness method. An electronic literature search in PubMed and EMBASE was performed using a combination of medical subject heading terms and text words related to mechanical ventilation and disease-specific terms.

RESULTS

The Paediatric Mechanical Ventilation Consensus Conference (PEMVECC) consisted of a panel of 15 experts who developed and voted on 152 recommendations related to the following topics: (1) general recommendations, (2) monitoring, (3) targets of oxygenation and ventilation, (4) supportive measures, (5) weaning and extubation readiness, (6) normal lungs, (7) obstructive diseases, (8) restrictive diseases, (9) mixed diseases, (10) chronically ventilated patients, (11) cardiac patients and (12) lung hypoplasia syndromes. There were 142 (93.4%) recommendations with "strong agreement". The final iteration of the recommendations had none with equipoise or disagreement.

CONCLUSIONS

These recommendations should help to harmonise the approach to paediatric mechanical ventilation and can be proposed as a standard-of-care applicable in daily clinical practice and clinical research.