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The Effect Of Pressure Rise Time On Ventilator Parameters And Gas Exchange During Time-Cycled And Flow-Cycled Neonatal Ventilation

David Chong, Sabrina Kayser, Eniko Szakmar MD, Colin J Morley MD, FRCPCH & Gusztav Belteki MD, PhD, FRCPCH

Neonatal Intensive Care Unit, The Rosie Hospital, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK; University of Cambridge, St. Edmund’s College, Cambridge, UK; First Department of Paediatrics, Semmelweis University, Budapest, Hungary

Pressure rise time (PRT), also known as slope time can be set on some modern neonatal ventilators. On other ventilators, PRT is determined by circuit flow implicitly. Changing slope time is thought to affect mean airway pressure, oxygenation and carbon dioxide elimination. We hypothesized that while changing the PRT may affect some ventilator parameters, it does not affect delivery of the target tidal volume and gas exchange.

In a cross-over study, 12 infants weighing <2 kg were ventilated with synchronized intermittent positive pressure ventilation with volume guarantee (SIPPV-VG) and pressure support ventilation with volume guarantee (PSV-VG). During both modes PRTs ranging between 0.08 and 0.40 seconds were used in 15-minute epochs. Data from the ventilator and from patient monitors were downloaded with 1 Hz sampling rate and analysed using the Python computer language. Ethical approval and informed consent was obtained. The study was funded by Evelyn Trust.

All participants completed the study without adverse events. During PSV-VG, a shorter PRT was associated with higher flow rate in the ventilator circuit (p=0.003); this was not seen during SIPPV-VG. During PSV-VG, a longer PRT was associated with longer inspiratory time (p<0.0001) and with lower PIP (p=0.003), but the MAP was not different. During SIPPV-VG the PIP was not significantly different in case of different PRTs; however, MAP was lower with a of longer PRT (p=0.001). In case of a short PRT (0.08 sec), the PIP was higher during PSV-VG than during SIPPV-VG (19.8 mbar versus 16.5 mbar, p=0.042). There was no significant difference in delivery of the targeted tidal volume, in respiratory rate or in minute volume among the epochs using different PRTs. Oxygen saturations (SpO2) increased slightly with increasing PRT but this was not significant (p=0.092). Similarly, there was no signifcant difference in endtidal CO2 levels with any PRTs in any of the two modes.

During SIPPV-VG or PSV-VG, using short or long PRTs affects some ventilation parameters but does not significantly change oxygenation or carbon dioxide elimination.

Bach KP, Kuschel CA, Oliver MH, Bloomfield FH. Ventilator gas flow rates affect inspiratory time and ventilator efficiency index in term lambs. Neonatology. 2009; 96(4):259-64.

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