1. Field of the Invention
The present invention relates to a high frequency oscillator (HFO) ventilator.
2. Description of the Prior Art
High-frequency (HF) ventilators may be classified as belonging to one of two categories, namely the high-frequency jet (HFJ) ventilator and the HFO ventilator.
The HFJ ventilator operates to fully ventilate a patient by supplying jet pulses of breathing gas to a patient's airways. These jet pulses are typically supplied through a narrow cannula at a frequency of between 2.5 Hz and 10 Hz, at a pressure of between 0.2 bar and 2.7 bar and with a tidal volume of around 2 to 5 milliliters (ml) per kilogram (kg) body weight of a patient. This high pressure jet pulse causes the lungs to expand during an inspiration phase in which the desired tidal volume is supplied. The expiration phase is essentially passive and results from the natural compliance of the lungs which tends to collapse them and expel the gas. In a modification to this basic HFJ ventilator it is known to provide a Venturi vacuum device in communication with the patient's airways on the expiration side of the ventilator. This device creates a vacuum of typically between 0.002 and 0.025 bar during expiration to promote the natural collapse of the lungs. However expiration is still effectively passive, relying on the compliance of the lungs to push out the supplied gas.
The HFO ventilator operates to fully ventilate a patient by introducing pressure oscillations to a column of gas in communication with a patient's airways. These oscillations cause the supply of breathing gas to and the active extraction of at least the supplied volume of gas from the airways of the patient, in alternation. It is this active extraction of the supplied volume that is the primary difference between HFO and HFJ ventilation systems. The peak-to-peak pressure amplitude about an average airway pressure is typically between 0.05 and 0.2 bar and oscillates at a typical frequency of between 10 Hz and 50 Hz to supply a tidal volume significantly less than required during spontaneous breathing, typically at or around anatomical dead-space volumes, and is usually less than that typically supplied by the jet device during HFJ ventilation.
Both types of HF ventilators operate in marked contrast to a conventional mechanical ventilator. The conventional ventilator operates to fully ventilate a patient by supplying breathing gas to the patient's airways in an amount and at a frequency substantially equal to those of a spontaneously breathing patient. Typically then, for an adult, the conventional mechanical ventilator will provide a tidal volume of around 500 milliliters at a frequency of around 0.2 Hz.
The HFO ventilator generally has a gas conduit with an opening at one end for connection to the patient's airways and an opposite end in gaseous communication with an oscillator. The oscillator includes a reciprocally moveable element, such as a membrane or a piston, as part of a variable gas holding volume to which the end of the conduit is in gaseous communication. A drive unit is provided to reciprocate the moveable element at a predetermined high-frequency to alternately remove a volume of gas from and return it to the gas conduit. Over-pressure and under-pressure pulses are thereby supplied to gas within the conduit at that frequency. This causes a column of gas, the volume of which is dependent on the volume change of the oscillator, to be moved along the gas conduit into and out of the patient's airway and thereby to provide ventilation. A continuous so-called “bias” flow of fresh breathing gas moves along a flow path between an inlet and an outlet and intersects the path of the moving column within the conduit to flush through the outlet carbon dioxide-rich gas that has passed from the patient's lungs. This bias flow also maintains an average positive airway pressure (or bias) about which pressure the high-frequency pressure pulses oscillate. A disadvantage of the known HFO ventilator is that a large percentage (typically over 70%) of the volume of gas moved by the variation in oscillator volume never reaches the patient and is lost from the conduit through the outlet. The volume change of the oscillator therefore must be made commensurately larger in order to supply an adequate tidal volume to the patient. As a result it becomes increasingly difficult to maintain the necessary volume changes as the oscillation frequency increases and tidal volumes may then become insufficient. An additional problem is that the gas conduit itself must be made of a relatively stiff material so that the energy of the pressure pulses generated by the oscillator is not reduced through work done in expanding and contracting the conduit. Such a length of stiff conduit makes the HFO ventilator cumbersome to deploy.