29-09-22 - Interactive interpretation of the ECG Flashcards
Learning outcomes
- To be able to interpret the ECG
Describe the aspects of an ECG trace?
What are the 5 different waves of the ECG trace?
Where do we measure waves from?
Describe the 3 different important timing intervals and how long they each last for?
- 5 different waves of the electrocardiogram:
1) P wave – atrial depolarisation
2) Q wave – depolarisation in the septum
3) R wave – Depolarisation of most of the mass of the ventricles
4) S wave – final depolarisation of the ventricles
5) T wave – ventricular repolarisation - We measure waves from the isoelectric line, which is time at which there is no electrical activity in the heart
- 3 different important timing intervals:
1) P-R interval – (0.12-0.2s)
2) QRS complex width (0.06-0.12s)
3) Q-T interval (0.25-0.35s)
What does the electrocardiogram (ECG) measure?
How does it measure this?
- The electrocardiogram (ECG) measures the electrical activity of the heart over time
- The 12 leads of the ECG represent 12 electrical views of the heart from 12 different angles
- The ECG uses 10 electrodes (4 for the limbs, 6 for the chest)
- Leads are not electrodes
- A lead is a certain perspective or view of the heart function
- An electrode is the physical tool used to provides these views, which is why 10 electrodes can provide 12 views
1) 4 electrodes on the limbs and 6 limb leads:
* The 6 limb leads look at the heart in a vertical plane, and are obtained through 3 electrodes
* 1 electrode is an earth, used to remove background noise (placed on right leg)
* The other 3 electrodes are placed on the left leg, right arm, and left arm
* 3 electrodes are used to create virtual leads between each pair of electrodes
* The 3 limb Leeds between the electrodes on the limbs form the Einthoven triangle
* The limb leads (I, II, and III) measure the sum of the electrical activity of the heart and the direction that electrical activity is moving in
* One end of each lead is designated positive and negative, meaning these 3 limbs I, II, and III are bipolar
* Lead I represent voltage between the right arm (negative pole) to the left arm (positive pole), thus looks at the heart from the left
* Lead II represent voltage between the right arm (negative pole) to the left leg (positive pole), thus giving an inferior left view
* Lead III measures voltage between the left arm (negative pole) and the left leg (positive pole), looking at the heart from an inferior right angle
2) There are also 3 augments limb leads that are unipolar (aVR, aVL, aVF)
* These use 1 limb electrode as a positive pole, and take the average of inputs from the other 2 as the zero reference
* aVR looks at the upper right side of the heart
* aVL looks at the upper left side of the heart
* aVF looks at eh inferior wall of the heart
3) 6 electrodes across the chest (V1 – V6) to give more specific localised information about areas of the heart
* The chest electrodes form Precordial leads, which allow the heart to be viewed in a horizontal plain
* These are unipolar leads, with the corresponding chest electrodes serving as the positive poles and the reference negative value being the same for all chest leads
* This reference negative value can be calculated as the average of inputs from the 3 limb electrodes
4) In the cases of all leads:
* Depolarisation moving towards the positive causes the trace to go up (positive deflection)
* Depolarisation moving away from the positive causes the trace to go down (negative deflection)
* The reverse is true for repolarisation
What do normal ECG traces look like?
- Normal ECG traces:
What are the 12 steps of the systemic approach for analysing an ECG?
- Systemic approach for analysing an ECG:
1) Identity, Standardisation
2) Rate
3) Rhythm
4) P Wave
5) P–R Interval
6) QRS Complex
7) QT Interval
8) ST Segment
9) T Wave
10) Axis
11) Other Abnormal Components
12) Formulate an Interpretation
What do we do during Identity?
What 2 things do we conform during standardisation?
What do big and small boxes represent in terms of time, length, and, voltage?
- Identity:
- Confirm patients name, age, and ECG date (should all be written on ECG)
- Standardisation:
- Make sure that 1cm = 1mV and paper speed is 25mm/seconds
- 1 small square is 0.04S (40ms), 1mm (0.1cm), 0.1mV
- 1 big square is 0.2s (200ms), 5mm (0.5cm), 0.5mV
What do we do during Rate (bpm)?
How long is 1 small square?
How long is 1 big square?
How can we calculate rate?
What lead do we use to calculate rate?
- Rate (bpm):
- 1 small square is 0.04S (40ms)
- 1 big square is 0.2s (200ms)
- Rate calculation:
- 300 divide by the number of big squares per R – R interval.
- Calculate this in the second lead (II)
- Normal rate 60-100 b.p.m
- Bradycardia <60
- Tachycardia >100
What do we assess when looking at rhythm?
What is sinus rhythm?
How 4 things can we check when looking for sinus rhythm?
- Rhythm:
- What we assess when looking at rhythm:
1) Are the P Waves present? (<0.25mV and upright in II, III, and AVF)
2) Are the QRS complex narrow – normal (<0.12s/120ms) or wide (>0.12s/120ms)
3) Relationship between the P waves and QRS complexes (one P wave followed by one QRS complex)
4) Is the rhythm regular or irregular? - Sinus rhythm is the name given to the normal rhythm of the heart where electrical stimuli are initiated in the SA node, and are then conducted through the AV node and bundle of His, bundle branches and Purkinje fibres.
- Checking for sinus (regular) rhythm
1) Normal P waves
2) Normal QRS complexes
3) One P wave followed by one QRS wave
4) Mark the position of 3 successive R waves, and slide the mark forward and check if the intervals are equal
What is an arrhythmia?
What are 4 heart conditions associated with rhythm?
How does their trace appear differently on an ECG?
What are the only 2 shockable arrhythmias?
- An arrhythmia is an abnormality of the heart’s rhythm
- 4 heart conditions associated with rhythm:
1) Atrial fibrillation
* No discernible P Waves
* Either a lot of P waves, or just not visible
* Irregular QRS complexes
* Irregularly irregular
2) Atrial flutter (4:1)
* P waves seen at a rate of 300 per minute, giving a saw-toothed appearance
* 4 P waves per QRS complex
* Ventricular activation is perfectly regular at 75 per minute
3) Junctional (Nodal) tachycardia
* Normal QRS complex, but absent P waves
* P waves can be present but inverted in leads II, III, and aVF (inferior leads), and upright in V1 and aVR
* Produces heart rate of >100
* No P waves, so atria are not contracting
4) Ventricular tachycardia
* After 2 sinus beats, the rate increases to 150bpm
* QRS complexes become broad
* T waves difficult to identify
* Final beat on this ECG diagram shows a return to sinus rhythm
- The only 2 shockable arrhythmias are Ventricular tachycardia and Ventricular Fibrillation
What 3 things are we looking for when analysing P wave on ECGs?
What 2 conditions can absent P waves represent?
- P waves:
- Analysing P waves on ECGs:
1) P waves Is <0.25mV
2) Upright in II, III, and AVF
3) Normally precedes QRS complex - Absent P waves can represent:
1) Atrial fibrillation
2) Nodal (junctional) rhythm
What conditions are abnormal P waves associated with?
- Conditions abnormal P waves are associated with:
1) Left atrial hypertrophy (P mitrale)
* Bifid P wave (M shape)
2) Right atrial hypertrophy (P-pulmonale)
* Peaked P wave bigger than 2.5 small boxes
What is the P-R interval?
How is it measured?
What is the normal range for P-R interval?
What can a prolonged P-R interval indicate?
- P-R interval
- The P-R interval is the time between atrial and ventricular depolarisation
- We measure P-R interval from the beginning of P to the beginning of Q wave
- Normal range for P-R interval
- Normal range for P-R interval is 0.12-0.2s (120-200ms)
- Prolonged >0.2s (200ms) implies delayed AV conduction from SA node to AV node, which can be a sign of heart block
What is occurring during the QRS complex?
How do we measure the QRS complex?
What is the normal duration for a QR complex?
What is a normal Q wave size?
- QRS complex
- Ventricular depolarisation is occurring during the QRS complex
- To measure QRS complex, we measure from the beginning of Q to the end of S wave
- Normal QRS complex duration is <0.12ms (3 boxes)
- Normal Q wave size is <0.04s (1 small box) and <2mm in depth (2 small boxes)
What conditions can abnormal QRS complex in terms of length (3) and voltage (4) indicate?
- Conditions associated with abnormal QRS complex length and voltage
- If the QRS complex is >0.12s, this can indicate:
1) Ventricular conduction defects
2) Bundle branch block
3) Left and right bundle branch block - If the voltage of the QRS complex is low (<5mm/0.5mV), this can indicate:
1) Hypothyroidism
2) COAD – chronic obstructive airways disease
3) Myocarditis
4) Pericarditis and pericardial effusion (build-up of fluid in pericardium)
How are the QRS complexes affected in left and right ventricular hypertrophy?
- How the QRS complexes are affected in left and right ventricular hypertrophy:
1) Left ventricular hypertrophy
* R wave in V5 is >25mm (2.5 big boxes) or
* Sum of the S wave in V1 and R wave in V5 or V6 >35mm (3.5 big boxes)
* This sum is referred to as the Sokolow-Lyon index
2) Right ventricular hypertrophy
* Dominant R wave in V1
* T wave inversion in V1 – V3 or V4
* Deep S wave in V6
What length and depth would make a Q wave significant?
When can a significant Q wave be present?
When might it be present in Lead III?
- Significant Q wave
- A significant Q wave’s length would be >0.04s
- A significant Q wave’s depth would be >2mm (2 small boxes)
- A significant Q wave may be present a couple hours/days after MI and can stay forever
- If a significant Q wave is present in lead III, we should consider pulmonary embolism
Where do we measure QT interval from?
Why do we need to have a corrected value for the QT interval?
What is the formula for this?
What is a normal value for QT?
- QT interval
- QT interval is measured from start of the QRS complex to the end of the T wave
- Since QT interval varies with rate, we need to have a corrected QT interval (QTc)
- QTc = QT / square route of RR (RR interval), which can be calculated on the ECG
- A normal value for QT is 0.38-0.42 seconds
- A prolonged QT interval can be an indication of the following conditions:
1) Acute Myocardial Ischaemia
2) Myocarditis
3) Bradycardia
4) Head Injury
5) Hypothermia
6) U&E (electrolyte) Imbalance (decreased K+ Ca2+ Mg2+)
7) Congenital
8) Drugs (Quinidine, Antihistamines, Macrolides, Amiodarone, Phenothiazines)
What is occurring during the T wave?
How does it normally present?
How can it present abnormally to indicated ischaemia/infarction?
How is the T wave affected by Digoxin?
- T wave
- The T wave is ventricular repolarisation
- The T wave is normally inverted in aVR and V1 (and V2 in young)
- The T wave can present inverted in I, II, and V4 – V6 to indicate ischaemia/infarction
- Digoxin effects on T wave:
1) T wave inversion
2) ST segment sloping depression
What is the axis?
What does a normal axis look like on an ECG?
What does a left axis deviation look like on an ECG?
What conditions is it associated with?
What does a right axis deviation look like on an ECG?
What conditions is it associated with?
- Axis
- The axis is the sum of all the ventricular forces during ventricular depolarisation
1) Normal axis
* Between -30° and +90°
* Predominantly upwards deflections in leads I, II, III
2) Left axis deviation
* Between -30° and -90°
* Negative QRS deflections in leads II and III
* Positive QRS deflections in I
* Associated with Left ventricular hypertrophy and MI
3) Right axis deviation
* Negative QRS deflections in lead I
* Positive QRS deflections in lead II and III
* Associated with RV hypertrophy, pulmonary embolism (PE). and MI
How does acute MI affect an ECG over time (6 stages)?
What permanent changes can MI cause to ECGs?
- During an acute MI, the ECG evolves through stages:
1) Normal
2) Acute - ST elevation
3) Hours – decreased R wave and new Q wave
4) Day 1-2 T wave inversion and Q wave deeper
5) Days later – ST normalises and T wave goes back to normal
6) Weeks later – ST and T normal, Q wave persists - If the patient survives, there is a bigger (deeper) Q wave and an inverted (negative) T wave
- These are from scars of MI
- The T wave may go back to normal, but the Q wave will stay
How can anterior, anterolateral, lateral, inferior, and posterior infarct be localised on an ECG?
- How anterior, anterolateral, lateral, inferior, and posterior infarct can be localised on an ECG:
1) Anterior infarction
* Any of the precordial leads (V1 – V6)
* Presents with sinus rhythm
* Q waves in leads V2 – V4 are larger (deeper)
* Inverted T waves in leads V4 – V6
2) Anterolateral infarction
* Sinus rhythm
* Q waves in leads I, II, AVL, V3 – V5 are deeper
* Raised ST segments in leads V2 – V6
3) Lateral infarctions
* Leads I, AVL, V5 and V6
4) Inferior infarction
* Sinus rhythm
* Q Waves in leads III and AVF
* Depressed (ischaemic) ST segments in leads AVL and V6
5) Posterior infarction
* Reciprocal changes in lead V1 (ST-segment depression, tall R wave)
Locations of MI diagram
Locations of MI diagram
What 3 ways does Pulmonary Embolism change ECG readings?
How do small PE alter ECG readings?
- 3 ways does Pulmonary Embolism change ECG readings:
1) Large S wave in lead I (S1)
2) Deep Q wave in lead III (Q3)
3) Inverted T wave in lead III (T3) - Small pulmonary emboli won’t show changes in ECG
Example ECG
1) How old is the patient?
2) What is the rate on the ECG? (bmp)
3) What is the rhythm on the ECG?
4) Is the ECG P wave normal?
5) Duration of P-R interval on the ECG is?
6) Duration of the QRS complex on the ECG is?
7) Is there any evidence of abnormal Q waves on the ECG?
8) Does this ECG show signs of ventricular hypertrophy?
9) The duration of QT interval on this ECG is?
10) Is the ST segment is isoelectric?
11) Where is the T wave inverted on this ECG?
12) What is the axis like on this ECG?
13) What conditions did this ECG show signs of?
1) How old is the patient?
* 21
2) What is the rate on the ECG? (bmp)
* 50-60bpm, as the R-R intervals are not equal
* Can take an average
3) What is the rhythm on the ECG?
* Irregular, as the R waves are not in line with each other
4) Is the ECG P wave normal?
* Yes, looking at the 2nd lead
5) Duration of P-R interval on the ECG is?
* About 0.14s (3.5 boxes)
6) Duration of the QRS complex on the ECG is?
* 0.1s – 2.5 boxes
7) Is there any evidence of abnormal Q waves on the ECG?
* No
* It is Ok if the Q wave is bigger in 1 or 2 leads
8) Does this ECG show signs of ventricular hypertrophy?
* Yes, as R wave in 5th lead is >25mm
9) The duration of QT interval on this ECG is?
* Mostly 0.38s
10) Is the ST segment is isoelectric?
* No
11) Where is the T wave inverted on this ECG?
* T wave is inverted in aVR lead
12) What is the axis like on this ECG?
* Normal axis
13) What conditions did this ECG show signs of?
* Signs of:
* Irregular sinus rhythm – synonym for sinus arrythmia
* Sinus arrhythmia
* Sinus bradycardia (rate below 60bpm)
* Normal ECG – normal for young person
* Ventricular hypertrophy
