ECGs Flashcards
what is an ECG?
An electrocardiogram is a representation of the electrical events of the cardiac cycle.
Each event has a distinctive waveform and the study of waveform can lead to greater insight into a patient’s cardiac pathophysiology.
what produces an upright positive deflection?
Electrical impulse that travels towards the electrode
what can ECGs identify?
-Arrhythmias
-Myocardial ischaemia and infarction
-Pericarditis
-Chamber hypertrophy
-Electrolyte disturbances (hyper/hypokalaemia)
-Drug toxicity
describe SAN
– dominant pacemaker with an intrinsic rate of 60-100bpm
describe AVN
back up pacemaker with an intrinsic rate of 40-60 bpm
describe ventricular cells
back up pacemaker with an intrinsic rate of 20-45 bpm
what are the three pacemakers of the heart?
- SAN
- AVN
- Ventricular cells
what is the standard calibration?
25mm/s
0.1mV/mm
what are the impulse conduction points in an ECG?
- SAN
- AVN
- Bundle of His
- Bundle branches
- Purkinje fibres
describe p waves
atrial depolarisation
describe the QRS complex
ventricular depolarisation
describe the t waves
ventricular repolarisation
describe the PR interval
atrial depolarisation and delay in AV junction (delay allows time for the atria to contract before the ventricles contract)
what do the ECG leads measure?
The leads measure the difference in electrical potential between two points
what are the two leads called?
- bipolar leads
- unipolar leads
describe bipolar leads
two different points on the body (positive and negative)
describe unipolar leads
one point on the body and a virtual reference point with zero electrical potential, located in the centre of the heart (only require a positive electrode for monitoring)
how many electrodes, precordial, limbs are used?
There are 12 leads calculated using only 10 electrodes: 6 precordial, 4 limbs (1 neutral).
what do the chest electrodes do?
The 6 chest electrodes give 6 chest leads which provide information about the heart’s horizontal plane
what do the 4 limb electrode do?
The 4 limb electrodes give 6 frontal leads that provide information about the heart’s vertical plane. The placement can be explained as Einthoven’s triangle.
name the 6 leads
Lead I
Lead II
Lead III
Augmented vector right (aVR)
Augmented vector left (aVL)
Augmented vector foot (aVF)
which leads are bipolar
Leads I, II, and III require bipolar leads
which leads are unipolar?
all the augmented leads are unipolar.
what are common p wave abnormalities?
Right atrial enlargement – tall >2.5mm – P pulmonale
Left atrial enlargement – notched (M-shaped) – P mitrale
what are common QRS complex abnormalities?
Depth of the S wave should not excess 30mm
Pathological Q wave:
>2mm deep and >1mm wide
>25% amplitude of the subsequent R wave
what does the QRS complex represent?
This represents the overall direction of the heart’s electrical activity, and abnormalities of the QRS axis hint at ventricular enlargement or conduction blocks.
what are common ST segment abnormalities?
ST segment is usually flat (isoelectric), elevation or depression of ST segment by 1mm or more can be pathological.
what are common T wave abnormalities?
Should be at least 1/8 but less than 2/3 of the amplitude of R
Abnormal T waves are symmetrical, tall, peaked, biphasic or inverted
T wave amplitude rarely exceeds 10mm
what are common U wave abnormalities?
Small, round, symmetrical and positive in lead II, with amplitude <2mm (regular)
U wave should be same direction as T wave
what are common QT interval abnormalities?
QT interval decreases when heart rate increases
Regular interval is 0.35-0.45s
Should not be more than half of the interval between adjacent R waves
how do you calculate heart rate from a ECG?
Rule of 300/1500 – regular rhythms
Count the number of big/small boxes between two QRS complexes and divide this into 300/1500 for regular rhythms.
10 second rule – irregular rhythms
Count the number of beats present on the ECG and multiply by 6.
describe the quadrant approach for the QRS axis
- QRS complex in leads I and aVF
- Determine if they are predominantly positive or negative
- The combination should place the axis into one of the 4 quadrants below
LAD = left axis deviation
RAD = right axis deviation
