An acoustic model of the patient undergoing artificial ventilation

The difficulty of applying conventional concepts of respiratory mechanics to high frequency ventilation (HFV) has so far inhibited its general acceptance by clinicians. The basis of an approach derived from an acoustic analysis of the ventilated patient is presented which may help to clarify the mechanism of action of HFV and offer some guidelines to its clinical application. The ventilator is considered as an acoustic source, and the patient's acoustic response to the ventilator is analysed. A response equation for the patient's respiratory system at low frequencies is derived, from which an expression for the fundamental resonant frequency is obtained. The high frequency acoustic response of the patient is examined with particular reference to the possibility of exciting multiple resonant modes within the airways. Following this, we have discussed the significance of the acoustic response and acoustic resonance in relation to gas exchange. It is suggested that these resonances may be used in adjusting indices of HFV in order to obtain optimum ventilation.