The Manley ventilator

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1 Anaesthesia, 1995, Volume 50, pages 6471 CLASSIC PAPER The Manley ventilator Roger Manley was senior house officer (SHO) in the anaesthetic department of Westminster Hospital when he made and tested his Manley ventilator. By 1961, the educated hand was giving way to the use of mechanical ventilators as a means of ventilating the lungs of paralysed patients. The machines available in the UK, the Blease pulmoflator and the Barnet ventilator, were large, expensive, noisy apparatus using electrical pumps as their motive force, whilst in the USA the pressure driven Bird ventilator was finding favour. The teaching of the early pioneers, especially in the USA where modest doses of muscle relaxants were used to produce relaxation and not complete paralysis, had been that it was safer to assess the state of partial paralysis using manual ventilation to assist the patients ventilation rather than mechanical IPPV to replace it. By 1958 prolonged artificial ventilation by IPPV was being used increasingly in the treatment of ventilatory failure. This followed the demonstration of the value of positive pressure ventilation as an alternative to the iron lung for the treatment of respiratory paralysis caused by polio. The ventilators used for this purpose in the UK were the Beaver and the East Radcliffe, both driven by electric motors; however, neither were well suited to use in the operating theatre, where cyclopropane was still in favour. On a Saturday in 1959, a study day was organized by the Faculty of Anaesthetists at the Royal College of Surgeons in London on the treatment of ventilatory failure by artificial ventilation. On the way back to the Westminster Hospital after that symposium, Roger Manley suggested the possibility of using the pressure of the gases from the anaesthetic machine as the motive power for a simple apparatus to ventilate the lungs of patients in the operating theatre. We discussed the principle of the Bird ventilator and the Stephenson pressure valve system used by the American Paramedical Services to administer oxygen for resuscitation. We puzzled as to whether the reduced gas pressure emerging from the Rotameters of an anaesthetic machine would suffice to drive a ventilator. By the following Monday, Roger had made a working prototype and demonstrated that the gas pressure was sufficient to fill a reservoir bag, raising a weight to a point where it opened a valve an triggered off the compression of the gas bag delivering its contents to the patient. It was a mark 2 version of this apparatus that he took to Blease who refined it and developed it as the Manley ventilator. With the advent of this ventilator a cheap, simple effective means of IPPV became available in every operating theatre. It soon replaced the educated hand as a means of ventilating patients and hastened the acceptance of total paralysis and artificial ventilation rather than partial paralysis and assisted ventilation. Roger Manley later became Managing Director of Blease and later Cape Engineering, before joining the British Oxygen Company as a consultant. STANLEY FELDMAN X/95/ The European Society of Cardiology 64

2 The Manley ventilator 65 VOL 16 NO 3 ANESTHESIA JULY 1961 A new mechanical ventilator ROGER W. MANLEY, DA Senior House Officer, Department of Clinical Measurement Westminster Hospital Since the discovery of the relaxant drugs, many techniques requiring controlled ventilation have been introduced. The method most commonly used is the rhythmical squeezing of a bag by the anasthetist. This paper describes the development of an automatic bag squeezer which fulfils the following requirements. (1) The ventilation should be designed to minimise the cardiovascular effects of intermittent positive pressure on the anasthetised patient. (2) The machine shouid provide the anasthetist with as much information as possible about the ventilation and the degree of relaxation of the patient. This will then largely compensate for the loss of contact with the bag. (3) The machine itself should be sufficiently small to stand on an anaesthetic apparatus. (4) It should operate from the gases supplied without ancillary sources of energy. When a Boyle s or similar anaesthetic apparatus is used with a nonrebreathing circuit, the flow of gases from the apparatus will be equal to the minute volume delivered to the patient provided no loss of gas occurs. The gases leave the reducing valves on a modern continuous flow apparatus at a pressure in excess of Slb/square in (352cm water) and pass into the flowmeter through a needle valve. From this, gas at a pressure below 3Ocm water is delivered to the patient. Hencea pressure difference of over 300cm water is available as a source of energy. The mechanism and valves of the automatic ventilator are operated entirely by a part of this pressure difference. The valves are of a type that needs no lubrication and they are not affected by condensation of water vapour from the expired gas. The expiratory valve, together with the whole of the expiratory side of the apparatus, has been made readily detachable for cleaning and sterilisation. 317

3 66 The Manley ventilator 318 ANlESTHESlA PIG. 1 The ventilator E 12 W N FIG. 2 Diagram of the ventilator showing mechanism INFLATION &.=F c c PATIENT PIG. 3 Diagram of the ventilator showing gas flow during the inspiratory phase V

4 The Manley ventilator 67 ANESTHESIA 319 DEFLATION PATIENT FIG. 4 Diagram of the ventilator showing gas flow during the expiratory phase PATIENT FIG. 5 Diagram of the ventilator showing gas flow with manual ventilation The ventilator produces inflation with a variable weight-loaded bellows (B2) delivering a pre-set tidal volume through inspiratory and expiratory disc valves (V2, V3) in a non-rebreathingcircuit. The weight (W) is movable along a graduated scale (P) to give pressures of 8-30cm water. In addition, it has a second stage consisting of a bellows and valve (B1 and V1) to provide time cycling for the end of the inspiratory phase and to provide the operating pressure for the inspiratory and expiratory disc valves. It also has a tap (Tl) to divert the gas flow direct to the patient s lungs for manual ventilation with a bag. The gas flow from the anesthetic apparatus is admitted to the bellows (BI) and kept at a constant pressure of loocm water by spring loading. The pressure in the bellows is maintained when it is empty by the valve (N) that regulates the outflow when the two end plates of the bellows converge on each other. A constant back pressure on the rotameters is thereby provided to prevent the bobbins from jumping up and down.

5 68 The ManIey ventilator 320 ANESTHESIA The gases pass from the bellows to the tap (Tl) which diverts the flow direct to the patient for manual ventilation or to the valve (Vl) for automatic ventilation. The gas passes via valve (Vl) to the weightloaded bellows (B2) and thence to the patient s lungs through valve (V2). From the patient s lungs gas reaches valve (V3) and proceeds via tap (T2) to the bag and expiratory valve for manual ventilation or to the atmosphere during automatic ventilation. The valves The valve (Vl) is opened and closed by a toggle mechanism (M) and a lever system (L) that is acted on by the alternate filling of the two bellows. Valves (V2 and V3) are operated automatically by the changes in pressure in the tube from bellows (Bl) to valve (Vl) which occur when valve (Vl) is opened and closed. This is achieved by spring biased diaphragms in small pressure chambers attached to the valves (V2 and V3). These are set so that valve (V2) opens at a pressure of 75cm water and valve (V3) closes at a pressure of 5Ocm water. When valve (VI) is closed, the pressure in the chambers rises to that in bellows (Bl) (loocm water) and when valve (Vl) opens, the pressure falls to that in bellows (B2) (8-30cm water). A safety blow-off valve (S) set at 35cm water, is incorporated in the patient circuit as an added protection. Inspiratory phase Valve (Vl) is closed. The fresh gas is retained in the bellows (Bl) which fills during this phase. Valve (V2) is open and valve (V3) is closed. The weight (W) forces the gas from the bellows into the patient s lungs and produces inflation. The inflation pressure depends on the position of the weight (W) on the graduated scale (P). The tidal volume depends on the setting of the stop on the tidal volume scale (TV) and the duration of this phase depends on the height reached by the bellows (Bl) before it causes the lever system (L) to open valve (Vl). This is set on the duration of inflation control which alters the height of the fulcrum (F). End of inspiratory phase When valve (Vl) opens, the pressure in the chamber falls and valve (V2) closes and valve (V3) opens. Expiratory phase The patient s lungs are now open to the atmosphere and expiration takes place. The gas in the bellows (Bl) together with the flow of fresh gases and the small volumes from the pressure chambers now pass through valve (Vl) into bellows (B2) and fills it. The duration of this phase depends on the height reached by the bellows (B2) before the lever system closes valve (Vl).

6 The Manley ventilator 69 ANESTHESIA 321 End of expiratory phase When valve (Vl) closes, the pressure in the chambers rises, closing valve (V3) and opening valve (V2), enabling inflation to take place. Manual ventilation Taps (TI) and (T2) are turned to the position for manual ventilation. The patient circuit is now a Mapleson D semi-closed system (FIG. 5) allowing partial rebreathing to occur. THE VENTILATOR IN USE The minute volume required is set on the rotameters of the anasthetic apparatus and the desired tidal volume is set on the tidal volume scale. The frequency of ventilation will be equal to the minute volume divided by the tidal volume. The weight is adjusted to provide just enough pressure to enable the tidal volume to be inflated, which depends on the compliance of the patient s lung and chest wall. Compliance is partly dependent on relaxation and any alteration will show as a change in the volume inflated as the pressure remains constant. Such a volume change is immediately evident to the anasthetist who may wish to compensate the change by varying either the inflation pressure or the degree of relaxation of the patient. The duration of inflation control is set to provide sufficient time for inflation to occur, which depends on the airway resistance. Increase of resistance during anasthesia will show as a failure of the tidal volume to be inflatedin the time set. Clearly the machine allows the anresthetist to distinguishchangesin theairway resistancearising due to obstruction of the airway or accumulation of secretions, from changes of compliance associated with the degree of relaxation. At all times information is immediately available concerning the minute and tidal volumes, the inflation pressure, the compliance and airway resistance during anasthesia. The anzsthetist may at any moment change to manual ventilation with a bag to feel conditions in the patient. A mounting for a Wright Anemometer is provided on the expiratory valve to measure the volume expired if this is needed. Ventilator performance One object of the present machine was to produce adequate positive pressure ventilation with the least physiological disturbance of the patient. It is desirable to provide a high flow to the patient early in the inspiratory phase to enable inflation to take place in a relatively short time. The result of this characteristic is twofold. Firstly, pulmonary blood flow suffers least interference as the intra-alveolar pressure is then atmospheric for a relatively large proportion of the respiratory

7 70 rhe Manley ventilator 322 ANESTHESIA cycle. Secondly, the venous return to the right side of the heart is not impeded. The performance of the ventilator was investigated by measuring tracheal flow, tidalvolume andmouth pressurein anzsthetised patients. Flow rates were measured by using a Fleisch pneumotachograph with a Statham PM 197TC differential pressure transducer of range f0.7cm water connected to a carrier amplifier (Southern Instruments M R 501). Tidal volume was derived from the flow signal by electrical integration. Mouth pressures were measured by a Statham P23G transducer and carrier amplifier (as above). The pen recorder was Siemens Ediswan Pen Oscillograph M K 111. RESULTS FIG. 6 shows the results from a female patient aged forty-eight years undergoing a cholecystectomy. The patient was apnceic, the abdomen was open and relaxation was complete while the recordings were made. FIG. Trace I is a time tracing in seconds Trace I1 is the tracheal flow. Scale lcm=200 I/min flow Trace 111 is the tidal volume. Scale lcm= 1 litre Trace IV is the mouth pressure. Scale lcm= locm water Consider tracing 11. Flow into the patient corresponds with a downward deflection of the pen and a tracheal flow rate of over 120 l/min is reached at the commencement of the inspiratory phase. The flow rise

8 The Manley ventilator 71 ANESTHESIA 323 time is 0.2secs. This rapid rise in flow rate results from the low resistance of the inspiratory valve. The high flow delivered by the machine causes a rapid rise in the mouth pressure (IV) which produces a tidal volume (111) of 7OOml in 0.7 sec. i.e. approximately 20 per cent of the respiratory cycle. It may be noted that the extra dead space introduced by the pneumotachograph was 2OOml and that under normal operation the tidal volume inflated would have been 5OOml and this would have been inflated in 0.4secs or approximately 12 per cent of the respiratory cycle. Trace II also shows that a flow rate in excess of 120 l/min was achieved at the commencement of expiration. With these high flow rates it is seen that the alveolar pressure is held at atmospheric for a duration of two seconds in a respiratory cycle of three and a half seconds as shown on traces 11 and IV when mouth pressure is atmospheric and there is no tracheal flow. This form of ventilation causes little physiological disturbance and performs controlled ventilation in a way not possible with a hand squeezing a bag. The prototype shown in FIG. 1 has been in routine use for some months in the Department of Anaesthetics at Westminster Hospital. Over one hundred cases ranging in age from eight to eighty-six years have been ventilated with this machine and no disadvantages have been found. A production model is undergoing final clinical trials. VENTILATOR SPECIFICATION Minute volume 2-20 I/min Tidal volume 200-8oOml Frequency of ventilations per rnin Inflation pressure 8-3Ckm water Expiratory/inspiratory time ratio 2-1 to 10-1 Resistance of expiratory valve and tap less than lcrn water for 60 l/min flow Peak inspiratory flow rate (Wright peak flow meter) 110 I/min at locm water 140 I/min at 2Ocm water 180 I/min at 3Ocm water SUMMARY A new mechanical ventilator is described for intermittent positive pressure ventilation during anaesthesia. It is operated by the gas flow from a Boyle s or similar anaesthetic apparatus and requires no ancillary source of energy. Results are presented to show how the requirements for this type of ventilator may be realised in clinical practice. Acknowledgements I wish to thank Dr G. S. W. Organe, Director of the Department of Anesthetics for allowing the ventilator to be used by members of his department at Westminster Hospital and Dr P. Cliffe, Director of the Department of Clinical Measurement, for his help in the investigation of the ventilator s performance. I also wish to thank Mr J. Blase (Blase Anesthetic Equipment Ltd) for his continued help in developing the production model of the ventilator.

Ventilators. Dr Simon Walton Consultant Anaesthetist Eastbourne DGH KSS Basic Science Course

Ventilators. Dr Simon Walton Consultant Anaesthetist Eastbourne DGH KSS Basic Science Course Ventilators Dr Simon Walton Consultant Anaesthetist Eastbourne DGH KSS Basic Science Course Objectives Discuss Classification/ terminology Look at Modes of ventilation How some specific ventilators work