2.2 Systematic Interpretation of ECG Flashcards
Outline the step-by-step approach for interpreting an ECG.
I would also add Step 9: check the T-wave.
Step 1: What is the normal calibration for an ECG?
Standard Calibration
- X-axis: Paper speed is 25mm/second
25 mm = 1 sec, i.e. 1 little box is 0.04 seconds, or 40ms. 1 large box is 0.2 seconds.
- Y-axis: 10mm = 1mV
1 mm is 1 little red square (=0.1mV)
The ‘standard’ paper has 5 little boxes in each large box, i.e. 10 little boxes for 2 large boxes.
Half-Standard Calibration
- 5mm = 1mV
- 1mm = 0.2mV
Notice where each of the leads are located on an ECG print-out.
Frontal Plane
First column: I, II, III
- I: 0 degrees
- II: +60 degrees
- III: +120 degrees
Second column: aVR, aVL, aVF
- aVR: +210 degrees
- aVL: -30 degrees
- aVF: +90 degrees
Transverse Plane
Third column: V1-3
Fourth column: V4-6
Step 2: Determine rhythm of the heart. How do you determine if sinus rhythm is present?
Sinus rhythm is present if:
- P-waves are all present
- Each P-wave is followed by a QRS.
- Each QRS is preceded by a P-wave.
- P-waves are identical and upright in II and aVF.
P-waves are all Present/Identical/Upright
- Normal depolarisation of atria, not prolonged
Each P-wave is followed by QRS
- There’s no blockage of the conduction as it travels to the ventricles from the atria.
Step 3: Determine the rate.
The bottom strip of the ECG - made of complexes from III, aVF, V3 and V6 are a continuous succession of P-waves and can be used to calculate rate even though they are different leads.
Otherwise, you can also use Lead II.
4 QRSes in 12 boxes means 4 beats in 6 seconds, that is 20 beats in 1 minute?
Method 1: Count Big Boxes
- Count the number of big boxes between 2 QRS complexes and divide 300 by that number.
Method 2: 3 seconds x 20 Rule
- Count the number of QRS complexes in 3 seconds and multiply by 60.
Note - at a paper speed of 25mm/s, there would be 15 large boxes in 3 seconds.
Step 4: Calculate timing intervals.
There are 3 timing intervals you need to check:
- PR interval 0.12-0.2s - 3-5 little boxes
- QRS interval less than 0.1s - 2.5 little boxes
- Normal corrected QT interval 0.30-0.46s - 1.5 big boxes to 2 big boxes + 3 little boxes.
Remember, 1 little square is 0.04 s, 1 big square is 0.2s.
Step 5: Determine the axis using lead shortcuts.
Method 1: Leads I and II
Normal Axis (-30 to +90 degrees)
- QRS complex is upright in leads I and II
Method 2: Leads I and aVF
Normal Axis
- +ve in Lead I
- +ve in aVF
Left Axis Deviation
- +ve in Lead I
- -ve in aVF
Right Axis Deviation
- -ve in Lead I
- +ve in aVF
(Source: https://www.healio.com/cardiology/learn-the-heart/ecg-review/ecg-interpretation-tutorial/determining-axis)
If the deviation of the ECG does not fit any of these above, the axis is termed ‘indeterminate’.
Step 5: Determine the axis using the ‘Star Method’.
https://sites.google.com/site/electrocardiologyinstruction/home/the-star-method-of-axis-determination
Step 5: List the degrees of each type of axis deviation.
The normal QRS axis should be between -30 and +90 degrees. Left axis deviation is defined as the major QRS vector, falling between -30 and -90 degrees. Right axis deviation occurs with the QRS axis and is between +90 and +180 degrees. Indeterminate axis is between +/- 180 and -90 degrees. This is summarized in the image below.
Step 6: Check P-wave morphology and voltage.
Normal Characteristics of a P-wave
- < 0.12 s duration (3 little boxes)
- Upright in lead I and II and negative deflection less than 1 box wide and 1 box deep in V1
Note that changes to these probably indicate significant atrial deformity, and is prognostically important, as ECG is less sensitive than other methods, e.g. MRI.
Describe what ECG changes indicate L atrial enlargement.
Note that if negatively deflected P-wave in V1 is more than 1 box wide and 1 box deep, it means that there is left atrial enlargement.
Describe what ECG changes indicate R atrial enlargement.
If the P-wave in lead II is pointed and higher than 2mm, then that indicates right atrial enlargement.
Step 7: Check QRS morphology and voltage.
Features of a Normal R Wave
Normal R wave progression across praecordial leads V1-V6:
- Usually the R wave starts small in V1, then gets progressively bigger V1-3
- Around V3-V4, there is a dominant R wave with minimal S wave, usually the transition from negative R wave to positive R wave occurs between V3-V4
- Then the maximal R wave will occur somewhere between V4-V6
Small Q’s
- Q waves should be downward deflection of 1 little box. Should not be more than 1 little box wide - if so, think damage to the myocardium.
Voltage of QRS
- High voltage - Sokolov-Lyon criteria (S wave depth in V1 + tallest R wave height in V5-V6 > 35 mm)
- Low voltage
- The amplitudes of all the QRS complexes in the limb leads are < 5 mm; or,
- The amplitudes of all the QRS complexes in the precordial leads are < 10 mm
What is a physiological Q wave?
A normal Q wave is any negative deflection that precedes an R-wave.
It represents the normal depolarisation from left-to-right of the interventricular septum.
Small ‘septal’ Q waves are often seen in the L-sided leads (I, aVL, V5 and V6).
This is because the wave of depolarisation is moving away from the leads.
Small Q waves <2mm are normal in most leads, can sometimes be a bit larger in I and aVR.
Q waves are therefore not usually seen in R-sided leads.
https://litfl.com/q-wave-ecg-library/
What is a pathological Q wave?
Q waves are considered pathological if:
- > 40 ms (1 mm) wide
- > 2 mm deep
- > 25% of depth of QRS complex
Or, if they are seen in leads V1-3.
Pathological Q waves usually indicate current or prior myocardial infarction.
https://litfl.com/q-wave-ecg-library/
Step 8: Check ST segment for elevation or depression.
Part 1: Explain the common causes of ST segment depression.
Up to 0.5mm ST depression in limb leads is acceptable.
