this invention applies , amongst other things , to nebulizers of the type which generate pulses of atomisation , as in the prior art nebuliser described above . the invention is not , however , limited to any particular nebulizer , but may be applied to other nebulizers or to other drug delivery devices such as spacers . extending the proportion of the inhalation of the patient in which atomisation takes place above 50 %, as is used in the halolite , results in the patient receiving their treatment faster since it will take fewer breaths to deliver the required volume of medication . however , to maintain a satisfactory lung residence time for the medication , delivery of the medication must stop some time before the patient stops inhaling so that the medication has a minimum residence time , preferably of at least half a second , most preferably about one second in duration . this residence time is referred to as the aerosol hold period . however , it is preferred that the aerosol hold period is never shorter than 50 % of the predicted duration of inhalation . treatment time can therefore be minimised while the therapeutic effects are maintained . fig4 and 5 of this application show a suitable nebuliser based on the halolite . referring to fig4 , a mouthpiece 1 is shown through which a patient inhales the direction of arrow 2 . below the mouthpiece 1 is a removable atomising section 3 which , in turn , rests on a base 4 . the base 4 is shown in more detail in fig5 . the base 4 includes an inlet 5 through which air is supplied under pressure from a compressor ( not shown ). the pressurized air is led via a tube 6 to a manifold 7 which controls the flow of pressurized air to an air outlet 8 which directs air into the atomising section 3 shown in fig4 . the base 4 also includes a pressure sensor 9 which detects the pressure within the atomising section 3 via aport 10 . referring again to fig4 , air under pressure passes through the air outlet 8 of the base 4 and is conducted through a tubular post 11 to an atomiser nozzle 12 out of which the air issues under pressure . a deflector 13 is located in the path of the pressurised air issuing from the nozzle 12 so that the pressurized air is deflected laterally so as to pass beneath a baffle 14 . the passage of the pressurized air across the top of the tubular post 11 causes medication 15 to be drawn up between the outer surface of the tubular post 11 and the inner surface of a sleeve 16 which surrounds the tubular post 11 . the medication 15 is atomised in the stream of air , and carried away in the stream of air below the rim of the baffle and up through the mouthpiece 1 to a patient . the pressure sensor 9 in the base 4 monitors the breathing pattern of a patient , and on the basis of the breathing pattern , the manifold 7 is controlled by a processor ( not shown ) to supply pressurized air to the atomising section 3 only during part of an inhalation of a user . the processor calculates the length of the pulse of atomization . a preferred example of the calculations carried out will now be described . where there is adequate tidal volume , and sufficient time between the end of aerosol generation and the start of exhalation , the pulse duration can be extended from the 50 % of the prior art halolite product without significantly affecting the amount aerosol exhaled . firstly , a calculation can be made to find the tidal volume ( tv calc ). one way in which this can be done is by multiplying the duration of inspiration measured by the sensor 9 by the peak inhalation flow measured by the sensor 9 and then by a constant which is set to be about 0 . 7 . another , preferred , way in which the tidal volume ( tv calc ) is found is carried out on a breath by breath basis so as to calculate , for example from the previous three breaths , an average inhalation volume for the next breath . the patient &# 39 ; s inspiratory flow is monitored continuously , typically every 10 ms and this is integrated over the duration of inspiration of the patient to derive the tidal volume of the patient . gb 2343122a discloses a nebulizer and also discloses these two calculations of the volume of inspiration . we hereby incorporate all of the details of these calculations from that document herein by this reference . if the calculated tidal volume ( tv calc ) is less than a tidal volume limit , then the processor will control the aerosol generator to generate an aerosol simply during the first 50 % of the inhalation phase . in this case , the tidal volume limit is the minimum tidal volume required to extend to the pulse time above 50 % of the inhalation period , and is set to about 1 litre in this embodiment , although other volumes could be selected . if the calculated tidal volume ( tv calc ) is more than or equal to the tidal volume limit , then the processor will carry out a calculation whereby the pre - set time before the person is expected to inhale is subtracted from the expected duration of inhalation to define the pulse length by which the processor controls the aerosol generator to generate an aerosol . for example , if the pre - set time is one second , and the expected duration of inhalation of the person is four seconds , then an aerosol will be generated for three seconds . other pre - set times may also be appropriate . for example it could be in the range of 0 . 5 to 2 seconds , or in a narrower range at 0 . 75 to 1 . 5 seconds . the invention also relates to other drug delivery apparatus , such as spacers in which a dose of a drug in droplet or powder form is released into a spacer chamber or holding chamber from which the patient inhales . these are most appropriate for elderly patients or children who have difficulty in using a multi - dose inhaler or dry powder inhaler , for example , because they have trouble co - ordinating the release of the drug with the beginning of inhalation , or because their inhalation flow rates are too small . for example , spacers are disclosed in international patent publication number wo 96 / 13294 , the contents of which are hereby imported by reference . the length of the atomisation pulse is dependent upon the patient &# 39 ; s inspiratory tidal volume . the nebuliser must therefore measure the patient &# 39 ; s tidal volume , preferably on a breath by breath basis so as to calculate , for example from the previous three or four breaths , an average inhalation volume for the next breath . a timer is included in the nebuliser connected to the pressure sensor 9 ( shown in fig5 ) in order to measure the duration of inspiration . a memory is also included in the nebulizer in which is stored one or more of the following : according to one form of the invention , the patient &# 39 ; s inspiratory flow is monitored continuously , typically every ten milliseconds , and this is integrated over the inspiratory duration . in view of the fact that the nebuliser adapts to the breathing pattern of a patient , when the patient starts breathing , no atomization takes place during the first three or four breaths . those first three or four breaths are used to analyse the breathing pattern of the patient . the flow rate of those breaths are measured , and from this , the average duration of the inhalation phase of the breaths is calculated . the average duration of inhalation is then used in the calculation to determine the pulse length of atomisation during the subsequent breath . in addition , as the patient continues to breathe in and out , the average duration of inhalation is calculated using the breaths immediately proceeding the breath that is being predicted . thus , if a patient &# 39 ; s breathing pattern improves during treatment , the nebuliser will adapt to this change in order to optimise the dose administered during each breath . referring now to fig6 , a drug delivery apparatus is shown which is a mesh type nebulizer 51 for generating an aerosol indicated generally at 52 in a passageway 53 . the passageway 53 has an inlet 54 through which air enters it , and at its opposite end 55 the air passing through the passageway 53 is led to a mouthpiece or the like ( not shown ). during operation of the nebulizer 51 , the aerosol 52 is entrained in the airflow leading to the mouthpiece . nebulization takes place by a drug being forced through a mesh plate 56 by using an ultrasonic transducer 57 which drives a horn 58 to vibrate in the region of the mesh plate 56 . the horn 58 is located close to the rear face of the mesh plate 56 and is caused to vibrate by the ultrasonic transducer 57 , whereby the aerosol 52 is generated from the front face of the mesh plate 56 . the substance to be atomised into an aerosol 52 is in fluid contact with the rear face of the mesh plate 56 and it is this that is driven through the holes of the mesh plate 56 by the vibrating horn 58 . during each treatment , a certain volume of the substance to be atomised is located in a reservoir 59 which is located above the mesh plate 56 in which to feed the substance to be atomised to its rear face . a fluid sensor 60 is located between the reservoir 59 and the mesh plate 56 such that once the substance to be atomised has substantially all been aerosolized , this is detected so that the ultrasonic transducer 57 may be switched off at the end of treatment . a power supply 61 is used to power the atomiser since power is required to drive aerosolization . an electronic controller 62 controls the ultrasonic transducer 57 so that , for example , once the fluid sensor 60 senses that there is no liquid remaining to be atomised , the ultrasonic transducer 57 will be switched off . in addition , a more sophisticated control device can be used here such that the patient &# 39 ; s breathing is measured , and atomisation only occurs during the inhalation part of a patient &# 39 ; s breathing pattern . an airflow regulator 63 is located in the passageway 53 . this is shown in more detail in fig7 from which it will be seen that the regulator 63 includes a frame 64 having an interior edge 65 ( shown in dotted lines ) which defines an aperture through which air must pass if it is to enter the passageway 53 . a resilient flap 66 is located in front of the aperture located in the frame 64 , and a rib 67 lying on the frame 64 acts as a spacer to prevent the flap 66 from completely closing the aperture . the flap 66 is typically made of a resilient silicone material . this means that any airflow through the regulator 63 which passes through the aperture and then against the flap 66 will cause the flap 66 to be deflected away from the frame 64 allowing the air to pass relatively freely . however , airflow passing the opposite way will cause the flap 66 to close , and the aperture will be severely restricted allowing a limited airflow to pass . the resilient nature of the flap will tend to offer more resistance to the airflow the greater the pressure difference on the opposite sides of the frame 64 . this airflow regulator , therefore , limits the rate at which air passes through the passageway 53 towards the mouthpiece and has the effect of lengthening the duration of inhalation which allows longer pulses of aerosol to be delivered . this will reduce the time it takes to deliver a volume of a drug . the nebulizer also includes an airflow detector 68 which is able to measure both the direction of airflow through the passageway 53 and the velocity of the airflow . in this embodiment , it is indicated to be located within the passageway 53 , but could be located in various other positions , even in the mouthpiece . the detector 68 may be any one of a variety of different types of detector , such as a temperature sensor , a pressure sensor , a microphone type sensor or a mechanical sensor which is deflected by the airflow . the type of sensor used is not an important factor in this invention . fig8 shows the arrangement of the nebulizer in block diagram form . from it , one block refers to the airflow detector 68 shown in fig6 . the output from the detector 68 is passed to a processor 70 which controls the aerosol generator 72 and a patient signalling device 73 . the processor 70 will include the electronic control 62 shown in fig6 , as well as other elements . the aerosol generator 72 refers to a combination of the mesh plate 56 , the ultrasonic transducer 57 , the horn 58 and the reservoir 59 in the nebulizer 51 shown in fig6 . the patient signalling device 73 is not shown in fig6 , but is some form of device which generates feedback signals for the patient when inhalation is detected and when treatment is complete . according to one arrangement , this could be a vibrator device which causes the nebulizer 51 to gently vibrate . alternatively , it can be an audio device which uses sounds to signal to the patient . it could even be a visual device where the patient is signalled on the basis of visual signals which may be lights or an lcd screen . the signalling device 73 could be a combination of these systems . the basic operation of this nebulizer will now be described . firstly , the patient will pour a certain volume of the substance to be atomised into the reservoir 59 . the reservoir 59 may be sized such that it will exactly hold the appropriate volume of the substance that is required . the patient can then begin to breathe in and out through the mouthpiece . upon commencement of inhalation , the airflow detector 68 will detect the commencement of inhalation , and the electronic control 62 will cause the ultrasonic transducer 57 to vibrate , thereby driving the horn 58 to cause aerosolization of the substance to be atomised . as the substance is aerosolized , the reservoir 59 empties , and once the level of the substance drops below the fluid sensor 60 , the electronic control 62 switches off the ultrasonic transducer . during inhalation , the airflow regulator 63 operates to regulate the speed of air passing through the passageway 53 and to the patient , thereby lengthening the patient &# 39 ; s inhalation phase . the nebulizer will only deliver aerosol laden with the aerosol 52 during part of each inhalation by the patient . the nebulizer will be arranged such that there is a pre - set period of time at the end of each inhalation of the patient during which the drug is not delivered . it is , therefore , important that the nebulizer is able to estimate the duration of each inhalation phase . therefore , when the patient first uses the device , he will breathe in and out , and the commencement and end of inhalation will be measured over a number of breaths typically three or four , and an average breath length calculated . this average inhalation length is used as the estimate for the subsequent duration of inhalation . a calculation can then be made by the processor 70 to subtract the aerosol hold period from the estimated inhalation length . this will give the period over which the aerosol can be delivered to the patient such that the aerosol hold period during which the patient continues to inhale without any aerosol being inhaled , should approximate the desired pre - set aerosol hold period . as such , the processor 70 will control the aerosol generator 72 to generate an aerosol of the drug from the commencement of the inhalation to the beginning of the aerosol hold period when it is switched off by the processor 70 . the full calculation is explained in more detail above with respect to the preferred embodiment . since the symptoms of the patient can vary over time , both from treatment to treatment , and even during a single treatment , by regularly recalculating the estimated period of inhalation , the nebulizer remains optimised for that patient at that particular time . in fact , in the preferred embodiment , the estimated inhalation period is recalculated on every breath by taking an average of the last few inhalation periods . during exhalation , the exhaled air might be exhausted from an outlet in the mouthpiece , or alternatively might flow back up the passageway 53 towards the airflow regulator 63 which will open to allow the air to be exhausted freely . it is preferable to locate the airflow detector 68 as close to the mouthpiece as possible , and where the exhaled air is exhausted from an outlet in the mouthpiece , it will normally be appropriate to locate the airflow detector 68 within the mouthpiece . the use of an airflow regulator may be preferred since it lengthens the period of inhalation which allows longer pulses of aerosol to be delivered , and this reduces treatment times . also , the airflow regulator 63 may be located anywhere in the device where it will restrict the airflow leading to the patient . however , it is preferred that it is located upstream of the point at which the aerosol is generated during inhalation . that way , the aerosol will not be removed from the airstream by the constriction caused by the airflow regulator 63 . it will be appreciated that , although a mesh type nebulizer is used in the device shown in fig6 and 7 , other types of nebulizer may be equally appropriate . what is important is the adaptation of the nebulizer to the patient &# 39 ; s actual breathing pattern .
a drug delivery apparatus of the type which releases a drug in aerosolized form into the inhaled airstream of a person , programmed to release the drug in a pulse which ends a pre - set time before the person is expected to stop inhaling . in doing so , a minimum residence time within the lungs is created , and drug treatment time can be reduced . a corresponding method of controlling a drug delivery apparatus comprises the step of controlling the apparatus to release the drug in aerosolized form into the inhaled airstream of a person in a pulse which ends a pre - set time before the person is expected to stop inhaling .