Patent GB-441057-A (GB)
Title: Improvements in or relating to electro-cardiographic apparatus
IPC: C09K11/56
Filed: 1934-04-12
Abstract: 441,057. Current and pressure meters. COSSOR, Ltd., A. C., and BEDFORD, L. H., Highbury Grove, London, and ROBERTSON, D., The Grange, Bletchingley, Surrey. April 12, 1934, No. 11073. [Class 37] [See also Group XL] An electro-cardiograph comprises a valve amplifying unit for amplifying the patient voltage, a cathode ray oscillograph employed to indicate visually or record photographically the amplified oscillations, a time-base circuit employed, when visual indications are given, to effect displacements of the ray at right angles to those due to the amplified oscillations, and a fluorescent screen having an afterglow of several cardiac cycles. Provision is made to calibrate the record, to prevent interference by high frequency or mains frequency oscillations and to avoid distortion of the record by the time base circuit. As shown the amplifier unit comprises two triode valves V1, V2, Fig. 1, and a screen-grid output valve V3, all supplied with low-tension current from a battery and leads including a switch LT. High tension supply to the valves V1, V2, is taken through resistances R2, R5, from a battery B which also supplies the screen electrode of the output valve. The high-tension supply for the valve V3 is taken through a socket HS from an anode converter or mains rectifier unit. The valves are coupled by long time-constant resistance-capacity couplings C12R4, and C23R4, passing, without attenuation, frequencies as low as 0À1 cycles per second, and by couplings R2C3, R3C4, and R5C5, the time-constants of which are chosen to attenuate frequencies above 100 cycles. The voltage of the low-tension and high-tension batteries may be read on a volt-meter VM having a suitable switch SV and resistance R7. The cathode-ray oscillograph CRO, Fig. 3, comprises a fluorescent screen SC, a filament connected to a battery b through a resistance R22 and variable resistance R2 and a focusing shield SH, an anode GN, and two pairs of deflecting plates Px1, Px2, and Pyl, Py2 all connected to a six-point socket. The plates Py2, Px2, are connected to the anode GN through condensers Co3, Co2, and the anode and focusing shield are connected to the filament circuit through resistance R23 and variable resistance R1 respectively. The six-point socket is connectable with a plug, Fig. 1, correspondingly lettered, the pin SH, and therefore the focusing shield and the filament, and also the patient, being connected to the negative high-tension terminal HS, which is earthed. The plates Py1, Py2, receive the amplified oscillations from the patient through a condenser C34, connected to the anode of valve V3, and a tapping y of a potentiometer R11, R8 across HS. The anode is connected to a tapping point q of the potentiometer R8. The plates Px1, Px2, receive a periodic applied potential difference through a tapping x of potentiometer R10, and a time-base circuit comprising a Neon tube N, a condenser C, a variable resistance R12, VL, and a switch So1, the circuit producing a deflector of the cathode ray, increasing at a rate variable by the resistance VL, followed by a fly-back as the Neon tube N discharges. To avoid distortion of the record due to the discharge of the tube, the time-base circuit is connected to the valve V3 through a condenser C6, the impulse through which during discharge deflects the recording ray off the screen, so producing a recognisable discontinuity. Preferably two cathode ray oscillographs are employed for visual indications and recording respectively, the amplifier having two six-pin plugs as shown, the connections to which are in parallel. The recording oscillograph connections embody a switch s2 for suppressing the time-base deflection. The record is taken by an attachable fixed-focus camera on a film, moved at right angles to the deflections recorded, by a constant-speed motor. The fluorescent screen employed embodies zinc sulphide giving a blue instantaneous fluorescence, employed for the photographic record, and also a green after-glow utilized in viewing. The connections to the patient comprise a fourpoint socket IS, receiving a plug carrying connections to the left and right arms and the left leg of the patient enclosed in sheathing earthed through the socket E. The sockets LA, RA, LL, corresponding to the arm and leg connections are connected to tongues 1, 3, 4, 6, of a key K2, tongues 2, 5, of which are connected respectively to the grid of valve V1 and to the filament of this valve through a resistance Rx ; this arrangement permits of the potential difference between any two of the three body connections being applied to the valve. The resistance Rx is so chosen as to give a standard potential difference, e.g. one millivolt, when placed in series with resistances Ry, Rz, across the low-tension terminals. A key, K1 having tongues 1-5 and three positions, permits, (1) the connection of the grid of valve V1 to its filament through the resistance Rx to shortcircuit the input to the valve, (the position shown), (2) the breaking of contact between tongues 4, 5 during ordinary use of the apparatus, and (3) the momentary shortcircuiting of resistances Rx, Ry, thereby imparting a 1 millivolt impulse from the resistance Rx to the grid of valve V1, which is used to calibrate the oscillograph, the resistance, which is large compared with Rx, being included to eliminate the effect due to the contact resistance of the key. A high-tension supply circuit, Fig. 2, comprises a mains transformer T, a full-wave rectifier and filter circuit RV, F, connected to a plug OP. An eliminating circuit for mainsfrequency oscillations, also provided, comprises a secondary winding Q having across it a choke CH and a resistance R40 constituting a circuit giving a voltage of lagging phase, and a potentiometer NR1 and variable resistance NR2 constituting, with a condenser C, a circuit giving a variable phase advance. The central point of a resistance R60 also across Q is connected to the negative terminal of plug OP, and the contact of potentiometer NR1 is connected to a terminal C1 in the plug and thence, via socket HS, Fig. 1, through a condenser C2 to the grid of valve Vl thereby applying to the valve a voltage adjustable in amplitude and phase to balance out mains-frequency oscillations picked up by the patient.