A cardiac pacemaker is normally used to produce pulses that force the heart to beat at higher rate; this is particularly important when blockage occurs to the biological signals that stimulate the heart to beat. A demand pacemaker is specifically helpful to the patient with partial blockage, in which case an electric pulse to the heart muscle is only applied when a heartbeat does not happen within a predetermined time interval.
Basic Features of a Demand Cardiac Pacemaker
The pacemaker circuitry along with a lithium battery is enclosed in a metal case, which is implanted under the skin on the subject’s chest. The metal case and the tip of the catheter (a wire enclosed in an insulating tube) form the electrical terminals of the pacemaker. A block diagram of a demand cardiac pacemaker is shown below:
The Working of a Demand Cardiac Pacemaker
In reference to the above figure, the input amplifier amplifies the natural heart signals, which have small amplitude on the order of 1 mV. Filtering eliminates certain frequency components so that heartbeats can be better detected. A comparative circuit is then used to compare the amplified and filtered signal with a threshold value and detect either a natural heartbeat or an output pacing pulse. This detection decision is passed on to the counting and timing circuitry through an AND gate. The second input to the AND gate comes from the counter circuit such as the input signals for 0.4 s after the start of a natural or forced beat are ignored.
The timing functions are done by counting the output cycles of a timing oscillator, which generates a square wave with a precise period of 0.1 s. The digital signals produced by the counter are passed on to a digital comparator, which compares them with signals from a reference count generator and decides when the pulse generator is to deliver an output pulse of a specified amplitude and duration.
Also read:
- Cardiac Pacemakers
- Electrocardiogram (ECG)
- Cardiac Defibrillators
- Key Features of Modern ECG Systems
- Cardiac Monitor
Comments