Measurement and Assessment of Work of Breathing in Neonates During Nasal Continuous Positive Airway Pressure Therapy

Introduction: Nasal continuous positive airway pressure (nCPAP) is a widely used form of non-invasive respiratory support in neonates. The general aim of nCPAP therapy is to improve lung volume, oxygenation and decrease work of breathing. There is little data to guide clinicians on how to adjust the pressure and what parameter nCPAP should be titrated against. This is due to the lack of commercially available equipment to measure lung mechanics in patients receiving non-invasive respiratory support. In mechanically ventilated patients, measurements of work of breathing have been found to be useful in optimising ventilation strategies. An indicator of work of breathing or estimate of the metabolic and oxygen cost of breathing is the pressure time product (PTP). Objectives: To develop a monitoring system for the measurement of work of breathing in neonates on nCPAP, to investigate the effects of different nCPAP levels on PTP and to identify non-invasively determined predictors of PTP. Methods: PTP’s (PTPoe=oesophageal, PTPga=gastric and PTPdi=diaphragmatic pressure time product) derived by oesophageal gastric pressure transducer was compared with parameters derived by respiratory inductance plethysmography. Subjects were randomised to receive nCPAP level sequences of 2, 4, 6 and 8 cmH2O. Main results 37 of 57 subjects were analysed. Median gestational age 30 ± 4.9 weeks, median birth weight 1234 ± 443 g, chronological age ≤ 24 hours 24 subjects (64.9%) and baseline FiO2 ≤ 0.35. PTP’s decreased with increasing nCPAP level (p < 0.05). “Optimal” nCPAP as determined by the lowest PTPoe and PTPdi occurred between 6-8 cmH2O for 56% of the subjects. “Optimal” nCPAP as defined by the lowest PTPga occurred between 6-8 cmH2O for 39% of the subjects. Respiratory rate, abdominal excursion decreased, Te increased and heart rate remained unchanged with increased nCPAP level. PTPoe correlated best out of all three PTP’s with selected variables derived by non-invasive techniques. Respiratory rate explained 36.7% of the variance of PTPoe and abdominal excursion explained 45.5% of the variance of PTPoe. Best fitted prediction model for PTPoe included respiratory rate, phase angle, abdominal excursion, birth weight, gestational age and applied nCPAP level and explained 65.8% of the variance of PTPoe. One suspected pneumothorax was reported (1.7%). Conclusion: Respiratory muscle work load is affected by changes in nCPAP levels and can be predicted more accurately by a model consisting of respiratory rate, phase angle, abdominal excursion, gestational age and birth weight than by clinical parameters alone