It is well known that the earlier cardiopulmonary resuscitation (CPR) is initiated, the better the outcome. Practical limitations are common in the setting of out of hospital cardiac arrest (OHCA). Bystander initiated CPR is the best way to start CPR at the earliest. Leveraging the widely available mobile phones with positioning systems in place has […]
Electrophysiology study is an advanced evaluation of heart rhythm disorders. It is usually resorted to in cases where simple medications are not effective in controlling the rhythm abnormalities. Generally it is done without any sedation or general anaesthesia as it is not very painful. Local anaesthesia is given at the sites where the blood vessels are punctured to introduce small electrodes into the various chambers of the heart for recording the electrical activity of the heart internally. Each electrode has multiple recording poles – bipolar with two recording poles, quadripolar with four recording poles, decapolar with ten recording poles and duo-decapolar with 20 electrode poles.
The signals from the electrodes reach a junction box from where it reaches recorder which also displays multiple simultaneous channels on a screen. The signals can be electronically archived on a hard disc or printed on a calibrated recording paper. By analysing the sequence in which signals arrive at each electrode, the pattern in which the signals are conducted within the heart can be assessed. This gives valuable information on the origin of the heart rhythm disorder. The information gathered will guide further treatment in the form of medications or catheter ablation. Catheter ablation is the procedure by which a small electrode delivers an energy to produce localized damage a disease focus, curing the rhythm abnormality. Most commonly used source of energy is radiofrequency, while cooling with a cryo probe and microwave energy are other options.
Disorders of heart rhythm are technically known as cardiac arrhythmia. Rhythm disorders can be irregularity in the rhythm or change in the heart rate (increase or decrease). Commonest irregularity in the heart rhythm is due to abnormal beats originating from the lower chambers known as ventricular ectopic beats (VPB or VPC – ventricular premature complexes). Increase in heart rate can be due to physiological causes like exercise or due to simple disease states like fever. A decrease in heart rate can occur during deep sleep and when the body temperature falls below normal (hypothermia). There are several disease states which can reduce heart rate like a decrease in the function of the thyroid gland (hypothyroidism) and complete heart block. In complete heart block, the signals generate by the natural pacemaker of the heart, namely sinoatrial node, does not get conducted down to the lower chambers of the heart. The study of heart rhythm disorders is known as cardiac electrophysiology.
Disorders of the heart rhythm are initially documented by an electrocardiogram (ECG), which the technique used to record the electrical activity of the heart using multiple electrodes attached to the body. The device used to record the ECG is known as the electrocardiograph. ECG machines were very huge to begin with, when they were invented in the beginning of twentieth century. But now they are quite compact and portable, enabling recording at the bedside.
Heart rhythm disorders can be further documented by long term recorders like Holter monitors and event monitors. Continuous ECG monitoring for evaluation of cardiac arrhythmia is now commonplace in intensive care units and operating rooms.
Apart from recording of the electrical activity from the body surface as discussed, above, it is also possible to record electrical signals from within the heart using electrodes introduced into the heart chambers through the blood vessels. This procedure is known as electrophysiology study, and is used for getting advanced details regarding heart rhythm disorders for making important treatment decisions.
Electrical system of the heart ensures its rhythmic contractions at normal rate of 60-100 per minute. The signal generator of the heart is known as the sinoatrial node (SA node) and the relay station is known as the atrioventricular node (AV node). Signal conduction between the nodes is through internodal pathways and that from the AV node into the ventricles through the bundle of His and its branches.
SA node is the natural pacemaker of the heart which controls the heart rate. It is in turn under the control of the brain by the autonomic nervous system. SA node increases and decreases its rate in relation to the physiological need of the body. When we take rest, the rate is low and it increases gradually with progressive exertion. Heart rate increases with emotional stress as well. SA node is located in the upper part of the right atrium (upper chamber of the heart), near the entry of the superior vena cava (blood vessel draining deoxygenated blood from the upper part of the body).
There are three signal pathways within the right atrium which connects the SA node to the AV node. One of them gives a branch to the left atrium (left upper chamber of the heart) for giving electrical signals to that chamber.
AV node is situated at the junction of the upper and lower chambers of the heart (AV junction), almost in the middle, at the lower part of right atrium. It delays the signals coming from the SA node so that contraction of the upper chambers are fully over before the lower chambers start contracting.
Bundle of His
Bundle of His is the electrical conductions system of the heart which crosses the AV junction and passess into the wall between the right and left lower chambers (ventricles) of the heart. It divides into two bundle branches – right and left for each ventricle. Left bundle further divides into two divisions, one of which goes to the front region and the other to the back region of the left ventricle (left anterior and left posterior fascicles).
The bundle branches divide further into numerous small branches which take the electrical signals to all the muscle cells of the ventricles. These tiny branches are known as Purkinje fibres.
When the electrical system of the heart fails, the heart beats slow down and the person can get a black out. If the slowing is due to a permanent cause, an artificial electronic pacemaker is implanted under the skin and connected to the heart by a lead wire to restore the heart rhythm. If the cause is temporary (reversible), a temporary external pacemaker is used instead, which can be removed after the spontaneous rhythm of the heart is restored.