A study of neonatal volume-targeted ventilation

2017-02-15T04:33:58Z (GMT) by Wheeler, Kevin Iain
Background Preterm babies may need the help of a ventilator to breathe. Although the assistance of a ventilator can be life-saving, ventilators may also injure the baby’s immature lungs. Traditionally pressure-limited ventilation (PLV) techniques have been used which do not control the tidal volume (VT) entering the lungs. New volume-targeted ventilation (VTV) modes have been developed which aim to reduce lung injury by controlling the amount of air entering the lungs with each breath. This thesis studied clinical and physiological aspects of neonatal volume-targeted ventilation. Aims 1. To perform an up-to-date systematic literature review examining the effect of VTV upon clinical outcomes. 2. To examine the combined effects of VT and positive end-expiratory pressure (PEEP) during aeration of the preterm lung. 3. To report observations of Dräger Babylog 8000 plus VTV in clinical practice during specific clinical situations (obstructed flow and during surfactant administration). 4. To examine the effects of specific ventilation back-up rate, circuit flow and maximum pressure (Pmax) setting during VTV with the Dräger Babylog 8000 ventilator. Methods These included systematic review, meta-analysis, physiological and synchrotron-based imaging in a preterm rabbit model, and observational and interventional studies in humans. Results 1. Compared with PLV, VTV reduced the combined outcome of death or bronchopulmonary dysplasia, pneumothorax, days of ventilation, hypocarbia, and the combined outcome of periventricular leukomalacia or grade 3-4 intraventricular haemorrhage. 2. During aeration of the preterm lung, VT and PEEP had additive effects upon functional residual capacity (FRC) and lung compliance. With PEEP 10 cm H2O, using a higher VT increased the proportion of gas entering the lung bases, improving homogeneity of ventilation. However at lower PEEP, increasing VT disproportionately increased the proportion of gas entering the apical basal regions. 3. During clinical practice where VTV was provided using the Dräger Babylog 8000 plus volume guarantee (VG) mode, the ventilator responded to completely obstructed gas flow by reducing the peak inflating pressure (PIP) to half-way between the maximum inflating pressure and the positive end-expiratory pressure despite not achieving the set expired tidal volume. This event was observed in most babies who received surfactant, following which PIP increased for 30-60 minutes. Target VT was less likely to be delivered if Pmax was set <10 cm H2O above the pre-surfactant PIP. 4. In stable ventilated babies, reducing the back-up ventilation rate from 50 to 30 inflations/min increased the proportions of triggered inflations while maintaining patient stability and having a minimal effect on the overall ventilator rate. Compared with using a circuit flow of 6 or 8 L/min, undesirable effects were observed with a circuit flow of 4 L/min. Setting the Pmax 5 cm H2O above the working PIP reduced the proportion of high volume inflations (>10 mL/kg), compared with using Pmax 30 cm H2O for all babies, and did not impact patient stability or the number of ventilator alarms. Conclusions VTV strategies improve neonatal survival free of lung injury. The use of appropriate VT and PEEP during neonatal resuscitation may assist lung aeration; however, further studies are needed to assess lung injury. In clinical practice, the selection of appropriate ventilator parameters is important and affects how the baby and ventilator interact.