PATIENT CYCLED RESPIRATOR

CORRESPONDENCE First, some modifications were made to the respirator described in the paper. It had rarely Sir,—I have read with great interest Dr. Murray been found necessary to use a stroke volume in Lawson's reappraisal of decamethonium iodide, excess of 500 ml, so the 1^ gallon bag was replaced particularly as my colleagues and I have recently by one of 750 ml capacity—this being two airhad striking proof of the usefulness of this drug bed inflators overlaid by one corrugation and in the treatment of a case of tetanus, a full report cemented. The latching relay was replaced by a of which we hope to publish shortly. bi-stable relay operated directly from the trigger We started to treat the child with chlorpro- and a transistor timer used. mazine, but although major spasms were preThese modifications allowed the whole asvented, the patient's muscles were still tonically sembly to be mounted in a single case and it was contracted. He lay in obvious discomfort, with back arched and head retracted. With the ad- therefore a second, and much more compact, ministration of decamethonium iodide, the hyper- model of the respirator that was demonstrated to tonus was completely abolished and the patient, the Symposium at Leeds in February. At the same time the possibility was investia boy of four, lay comfortably in a prone position. gated of designing an infant respirator of variable One of the most satisfactory features of time and pressure using transistors, and an experidecamethonium iodide is that it is possible, with mental model incorporating two midget transistor this drug, to achieve relaxation of the patient's timing units which operated alternately and muscles without significantly depressing respiration. The patient in this case was treated with governed the inspiratory and expiratory periods decamethonium iodide for fourteen days. Spon- respectively was also shown at Leeds. This again ' taneous respiration was maintained throughout used a trigger in exactly the same manner as that and there were no chest complications. I know of described in the paper. From these preliminary experiments it seems no other drug with which this could have been clear that transistors of the types now coming on achieved with such ease and freedom from unthe market will prove extremely valuable in the toward effects. design of riming units for such purposes as resI join with Dr. Murray Lawson in hoping that pirators and two types of circuit in particular are the manufacturers will continue to market this being studied at the moment. most useful drug, and that it will find its true The first type of circuit uses two power tranplace in anaesthesia. sistors in what is, in effect, a very low frequency R. McD. S. KEIR Royal Infirmary, Dumfries multivibrator, provision being made for a photoelectric trigger to cause one of the transistors to conduct immediately on inspiration irrespective of the state of charge of the condensers in the PATIENT CYCLED RESPIRATOR circuit. The second type of circuit uses two flipSir,—After we submitted our paper on a "Patient- flops in a ring of two circuit, with a third operaCycled Respirator" (Brit. J. Anaesth. (1958), 30, ted by the trigger. The two timing flip-flops are 32) we started investigating the possibility of so arranged that they operate alternately, the one designing a transistor timing circuit in place of controlling the inspiration time, during which the the commercial timer referred to, and work upon solenoid valve is open, and the other governing this and on various projects arising out of it have the interval time, i.e. the period during which the been so promising that although it is hoped to solenoid normally remains closed. If, however, publish full details in due course we felt that the trigger operates during this period it oversome comments on our more recent work might rides the delay timer, and immediately initiates the inspiratory phase. The period of operations of be of interest. 347 DECAMETHONIUM IODIDE: A REAPPRAISAL

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500 mA while the solenoid valve is open. For a normal inspiratory ratio this implies a mean current of 200 mA, at 18V which for emergency use can easily be obtained from dry batteries, as can the supply for the bulb in the trigger (200 mA at 3V). A self-powered attache case respirator has therefore been built, with an improved form of trigger, and &coprating a s i l l cylinder, with provision for using external gas and mains supplies as available. This trigger operates on an instantaneous flow rate of less than 1 litre per minute and does not present any obsuucaon to breathing so, as the expiratory valve "fails safe", even if both normal and emergency supplies of either gas or electricity fail, volun& breathing is in no way impeded. Preliminary clinical trials suggest that this instrument will be very useful., J. R. GREER W e s t m Infirmay, Glasgow, W . l -

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the two flipflops is determined by the time taken by a condenser across the input to discharge through a variable resistor. The respective advantages of these two circuits are in general that, in the fist, the number of components is minimal and more particularly no relays are required (the solenoid valve being operated directly by a power transistor), thus the device is free from any possibility of sparking atcontacts. The range of variation of periods is, however, rather limited. In the second circuit in which separate timing units are used, a very wide range of inspiratory and interval periods may easily be obtained and there is, of course, no interaction between them. At the moment relays are used as a matter of conveni&ce but this is obviously not fundamental. In a typical respirator of either type the power consumption may be about 25 mA, plus perhaps

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