ECG Flashcards
Describe the standard calibration settings of an 12 lead ECG
X - axis is time: 25 mm = 1 second
- each small squae is 40 ms
- each large square is 0.2 s
Y-axis is voltage: 10 mm = 1 mV
How can a deep breath change the ECG
It can change the orientation of the T wave in leads III and aVF but has little effect in the rest of the ECG
What usually implies incorrect lead placement rather than pathology
North West axis
What is the normal PR interval
0.12 to 0.2 seconds
What is the normal duration of the QRS complex
0.06 to 0.1 seconds
What is the normal QT interval
< 0.44 seconds
Describe the electrical vector measured by each of the vertical leads
Lead 1: 0 degrees Lead 2: + 60 degrees Lead 3: + 120 degrees aVF: + 90 degrees aVL: - 30 degrees aVR: - 150 degrees
Which index is the most commonly used for assessment of Left Ventricular Hypertrophy
Sokolow-Lyon index
- S wave voltage in V1 plus R wave voltage in V5 or V6, whichever is the larger, greater than or equal to 35 mm.
Which is a more common cause of Left Axis Deviation: Left ventricular hypertrophy or left anterior hemiblock?
Left anterior hemiblock
What is left anterior hemiblock and state the ECG criteria
= Left anterior fascicular block (LAFB)
Electrical impulses are conducted to the left ventricle via the left posterior fascicle, which inserts into the left infero-septal wall of the left ventricle along its endocardial surface.
Initial electrical vector –> down and right followed by up and left to the left ventricle
Therefore: delay !
ECG CRITERIA:
QRS > 110ms
Prolonged R wave peak time >45ms in aVL
Increased QRS voltage in the limb leads
Left axis deviation (-45 to -90 degrees)
qR in I and aVL
rS in II, III, aVF
In LAFB, the QRS voltage in lead aVL may meet voltage criteria for LVH (R wave height > 11 mm), but there will be no LV strain pattern.
What is Left Ventricular Strain pattern
ST segment depression and T wave inversion in the left-sided leads
What are the criteria for diagnosing LVH
Voltage criteria must be accompanied by non-voltage criteria to be considered diagnostic of LVH.
VOLTAGE CRITERIA
- S (V1) + R (V5 or V6) > 35 mm
- R in aVL > 11 mm
NON-VOLTAGE CRITERIA
- Left ventricular strain pattern
- Increased R wave peak time in V5 or V6
What are the additional changes seen in patients with LVH
Left atrial enlargement
Left axis deviation
ST elevation in the right precordial leads V1-3 (“discordant” to the deep S waves).
What are the ECG criteria for left atrial enlargement
P-mitrale:
Lead II
- Bifid p wave > 40 ms between the peaks
- Total p wave duration> 110 ms
V1
- Biphasic p wave with terminal negative portion> 40 ms duration OR > 1 mm deep
What are the ECG criteria for right atrial enlargement
P-pulmonale
Lead II
- p wave amplitude > 2.5 mm
V1
- p wave amplitude > 1.5 mm
What are the ECG criteria for right ventricular hypertrophy
RAD (> +110 degrees)
V1
- R wave > 7mm tall or R/S ratio > 1
V5/V6
- S wave > 7mm deep or R/S ration < 1
QRS duration> 120 ms
Associated
- Right atrial enlargement
- RV strain ( ST depression/T-wave inversion V1 - 4 and inferior leads)
What are the ECG criteria for RBBB
Broad QRS > 120 ms
RSR’ pattern in V1-3 (‘M-shaped’ QRS complex)
Wide, slurred S wave in the lateral leads (I, aVL, V5-6)
Associated Features
ST depression and T wave inversion in the right precordial leads (V1-3)
Is Right Axis Deviation always abnormal
A minor degree of right axis deviation is not uncommon in tall thin subjects in whom the heart lies more vertically
What is a normal q wave?
Small Q waves are normal in most leads
Deeper Q waves (>2 mm) may be seen in leads III and aVR as a normal variant
Under normal circumstances, Q waves are not seen in the right-sided leads (V1-3)
What is a pathological q wave
> 40 ms (1 mm) wide
2 mm deep
25% of depth of QRS complex
Seen in leads V1-3
What is aVR used for
If aVR QRS complex is upright the limb leads have been placed incorrectly
What is Wellen’s syndrome
Wellen’s syndrome is a pattern of deeply inverted or biphasic T waves in V2-3, which is highly specific for a critical stenosis of the left anterior descending artery (LAD).
- Patients may be pain free by the time the ECG is taken and have normally or minimally elevated cardiac enzymes; however, they are at extremely high risk for extensive anterior wall MI within the next few days to weeks.
Due to the critical LAD stenosis, these patients usually require invasive therapy; do poorly with medical management; and may suffer MI or cardiac arrest if inappropriately stress tested
How long should elective surgery be delayed in the instance of an MI prior to surgery
At least 3 months
What physiological effect does potassium have on the heart
Progressively worsening hyperkalaemia leads to suppression of impulse generation by the SA node and reduced conduction by the AV node and His-Purkinje system, resulting in bradycardia and conduction blocks and ultimately cardiac arrest.
(Increased EC K+ mean lower gradient for K+ efflux during repolarization)
K > 5.5 –> repolarization abN = peaked t waves
K > 6.5 –> Paralysis of atria: Wide/flat/absent p wave
K > 7.0 –> Conduction abN and bradycardia: QRS wide + AV block with junctional/ventricular escape rhythms BBB/Fascicular blocks/Sinus brady/Slow AF –> Sine wave is a pre-terminal rhythm.
K > 9.0 –> Asystole/VF/PEA with bizarre wide complex rhythm
What is the difference between hyperacute t waves and peaked t waves in hyperkalaemia
Hyperkalaemia –> tall, narrow, symmetrical
Hyperacute –> broad and asymmetrically peaked
causes of Mobitz type I AV block include:
Increased vagal tone (as in athletes)
Acute myocardial infarction (especially inferior infarction)
Myocarditis
Drugs such as beta-blockers, calcium channel blockers, digoxin and amiodarone
How do you distinguish SVT from sinus tachycardia?
An SVT is not affected by deep inspiration as the sinus node is not active during SVT. If the patient cannot comply with your request to take a deep breath, just watch the monitor for a few minutes. The rate of SVT changes very little, by 1 or 2 bpm. Sinus tachycardia varies a lot more than that.
How is WPW relevant to the anaesthetist?
These patients may develop paroxysmal tachyarrhythmias during anaesthesia. Patients who give a history suggestive of frequent arrhythmias should be investigated by a cardiologist and put on appropriate treatment prior to elective surgery.
In what % of the population is RBBB a normal variant
10 to 15%
What causes LBBB
Always pathological (unlike RBBB)
- IHD
- HPT
- Dilated CMO
- Isolated conduction system disease
Why is it important that definitive treatment is concluded in patients with Mobitz type 2 and complete heart block prior to surgery
Perioperative hypoxia/electrolyte imbalance/hypotension/ischaemia can lead to temporary worsening of existing atrioventricular block
