3. Characteristics of the Normal ECG
It is important to remember that there is a wide range of normal variability in the 12 lead ECG. The following "normal" ECG characteristics, therefore, are not absolute. It takes considerable ECG reading experience to discover all the normal variants. Only by following a structured "Method of ECG Interpretation" (Lesson II) and correlating the various ECG findings with the particular patient's clinical status will the ECG become a valuable clinical tool.
Topics for Study:
- Heart Rate: 60 - 90 bpm
- PR Interval: 0.12 - 0.20 sec
- QRS Duration: 0.06 - 0.10 sec
- QT Interval (QTc ≤ 0.40 sec)
- Bazett's Formula: QTc = (QT)/SqRoot RR (in seconds)
- Poor Man's Guide to upper limits of QT: For HR = 70 bpm, QT ≤ 0.40 sec; for every 10 bpm increase above 70 subtract 0.02 sec, and for every 10 bpm decrease below 70 add 0.02 sec. For example:
- QT ≤ 0.38 @ 80 bpm
- QT ≤ 0.42 @ 60 bpm
- Frontal Plane QRS Axis: +90° to -30° (in the adult)
Normal sinus rhythm
The P waves in leads I and II must be upright (positive) if the rhythm is coming from the sinus node.
Normal Sino-atrial (SA), Atrio-ventricular (AV), and Intraventricular (IV) conduction
Both the PR interval and QRS duration should be within the limits specified above.
(Normal ECG is shown below - Compare its waveforms to the descriptions below)
It is important to remember that the P wave represents the sequential activation of the right and left atria, and it is common to see notched or biphasic P waves of right and left atrial activation.
- P duration < 0.12 sec
- P amplitude < 2.5 mm
- Frontal plane P wave axis: 0° to +75°
- May see notched P waves in frontal plane
The QRS represents the simultaneous activation of the right and left ventricles, although most of the QRS waveform is derived from the larger left ventricular musculature.
- QRS duration ≤ 0.10 sec
- QRS amplitude is quite variable from lead to lead and from person to person. Two determinates of QRS voltages are:
- Size of the ventricular chambers (i.e., the larger the chamber, the larger the voltage)
- Proximity of chest electrodes to ventricular chamber (the closer, the larger the voltage)
- Frontal plane leads:
- The normal QRS axis range (+90° to -30° ); this implies that the QRS be mostly positive (upright) in leads II and I.
- Normal q-waves reflect normal septal activation (beginning on the LV septum); they are narrow (<0.04s duration) and small (<25% the amplitude of the R wave). They are often seen in leads I and aVL when the QRS axis is to the left of +60°, and in leads II, III, aVF when the QRS axis is to the right of +60°. Septal q waves should not be confused with the pathologic Q waves of myocardial infarction.
- Precordial leads: (see Normal ECG)
- Small r-waves begin in V1 or V2 and progress in size to V5. The R-V6 is usually smaller than R-V5.
- In reverse, the s-waves begin in V6 or V5 and progress in size to V2. S-V1 is usually smaller than S-V2.
- The usual transition from S>R in the right precordial leads to R>S in the left precordial leads is V3 or V4.
- Small "septal" q-waves may be seen in leads V5 and V6.
ST Segment and T wave
In a sense, the term "ST segment" is a misnomer, because a discrete ST segment distinct from the T wave is usually absent. More often the ST-T wave is a smooth, continuous waveform beginning with the J-point (end of QRS), slowly rising to the peak of the T and followed by a rapid descent to the isoelectric baseline or the onset of the U wave. This gives rise to an asymmetrical T wave. In some normal individuals, particularly women, the T wave is symmetrical and a distinct, horizontal ST segment is present.
The normal T wave is usually in the same direction as the QRS except in the right precordial leads. In the normal ECG the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR.
Normal ST segment elevation: this occurs in leads with large S waves (e.g., V1-3), and the normal configuration is concave upward. ST segment elevation with concave upward appearance may also be seen in other leads; this is often called early repolarization, although it's a term with little physiologic meaning (see example of "early repolarization" in leads V4-6):
Convex or straight upward ST segment elevation (e.g., leads II, III, aVF) is abnormal and suggests transmural injury or infarction:
ST segment depression is always an abnormal finding, although often nonspecific (see ECG below)
ST segment depression is often characterized as "upsloping", "horizontal", or "downsloping".
The normal U Wave: (the most neglected of the ECG waveforms)
- U wave amplitude is usually < 1/3 T wave amplitude in same lead.
- U wave direction is the same as T wave direction in that lead.
- U waves are more prominent at slow heart rates and usually best seen in the right precordial leads.
- Origin of the U wave is thought to be related to after depolarizations which interrupt or follow repolarization.