Presented at the Neonatal Society 2018 Spring Meeting.
Mitra N, Morley CJ, Belteki G
Department of Neonatology, The Rosie Hospital, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK
Background: High-frequency oscillatory ventilation (HFOV) is widely used in neonatal intensive care. Some ventilators, such as the Dräger Babylog VN500 ventilator offer volume guaranteed mode (HFOV-VG) when the tidal volume to be delivered (VThf) can be set by the clinician. We investigated how HFOV-VG maintains VThf in the short and longer term and how it affects other ventilator parameters and blood gases.
Methods: Ventilator data were downloaded with 1Hz sampling rate as part of a quality improvement project aiming at reducing the unnecessary ventilator alarms (1). In this study, we re-analyzed a total of ~3.1 million seconds (36.7 days) of ventilation data from 17 infants receiving HFOV-VG during clinical care. To process the several million data points we used the Python computer language. We studied the average value and variability of VThf and other ventilator parameters and also determined their correlation with blood gas pCO2 levels.
Results: Overall, the median VThf was 1.93 mL/kg (IQR: 1.64-2.45 mL/kg). The difference between set and delivered tidal volume was <0.2 mL/kg in 83% of time. In the individual recordings the median VThf ranged between 1.44 and 3.31 mL/kg (group median 1.94 mL/kg). The significant short-term variability of VThf occurred mostly in babies not receiving high dose of sedative medication or muscle relaxant, and was due to ventilator-patient interactions. When averaged over 5-minute or longer periods, VThf was very close to the targeted value. After correction to body weight the VThf or the diffusion coefficient of CO2 (DCO2) showed weak but significant inverse correlation with pCO2 (for VThf, r = -0.162, confidence interval: -0.282, -0.037, p=0.01). Uncorrected values showed no correlation. Of the 53 gases when VThf was >2.5 mL/kg there were only 6 (11%) with a pCO2 >8 kPa.
Conclusion: During HFOV-VG the tidal volume of oscillations varies in the short term but is maintained very close to the target over the long term. VThf or DCO2 have poor correlation with CO2 levels across patients even after weight correction but more than 2.5 mL/kg VThf is rarely needed.
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1. Belteki G, Morley CJ. Frequency, duration and cause of ventilator alarms on a neonatal intensive care unit. Arch Dis Child Fetal Neonatal Ed. 2017 Oct 27. PubMed PMID: 29079651.