Electrocardiogram (ECG or EKG) records the electrical activity of the heart. Performing an ECG and interpreting it can be challenging, but it can be used to detect Acute coronary syndrome or arrhythmias. In this guide, we teach how to interpret the ECG, made easy.
Physiology of the ECG recording
What Causes What
- P wave is caused by the sequential depolarisation of the left and right atria.
- QRS complex: Simultaneous depolarisation of right and left ventricle (big spike).
- ST segment and T wave is the repolarisation of the ventricles (returning to iso electric line).
- Lub Dub: Lub is the sound of the atrioventricular valves snapping shut and the dub is the sound of the semi lunar valves snapping shut.
A lead is the angle at which the heart is viewed. The Earth lead is always on the right hip or leg. Not it does not matter how proximal or distal the lead is attached but do it uniformly (i.e. have all of them on the shoulder or hip or all on the limbs etc).
- Lead I: Positive electrode on left wrist or shoulder, negative on right wrist or shoulder
- Lead II: Left leg is positive, right arm is negative
- Lead III: Left leg is positive, left arm is negative
- aVR: Right arm positive, left leg and left arm are negative
- aVL: Left arm positive, left leg and right arm are negative
- aVF: Left leg positive, left arm and right arm negative
View for leads is from positive looking toward negative so view point is the positive electrode.
Augmented limb lead looks in between two negatives, taking an average.
Wave of depolarision towards view point causes upward deflection, away from causes downward. Essentially positive to positive goes up, positive from positive goes down. Opposite for repolarisation. Height of deflection is measurement of strength of depolarisation and is influence by if it is moving directly towards view point, lead II is biggest as the angle of it is the same as the angle of the heart.
Individual wave causes
- SAN is too small to be looked at on an ECG.
- Atrial then get depolarised which moves towards viewpoint on lead II and gives the p wave.
- AVN is too small to be detected but it's delay causes the P-R interval when the ECG is at iso-electric line
- Left and right bundle of his, left is faster so charge moves from right to left down the septum, this is away from the viewpoint, kind of, and quite small so gives the small q wave.
- Ventricular depolarisation causes r wave, big upwards deflection.
- Last part of ventricles to depolarise is at the top of the ventricles(fibre annular cap) and as left predominates over right you get a slight shift from left to right which produces the S wave, a small downwards deflection.
- Either due to it being QRS complex or due or the fact it is a small bit of tissue whose cells repolarise at different times you don't see atrial repolarisation.
- Outer ventricular muscles start to repolarise first so you get a wave of repolarisation moving away from the view point creating the upwards deflection of the t wave.
R waves are close together on inspiration compared to expiration, this is sinus arrhythmia and totally normal. This is due to the increased pressure from the thorax sucking in more blood and causing heart to beat faster. Vagus nerve decreases heart rate and constantly fires on the heart to reduce bpm. Vagus nerve activity also reduced as it is compressed during sinus arrhythmia.
Performing an ECG
12 lead ECG electrode placement:
- Introductions, consent and handwashing
- Expose patients chest
- Clean with alcohol if necessary
- Shave patient if necessary to ensure good contact
- Limb Leads:
- R (red) - right arm electrode (the ulnar styloid or arcromioclavicular joint are good sites)
- L (yellow) - left arm electrode (the ulnar styloid or arcromioclavicular joint are good sites)
- N (black) - neutral electrode - right leg (the malleoli or ASIS are good sites)
- F (green) - foot electrode - left leg (the malleoli or ASIS are good sites)
- Chest leads
- V1 - 4th intercostal space, right side of sternum
- V2 - 4th intercostal space, left side of sternum
- V3 - Between V2 and V4
- V4 - 5th intercostal space, midclavicular line
- V5 - horizontally in line with V4, anterior axillary line
- V6 - horzontally in line with V4 and V5, midaxillary line
- Are cables correctly connected?
- Is the equipment functioning correctly?
- Is there any external electrical interference?
- Was skin prepared adequately?
- Are the pads correctly adhered? Are they dried out?
- Is the patient moving, twitching or shivering?
How to read an ECG
Divide 300 (the number of large squares representing a minute) by the number of squares between 2 R waves. If patient has AF or type 2/3 heart block, count the number of QRS in 5/10 seconds and multiply up appropriately.
- Tachycardia (>100bpm)
- Bradycardia (<60bpm)
- Regularly irregular
- Irregularly irregular
- Atrial fibrillation
- Left axial deviation - leads I & II "leaving" eachother (lead I R wave points up, lead II R wave points down)
- Right axial deviation - leads I & II "returning" to eachother (lead I R wave points down, lead II R waves points up)
- PR interval- Beginning of P to start QRS
- Does each P wave precede a QRS complex?
- QRS Axis (mean of electrical signals produced). This is calculated by adding the sum total of all +ve deflections in I and aVF, and then working out their vector (i.e. draw a graph... I is the x axis, and aVF is the y axis going down). Angle of the resultant vector is heart axis. -30 to 90 degrees is acceptable, less than -30 is left deviation and more than 90 is right deviation.
- Amplitude of Q Wave
- Depressed (ischaemia, digoxin)
- Elevated (infarct)
- Peaked (hyperkalaemia)
- Inverted (infarct)
Intervals: P-R & Q-T
Identifying type of Heart Block; a poem:
- If the R is far from P, then you have first degree
- Longer, longer, longer, drop; then you have a Wenchebach (second degree)
- If some P's just don't get through, then you have a Mobitz II
- If Ps and Qs just don't agree, then you have a Third Degree