CHAPTER 4 : BASIC WAVES , HEART RATE

QRS WIDTH (INTERVAL)
The QRS width, or interval, represents the time required for a stimulus to spread through the ventricles (ventricular depolarization) and is normally
0.1 second or less

ST SEGMENT
The ST segment is that portion of the ECG cycle from the end of the QRS complex to the beginning of the T wave . It represents the beginning of ventricular repolarization. The normal ST segment is usually isoelectric, but it may be slightly elevated or depressed normally (usually by less than 1 mm). Some pathologic conditions such as myocardial infarction (MI) produce characteristic abnormal deviations of the ST segment.The very beginning of the ST segment (actually the junction between the end of the QRS complex and the beginning of the ST segment) is sometimes called the J point.


T WAVE
The T wave represents part of ventricular repolarization. A normal T wave has an asymmetric shape; that is, its peak is closer to the end of the wave than to the beginning. When the T wave is positive, it normally rises slowly and
then abruptly returns to the baseline. * When it is negative, it descends slowly and abruptly rises to the baseline. The asymmetry of the normal T wave contrasts with the symmetry of T waves in certain abnormal conditions such a MI and a high serum potassium level

QT INTERVAL
The QT interval is measured from the beginning of the QRS complex to the end of the T wave . It primarily represents the return of stimulated ventricles to their resting state (ventricular repolarization). The normal values
for the QT interval depend on the heart rate. As the heart rate increases (RR interval shortens), the QT normally shortens; as the heart rate decreases (RR interval lengthens), the QT interval lengthens.
The QT interval should be measured in the ECG leads that show the largest-amplitude T waves

There is no simple rule for calculating the normal limits of the QT interval.
Because of this problem, another index of the QT has been devised. It is the rate-corrected QT or QTc . The rate-corrected QT is obtained by dividing the actual QT by the square root of the RR interval (both measured in seconds):

QTc = QT / squre root RR

Normally the QTc is less than or equal to 0.44 second.

This is altered in many conditions which will be cliniching points in identifying the underlying condition




U WAVE
The U wave is a small, rounded deflection sometimes seen after the T wave. Functionally, U waves represent the last phase of ventricular repolarization. Prominent U waves are characteristic of hypokalemia

CALCULATION OF HEART RATE
Two simple methods can be used to measure the heart rate (number of heartbeats per minute) from the ECG:

1.The easier way, when the heart rate is regular, is to count the number of large (0.2-sec) boxes between two successive QRS complexes and divide a constant (300) by this. (The number of large time boxes is divided into 300 because 300 × 0.2 = 60 and the heart rate is being calculated in beats per minute or 60 seconds.)

When the heart rate must be measured very accurately from the ECG, you can modify the approach as follows: Count the number of small (0.04 sec) boxes between successive R waves and divide a constant (1500) by this number

2.If the heart rate is irregular, the first method will not be accurate because the intervals between QRS complexes vary from beat to beat. In such cases you can determine an average rate simply by counting the number of cardiac cycles every 6 seconds and multiplying this number by 10 . (A cardiac cycle is the interval between two successive R waves.) Counting the number of cardiac cycles every 6 seconds can be easily done because the top of the ECG paper is generally scored with vertical marks every 3 seconds. SEE BELOW

SELF ASSESSMENT :

1) CALCULATE QTc ?




2) CALCULATE HEART RATE IN THE FOLLOWING ?

A)


B)

3) Name the component waves of the QRS complexes shown below.

Hey doing a good job. Making things easier. Thanks Ranjith