Question

On ECG, wich intervals do not change with increasing heart rate?

Answer 1

Ans: QT- Interval

Brief Explanation:

Heart rate is defined as the number of ventricular systoles per minute. Normal heart rate is about 60 - 90 beats per minute. A decreased heart rate is called bradycardia, an increased heart rate is tachycardia.

An ECG Contains below waves and intervals:

P wave: It is caused by atrial depolarization and its duration is upto 0.1 sec.

PQ (PR) interval: This is the time interval between the beginning of P wave to the onset of QRS complex. If QRS complex starts with Q wave it PQ interval, if QRS complex starts with R wave it PR interval. PQ interval represents the period from the beginning of atrial depolarization to the beginning of ventricular one, it is normally lasts for 0.12 – 0.2 sec. PQ (QR) interval is inversely proportional to the heart rate.

QRS complex: This is produced by the ventricular depolarization. QRS complex consists of three waves Q, R and S. As heart rate increases, QRS duration decreases.

Q- Produced by depolarization of the ventricular septum and papillary muscles.

R - Produced by free ventricular walls depolarization.

ST segment: Time interval from the end of ventricular depolarization to the beginning of ventricular repolarization.

T wave: The ventricular repolarization spreads from the atrioventricular groove through the cardiac apex to the ventricular septum to produce T wave on ECG.

U wave: U wave is a small positive wave sometimes visible after T wave.

Q-T interval: QT interval is defined as the time interval between the onset of QRS complex and the end of T wave. This interval comprises the depolarization and repolarization of ventricles (so-called an electric systole of ventricles). Many hypothesis states that the duration of QT interval is significantly influenced by changes in heart rate. The duration of QT interval is corrected for the heart rate and the corrected value is marked QTc. All the measurements are doing by using QTc value, but not the QT.

In practice, the Q-T interval is expressed as a "corrected Q-T (QTc)" by taking the Q-T interval and dividing it by the square root of the R-R interval (interval between ventricular depolarizations). This allows an assessment that the Q-T interval that is independent of heart rate. Normal corrected Q-Tc intervals are 0.44 seconds or less.

Recent studies proved that the QT interval is governed mainly by extrinsic factors but not intrinsically rate related. The physiological control of QT interval is being used now to construct a cardiac pacemaker.