Heart Tracings and Monitoring

An electrocardiogram is more than a jagged line, but it is not a picture of the heartbeat itself. A standard ECG gives several electrical views during a brief recording. Longer monitors use different recording methods to investigate problems that come and go.

What an ECG measures An electrocardiogram, usually shortened to ECG, records small voltage differences at the skin that are generated by the heart's electrical activity. It does not send electricity into the body. The recorded pattern mainly reflects depolarisation, when heart muscle cells become electrically activated, and repolarisation, when they electrically recover. Electrical activation normally leads to contraction, but the ECG does not record contraction itself. An ECG therefore cannot directly measure pumping strength, valve movement or heart structure. An echocardiogram and other imaging tests answer those questions more directly. Electrodes and leads are different An electrode is a sticky sensor placed on the skin. A lead is an electrical view calculated from one or more electrodes. A standard 12 lead ECG usually uses 10 electrodes placed on the limbs and chest. These produce 12 views of the same electrical activity from different directions. The recording itself is commonly about 10 seconds long. The appointment takes longer because the skin must be prepared and the electrodes placed correctly. What the main waves represent The P wave mainly represents atrial depolarisation. The QRS complex mainly represents ventricular depolarisation, and the T wave mainly represents ventricular repolarisation. These waves describe electrical events rather than chamber squeezing. Mechanical contraction follows electrical activation, but an ECG cannot show how much blood the heart pumps. The timing, size and shape of the waves vary between leads and between people. A normal tracing is therefore a range of acceptable patterns rather than one identical shape. What clinicians can learn An ECG can show heart rate and rhythm. It can also reveal delays or blocks in electrical conduction and changes in electrical intervals such as the QT interval. Some patterns may suggest acute reduced blood supply, previous heart muscle injury, electrolyte disturbance, medicine effects or chamber enlargement. These patterns are clues and are not always specific to one cause. An ECG can be abnormal in a person without important heart disease. It can also appear normal when heart disease is present. Interpretation must include symptoms, previous tracings, medicines and other investigations. A normal ECG has important limits A resting 12 lead ECG records only a brief period. It may miss an arrhythmia that is not present during those seconds. A normal ECG also does not exclude an acute coronary syndrome when symptoms suggest one. Assessment may require repeated ECGs, blood troponin tests and urgent clinical review. A normal tracing should therefore not be used alone to dismiss concerning chest pain, fainting or persistent palpitations. Recording quality can change the tracing Movement, muscle activity, poor skin contact and electrical interference can create artefact. Incorrect electrode placement can also alter the recorded pattern. Artefact may hide a real abnormality or imitate one. The person recording the ECG checks the trace quality and may repeat the recording before interpretation. Computer generated measurements and interpretations can support review. They do not replace assessment by a healthcare professional trained to interpret the tracing in its clinical context. Longer monitoring is not one single test An ambulatory ECG monitor records heart rhythm while a person continues normal daily activities. The device and duration are selected according to symptom frequency, clinical risk and the suspected rhythm problem. A Holter monitor or patch monitor usually records continuously for a defined period. Continuous recording can capture events that cause no symptoms as well as events marked in a diary. An event recorder stores selected episodes. The person may activate it during symptoms, or the device may automatically store a rhythm that meets programmed criteria. An implantable loop recorder is placed under the skin and can monitor for much longer. It stores selected automatically detected or person activated episodes rather than producing one endlessly reviewed tracing. Matching the monitor to the symptom pattern When suspected atrial fibrillation episodes occur less than 24 hours apart, NICE recommends a 24 hour ambulatory monitor. Less frequent episodes usually need monitoring for a longer appropriate period. Monitoring for longer increases the chance of detecting an intermittent arrhythmia but does not guarantee detection. Poor contact, artefact, limited leads or an episode outside the monitoring period can still leave uncertainty. A symptom diary can show whether a recorded rhythm happened when symptoms occurred. A normal rhythm during symptoms may be informative, while an important arrhythmia without symptoms can also require attention. Wearable and single lead recordings have limits Some watches and handheld devices can record one or a few electrical leads. They may help capture a rhythm during symptoms or flag possible atrial fibrillation. A device alert is not a complete diagnosis. A limited lead tracing cannot answer every question that a correctly recorded 12 lead ECG can answer, and professional review may be needed. When urgent help is needed Call 999 if you currently have palpitations that do not go away, or palpitations with chest pain, shortness of breath, feeling faint or fainting. Call 999 for tight or squeezing chest pain, pain spreading to the arms, neck or jaw, severe breathing difficulty, or collapse with abnormal responsiveness. Do not wait to obtain or interpret an ECG yourself. If these symptoms have stopped, seek urgent advice from your GP surgery or NHS 111. This lesson explains general ECG principles. It cannot interpret an individual tracing or decide which monitor or investigation a person needs.

An ECG is a surface recording of heart generated voltage changes, not a direct picture of contraction or structure. Its meaning depends on the leads, recording quality, timing, clinical context and appropriate professional interpretation.