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 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 repolarization is typically hidden in the much more prominent QRS complex and normally cannot be seen without additional, specialized 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 (or 40ms per mm): • 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. 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 at 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. 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. 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.
Time-frequency analysis in ECG signal processing In electrocardiogram (ECG) signal processing, time-frequency analysis (TFA) is an important technique used to reveal how the frequency characteristics of ECG signals change over time, especially in non-stationary signals such as arrhythmias or transient cardiac events.
Common methods Steps Step 1: Preprocessing • Signal Denoising: Use wavelet denoising, band-pass filtering (0.5–50 Hz), or Principal Component Analysis (PCA) to remove electromyographic (EMG) noise. • Signal Segmentation: Segment the signal based on heartbeat cycles (e.g., R-wave detection). Step 2: Select an Appropriate TFA Method • Choose methods such as STFT, WT, or HHT based on the application requirements. Step 3: Compute the Time-Frequency Spectrum • Calculate the time-frequency distribution using the selected method to generate a time-frequency representation. Step 4: Feature Extraction • Extract power features from specific frequency bands, such as low-frequency (LF: 0.04–0.15 Hz) and high-frequency (HF: 0.15–0.4 Hz) components. Step 5: Pattern Recognition or Diagnosis • Apply machine learning or deep learning models to detect or classify cardiac events based on the time-frequency features.
Application scenarios Heart Rate Variability Analysis (HRV): • Time-frequency analysis helps to separate sympathetic and parasympathetic nervous system activity. Atrial Fibrillation Detection: • Analyze the time-frequency characteristics of atrial activity. Ventricular Fibrillation Analysis: • Detect time-frequency changes in high-frequency abnormal components.
Limb leads and electrical conduction through the heart The animation shown to the right illustrates how the path of electrical conduction gives rise to the ECG waves in the limb leads. What is green zone ? Recall that a positive current (as created by depolarization of cardiac cells) traveling towards the positive electrode and away from the negative electrode creates a positive deflection on the ECG. Likewise, a positive current traveling away from the positive electrode and towards the negative electrode creates a negative deflection on the ECG. The red arrow represents the overall direction of travel of the depolarization. The magnitude of the red arrow is proportional to the amount of tissue being depolarized at that instance. The red arrow is simultaneously shown on the axis of each of the 3 limb leads. Both the direction and the magnitude of the red arrow's projection onto the axis of each limb lead is shown with blue arrows. Then, the direction and magnitude of the blue arrows are what theoretically determine the deflections on the ECG. For example, as a blue arrow on the axis for Lead I moves from the negative electrode, to the right, towards the positive electrode, the ECG line rises, creating an upward wave. As the blue arrow on the axis for Lead I moves to the left, a downward wave is created. The greater the magnitude of the blue arrow, the greater the deflection on the ECG for that particular limb lead. Frames 1–3 depict the depolarization being generated in and spreading through the
sinoatrial node. The SA node is too small for its depolarization to be detected on most ECGs. Frames 4–10 depict the depolarization traveling through the atria, towards the
atrioventricular node. During frame 7, the depolarization is traveling through the largest amount of tissue in the atria, which creates the highest point in the P wave. Frames 11–12 depict the depolarization traveling through the AV node. Like the SA node, the AV node is too small for the depolarization of its tissue to be detected on most ECGs. This creates the flat PR segment. Frame 13 depicts an interesting phenomenon in an over-simplified fashion. It depicts the depolarization as it starts to travel down the interventricular septum, through the
bundle of His and
bundle branches. After the Bundle of His, the conduction system splits into the left bundle branch and the right bundle branch. Both branches conduct action potentials at about 1 m/s. However, the action potential starts traveling down the left bundle branch about 5 milliseconds before it starts traveling down the right bundle branch, as depicted by frame 13. This causes the depolarization of the interventricular septum tissue to spread from left to right, as depicted by the red arrow in frame 14. In some cases, this gives rise to a negative deflection after the PR interval, creating a Q wave such as the one seen in lead I in the animation to the right. Depending on the mean electrical axis of the heart, this phenomenon can result in a Q wave in lead II as well. Following depolarization of the interventricular septum, the depolarization travels towards the apex of the heart. This is depicted by frames 15–17 and results in a positive deflection on all three limb leads, which creates the R wave. Frames 18–21 then depict the depolarization as it travels throughout both ventricles from the apex of the heart, following the action potential in the
Purkinje fibers. This phenomenon creates a negative deflection in all three limb leads, forming the S wave on the ECG. Repolarization of the atria occurs at the same time as the generation of the QRS complex, but it is not detected by the ECG since the tissue mass of the ventricles is so much larger than that of the atria. Ventricular contraction occurs between ventricular depolarization and repolarization. During this time, there is no movement of charge, so no deflection is created on the ECG. This results in the flat ST segment after the S wave. Frames 24–28 in the animation depict repolarization of the ventricles. The epicardium is the first layer of the ventricles to repolarize, followed by the myocardium. The endocardium is the last layer to repolarize. The plateau phase of depolarization has been shown to last longer in endocardial cells than in epicardial cells. This causes repolarization to start from the apex of the heart and move upwards. Since repolarization is the spread of negative current as membrane potentials decrease back down to the resting membrane potential, the red arrow in the animation is pointing in the direction opposite of the repolarization. This therefore creates a positive deflection in the ECG, and creates the T wave.
Ischemia and infarction Ischemia or
non-ST elevation myocardial infarctions (non-STEMIs) may manifest as
ST depression or inversion of
T waves. It may also affect the
high frequency band of the QRS.
ST elevation myocardial infarctions (STEMIs) have different characteristic ECG findings based on the amount of time elapsed since the MI first occurred. The earliest sign is
hyperacute T waves, peaked T waves due to local
hyperkalemia in ischemic myocardium. This then progresses over a period of minutes to elevations of the
ST segment by at least 1 mm. Over a period of hours, a pathologic
Q wave may appear and the T wave will invert. Over a period of days the ST elevation will resolve. Pathologic Q waves generally will remain permanently. The
coronary artery that has been occluded can be identified in an STEMI based on the location of ST elevation. The
left anterior descending (LAD) artery supplies the anterior wall of the heart, and therefore causes ST elevations in anterior leads (V1 and V2). The
LCx supplies the lateral aspect of the heart and therefore causes ST elevations in lateral leads (I, aVL and V6). The
right coronary artery (RCA) usually supplies the inferior aspect of the heart, and therefore causes ST elevations in inferior leads (II, III and aVF).
Artifacts An ECG tracing is affected by patient motion. Some rhythmic motions (such as shivering or
tremors) can create the illusion of cardiac arrhythmia. Artifacts are distorted signals caused by a secondary internal or external sources, such as muscle movement or interference from an electrical device. Distortion poses significant challenges to healthcare providers, and strategies to safely recognize these false signals. Accurately separating the ECG artifact from the true ECG signal can have a significant impact on patient outcomes and
legal liabilities. Improper lead placement (for example, reversing two of the limb leads) has been estimated to occur in 0.4% to 4% of all ECG recordings, and has resulted in improper diagnosis and treatment including unnecessary use of
thrombolytic therapy.
Interpretation Whitbread, consultant nurse and paramedic, suggests ten rules of the normal ECG, deviation from which is likely to indicate pathology. These have been added to, creating the 15 rules for 12-lead (and 15- or 18-lead) interpretation. Rule 1: All waves in aVR are negative. Rule 2: The ST segment (J point) starts on the isoelectric line (except in V1 & V2 where it may be elevated by not greater than 1 mm). Rule 3: The PR interval should be 0.12–0.2 seconds long. Rule 4: The QRS complex should not exceed 0.11–0.12 seconds. Rule 5: The QRS and T waves tend to have the same general direction in the limb leads. Rule 6: The R wave in the precordial (chest) leads grows from V1 to at least V4 where it may or may not decline again. Rule 7: The QRS is mainly upright in I and II. Rule 8: The P wave is upright in I II and V2 to V6. Rule 9: There is no Q wave or only a small q (35 mm? Rule 12: Is there an
Epsilon wave? Rule 13: Is there an J wave? Rule 14: Is there a
Delta wave? Rule 15: Are there any patterns representing an
occlusive myocardial infarction (OMI)? ==Diagnosis==