Patent GB-1470432-A (GB)
Title: Apparatus for use in stimulating responses in a physiological system
IPC: A61B5/0402
Filed: 1974-04-10
Abstract: 1470432 Pacemaker pulse monitoring VITATRON MEDICAL BV 10 April 1974 [16 April 1973] 15986/74 Heading G1N [Also in Division A5] A physiological stimulator such as a pacer producing heart stimulating pulses comprises means for reducing the pulse amplitude until it is ineffective, means for producing back-up pulses of increased amplitude, means for memorizing the amplitude of an effective back-up pulse and means for setting the level of the next stimulating pulse at a safe margin above the effective back-up pulse. In the pacer shown, with normal functioning of the heart 1 (Fig. lb), the pick-up electrodes 4 and RE pass the beats through blocks 9, 13, 16 and 18 to a threshold detector which, providing the heart pulse amplitude is above a threshold level, produces an output pulse irrespective of the polarity of the pulse picked up and having a width depending upon the pulse amplitude. This pulse is fed to a stimulating pulse generator network 30 to effect reset of the generator timer and thus prevent the production of a stimulating pulse. Blocks 9 and 16 interrupt the pick-up channel under the control of a logic sequencer 52 between heart beats to remove spurious natural or external interfering pulses. The block 24 provides inhibiting pulses RI during a refractory period as do interference pulses detected in a gate 26. In the event of a beat not occurring within the predetermined time the pulse generator 30 produces a pulse GP which, via a logic sequencer 52 provides a stimulating pulse SP which passes to the heart electrode 3, Fig. 1b. If this stimulates a beat the pulse generator is reset as already described but if there is no response the logic sequencer 52 produces via gate 67 a pre-back up pulse PB which steps an up/down counter at the bottom of Fig. 1b and this steps up a stimulus current memory, the output of which controls an analogue to digital converter and stimulus current memory 136, so that a subsequent back-up pulse BU produced by the logic sequencer operates via gates 61, 70 and the pulse logic circuit 172 to produce a correspondingly higher level pulse of longer duration than the initial stimulating pulse. Such back-up pulses are produced in successively increasing amplitude until either a heart beat is detected or until the number of backup pulses exceeds a value determined by the position of a switch 66, Fig. 1a. The value stored in the memory 125 determines the level of the next stimulating pulse when a further heart beat is missing. The level at which the memory is set may be reset at will by operating a button 93, Fig. 1b. This causes each stimulating pulse to step down the counter until a pulse is of insufficient level to stimulate the beat. The pre-back-up pulse switches the counter to up control whereupon successive back-up pulses step the counter up until a response is detected. Further stepping occurs to increase the level by a safety margin, the value of this margin being selected by switch 104, Fig 1b. In an alternative mode of operation switch 94 is closed and the safety margin set to zero in which case the pulses alternate at each beat to be above and below the stimulus threshold. In a further alternative a timer periodically causes the level of the pulses to be reset. If the counter controlling the back-up pulse level becomes full it is switched to count down to prevent a sudden change from maximum to minimum values. In the bottom right hand corner of Fig. 1b is  shown external display apparatus which can give readings of the amplitude of the stimulus pulse and the back-up pulse, of above or below stimulation threshold value, of the safety margin and of the average threshold reading.