Interpretation of the ECG is fundamentally about understanding the electrical conduction system of the heart. Normal conduction starts and propagates in a predictable pattern, and deviation from this pattern can be a normal variation or be pathological. An ECG does not equate with mechanical pumping activity of the heart, for example, pulseless electrical activity produces an ECG that should pump blood but no pulses are felt (and constitutes a medical emergency and CPR should be performed). Ventricular fibrillation produces an ECG but is too dysfunctional to produce a life-sustaining cardiac output. Certain rhythms are known to have good cardiac output and some are known to have bad cardiac output. Ultimately, an echocardiogram or other anatomical imaging modality is useful in assessing the mechanical function of the heart. Like all medical tests, what constitutes "normal" is based on population studies. The heartrate range of between 60 and 100 beats per minute (bpm) is considered normal since data shows this to be the usual resting heart rate. Theory vector Schematic representation of a normal ECG Interpretation of the ECG is ultimately that of pattern recognition. In order to understand the patterns found, it is helpful to understand the theory of what ECGs represent. The theory is rooted in electromagnetics and boils down to the four following points: * depolarization of the heart toward the positive electrode produces a positive deflection * depolarization of the heart away from the positive electrode produces a negative deflection * repolarization of the heart toward the positive electrode produces a negative deflection * repolarization of the heart away from the positive electrode produces a positive deflection Thus, the overall direction of depolarization and repolarization produces positive or negative deflection on each lead's trace. For example, depolarizing from right to left would produce a positive deflection in lead I because the two vectors point in the same direction. In contrast, that same depolarization would produce minimal deflection in V1 and V2 because the vectors are perpendicular, and this phenomenon is called isoelectric. Normal rhythm produces four entities – a P wave, a QRS complex, a T wave, and a U wave – that each have a fairly unique pattern. * The P wave represents atrial depolarization. * The QRS complex represents ventricular depolarization. * The T wave represents ventricular repolarization. * The U wave represents papillary muscle repolarization. Changes in the structure of the heart and its surroundings (including blood composition) change the patterns of these four entities. The U wave is not typically seen and its absence is generally ignored. Atrial repolarisation is typically hidden in the much more prominent QRS complex and normally cannot be seen without additional, specialised electrodes. Background grid ECGs are normally printed on a grid. The horizontal axis represents time and the vertical axis represents voltage. The standard values on this grid are shown in the adjacent image at 25mm/sec: * A small box is 1 mm × 1 mm and represents 0.1 mV × 0.04 seconds. * A large box is 5 mm × 5 mm and represents 0.5 mV × 0.20 seconds. The "large" box is represented by a heavier line weight than the small boxes. Measuring time and voltage with ECG graph paper The standard printing speed in the United States is 25 mm per sec (5 big boxes per second), but in other countries it can be 50 mm per sec. Faster speeds such as 100 and 200 mm per sec are used during electrophysiology studies. Not all aspects of an ECG rely on precise recordings or having a known scaling of amplitude or time. For example, determining if the tracing is a sinus rhythm only requires feature recognition and matching, and not measurement of amplitudes or times (i.e., the scale of the grids are irrelevant). An example to the contrary, the voltage requirements of left ventricular hypertrophy require knowing the grid scale. Rate and rhythm In a normal heart, the heart rate is the rate in which the sinoatrial node depolarizes since it is the source of depolarization of the heart. Heart rate, like other vital signs such as blood pressure and respiratory rate, change with age. In adults, a normal heart rate is between 60 and 100 bpm (normocardic), whereas it is higher in children. A heart rate below normal is called "bradycardia" (100 in adults). A complication of this is when the atria and ventricles are not in synchrony and the "heart rate" must be specified as atrial or ventricular (e.g., the ventricular rate in ventricular fibrillation is 300–600 bpm, whereas the atrial rate can be normal 60–100 or faster 100–150). In normal resting hearts, the physiologic rhythm of the heart is normal sinus rhythm (NSR). Normal sinus rhythm produces the prototypical pattern of P wave, QRS complex, and T wave. Generally, deviation from normal sinus rhythm is considered a cardiac arrhythmia. Thus, the first question in interpreting an ECG is whether or not there is a sinus rhythm. A criterion for sinus rhythm is that P waves and QRS complexes appear 1-to-1, thus implying that the P wave causes the QRS complex. Once sinus rhythm is established, or not, the second question is the rate. For a sinus rhythm, this is either the rate of P waves or QRS complexes since they are 1-to-1. If the rate is too fast, then it is sinus tachycardia, and if it is too slow, then it is sinus bradycardia. If it is not a sinus rhythm, then determining the rhythm is necessary before proceeding with further interpretation. Some arrhythmias with characteristic findings: * Absent P waves with "irregularly irregular" QRS complexes is the hallmark of atrial fibrillation. * A "saw tooth" pattern with QRS complexes is the hallmark of atrial flutter. * A sine wave pattern is the hallmark of ventricular flutter. * Absent P waves with wide QRS complexes and a fast heart rate is ventricular tachycardia. Determination of rate and rhythm is necessary in order to make sense of further interpretation. Axis The heart has several axes, but the most common by far is the axis of the QRS complex (references to "the axis" imply the QRS axis). Each axis can be computationally determined to result in a number representing degrees of deviation from zero, or it can be categorized into a few types. The QRS axis is the general direction of the ventricular depolarization wavefront (or mean electrical vector) in the frontal plane. It is often sufficient to classify the axis as one of three types: normal, left deviated, or right deviated. Population data shows that a normal QRS axis is from −30° to 105°, with 0° being along lead I and positive being inferior and negative being superior (best understood graphically as the hexaxial reference system). Beyond +105° is right axis deviation and beyond −30° is left axis deviation (the third quadrant of −90° to −180° is very rare and is an indeterminate axis). A shortcut for determining if the QRS axis is normal is if the QRS complex is mostly positive in lead I and lead II (or lead I and aVF if +90° is the upper limit of normal). The normal QRS axis is generally down and to the left, following the anatomical orientation of the heart within the chest. An abnormal axis suggests a change in the physical shape and orientation of the heart or a defect in its conduction system that causes the ventricles to depolarize in an abnormal way. Classification Angle Notes Normal −30° to 105° Normal Left axis deviation −30° to −90° May indicate left ventricular hypertrophy, left anterior fascicular block, or an old inferior STEMI Right axis deviation +105° to +180° May indicate right ventricular hypertrophy, left posterior fascicular block, or an old lateral STEMI Indeterminate axis +180° to −90° Rarely seen; considered an 'electrical no-man's land' The extent of a normal axis can be +90° or 105° depending on the source. Amplitudes and intervals Animation of a normal ECG wave All of the waves on an ECG tracing and the intervals between them have a predictable time duration, a range of acceptable amplitudes (voltages), and a typical morphology. Any deviation from the normal tracing is potentially pathological and therefore of clinical significance. For ease of measuring the amplitudes and intervals, an ECG is printed on graph paper at a standard scale: each 1 mm (one small box on the standard 25mm/s ECG paper) represents 40 milliseconds of time on the x-axis, and 0.1 millivolts on the y-axis. Feature Description Pathology Duration P wave The P wave represents depolarization of the atria. Atrial depolarization spreads from the SA node towards the AV node, and from the right atrium to the left atrium. The P wave is typically upright in most leads except for aVR; an unusual P wave axis (inverted in other leads) can indicate an ectopic atrial pacemaker. If the P wave is of unusually long duration, it may represent atrial enlargement. Typically a large right atrium gives a tall, peaked P wave while a
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Magnetocardiography
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Impedance cardiography
Holter monitor