ECG interpretation Flashcards
How does the ECG work?
Voltmeter recoding electrical voltages at the skin surface, generated by the electrical activity of cardiac cells and caused by extracellular currents.
How many electrodes are placed on the body?
10
6 on the chest
4 on the limbs
How many leads does an ECG have?
12 leads.
6 chest/precordial leads: transverse plane.
6 limb leads: frontal plane.
What is lead I (electrodes)?
Lead between electrode at right arm (-) and electrode at left arm (+)
What is the lead that goes from top to bottom?
aVF (unipolar lead), +90 degrees
CT (-) towards left leg (+)
How many unipolar vs bipolar leads are there?
9 unipolar (CT is -)
3 bipolar
Does the voltmeter record electrical impulses outside or inside of the cell?
OUTSIDE.
When do we see a positive deflection on the ECG?
When the current outside (which goes from - to + by convention), travels towards the positive electrode.
When a depolarization current is directed towards the + electrode, an upward deflection is recorded!!
Vice versa
What is an equiphasic deflection?
An equiphasic deflection occurs when the current travels perpendicularly to the lead. If it is perfectly perpendicular, no deflection is seen. Otherwise, there is an equiphasic deflection, which is equally positive and negative.
Why is the T-wave upright despite being a repolarization?
Because the last cell to have been depolarized is the first cell to be repolarized. Therefore, the repolarization current still moves towards the + electrode.
What is the nomenclature of the QRS complex?
Q wave: when the first deflection of the QRS complex is downward
R wave: the first upward deflection (whether or not a Q wave is present)
S wave: any downward deflection following the R wave.
Additional deflections might be described (ie. R and R’).
We can use capital letters to indicate the dominant waves and lower cases to name the smaller waves.
What is the ST segment?
Line between QRS complex and T wave.
Normally isoelectric (same level as baseline).
What is the PR interval?
Time from start of P wave to start of QRS complex.
What is the QT interval?
Time from the start of the QRS to the end of the t-wave.
Time equivalent to 1 small box and 1 large box on an ECG, and paper speed?
1 small box (1 mm) = 40 ms
1 large box (5 mm) = 0.2s
Paper speed = 25 mm /s
Axes of ECG
Vertical: voltage (mV). 1mm = 0.1 mV
Horizontal: time.
Sequence of analysis of an ECG
1) Sinus rhythm or not?
2) Heart rate
3) Intervals (PR and QT)
4) Mean QRS axis
5) P-wave abnormalities
6) QRS abnormalities
7) ST and T wave abnormalities
How to assess sinus rhythm?
Upright P waves in leads I and II.
Normal PR interval
3-5 small boxes (0.12s-0.20s).
How to measure QT interval? (2 ways)
- Bazett’s formula (tachy or brady use):
Qtc = (QT interval in ms /sqr(RR interval in s))
Normal Qtc for women <0.46
Normal Qtc for men < 0.44 - Normal if QT is <1/2 of RR.
Mean QRS axis: negative lead 1 and negative lead 2
Can’t know, need to look at aVF.
Mean QRS axis: positive lead 1 and negative lead 2
left axis deviation
Mean QRS axis: positive lead 1 and positive lead 2 and negative aVF
normal
Mean QRS axis: negative lead 1 and negative aVF
extreme axis deviation
Mean QRS axis: negative lead 1 and positive lead aVF
right axis deviation
Which leads to look at for P-wave atrial enlargement?
leads II and V1
Right atrial enlargement characteristics.
Tall P-Wave in lead II (>2.5 mm)
Taller upwards than downwards deflection in lead V1.
Left atrial enlargement characteristics.
Large and deep negative P wave in V1 (>1 mm wide and > 1 mm deep).
How to assess right ventricular hypertrophy
Look at V1 and V2
Right axis deviation
R >S in lead V1
How to assess left ventricular hypertrophy
Look at V5 and V6 and aVL and 1
Left axis deviation suggestive.
- Big S wave in V1 and R wave in V5 or V6 >35 mm
OR - R in aVL >11 mm
OR - R in lead I >15 mm
How to assess right bundle branch block
V1 and V6
Wide QRS
MarRooN or MaRroW
M in V1
N or W in V6
How to assess left bundle branch block
V1 and V6
Wide QRS
WilLiam or VilLheiM
W or V in V1
M in V6
How to assess pathologic Q waves
wide (>1mm) and deep (>25% of QRS)
Anteroseptal leads
V1 and V2
Anterior leads
V3 and V4
Lateral leads
1, aVL, V5 and V6
inferior leads
2, 3, and aVF
Transient MI characteristics
ST depression
T wave inversion
Reversible once ischemia is gone.
STEMI characteristics
Complete obstruction of the coronary artery
Elevation of ST
NSTEMI characteristics
Partial obstruction of coronary artery
No Q wave
T wave inversion
ST depression
