Overview Of Cardiac Pacemaker, Cardiac Pacemaker Types, Precautions and Life
The rhythmic beating of the heart is due to the triggering pulses that originate in an area of specialized tissue in the right atrium of the heart. This area is known as the sino-atrial node.
By giving external electrical stimulation impulses (Fig 1.1) to the heart muscle, it is possible to regulate the heart rate. These impulses are given by an electronic instrument called a “Pacemaker”.
Fig 1.1 Pacemaker pulses followed by QRS complex of the heart
A pacemaker basically consist of two parts:
(i) Pulse generator
The waveforms used for pacing are around-topped rectangular pulses of 1-3 ms duration with rates adjustable from 50-150 pulses per minute.
The electrode arrangements for use with cardiac pacemaker can be in the form of bipolar or unipolar system. In the bipolar system, two electrodes are placed on the heart, whereas in the unipolar system, one electrode is placed on the heart and the other electrode is positioned elsewhere in the body.
1.1 Types of Pacemakers
There are different types of pacemakers, External pacemakers are used when the heart block presents as an emergency and when it is expected to be present for a short time. Internal pacemakers are used in cases requiring long-term pacing because of permanent damage that prevents normal self-triggering of the heart.
1.1.1 External Pacemaker
External pacemaker are employed to restart the normal rhythm of the heart in case of cardiac standstill, in situation where short-term pacing is considered adequate.
The pacing impulse is applied through metal electrodes placed on the surface of the body. Electrode jelly is used for better contact and to avoid burning of the skin. An external pacemaker may apply up to 80-mA pulses through 50-cm2 electrode on the chest. This procedure is painful and therefore is used only in an emergency or a temporary situation.
The pulses may be delivered:
(i) Continuously: When the heart rate is below the pre-set value. The impulse frequency is independent of the electrical activity of the heart.
(ii) On demand: Normally the pacemaker is inoperative but it is activated when the heart rate falls below the normal or pre-set value. When interval exceeds the pre-set value, the pacemaker comes into operation.
Pacing with external pacemaker through the chest requires a maximum of 150V pulses across an impedance of the order 1 kΩ. External pacing has the disadvantage that the electrodes burn the skin and the electrical pulses become painful.
1.1.2 Internal Pacemaker
The stimulating pulses can be preferably applied to the heart through a pacing passing through a vein and connected to the heart. This is called internal pacing. The pacing current required is much less than when it is applied through the chest. The voltage output of internal pacemakers is about 0-15V and the available output current ranges from 1-20mA. For long-term pacing by this method, the electrodes can be inserted percutaneously into the myocardium of the left ventricle.
A bipolar or unipolar electrode is inserted through the jugular or cephalic vein, into the right ventricle.
1.2 Temporary Pacing
For temporary pacing, external pulse generators (Fig 1.2) are usually battery-powered. The generator can provide demand or asynchronous pacing. With their calibrated controls, they can be used to determine both stimulation threshold and approximate R-wave potentials sensed by the electrodes of a pacing lead system. They provide a pacing rate adjustable from 30 to 180 pulses per minute and constant current output from 0.1 to 20 mA to a maximum of 18V as measured at output terminals across 500Ω. The pulse width is around 2ms. They are provided with defibrillator shock damage protection to 400 Ws.
(Fig 1.2) External Pacemaker for temporary pacing
1.3 Pacemaker Based on the Type of Output Waveform
There are three type of pacemakers based on the type of output waveform, these are:
(i) Voltage Pacemakers: The voltage output from the pacemaker remains constant and changes of resistance in the circuit will change in current.
(ii) Current Pacemaker: In current pacemakers, throughout the impulse, the current in the circuit is determined by the internal resistance of the pacemaker.
(iii) Current Limited Voltage Pacemakers: This is primarily a voltage circuit, but the maximum current in the circuit is limited.
Battery life of external pulse generators will vary depending on rate and current output settings. At typical rate and output setting of 70 ppm and 10 mA respectively, the battery life should be around 500 h.
1.4 Implantable Pacemakers
The implantable pacemaker, along-with its electrodes, is designed to be entirely implanted beneath the skin. Its output leads are connected directly to the heart muscle. The pacemaker is powered by small batteries. The circuit is so designed that the batteries supply sufficient power for a long period.
For any implantable circuit, the basic requirements are:
– The components used in the circuit should be highly reliable;
– The power source should be in a position to supply sufficient power to the circuit over prolonged period of time;
– The circuit should be covered with a biological inert material so that the implant is not rejected by the body; and
– The unit should be covered in such a way that body fluids do not find a way inside the circuit
Life of Pacemaker
From 1968 to 1972, the average life of most pacemakers was 22 months, the average life of a pacemaker in 1974 was around 31 months. From 1975 onward, the life of pacemakers was increased to more than five years. Present day pacemaker manufacturers provide life time of pacing performance warranty.
1.4.1 Types of Implantable Pacemaker
Depending upon the clinical requirements, different types of implantable pacemakers are,
(i) Fixed rate pacemaker: This type of pacemaker is intended for patients having permanent heart blocks. The rate is pre-set, say at 70 bpm. The rate can be varied externally in implanted units by magnetically actuating a built-in relay.
(ii) Demand Pacemaker: The demand unit functions only when the R-R intervals of the natural rhythm exceed a pre-set limit.
(iii) R wave Triggered Pacemaker: The ventricular synchronized demand type (R wave triggered) pacemaker is meant for patients who are generally in heart block with occasional sinus rhythm. The pacemaker detects ventricular activity (R wave of ECG) and stimulates the ventricles after a very short delay time of some milliseconds.
(iv) R wave Blocked Pacemaker: Pacemaker is meant for patients who generally have sinus rhythm with occasional heart block. The circuitry detects spontaneous R wave potentials at the electrodes, in the case of ventricular activity the R wave does not trigger the output circuit of the pacemaker but blocks the output circuit and no stimulation impulse is given to the heart.
(v) Atrial Triggered Pacemaker: The pacemaker detects the atrial de-polarization and starts the pulse forming circuits after a delay so that the impulse to the ventricles is delivered after a suitable PR interval. The major advantage of this pacemaker is its ability to provide maximum augmentation of cardiac output at changing atrial rates to meet various physiological requirements.
(vi) Dual Chamber Pacemaker: These devices are capable of treating the majority of those patients who suffer from disease of the sino-atrial node by providing atrial stimulation whenever needed.
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