This invention relates to a system for determining the condition of a patient's respiratory passages.

Patients with respiratory system diseases, such as asthma, COPD, cystic fibrosis and the like, have a prominent pathophysiological feature in the form of hyper secretion of mucus, often accompanied by impaired mucus transport. This imbalance between mucus transport and secretion results in mucus being retained in the respiratory system. Positive expiratory pressure (PEP) apparatus, that is, apparatus that presents a resistance to expiration through the device, are now widely used to help treat patients suffering from a range of respiratory impairments. More recently, such apparatus that apply chest physiotherapy by providing an alternating resistance to flow have been found to be particularly effective. One example of such apparatus is sold under the trade mark Acapella (a registered trade mark of Smiths Medical) by Smiths Medical and is described in

US6581598, US6776159, US7059324 and US7699054. Other vibratory respiratory therapy (V-PEP) apparatus is available, such as "Quake" manufactured by Thayer, "AeroPEP" manufactured by Monaghan, "TheraPEP" manufactured by Smiths Medical and "IPV Percussionator" manufactured by Percussionaire Corp. The generated vibratory positive pressures mechanically reduce the

viscoelasticity of sputum by breaking down the bonds of mucus macromolecules which enhances mucociliary clearance. Alternative apparatus such as "CoughAssist" manufactured by Philips are also available. Respiratory therapy apparatus can instead provide an alternating resistance to flow during inhalation.

Although these devices can be very effective, it can be difficult to assess the extent of lung disease, the best manner of treatment and the effectiveness of any such treatment.

It is an object of the present invention to provide alternative respiratory analysis and therapy systems and methods for deriving information abou the condition of a patient's lungs. According to one aspect of the present invention there is provided a system of the above- specified kind, characterised in that the system includes a waveform generator for applying an alternating waveform to the patient's respiratory system at a first location, a sensor for monitoring the resultant waveform at a second location spaced from the first location, at least one of said locations being on the chest wall, and a processor for determining the phase difference, time delay or phase angle between the applied acoustic signal and the monitored resultant waveform and providing an output derived from the phase difference indicative of the condition of the patient's respiratory passages.

The applied waveform is preferably of a square, triangular, saw or sine wave form. The frequency of the applied waveform is preferably between about lHz and 2 kHz. The system preferably includes utilisation means connected to receive the output from the processor. The utilisation means may include a display. The waveform generator may be arranged to be mounted on the patient's chest. The sensor may be arranged to be positioned to receive the resultant waveform at the patient's mouth.

According to another aspect of the present invention there is provided a method of determining the condition of a patient's respiratory passages including the steps of applying an alternating waveform to the patient's respiratory system at a first location, monitoring the resultant waveform at a second location spaced from the first location, at least one of said locations being on the chest wall, and determining and correlating the phase difference, time delay or phase angle between the applied waveform and the monitored waveform and providing an output derived from the phase difference indicative of the condition of the patient's respiratory passages.

According to a further aspect of the present invention there is provided a system for use in carrying out a method according to the above other aspect of the present invention.

A system and method according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the system schematically;

Figure 2 is a graph showing the effect of an applied sine wave; and

Figure 3 is a graph showing the effect of an applied square wave;

With reference first to Figure 1 there is shown a patient 1 and a system 2 for determining the condition of his respiratory passages including the lungs 10. The system 2 includes a waveform generator 20, similar to a loudspeaker, connected to a driver unit 21 that is arranged to drive the loudspeaker to produce an output of alternating form with a frequency that may be somewhere in the range of lHz to 2kHz, predominantly in the acoustic or sub-acoustic range. The magnitude of oscillation is preferably equivalent to a pulse pressure up to 5 psi (34 kPa). The frequency could be fixed or could instead be arranged to sweep or to hop between different frequencies in this range. The applied waveform is preferably of one of the following forms: sine wave, square wave, triangular wave or sawtooth. The applied waveform could also be made up from a combination of different frequencies, magnitudes and phase angles.

The system 2 also includes an acoustic sensor 22 with an output connected to a processing unit 23 that also receives a synchronisation output from the driver 21. The processing unit 23 provides an output indicative of the condition of the respiratory passages to utilisation means 24, such as a display, recorder or transmitter.

It has been found that when an acoustic wave is applied to the chest the signal is transmitted through the chest wall, the lung tissue and into the airway such that the vibration or sound is transmitted through the mouth. Figure 2 illustrates a sine wave signal at a frequency of about 5Hz applied to the chest wall (upper trace) and a detected signal (lower trace) at the mouth that is of triangular form. It can be seen that the shape of the detected signal is substantially the same as that of the applied signal but that the signal at the mouth has undergone a phase angle change, as illustrated by the difference between the reference times Tl and T2, which is equivalent to 66°. The phase shift depends on the impedance presented by the chest and lungs and is also dependent on the frequency of the applied waveform.

Figure 3 shows the effect on an applied square wave signal (upper trace) and the resultant detected signal (lower trace). It can be seen that the received signal has been changed to a triangular shape; this is a result of certain frequency elements being removed from the square wave by passage through the respiratory system. There has also been a phase shift although this is less than that produced by the sine wave, being equivalent to 47°.

Instead of the continuous waveforms shown in Figures 2 and 3, pulsed or transient waveforms could be used.

The processing unit 23 receives at one input the synchronising signal from the driver 21 and at another input the signal from the sensor 22. The processing unit 23 correlates these two signals to compute the phase angle between them and, from this, provides an output to the utilisation means 24 indicative of the condition of the patient's lungs. With analogy to equivalent electronic impedance, any change in frequency and magnitude of the applied waveform can be thought to be a filter. The characteristics of the filter (generally thought to be either high-, low- and bandpass) are representative of the energy dissipated at any layer in the lung. When the impedance of that layer matches the frequency characteristics of the signal, then the signal is attenuated. The extent of maximum attenuation is thought to represent regions in the lung that contain disease, relative to normal lungs. The processing unit 23 could also be arranged to analyse the amplitude of the detected signal and the shape of the detected waveform in order further to characterise the condition of the patient's lungs.

Although the acoustic sensor 22 is shown located in the region of the patient's mouth, it could be positioned at other locations such as on the chest wall at a different location from that at which the acoustic generator 20 is placed. The acoustic generator 20 could also be located at various different locations across the chest. The acoustic sensor 22 and generator 20 should be spaced sufficiently to ensure that the sensor only receives signals that have passed through the lungs and does not receive signals directly from the generator. The applied acoustic signal can be applied at various different frequencies and at various different waveforms. In this respect, the driver 21 could be arranged to vary any or all of: the frequency, amplitude and waveform of the applied acoustic signal during a single patient diagnosis so that the maximum amount of information about the lung condition can be extracted. The acoustic energy could instead be applied at the mouth and detected at the wall of the chest, although this would produce different results.

The system can be used to assess the extent and severity of lung disease in the patient. It could also be used to determine which frequencies, magnitudes and phase angles would be best in treating certain diseases. The system could be used to target individual layers within the chest cavity during vibratory treatment, so as to increase the mobilisation of mucus secretions in specific layers, range of layers or in the lung as a whole. The system could also be used to decrease the viscoelasticity of mucus at specific layers in the respiratory system. The system could also be used to provide information that could be used in optimising the setting of oscillatory or vibratory therapy devices such as Acapella (Acapella is a registered trade mark of Smiths Medical).