Rhythm recognition ECG

 

In this section we will cover some basic physiology, how to systematically read a rhythm strip and some common cardiac rhythms. 

It does not cover how to interpret a 12 lead ECG, and does not cover all arrhythmias. Further management will be on review by a senior medical practitioner.

Basic electrophysiology:

The electrical system of the heart is the power source that causes contraction and relaxation of the atria and ventricles, pumping blood through the heart. In healthy individuals this electrical impulse originates from the sinoatrial (SA) node, a small bundle of specialised cells in the right atrium. At rest the cells are polarised, a shift of ions triggers depolarisation and creates an electrical signal that travels through the conduction system to the myocardial cells causing contraction. The diagram below shows the electrical pathway through the heart.

  • The electrical impulse starts in the SA node, the hearts natural pacemaker. Rate 60-100bpm. The electrical activity spreads rapidly through the walls of the atria (seen as p-wave on ECG) causing atrial contraction, forcing blood into the ventricles.

  • The signal reaches the AV node, a cluster of cells between the atria and ventricles, that acts like a gate to slow the electrical signal before it enters the ventricles. This gives the atria time to contract before the ventricles do. Seen on the ECG as the small isoelectric segment between the p-wave and QRS complex. The AV node can also act as a pacemaker if the SA node fails, with an intrinsic rate 40-60 bpm.

  • There is rapid conduction through the bundle of His and down the left and right bundle branches to the ventricular myocardium by the purkinje fibres. This causes co-ordinated contraction of the ventricles, forcing blood out to the lungs and body. Represented as QRS complex on ECG.

  • The t-wave represents recovery of the resting potential. The SA node then fires another impulse and the cycle begins again.

Each component of the conduction pathway has its own intrinsic rate, and can generate an impulse if needed. The further along the system the lower the heart rate. 

  • Bundle of His – 40 bpm

  • Purkinje fibres – 15 -40 bpm

This video explains about electrical activity through the heart.

Electrocardiogram (ECG) – Rhythm strips

The electrical activity through the heart can be recorded on an ECG. The rhythm strip is usually lead II on a 12 Lead ECG. Components of a normal ECG signal are shown below.

ECG paper in the UK is generally standard across all hospitals, its length represents time. 

One small square is equivalent to 0.04 seconds.

One large square is equivalent to 0.2 seconds.

  • First positive deflection represents atrial depolarization. It is a small wave as the atria does not generate a lot of force.

  • The time it takes the impulses to travel from the atria through the AV node. It is measured from the start of the P wave to the onset of the QRS complex and is usually no more than 5 small squares in duration (0.12-0.20 s)

  • This represents ventricular contraction. Measured from the onset of the Q to the end of the S wave.  Is generally no longer than 3 small squares (<0.12s)

  • Represents ventricular repolarization. Should follow the same direction as the main QRS complex.

  • 6 Stages to Analyse ECG Rhythm

    GO

  • Common Rhythm Strips

    GO

  • Management of Arrhythmias

    GO