ECG is one of the routine clinical examinations and has great value in the diagnosis of coronary heart disease (especially acute myocardial infarction) and arrhythmia, so it is essential for clinicians to master the interpretation of ECG. Therefore, it is essential for clinicians to master the interpretation of ECG. The author has compiled the “Six Steps” to help you to judge ECG quickly, please take a look!
01
The clinical significance of each ECG waveform
The normal cardiac conduction system consists of the sinus node, inter-nodal bundle, atrioventricular node, Hitchcock’s bundle, left and right bundle branches and Purkinje fibers, whose main function is to form and conduct impulses.
Pictures
Image source: Station Cool Helo
One of the important steps in reading an ECG is to measure the indicators of each ECG waveform to understand whether the ECG waveform is normal through the numerical parameters.
The main measurements of the ECG include two main components: Time and voltage amplitude.
- Transverse spacing represents time, 0.04 s per small frame and 0.20 s per large frame.
- The longitudinal direction is the voltage amplitude, and the standard voltage 1 mV = 10 mm, the longitudinal spacing is equal to 0.1 mV per small frame and 0.5 mV per large frame.
P wave
- Meaning: potential change of the left and right atrial depolarization
- Shape: obtuse circular
- Direction: Ⅰ, Ⅱ, aVF upward, aVR downward
- Amplitude: limb conduction ＜0.25 mv; thoracic conduction ＜0.20 mv
- Time limit: ≤ 0.12 s
P-R interval
- Meaning: time from the start of atrial depolarization to the start of ventricular depolarization
- Time: 0.12 to 0.20 s
QRS wave group
- Meaning: Depolarization waves of the right and left ventricles
- Time: 0.06～0.10 s
- Amplitude: RV1 ≤ 1.0 mv, RV1 ≤ 1.5 mv in complete right bundle branch block; RaVR < 0.5 mV; RV5, RV6 ≤ 2.5 mV; RⅠ < 1.5 mV; RaVL < 1.2 mv; RaVF < 2.0 mV
- Abnormal Q wave: the amplitude of the Q wave is greater than 1/4 of the amplitude of the R wave in the same lead, with a time limit of ≥ 0.04 s
ST segment
- Significance: ventricular slow repolarization
- Downward shift: ≤ 0.05 mV (in any lead)
- Upward shift: < 0.1 mV (V4 to V6); < 0.5 mV (V3); < 0.3 mV (V1 to V2)
- Time limit: 0.05 to 0.15 s
T wave
- Meaning: rapid ventricular repolarization
- Direction: The direction of T wave is mostly the same as the direction of the main QRS wave, T wave in Ⅰ, Ⅱ, V4 ~ V6 leads up, in aVR leads down, in V1 ~ V3 leads can be up or down, but if in V1 leads are upright, in V2, V3 leads should not be inverted
- Amplitude: not less than 1/10 of the R wave in the same lead
- Time limit: < 0.25 s
Q-T interval
- Meaning: The whole process of depolarization and repolarization of ventricular muscle
- Time: 0.32-0.44 s, female ＜0.46 s, male ＜0.45 s (ventricular rate: 60-100 bpm), QTc ＜0.44 s (corrected QT interval)
U wave (most pronounced in leads V2 to V4)
- Significance: effect of ventricular secondary potentials
- Direction: roughly the same direction as T wave, V3 is obvious; amplitude does not exceed 1/2 of the T wave in the same lead
- Increased: hypokalemia, hypercalcemia, drug effects, acute cerebrovascular accident, etc.
- Inversion: suggests anterior descending branch lesion, hypertension, right ventricular hypertrophy or overload
02
6-step method for rapid ECG evaluation
Step 1: Determine the basal rhythm
Under normal conditions, the rhythm of the heart is controlled by the sinus node, so it is called sinus rhythm.
Sinus rhythm is manifested on the ECG by
- Inversion of the P wave in the aVR leads.
- The P wave is inverted in the aVR leads; it is upright in leads I, II, avF, and V4 to V6.
However, when the electrical axis is left deviated, the P wave is inverted in leads Ⅱ, Ⅲ, and avF. In this case, sinus rhythm can be determined by the fact that the P wave is inverted in lead aVR and upright in leads Ⅰ and V4-V6.
Step 2: Measure the P-P or R-R interval to determine the heart rate
The heart rate is the number of heart beats per minute. When measuring heart rate on an electrocardiogram there are usually two scenarios.
- Heart rate = 60/RR or PP interval if the heart rhythm is regular. A normal adult sinus heart rate is 60 to 100 beats per minute, with more than 100 beats per minute classified as sinus tachycardia and less than 60 beats per minute classified as sinus bradycardia.
- When the heart rhythm is irregular (e.g., in atrial fibrillation), the heart rate can be calculated by counting the number of heartbeats per 10 seconds × 6.
Step 3: Look at the QRS waveform migration pattern
Limb conduction: Ⅰ, Ⅱ, aVL main wave upward, aVR main wave downward, aVF, Ⅲ main wave direction generally upward, but also downward.
The R/S of V1 < 1, R/S of V5 > 1, R/S of V3 and V4 ≈ 1.
Image Thoracic lead shift map
Source: Courtesy of the authors
Reverse clockwise transposition: When the position of the probe electrode does not move and the heart turns in a counterclockwise direction along its long axis, it is called reverse clockwise rotation.
The manifestation on the ECG is that the image with R/S ≈ 1, which should appear in leads V3 and V4, appears in leads V1 and V2 (below).
Image
Source: Courtesy of the authors
This phenomenon can be seen in normal subjects (so-called transverse heart), in early repolarization syndrome, in type A preexcitation and, in part, in left ventricular hypertrophy.
Clockwise transposition (below): When the position of the probing electrode does not move and the heart turns clockwise along its long axis, this is called clockwise transposition.
The specific electrocardiographic manifestations are V1 to V4 leads are dominated by S waves, i.e., R/S < 1, and usually require V5 leads R/S ≈ 1.
When the above requirements are met, V5 leads with R/S ≈ 1 are said to be mildly cis-locked, while V1 to V6 leads with rS pattern and R/S significantly < 1 are said to be significantly cis-locked.
The former is seen in young people in general, while the latter is commonly seen in pulmonary heart disease or precordial disease.
Pictures
V5, V6 leads R/S ≈ 1
Source: Provided by the authors
Step 4: Observe the voltage, shape, and direction of the P, QRS, T, and U waves in each lead, as well as the displacement of the ST segment
Observe the amplitude and time frame of P, QRS, T and U in each lead one by one, and measure the P-R interval and Q-T interval. If
- P wave time limit is not prolonged, the amplitude is increased ≥ 0.25 mv (limb leads), ≥ 0.20 mv (thoracic leads), most often seen in pulmonary heart disease, so it is also called “pulmonary P wave”.
- Prolonged P-wave time limit ≥ 0.12 s, P-wave is often bimodal with two peaks ≥ 0.04 s apart, which is called “mitral P-wave”.
- Increased R-wave voltage: RV5, RV6 > 2.5 mv, RV5 + SV1 > 4.0 mv (male) or > 3.5 mv (female), RⅠ > 1.5 mv, RavL > 1.2 mv, RavF > 2.0 mv, RⅠ + SIII > 2.5 mv The presence of each of the above items alone can diagnose LV hypervoltage, and the more the number of items met, the greater the reliability of the diagnosis.
- The presence of a wide QRS wave aberration can be seen in ventricular (premature ventricular, ventricular tachycardia, ventricular flutter, ventricular escape rhythm), infarction, hyperkalemia, and pre-excitation syndrome.
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In addition, the QRS waveforms of V1 and V5 should be observed to determine whether there is bundle branch block, etc.; whether the QRS waves are generated by supraventricular downward ventricular transmission or intraventricular ectopic excitation; whether there are abnormal Q waves; whether there is displacement of the ST segment; and whether the morphology and direction of T waves are abnormal.
Step 5: Look at the long leads (II or V1 leads) and observe the rhythm
The most commonly used is the II lead, which has obvious P waves and more accurate PR interval measurement; for arrhythmia analysis, the most suitable V1 lead is used because the main frequency of P waves in V1 lead is higher than other leads, while the frequency and amplitude of T waves are lower, so P will be easily recognized even if it overlaps on T.
Atrial fibrillation or atrial flutter is more clearly shown on V1. Ectopic P waves in this lead are easily distinguished from sinus P waves, which are usually positive and negative in both directions, and ectopic P waves, which are usually negative and positive or negative in both directions.
In addition, it is important to determine whether each P wave is followed by a QRS wave group; whether the temporal relationship between P waves and QRS waves is fixed, and whether there are early or delayed QRS waves. It is important to determine whether the P, QRS, and T patterns of each cardiac cycle are consistent, and whether they are pseudo-differential, premature, or escape beats.
Step 6: Make an ECG diagnosis by combining the ECG findings with the medical history
The normal range of the ECG is large, and the criteria are not absolute. It is important to avoid mistaking some normal variants for abnormalities, such as unstable T-wave changes.
In addition, some ECG changes are not specific, and the same ECG changes can be seen in a variety of heart diseases, which must be judged clinically. For example, ischemic ST-T changes can be seen in ventricular strain due to long-term hypertension, non-ST-segment elevation myocardial infarction, viral myocarditis, and many other diseases, and a more valuable diagnosis can only be made by considering the patient’s clinical situation.
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