Patent GB-1432572-A (GB)
Title: Respirators
IPC: A61M16/00
Filed: 1973-03-27
Abstract: 1432572 Automatic control of pressure and flow F HOFFMANN-LA ROCHE & CO AG 27 March 1973 [27 March 1972] 14560/73 Heading G3R [Also in Division A5 and G1] A respirator with automatic control for flow and pressure of respiratory gas comprises a device disposed adjacent a patient connection for measuring the flow and pressure of respiratory gas for converting these parameters into electrical signals; a valve system disposed between the respiratory gas source and the flow and pressure measuring device for controlling the flow and pressure of respiratory gas during inhalation and exhalation; and an electric control device, the measuring device, the valve system and the electric control device being inter-connected to constitute a control loop for comparing the electrical signals generated in use of the respirator with set values and for generating a correcting signal for influencing the valve system. General As shown in Fig. 1, respiratory gas is fed along duct 1 to a regulating valve 2, which feeds the gas to duct 3 during inhalation and to duct 4 during exhalation, with a pressure dependent on the degree of opening of the valve. Ducts 3, 4 lead to an inhalation/exhalation valve 5 which in turn leads either through duct 6 directly to the tracheal, tube 10 of the patient 11, or to atmosphere or, via ducts 7 and 8, to means for recovering the gas. Duct 6 extends through a measuring head 9 for measuring the flow and pressure of the gas in terms of electrical signals which are transmitted to an electronic system-13, and therein are compared with signals corresponding to desired values. Difference signals are used to control the regulating valve 2 to restore the flow and pressure to desired magnitudes. Pressure and flow are both measured by an arrangement of orifices and thermistors Fig. 5, (not shown). The cooling effect on the thermistors is a measure of flow; for pressure, the flow to atmosphere through a pair of orifices is measured, the actual signal being produced as the difference between a pair of thermistors. Electronic control device As shown in Fig. 7, the regulating valve 2, the I/E valve 5 and the measuring head 9 are connected to an electronic system, shown as a block 13 in Fig. 1. Actual flow is measured by a device 105 in head 9, and the corresponding electric signal is fed to a subtracting junction 103'. A generator 103 produces a periodic signal which at any given time represents the desired flow, and this signal is also fed to junction 103' in such a way that the output of junction 103' represents the difference, i.e. error, signal, which is fed to one input of a control circuit 101. Similarly, actual pressure is measured by a device 105', and the derived signal is fed to a subtracting junction 104'. A generator 104 produces a periodic desired pressure signal which is also fed to junction 104', from which the error signal is fed to another input of control circuit 101. Circuit 101 acts as a selector passing either the flow error signal or the pressure error signal to a control circuit 100, which is connected to operate regulating valve 2 through its servomotor. The generators 103, 104, which produce the signals corresponding to desired flow or pressure, are connected to a control 106 whereby the curve representing variation of signal with time can be altered in shape. They are also connected to a timing generator 102, which determines the period of the cyclic signal voltages. A control 107 connected to generator 103 determines the flow signal amplitude. In case of momentary disturbances due to e.g. spontaneous respiration, coughing, or excessive pressure or pressure rise, control is changed to pressure control and the respiratory cycle is interrupted and resumed at the end of the disturbance. The signals due to a disturbance are fed to a logic circuit 108 which is connected to timing generator 102 and to the generator 104. A measuring device 109 measures the final inhalation pressure, and a corresponding signal is fed to generator 104 and governs that starting level of the exhalation pressure signal. Respiratory frequency is adjusted by control 110, connected to generator 102, and the ratio of the inhalation and exhalation periods is adjusted by a control 112 and a correction circuit 111, connected to timing generator 102. For this purpose a circuit 113 compares a desired volume signal (from control 107) with the actual rate of flow signal produced by an integrator circuit 114 from the measured rate of flow. Circuit 114 also feeds an indicating or recording device 115. Logic circuit 108 is connected to a comparison circuit 116 which compares the actual pressure signal (from pressure measuring device 105') with the maximum permissible pressure signal, set by control 117. The circuit 116 also compares the actual rate of rise of pressure signal with the maximum permissible rate of rise of pressure signal, as set by control 118. Pressure is recorded or indicated by a unit 119 connected to pressure measuring device 105'. A circuit is described Figs. 8 and 9 (not shown), for establishing a zero level for signals derived from thermistor 62, and representing inhalation flow, or from thermistor 63, and representing exhalation flow.