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
