Patent GB-1216241-A (GB)
Title: Apparatus for measuring blood pressure
Inventor: EDWARDS WILLIAM CLAYTON
IPC: A61B5/022
Filed: 1968-02-29
Abstract: 1,216,241. Measuring blood pressure. SMITH KLINE & FRENCH LABORATORIES. 29 Feb., 1968 No. 9922/68. Heading ASK. Apparatus for connection to an inflatable cuff 21 for measuring blood pressure comprises pressure indicating means 71, 85 with a pressure inlet, a mechanical-to-electrical transducer 31, and distinguishing means arranged to respond to Korotkoff sound received by the transducer to control the pressure indicating means and for this purpose to provide a pulse at an output when there appears at an input a pulse the amplitude of which exceeds a minimum amplitude which is arranged to decrease with time from a value determined by amplitudes of preceeding pulses received at the input. As shown (Figure 2), an inflatable cuff 21 communicates with a metal bellows 39 which expands and contracts as the pressure in the cuff varies, the movement of the bellows being applied through a linkage 43, 51, 55 to rods 57, 73 arranged to move pressure indicating pointers 71, 85 which may, however, be arrested against movement by solenoid operated abutment devices 106, 116. The Korotkoff sounds are detected by a microphone 31 in the cuff and the resultant signals are passed through first circuitry which processes and examines the sounds, and then to logic circuitry which selectively causes the indicating pointers to retain indications of systolic and diastolic pressure. The first circuitry comprises (Figure 1) a crystal or ceramic microphone 12 with a Darlington impedance matching pre-amplifier 13, 15 connected through shielded cable 17 (from the cuff) to an adjustable gain (potentiometer 20) amplifier 14, 16, a tuned circuit 24, 26, 28 acting as a band pass filter at 80 c.p.s., and a full wave rectifier 34, 36. An output line 38 leads to the logic circuitry which comprises a monostable multivibrator 52, 54 providing rectangular pulses, an emitter follower 58 connected through a diode 60 to a second monostable multivibrator 66, 68, a two stage amplifier 74, 76, a bi-stable multivibrator 88, 90, a gate circuit 120, 122, and amplifiers 102, 104 and 112, 114 leading to the solenoid devices 106, 116. In use, the cuff is first inflated with a pump 25 and a switch 4, 5 automatically activates the circuits, the transient current energizing both solenoids 106, 116 so that both gauges are free to indicate the pressure in the cuff. The cuff is inflated above systolic pressure (determined by the cessation of Korotkoff sounds-when a lamp 80 goes out) and the cuff is then allowed to gradually deflate through a valve 27. As the cuff pressure decreases through systolic a first Korotkoff sound causes a pulse to trigger in turn the multivibrators 52, 54; 66, 68,; 88, 90 whereby the solenoids 106, 116 are de-energized and the reading of systolic pressure is retained on both gauges. As each further sound is recorded a negative pulse appears at the base of transistor 58 and the diastolic solenoid 116 is intermittently energized so that as the cuff pressure decreases the diastolic pointer 85 moves in steps towards the diastolic pressure; diode 108 prevents systolic solenoid 106 from being energized while solenoid 116 is energized intermittently. An integrating capacitor 48 is charged as the first Korotkoff sound pulse is passed and sets decreasing amplitude level against which further pulses are compared to determine whether they originate from true Korotkoff sounds. Finally, when the cuff has deflated below diastolic pressure, the switch 4, 5 removes operating power from the apparatus. A modified logic circuit is illustrated in Figure 3 (not shown).