When pressure is applied by the ventilator to drive gas into the lungs, energy is expended to overcome airway resistance R to gas flow in the airways, in order to store gas in the alveoli, whose readiness to having their volume increased is represented by the concept of compliance, C. The storage of gas within individual compliances represents potential energy storage. The acceleration of gas and anatomical components within the system represent kinetic energy change, resisted by the inertance, I, of the system. At conventional ventilation frequencies, these kinetic energy changes are negligible compared with the other energy changes taking place. Inertance can be ignored and the system behaves like a flow resistor in series with a compliance. These variables determine the pressure and volume changes that take place within the lung. As ventilation frequency increases into the high range, inertance becomes significant and the frequency response of anatomical structures becomes important, with phase differences between pressure and volume signals occurring [Lin et al. 1989]. Mechanical resistance, R, in the system is largely due to resistance to gas flow down airways and is defined as pressure change per unit flow ΔP/Q, typically 4 cm H2O l−1 s. at 0.5 l s−1. However there is a contribution from viscous resistive forces in the lung and chest wall tissues. High resistance may require long inspiratory times, while expiratory times that are too short may lead to gas trapping in alveoli. Excessive resistance may mean that the power required to ventilate the patient may exceed that available to the ventilator. Compliance, C, is a measure of the capacitative properties of the alveoli and is defined as volume change per unit pressure change ΔV/ΔP. It is not uniform throughout the respiratory cycle and has values in the range 0.05–0.10 L (cmH2O)−1. Dynamic compliance is the value given to this variable throughout the inspiratory period to the end of inspiration, when airway pressure is highest. During the inspiratory pause, airway pressure falls to a plateau during which the static compliance can be measured, which is greater than the dynamic compliance.
Bronchial thermoplasty reduces gas trapping in severe asthma
