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.