Patent GB-1324163-A (GB)
Title: Respiratory apparatus
IPC: A61M16/00
Filed: 1969-09-27
Abstract: 1324163 Servo-controlled respirators V S ROBERTSON 23 Dec 1970 [27 Sept 1969] 47655/69 Heading A5T A respirator, Fig. 2, comprises a bellows unit 21 having an upper movable member 21E with a rod 21B fixed to the member and pivtoally connected at 25B to a lever 25 which passes through an adjustable fulcrum 26 and is slidingly, pivotally connected at 25A to a member 27 placed inside a solenoid S2, the member adapted to move downwards into the solenoid when the latter is energised, the fulcrum member 26 is threadedly engaged with a screw 29 extending between two upright members 20A, 20B, the screw being rotatable by means of a motor M via meshed gears 30, 31, to vary the mechanical advantage of the system. Air is admitted into the bellows unit 21 via an inlet port 21C fitted with a one-way valve (not shown) and is transmitted to a patient via a conduit 23 fitted to an outlet port 21D also containing a one way valve, exhaled air from the patient passing to an exhalation valve V1 via a conduit 24. A circuit of the type shown in Fig. 4 controls the respirator and in use, if the patient makes no attempt to breath, an astable multivibrator MV, the frequency of which is adjustable by means of a variable resistor VR1, produces an output pulse which is differentiated by the circuit D and fed to a monostable vibrator MS having a time constant variable by means of the variable resistor VR2, the output from which is taken to a switch circuit SW the latter being adapted to switch off the energising supply of the solenoid S2 only during the duration of the pulse. In the period whilst the solenoid is not energised, a weight 34, Fig. 2, causes the bellows unit 21 to collapse and air at a substantially constant pressure to be driven to the patient; in the period the solenoid S2 is energised the member 27 is pulled into the solenoid and air is taken into the bellows unit 21 and the patient may exhale through the valve V1 which is opened by a solenoid connected in parallel with S2. An anemometer S is connected to the inlet port 21C of the bellows unit, the output from which is shorted by a switch (not shown) at all times except when the member 27 is in its fully down position and the bellows are in a ready supply quiescent state. If the patient makes an effort to breathe, the flow of gas through the port 21C is detected and the output from the anemometer is fed to a variable gain amplifier A, Fig. 4, and to a Schmitt- Trigger ST which produces a pulse to suppress the output of the multivibrator MV and triggers the monostable MS after a delay caused in circuit DL, the duration of which is variable by means of resistor VR3. The output of the amplifier A is also fed to an integrator I and to a differential amplifier for comparison with a preset voltage determined by the resistor VR4, the output from which is used to drive the motor M and so alter the mechanical advantage of the bellows system to supply more or less a volume of air to the patient. In a modification, Fig. 1 (not shown), the automatic compensation described above is omitted and the fulcrum (5) may be moved along the lever (4) manually by sliding the solenoid unit (S1) along the base-plate (1). Resilient stops for the member 27 are provided to minimise noise and alternatively electronic apparatus may be used to reduce the voltage supplied to the solenoid when the member 27 is approaching the end of its path. The rate of respiration, i.e. the frequency of the multivibrator MV, may be varied according to the amount of carbon dioxide in the exhaled air detected by heated thermistors in a bridge circuit.