eLFH - The Electrocardiogram Part 1 Flashcards
Standard ECG paper speed
25 mm/s
= 5 large squares per second
Usual representation of one small square on ECG
0.04 s on x axis
0.1 mV on y axis
X and Y axis on ECG
X axis = time (s)
Y axis = Amplitude (mV)
Implication of negative deflection vs positive deflections on ECG
Negative deflection means electrical impulse is moving away from electrode
Positive deflection means electrical impulse is moving towards electrode
Each lead looks at electrical activity in different plane to the others
Directions of limb lead planes
Coronal plane
Direction of chest lead planes
Horizontal plane
Bipolar leads
Leads I, II and III
Form triangle called Einthoven’s triangle
Unipolar leads
Augmented limb leads (aVR, aVF and aVL) and chest leads (V1-V6)
Measured from imaginary reference point of zero potential called the ‘indifferent electrode’ (centre of limb lead electrodes)
Lower amplitude than bipolar leads and therefore need to be augmented
Normal cardiac axis
Between - 30 degrees and + 90 degrees
Left axis deviation
Between - 30 degrees and - 90 degrees
Causes of left axis deviation
Inferior MI
Left anterior hemiblock
LBBB
Pregnancy - mechanical displacement of the heart
Cardiomyopathy
Right axis deviation
Between + 90 degrees and + 180 degrees
Causes of right axis deviation
RBBB
Right ventricular hypertrophy
Normal variant in young
Causes of right ventricular hypertrophy
COPD
PE
ASD
Pulmonary stenosis
Net electrical potential calculation
For a particular lead:
Number of small squares positive - number of small squares negative = net electrical potential
Leads commonly used in anaesthesia and why
Usually lead II
Gives best visualisation of P wave and QRS to determine rhythm
CM5 configuration of leads use
More sensitive to identify ischaemia
Allows detection of 80% of LV ischaemia as it is exploring largest mass of LV muscle
CM5 electrode positions
Right Arm electrode placed on manubrium
Left Arm electrode placed on at V5
Left Leg electrode placed on Left Clavicle
Select lead I for monitoring (between Manubrium and V5)
Electrode positioning in cardiac anaesthesia
5 electrode ECG monitoring - allows monitoring of a limb lead and chest lead
Usual 3 lead electrodes placed + right leg electrode + choose one from any of V1 to V6
Elements of the ECG
P wave
Q wave
QRS complex
PR interval
ST segment
T wave
QT interval
P wave - represents
Atrial depolarisation
P wave - size
< 0.2 s
< 2.5 mm
P wave abnormalities
Absence - AF
Flutter waves (saw tooth) - atrial flutter
Hypertrophy:
- Right atrial (P pulmonale)
- Left atrial (P mitrale)
Q wave - represents
1st downward deflection after P wave
Abnormal in chest leads
Q wave - abnormalities
Q waves in any lead are abnormal - previous MI with full thickness myocardial loss
QRS complex - represents
Ventricular depolarisation
QRS complex - size
< 0.12 s
QRS complex - abnormalities
Widened QRS - Bundle branch block (right, left and hemiblocks)
Increased height QRS - ventricular hypertrophy
Small QRS - pericardial effusion
PR interval - represents
Normal delay at AV node
PR interval - duration
< 0.2 s
PR interval - abnormalities
Long PR - 1st degree heart block (AV conduction delay)
Short PR - Wolff-Parkinson-White (extra nodal conduction)
ST segment - abnormalities
ST depression - Reversible ischaemia
ST elevation (>1mm limb leads or >2mm chest leads) - Irreversible ischaemia
Saddle ST elevation - Pericarditis
T wave - represents
Ventricular repolarisation
T wave - size
< 5 mm in limb leads
< 10 mm in chest leads
T wave - abnormalities
T wave inversion (except aVR and V1) - non specific e.g. ichaemia, infection, LVH
Tented T waves - hyperkalaemia
QT interval - represents
Ventricular refractory period
QT interval - duration
350 to 430 ms
QT interval - correction
Bazett’s formula - corrects for HR
QTc = QT / square root RR (s)
QT interval - abnormalities (prolonged QT causes)
Congenital
Electrolyte disturbance - low K+, low Mg2+, low Ca2+
Drugs
Congenital causes of prolonged QT
Romano Ward syndrome
Lervell Lange Nielson syndrome
Drug causes of prolonged QT
Antiarrhythmics class III - amiodarone
Antibiotics - macrolides
Antihistamines
Antipsychotics - phenothiazines
Antidepressants - tricyclic antidepressants
Potential additional ECG waves
Delta waves
J waves
U waves
Delta waves
Up-stroking between P wave and QRS complex
Seen in WPW - pre excitatory syndrome
J waves
Extra deflection at end of QRS complex
Seen in hypothermia < 25 degrees Celsius
J waves alternative name
Osborne waves
U waves
Small deflection following T wave
Represents repolarisation of papillary muscles
Seen in hypokalaemia most commonly
Causes of U wave
Hypokalaemia
Hypercalcaemia
Hyperthyroidism
Digoxin therapy
