Investigating the Effect of Altering the Slope of the Rise in Pressure During Mechanical Ventilation of Preterm Babies

• Background of the project

About 1.5% of newborns require mechanical ventilation. Although mechanical ventilation can be life-saving for neonates with respiratory failure, it may cause lung injury due to excess airway pressure (barotrauma), delivery of high tidal volumes (volutrauma) and repetitive closing/re-opening of lung units (atelectotrauma)1. Very preterm neonates are especially vulnerable to ventilator induced lung injury. Ventilator associated lung injury is one of several factors contributing to the burden of chronic lung disease of infancy, also called bronchopulmonary dysplasia (BPD).

Ventilators have several different modes. The most frequently used mode is, time-cycled, pressure limited, where the doctor sets inspired oxygen level, the peak inflation pressure (PIP), rate, and inflation time of the ventilator. In this mode the inflations are usually synchronised with the baby's breathing. This is either Synchronised Intermittent Positive Pressure Ventilation (SIPPV), where the ventilator synchronises inflations with all the baby's breaths, or Synchronised Intermittent Mandatory Ventilation (SIMV), where the ventilator only synchronises with a set number of breaths. These can be delivered with or without targeting the tidal volume delivered to a set value by adjusting the peak inflating pressure. This mode is called volume guarantee (VG). A similar mode of ventilation is flow-cycled or Pressure Support Ventilation (PSV) and can be combined with tidal volume targeting 2. In this mode the inflation is terminated as soon as the flow rate falls to 15% of the maximum flow during inflation.

These modes have continuous gas flow through the ventilation circuit. Inflation starts and pressure rises when the expiratory valve is closed. The rate of circuit flow alters the ventilation waveforms. With time-cycled ventilation the higher the circuit flow the faster the pressure rises, the lung is distended and potentially injured, and the earlier the set PIP is achieved. A high flow, with a relatively long inflation time will result in a "pressure plateau", that is the PIP is sustained after the flow, into the baby, has stopped because the lung volume is maximal at the PIP used (Figure 1). In PSV inflation stops when the flow has decreased to ~15% of the peak inflation flow and so the inflation time is shorter. Increasing the device flow shortens the inflation time and thereby reduces the mean airway pressure.

Despite the effects on ventilation patterns and the speed of lung distension and injury, consideration has rarely been given to the circuit flow. By protocol, it is usually set at 7-10 L/min when the ventilator is turned on and it is not changed.