Conventional mechanical ventilation is known as positive pressure ventilation (PPV) because the machine delivers positive pressure directly into the airways to ventilate and expand the lungs. In a healthy lung, inflation occurs when the pressure inside the lung is greater than the pressure outside the lung. This transpulmonary pressure is the difference between the pressure inside the lung (in the alveolus) and the pressure just outside the lung (the pleural cavity). Since positive pressure ventilation delivers pressure directly into the airways, the transpulmonary pressure is increased by making the lung pressure more positive. This is markedly different than physiologic breathing and carries an increased risk of ventilator-associated lung injury1. In the setting of sick lungs, where the pressure required to open collapsed areas of lung may be more than areas of healthy lung, this higher pressure can in turn cause damage to the areas of healthy lung. The known effects of this ventilator-associated lung injury from positive pressure ventilation can be avoided with the use of negative pressure ventilation. In negative pressure ventilation (NPV), the transpulmonary pressure is increased by making the pleural pressure more negative. This is achieved by using a plastic shell that covers the chest and generates negative pressure between the plastic shell and the chest. This pressure is distributed more evenly across a large surface of the chest wall and results in more uniform lung expansion. As a result, NPV results in better oxygen delivery and less lung injury than positive pressure ventilation2.~However, despite the extensive use of NPV in other countries, there is little data available regarding the transpulmonary pressure that these machines can generate; i.e. how well does negative pressure in the plastic shell transmit to the pleural cavity to expand the lung. There is a large amount of data supporting the use of biphasic cuirass ventilation to minimize lung damage3 and improve hemodynamics4-9, but no studies have been done to date that look at the transpulmonary pressure and how it differs depending on age and size. It is still unclear what optimal pressure is required via the chest cuirass to expand and ventilate the lungs via. It is also unknown what maximum pressures can be used before the lung becomes overinflated and complications arise.~This study will examine the correlation between the transpulmonary pressure and the actual setting on the biphasic chest cuirass device is the primary study end point.
This study will examine the correlation between the transpulmonary pressure and the actual setting on the biphasic chest cuirass device is the primary study end point.