Role of ECG in understanding the heart rhythm

  • Cardiac cycle is the sequence of events that occurs in one complete heart beat.
  • The pumping phase of the cycle, also known as systole, occurs when heart muscle contracts. The filling phase, which is known as diastole, occurs when heart muscle relaxes.
  • The heart has an inbuilt rhythm of contraction and relaxation. A small group of heart muscle cells called the pacemaker help achieve this.
  • The electrical changes that spread through the heart can be detected at the surface of the body by an instrument called the electrocardiograph.
  • The body is an excellent conductor of electricity because tissue fluid contains a high concentration of ions that move in response to electrical impulses generated throughout the body.
  • Electrical impulses generated by the heart's intrinsic conduction system are thus conducted to the body surface, where they can be detected by surface electrodes placed on the skin.
  • By placing electrodes over the chest an observer can gain a complete picture of the heart's electrical events, which are monitored by an electrocardiograph and recorded as an electrocardiogram (ECG or EKG).
  • The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which is symbolized by the P wave on an electrocardiogram.
  • The SA node sends depolarization wave to the AV node which with a delay of about 100 milliseconds (to allow atria finish their contraction) causes contraction in both ventricles (seen in the QRS wave). At the same time, the atria are repolarized and relaxed.
  • P wave is of atrial origin, hence called ‘atrial complex’. Q, R, S, T being of ventricular origin, are collectively known as ‘ventricular complex’.
  • The ventricles are repolarized and relaxed symbolized by the T wave.

ECG Interpretation

ECG Interpretation in detail

P wave

  • A small upward wave indicating atrial depolarization (atrial depolarization is followed by atrial contraction); occurs when the electrical impulse spreads over the atria
  • Its average duration is 0.1 seconds; same as atrial systole
  • Normal P wave indicates-
    • Impulse is originating at the SA node
    • Impulse spreads over the atria in the usual direction
    • There is no defect of conduction
    • Strength of contraction, mass of atrial musculature and its nutrition is normal
  • If the P wave is inverted it indicates that the SA node fails to initiate impulse and the atrial muscle depolarizes by the impulse originating in AV node
  • In case of atrial fibrillation, the P wave is absent
  • In case of atrial hypertrophy, the P wave may be large and notched
  • In case of Nodal rhythm, the P wave is reversed

Q wave

  • This wave is produced when the impulse arrives at the muscular part of the septum and the septum contracts
  • Q wave is absent when there is a congenital defect of the septum

R and S wave

  • Each of these waves ‘R’ and ‘S’ is produced during ventricular depolarization on either sides
  • The shape, size and duration of RS is altered during ventricular abnormality

T wave

  • This wave indicates contraction of the basal part of the ventricles
  • It is the repolarization wave of the ventricles (ventricular repolarization is followed by ventricular relaxation)
  • T wave gets flattened with age
  • Exercise increases T wave amplitude in a healthy heart
  • Abnormalities in the size, shape, direction and duration of T wave indicate myocardial damage and cardiac hypoxia

U wave

  • This wave occurs due to the slow repolarization of intraventricular conducting system

QRS complex/ QRS interval

  • Downward deflection, then a large upward peak, ending as a downward deflection
  • ‘QRS complex’ represents ventricular depolarization which is followed by ventricular contraction
  • ‘QRS interval’ measures the total ventricular depolarization time
  • In a normal ECG tracing, atrial repolarization (followed by atrial relaxation) is hidden by the QRS complex

PP interval

  • This is the interval between two successive P waves
  • Equal intervals in next successive stages indicate rhythmical depolarization of the atrium

PR interval

  • Interval from onset of P wave to that of QRS complex, indicates the impulse conduction from AV node to ventricles
  • Measures the conduction time of impulse from SA node to ventricles
  • Variable PR interval in successive stages indicates dissociated impulse transmission between the atria and ventricles
  • Atrioventricular dissociation indicates that the ventricles are abnormally contracting without the influence of the SA node

RR interval

  • This is the interval between two successive R waves
  • If the RR interval in next successive stages are same then it indicates that the ventricles are depolarizing rhythmically

QT interval

  • measures ventricular total systolic time

RS-T segment

  • Elevation or sagging of this segment indicates myocardial damage/ hypoxia

TP interval

  • This interval measures the diastolic period of the heart
  • Alteration of this interval indicates a disturbance in heart rate
  • If the TP interval in successive stages are variable then it indicates atrioventricular dissociation