This guide demonstrates how to read an ECG using a systematic approach. If a patient has a regular heart rhythm their heart rate can be calculated using the following method:. As a result, you need to apply a different method:.
Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are similar.
If you are suspicious that there is some atrioventricular block AV block , map out the atrial rate and the ventricular rhythm separately i. As you move along the rhythm strip, you can then see if the PR interval changes , if QRS complexes are missing or if there is complete dissociation between the two.
Cardiac axis describes the overall direction of electrical spread within the heart. Read our cardiac axis guide to learn more.
If P waves are absent and there is an irregular rhythm it may suggest a diagnosis of atrial fibrillation. The PR interval should be between ms small squares. A prolonged PR interval suggests the presence of atrioventricular delay AV block. AV nodal conduction resumes with the next beat and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself.
The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd block or 4th block P wave. Third-degree complete AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.
Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other , due to the atria and ventricles functioning independently. Cardiac function is maintained by a junctional or ventricular pacemaker. To help remember the various types of AV block, it is useful to know the anatomical location of the block within the conducting system.
Analysis and Interpretation of the Electrocardiogram. Left ventricular hypertrophy. Premature ventricular complex. Supraventricular tachycardia - AV reentry. Electrolyte imbalances: Electrolytes are electricity-conducting particles in the body that help keep the heart muscle beating in rhythm.
If your electrolytes are imbalanced, you may have an abnormal ECG reading. Heart attack or ischemia: During a heart attack, blood flow in the heart is affected and heart tissue can begin to lose oxygen.
This tissue will not conduct electricity as well, which can cause an abnormal ECG. Ischemia, or lack of blood flow, may also cause an abnormal ECG.
Heart rhythm abnormalities: A heart typically beats in a steady rhythm. An EKG can reveal if the heart is beating out of rhythm or sequence. Such as beta-blockers, sodium channel blockers, and calcium channel blockers. Treatment response to an abnormal electrocardiogram result depends on the underlying cause.
This person may require a pacemaker , which helps restore the heart to a more normal rhythm. This allows:. The amplitude, or voltage, of the recorded electrical signal is expressed on an ECG in the vertical dimension and is measured in millivolts mV.
On standard ECG paper 1mV is represented by a deflection of 10 mm. An increase in the amount of muscle mass, such as with left ventricular hypertrophy LVH , usually results in a larger electrical depolarisation signal, and so a larger amplitude of vertical deflection on the ECG. An essential feature of the ECG is that the electrical activity of the heart is shown as it varies with time. In other words we can think of the ECG as a graph, plotting electrical activity on the vertical axis against time on the horizontal axis.
Standard ECG paper moves at 25 mm per second during real-time recording. This means that when looking at the printed ECG a distance of 25 mm along the horizontal axis represents 1 second in time. ECG paper is marked with a grid of small and large squares. Each small square represents 40 milliseconds ms in time along the horizontal axis and each larger square contains 5 small squares, thus representing ms. Standard paper speeds and square markings allow easy measurement of cardiac timing intervals.
This enables calculation of heart rates and identification of abnormal electrical conduction within the heart see Figure 4.
It will be clear from above that the first structure to be depolarised during normal sinus rhythm is the right atrium, closely followed by the left atrium.
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