ECG- A how to guide

An electrocardiogram (or ECG) is a machine used to measure the movement of electrical activity of the heart.

Schematic diagram of normal sinus rhythm for a...
Schematic diagram of normal sinus rhythm for a human heart as seen on ECG (with English labels). (Photo credit: Wikipedia)

The P wave represents the depolarisation of the atria.

The QRS complex represents the depolarisation of the ventricles.The T wave represents the repolarisation of the ventricles (note that repolarisation of the atria is masked by the QRS complex).The image shows a typical appearance of the ‘rate lead’ ecg (or lead II).  The rate lead is printed along the bottom of the ecg and is the first part you should look to interpret.

Reading the rate lead

There are a few things you should look at first.

  • P wave
    • Is there one?  Is it in the correct direction?
  • The P-R interval
    • It should be 0.12-0.2s (between 3-5 little boxes)
  • The QRS complex
    • Is there one?  Is it less than 0.12s (3 small squares)? (Broad QRS complexes are usually an indicator of problems of AV conduction)
    • Does the QRS complex always follow a P wave?  Are the QRS complexes and P waves in sync with, or completely independent of, each other?
  • The T wave
    • Is there one? Is it in the correct direction (inverted t-waves may be a sign of ischaemia: acute or chronic)?  Are they normal looking (hyperacute, large t-waves can be seen in hyperkalaemia)
  • The QT interval
    • There are multiple complicated formulae for calculating the corrected QT interval, but as a general rule of thumb, the QT interval should be less than half of the previous R-R interval.  Prolonged QT can be hereditary, but can also be a sign of electrolyte insufficiencies and can be a risk for arrhythmias (notably Torsades de Pointes).
    • Is there any elevation of the ST-segment (often an indication of MI)?
  • The rate
    • Each large box covers 0.2 seconds and each small box covers 0.04 seconds.
    • The typical rate lead is 30 large boxes long so you can either find the rate by:
      1. Counting all the QRS complexes and multiplying by 10 (0.2 x 30 x 10 = 60 seconds) OR
      2. Counting the number of large boxes between each QRS complex and dividing 300 by it (300 x 0.2 = 60 seconds)
        1. NB Process 1 is more accurate if heart rate is irregular, whereas process 2 is better if the heart is regular sinus rhythm
    • Normal is between 60 and 100 bpm.  (Tachycardia is faster; bradycardia slower)
  • Rhythm
    • Are the QRS complexes regular? Are there any skipped beats?

The 12-lead ECG

English: Diagram showing the connection of ECG...
English: Diagram showing the connection of ECG leads (Photo credit: Wikipedia)

Where do the wires go?

  • V1– 4th intercostal (IC) space on the right sternal edge
  • V2– 4th IC space, left sternal edge
  • V3- (half way between V2 and V4- on the 5th rib)
  • V4– 5th IC space, mid-clavicular line (apex)
  • V5– 5th IC space, anterior axillary line
  • V6– 5th IC space, mid-axillary line
  • RA– right arm (right- red)
  • RL- right leg (black)
  • LA- left arm (yellow- lemon left)
  • LL– left leg (green- spleen)

The concept of leads

Note that the electrodes (V1, RA, LL etc…) are not the same as ECG ‘leads’.  ECG leads are the tracings recorded measuring the change in voltage between the electrodes.

So, in essence, the 6 limb leads shown in the image above measure the direction of electrical flow in the coronal plane.  If the current moves in the direction of a lead, the ECG will hyperpolarise.  Therefore, it is not surprising that lead II is often used as the rate lead, since the normal direction of electrical activity is from the atria to ventricles (around 60° in the plane).

The precordial leads (V1-V6) measure electrical activity in the axial/horizontal plane (see image below).  Note that each electrode has a corresponding lead, unlike the limb leads.  This is because the precordial leads measure the electrical flow between the electrode and the average of all the limb electrodes (this gives an ‘average background activity’ which can be measured agaist).

I.e. V1 and V2 measure activity of the right ventricle, V3 looks at the septum and V4-6 look at the left ventricle.  These are important because the lead in which the transition point (the point where R and S waves are roughly equal) is found marks the location of the septum.  E.g. A shift from V3 to V5 could indicate right ventricular hypertrophy, shifting the septum to the left.  Used in combination, all 12 leads can give a good indication on how well different areas of the heart are conducting electrical activity.

The concept of the Mean Electrical axis

The mean electrical axis is the sum of all the electrical vectors (i.e. sum of the amount and direction of electrical activity) that is occurring in the heart.  In practice, this can be determined by locating the lead that has the lowest activity (or where R is closest to S in the QRS complex).  Note what appears to be a small peak/trough on an ECG may be the result of biphasic activity i.e. the electrical current is moving perpendicular to the lead.  Using this information, we can calculate the axis perpendicular to this to find the mean electrical axis.

Normally, the limb lead with biphasic activity is aVL (-30°).  Perpendicular to this (+/- 90°) is lead II (60°).  If there is a change in the size of the heart, the electrical axis can shift e.g. in right ventricular hypertrophy more current passes through the enlarged right ventricle: activity will be away from lead I (negative deflection) and more towards lead III (rather than II), and the mean electrical axis will shift towards lead III and aVF (90-120°) (right axis deviation).  (see ECG posts for more examples)



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