Cardiac Pacemaker – By Dr. Gnana Sankaralingam
Pacemaker is an artificial device that electrically stimulates the heart muscle to depolarise, producing contraction. This device can convert abnormal electrical rhythm of the heart in patients who have had a heart attack or open heart surgery. It may also be used to complete the cardiac cycle of contraction and relaxation, when the heart is unable to do so by itself. It can be done as an elective or emergency procedure and named according to the location of electrodes and pathway of stimulus to the heart. Pacemaker may be temporary or permanent depending on the condition of the patient. Temporary pacemaker used in an emergency situation, could also serve as a bridge until a permanent pacemaker is inserted. Temporary pacemaker which is not implanted is about the size of a small radio.
Pacemaker is typically used for these following conditions: Haemodynamically unstable bradycardia (slow heart rate) especially if patient does not respond to drug therapy with symptoms of hypotension (low blood pressure), blackouts and change in mental state and pulmonary oedema (fluid in the lungs); Bradycardia in which heart rate is so slow that it leads to wide complex ventricular rhythms that could precipitate ventricular tachycardia or ventricular fibrillation; Malignant pulseless tachycardia either ventricular or supraventicular; Cardiac arrest due to drug overdose or electrolyte imbalance. They are contraindicated in patients with severe hypothermia (low body temperature) with bradycardic rhythm due to which ventricles are more prone to fibrillation and are resistant to defibrillation.
Permanent pacemakers are self contained devices, designed to operate for 20 years. They are surgically implanted under local anaesthetic. Leads are routed transversely, positioned in the appropriate chambers and then anchored to the endocardium (inner layer of heart muscle). Pacing electrodes can be placed in atria, ventricles or both. The generator is implanted in a pocket made from subcutaneous tissue, normally under left clavicle (collar bone). They can be, commonly used synchronous (on demand) that monitors heart rhythm and paces the heart only when the heart fails to do so by itself or rarely used asynchronous (fixed rate) which fires at a pre-set rate regardless of intrinsic cycle of the heart. They work by generating an impulse from the power source and transmitting that impulse to the heart muscle which flows throughout the heart and causes the muscle to depolarise. It consists of two components: pulse generator which contains the power source and circuitry which is a microchip that is programmed to guide the pacing through leads attached to the electrodes. Lithium battery in a permanent or implanted pacemaker can last about 10 years.
Pacing leads have either one electrode (unipolar) or two (bipolar). Leads for a pacemaker may be designed to stimulate a single chamber of heart and are placed in either the atrium or the ventricle. For dual chamber or atrio-ventricular pacing, leads are placed in both chambers, generally on the right side of the heart. In a unipolar system, the electrical current moves from the pulse generator through the lead wire to the negative pole at the tip of the electrode, from where it stimulates the heart and returns to the metal surface of the pulse generator which is the positive pole, to complete the circuit. In a bipolar system, both positive and negative poles are at the tip of the electrode, and current flows from the pulse generator through the lead wire to the negative pole, at which point it stimulates the heart and then flows back to the positive pole at the tip to complete the circuit.
Temporary pacemakers are of different types: transcutanoeus, transvenous, transthoracic and epicardial. Transcutanoeus pacing, where electrodes are placed on skin is also known as external pacing is the best choice for life threatening situations when time is critical. Back electrode pad is placed on the back along left of the spine just below scapula (shoulder blade). Front electrode pad is placed over the level of heart (V2 to V5 position) on front of the chest and adjusted to get the best wave form. This placement ensures that the electrical stimulus must travel a short distance to the heart. It is a non-invasive method that works by sending electrical impulses from the pulse generator to the heart, by way of two electrodes placed on front and back of the patient. It is used only until transvenous pacing can be instituted. They are used in unconscious patients because most of the alert patients do not tolerate the sensations produced by high energy level needed for pacing.
Transvenous pacing is more comfortable to the patient than transcutaneous pacing. It is the most commonly used of the temporary pacemakers. An electrode catheter is threaded through a vein into right atrium or right ventricle and attached to pulse generator, which provides an electrical stimulus directly to the endocardium (Inner lining of the heart). Transthoracic pacing is done using a needle, where electrode is passed through chest wall into right ventricle, similar to doing pericardial tap. It carries a risk of laceration to coronary artery and bleeding into pericardial space causing tamponade. It is done only if there is no other option. Epicardial pacing is done during cardiac surgery, where electrodes are inserted through epicardium (outer layer of heart muscle) of right atrium or right ventricle.
Pacemaker capabilities are shown by coding system inscribed on it. First letter denotes chamber paced, second letter denotes chamber sensed and third letter signifies the response to heart activity (T for trigger pacing, I for inhibit pacing and D for dual action, that depends on the mode and site). Commonly used pacemaker codes are: AAI (single chamber one that paces and senses atrium), VVI (single chamber one that paces and senses ventricle – often used for complete heart block and for intermittent pacing), DVI (pace both chambers but sense ventricle only – used in AV block or sick sinus syndrome) and DDD (dual chamber one that paces and senses both chambers at same time – used in AV dissociation).
Common problems faced by pacemakers are failure to capture, failure to pace and failure to sense. Failure to capture indicates inability to stimulate the chamber as shown by a spike without a ventricular response (QRS complex). Failure to pace is indicated by the abscence of spike and complex in ECG recording as there is no pacemaker activity. Failure to sense is indicated by pacemaker spikes that occur abnormally when intrinsic cardiac activity is present (firing at the wrong time for the wrong reason). They are commonly the result of battery or pulse generator malfunction, lead misplacement or disconnection and electrolyte disturbance. Pacemaker should be regularly checked with ECG and cause rectified.
