ECG and interpretation

What is ECG and how the interpretation can be done .

Introduction

The electrocardiogram (ECG) is one of the simplest and oldest cardiac assessment of Heart condition With modern machines, surface ECGs are quick and easy to obtain at the bedside and are based on relatively simple electrophysiological concepts.

Basic principles

What is an ECG?

An ECG is simply a representation of the electrical activity of the heart muscle as it changes with time, usually printed on paper for easier analysis. Like other muscles, cardiac muscle contracts in response to electrical depolarisation of the muscle cells. It is the sum of this electrical activity, when amplified and recorded for just a few seconds that we know as an ECG.

The normal ECG

It will be clear from above that the first structure to be depolarised during normal sinus rhythm is the right atrium, closely followed by the left atrium. So the first electrical signal on a normal ECG originates from the atria and is known as the P wave. Although there is usually only one P wave in most leads of an ECG, the P wave is in fact the sum of the electrical signals from the two atria, which are usually superimposed.

There is then a short, physiological delay as the atrioventricular (AV) node slows the electrical depolarisation before it proceeds to the ventricles. This delay is responsible for the PR interval, a short period where no electrical activity is seen on the ECG, represented by a straight horizontal or ‘isoelectric’ line.

Depolarisation of the ventricles results in usually the largest part of the ECG signal (because of the greater muscle mass in the ventricles) and this is known as the QRS complex.

• The Q wave is the first initial downward or ‘negative’ deflection
• The R wave is then the next upward deflection (provided it crosses the isoelectric line and becomes ‘positive’)
• The S wave is then the next deflection downwards, provided it crosses the isoelectric line to become briefly negative before returning to the isoelectric baseline.

In the case of the ventricles, there is also an electrical signal reflecting repolarisation of the myocardium. This is shown as the ST segment and the T wave. The ST segment is normally isoelectric, and the T wave in most leads is an upright deflection of variable amplitude and duration 

Normal intervals

The recording of an ECG on standard paper allows the time taken for the various phases of electrical depolarisation to be measured, usually in milliseconds. There is a recognised normal range for such ‘intervals’:

• PR interval (measured from the beginning of the P wave to the first deflection of the QRS complex). Normal range 120 – 200 ms (3 – 5 small squares on ECG paper).
• QRS duration (measured from first deflection of QRS complex to end of QRS complex at isoelectric line). Normal range up to 120 ms (3 small squares on ECG paper).
• QT interval (measured from first deflection of QRS complex to end of T wave at isoelectric line). Normal range up to 440 ms (though varies with heart rate and may be slightly longer in females)

Heart rate estimation from the ECG

Standard ECG paper allows an approximate estimation of the heart rate (HR) from an ECG recording. Each second of time is represented by 250 mm (5 large squares) along the horizontal axis. So if the number of large squares between each QRS complex is:

• 5 - the HR is 60 beats per minute.
• 3 - the HR is 100 per minute.
• 2 - the HR is 150 per minute.A normal ECG is illustrated above. Note that the heart is beating in a regular sinus rhythm between 60 - 100 beats per minute (specifically 82 bpm). All the important intervals on this recording are within normal ranges.
  1.  P wave:
  • upright in leads I, aVF and V3 - V6
  • normal duration of less than or equal to 0.11 seconds
  • polarity is positive in leads I, II, aVF and V4 - V6; diphasic in leads V1 and V3; negative in aVR
  • shape is generally smooth, not notched or peaked
  2. PR interval:
  •  Normally between 0.12 and 0.20 seconds.
  3. QRS complex:
  • Duration less than or equal to 0.12 seconds, amplitude greater than 0.5 mV in at least one standard lead, and greater than 1.0 mV in at least one precordial lead. Upper limit of normal amplitude is 2.5 - 3.0 mV.
  • small septal Q waves in I, aVL, V5 and V6 (duration less than or equal to 0.04 seconds; amplitude less than 1/3 of the amplitude of the R wave in the same lead).
  • represented by a positive deflection with a large, upright R in leads I, II, V4 - V6 and a negative deflection with a large, deep S in aVR, V1 and V2
  • in general, proceeding from V1 to V6, the R waves get taller while the S waves get smaller. At V3 or V4, these waves are usually equal. This is called the transitional zone.
  4. ST segment:
  • isoelectric, slanting upwards to the T wave in the normal ECG
  • can be slightly elevated (up to 2.0 mm in some precordial leads)
  • never normally depressed greater than 0.5 mm in any lead
  • 
    

.5. T wave:

• T wave deflection should be in the same direction as the QRS complex in at least 5 of the 6 limb leads
• normally rounded and asymmetrical, with a more gradual ascent than descent
• should be upright in leads V2 - V6, inverted in aVR
• amplitude of at least 0.2 mV in leads V3 and V4 and at least 0.1 mV in leads V5 and V6
• isolated T wave inversion in an asymptomatic adult is generally a normal variant

6. QT interval:

•  Durations normally less than or equal to 0.40 seconds for males and 0.44 seconds for females.

Complete heart block

·         Complete heart block refers to a form of atrioventricular dissociation where no P wave produces a QRS complex. A sinus or ectopic atrial rhythm develops that fires independently of the ventricles. This rhythm may be junctional (as illustrated below) or ventricular in origin. The rhythm is usually regular, but may present irregularly as a result of intermittent premature ventricular beats. Patients presenting with complete heart block complain of symptoms resembling profound bradycardia (loss of atrial kick) and reduced cardiac output (syncope, angina, presyncope).