Occasionally, we like to feature a normal ECG. It is important to start with the characteristics of the normal ECG when learning to recognize “abnormal”. Of course, there are many variations in ECGs considered to be normal. Once a student recognizes the features of the normal ECG, it becomes possible to recognize “abnormal” and then learn the clinical ramifications of the abnormalities.
This strip includes a 12-lead ECG in standard format, as well as three rhythm strips in Leads V1, II, and V5. All six channels are run simultaneously, so it is easy to compare one heartbeat in multiple leads.
What are the features of this ECG that make it “normal”? It is a combination of features the ECG has, and features it does NOT have. Normal findings include:
* Normal sinus rhythm. The rhythm is regular, the rate is 80 bpm, and there is a P wave before every QRS complex. The P waves all look alike in each lead, and they are upright in the inferior leads (II, III, and aVF).
* The intervals are within normal limits. The PR interval is about .16 sec. (160 ms), the QRS complexes are narrow, and the QT interval is about 320 ms.
* The ST segments are not elevated or depressed. The T waves are upright, except in Leads III and aVR. Negative T waves are normal in these leads. The shape of the ST segments is concave upward, “smiling”.
* The frontal plane axis is within normal limits. It is slightly to the left of the center of normal, making Lead I taller than Lead II. But, when Lead II is positive, the axis is normal.
* R wave progression is correct in the precordial leads. The relative size of the R wave to the S wave in each lead is increasing as we progress from V1 to V6. Therefore, V1 is mostly negative, and V6 is mostly positive. The change occurs in regular increments as we move to the patient’s left. This is indicative of correct lead placement as well as normal direction of depolarization.
ABNORMAL FEATURES THAT ARE NOT PRESENT
* There are no ST elevations or depressions, and no pathological Q waves.
* There is no QRS widening. There is no PRI or QT prolongation.
* There is no dysrhythmia.
* There are no signs of hypertrophy.
Remember, what constitutes “normal” has many variations. For example, a normal rate in an infant would be considered tachycardia in an adult.