BJA/RCoA -CAI joint funded Collaborative Research Grant

The majority of critically ill patients on intensive care are unable to breathe sufficiently well for themselves and require artificial ventilation. This technique blows air into the lungs through a tube placed in the wind-pipe using a ventilator. Unfortunately, this process preferentially inflates the upper parts of the lungs which have less weight on them, whilst the more dependent parts are compressed by the heart, abdominal contents and the weight of the lung tissue itself. This leads to overexpansion and stretching of the nondependent areas and also exposes the lungs to shearing forces, whilst the dependent parts of the lung are squashed and close up, sometimes to the point where they cannot be re-opened; even with extra inflation pressures. This causes harm and reduces the lung surface area available for the gases to pass from the lung to the blood. Smaller size breaths from the ventilator, which reduce lung overstretching, do improve outcome and survival. Unfortunately there are other problems associated this technique, including the need for more sedation and possibly the elevation in carbon dioxide in the blood. Importantly because the patient does not breathe for themselves, their breathing muscles are more prone to wasting and so this may have serious consequences when the patient tries to get off ventilator support. In addition paralyzing drugs are often given, which avoid clashes between the ventilator breathing pattern and efforts made by the patient. Although this may help the ventilation, it worsens muscle wasting.

Strategies that allow the patient to do some 'spontaneous' breathing may help with these problems. This study will compare conventional "low tidal volume" ventilation to an alternative, where breathing is maintained most of the time, which is known as APRV. APRV involves applying a relatively high continuous background pressure most of the time, which helps hold the air passages open in the lung, interspersed with short periods of low pressure, which are thought to be too short to promote airway closure. Importantly the technique allows for a controlled lower level of sedation, which promotes spontaneous breathing around this pressure level. This means the patient is using their own breathing muscles to expand the lungs in a more natural pattern, so reducing the problem of much of each breath passing to the non-dependent parts of the lung, where it causes over-stretching of lung tissue in CMV and under or no expansion of the dependent parts of the lung. This more even expansion means that the lung is expanding in a more natural way and it is hoped better exchange of gases between the lung and the blood stream is achieved. The patients in this study will have moderate to severe ARDS which is a form of major lung damage that occurs in critical illness, where all the problems described above are magnified. The plan is to see if either ventilation technique offers an advantage in terms of survival or a reduction in the time on the ventilator. We will also measure muscle strength in these patients.