Important Normal Values for ECG

Transcript of the video: Normal values for ECG will depend on the age, body size, gender, as well as the population being assessed. So there is a wide variation. Still, in a normal adult population, some normal values are useful, while assessing an ECG.

These are the normal waves which you expect on the ECG and normal heart rate in an adult is 60 to 100 per minute. P wave, the atrial activation, PR segment, QRS complex – ventricular activation, ST segment, and T wave, ventricular repolarization. Atrial repolarization wave is usually not seen in a standard surface ECG. It will somewhere in the ST segment (error: PR segment) and part of the QRS, of low amplitude. Usually, the Ta wave, that is the atrial repolarization wave, is not seen in a surface ECG. This is P wave, this is QRS complex and T wave. A segment does not include a wave. PR segment, ST segment. But an interval includes one or more waves and a segment. PR interval will include P wave and PR segment. QT interval will include QRS complex and ST segment as well as the T wave. So interval includes one or more waves and one segment, while segment is without a wave. And there is another one which is not represented here. Part of it is seen here, that is the TP segment. Previous T to the current P. That is the true isolectrical interval, isoelectric interval in the ECG. True isoelectric interval in the ECG is TP segment. While PR and ST are not, even though it is shown as isoelectric in this diagram, they are not truly isoelectric, because, PR segment will contain a negative component of the Ta wave and ST segment is usually slowly upsloping so that it merges with the ascending limb of the T wave.

This is the RR interval, interval between peaks of two R waves. PP interval, from the onset of the P wave to the onset of the next P wave. From the PP interval, you an calculate the atrial rate. From the RR interval, you can calculate the ventricular rate. Usually we calculate the RR interval and calculate the ventricular rate and say that it is the heart rate, presuming that both are equal. But in conditions like complete heart block, it may be different. PP interval will be shorter and RR interval will be longer, in complete heart block. Otherwise, in most cases, they are equal, in normal sinus rhythm. And, this will vary with the heart rate. Heart rate increase, both PP and RR interval shortens. In general, all the intervals shorten with increase in heart rate, but the change, change in the intervals will be more for PP interval and RR interval, compared to the other intervals. For the same reason, these two intervals will be shorter in children, compared to adults. Younger the child, shorter the PP interval and RR interval, as the heart rate is higher. In newborn, heart rate may be about 140 per minute, while in an adult it may be only 70 per minute, and in a well trained athlete, it may be even as low as 40 per minute.

Normal P wave dimensions are easy to remember. Both the height of P wave or amplitude of P wave and width of P wave are 2.5 mm in a standard ECG, meaning that an ECG recorded at a paper speed of 25 mm per second. The P width of 2.5 mm will correspond to 100 milliseconds when you express as time, and if it is more than that, you can say that P is widened, and it occurs in left atrial enlargement. Height of the P wave more than 2.5 mm occurs in right atrial enlargement. If both height and width are increased, it is biatrial enlargement.

PR interval is from the onset of P wave, to the onset of QRS complex. If there is a Q wave, up to the onset of the Q wave. If there is no Q wave, up to the onset of the R wave. That is how, PR interval is calculated. It includes P wave and PR segment. So it is atrial conduction as well as conduction through the AV node. If intra atrial conduction is delayed, P wave widens. Then also PR interval widens. But most of it is contributed by atrioventricular conduction. PR interval is mostly by atrioventricular conduction. Short PR interval occurs in WPW syndrome and long PR interval occurs in atrioventricular, AV block.

Normal QRS width is also easy to remember. It is up to 100 milliseconds or two and a half divisions. It is taken as from the onset of the QRS complex to the end of the QRS complex. The end is also known as J point. The total width is the QRS duration. And QRS duration can be abnormal when there is a conduction abnormality, intraventricular conduction abnormality like a bundle branch block. It is also abnormal when the focus is from the ventricle. Wide QRS complex is a feature of a ventricular ectopic beat as well. So that is the QRS width, usually 100 milliseconds. When it is three divisions or 120 milliseconds, you can call it as wide QRS. Even 110 milliseconds can be taken as normal. So, when it is three divisions, it is taken as abnormal. 120 milliseconds or more of QRS width is taken as abnormal.

QT interval is from the onset of the QRS complex, to the end of the T wave. This represents the ventricular depolarization as well as repolarization. Measurement of QT interval is very important, as QT prolongation can predispose to serious, life threatening ventricular arrhythmias. That is why, it is now mandatory, it is a regulatory requirement for any new medication being introduced, for having QT testing. Should be shown that there is no QT prolongation, or if at all it is there, that should be minimal. So QT interval is assessed for any new drug and it is supposed to be a billion dollar industry, measurement of QT interval for the large number of drugs being tried. QT prolongation leads to torsades des pointes, which is a very serious arrhythmia. And QT interval normally, that range also varies with the heart rate, so we have to correct for the heart rate. One easy value which I remember is 0.34 to 0.43, normal range. But there is a difference between male and female, and also between ages. So you have to use a nomogram to assess the QT interval. More than that, QT interval shortens with increasing heart rate. So you have to correct the QT interval. Usual formula which we use is the Bazett’s formula, which is measured QT interval, divided by the root of the RR interval in seconds, square root of RR interval in seconds. That is the Bazett’s formula. But this formula also has limitations. There is another formula known as Fridericia’s formula, in which QT interval is divided by the cube root of the RR interval in seconds. That also has other limitations. Several other logarithmic formulas are also there, formulas like Framingham formula, Hodges formula, they are all difficult to calculate. So practically at the bedside, we take only Bazett’s formula.

Finally, we come to two segments in the ECG. PR segment, which is also known as PQ segment, as it is often from P to Q. Then ST segment. These two are nearly isoelectric, but not truly isoelectric. ST segment is usually mildly elevated, with upward concavity and it will merge towards the ascending limb of T wave. Especially in the younger individuals, there could be mild ST elevation in anterior leads normally. So in anterior leads, for diagnosis of ST elevation myocardial infarction, V1, the cutoff is usually 2 mm, while 1 mm is enought in other leads. When there is ST depression, even 0.5 mm is enough, to consider abormal ST. That is usually with angina and ventricular strain patterns. PR segment elevation and depression can occur in atrial infarction and pericarditis. Those are the two conditions in which PR segment can be abnormal.