LAD widowmaker artery ECG leads
Inferior wall MI leads RCA territory
P mitrale vs P pulmonale ECG criteria
Normal PR interval duration and segments
Wolff-Parkinson-White syndrome delta wave
ECG
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| Inferior Wall (RCA Territory) Anteroseptal Wall (LAD Territory / "Widowmaker") Lateral Wall (LCx Territory) |
LAD (Left Anterior Descending) is known as the widow maker artery / committed artery for MI → It supplies the antero-septal region i.e., V1, V2, V3, V4.
The lateral surface V5, V6, Lead I, aVL. These are supplied by the left circumflex artery.
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| The Math of ECG Grid Squares: Time and Amplitude |
1 large square has 25 small squares.
(For all calculations we use 1 small square.)
Small box:
Time = 0.04 sec
Amplitude = 0.1 mV
1 mm
Time taken by the wave to travel 1 small square = 0.04 sec.
So for 1 large square, duration: 0.04 × 5 = 0.20 sec
(because 1 large square has 5 small squares)
The amplitude in one small square is 0.1 mV.
For 1 large square the amplitude: 0.1 × 5 = 0.5 mV.
A biphasic P wave is seen in Lead V1 (one positive component followed by a negative component) reflecting right atrial depolarization followed by left atrial depolarization.
Sometimes U wave is seen.
Cause for P wave → Atrial depolarization
Cause for QRS wave → Ventricular depolarization
Cause for T wave → Ventricular repolarization
Cause for U wave → Unknown
T wave:
Ventricular muscle cells start repolarization across late.
Whenever you see a straight line in ECG, it is called a segment.
1. PR Segment
End of P & before QRS.
Since no current is flowing, it is a straight line.
It denotes normal AV nodal delay.
2. ST Segment
End of QRS & before T wave.
QRS denotes ventricular depolarization.
T wave denotes ventricular repolarization.
So exactly during ST segment ventricular depolarization is over and QRS complex & ventricular repolarization has not started yet.
As no current is flowing, this part is segment, hence iso-electric.
If current is produced by injured tissue in MI:
ST elevation can occur.
ST depression can occur.
If you combine a straight line & wave, it is called interval in ECG.
Straight line + wave = Interval.
Note:
Ventricular repolarization wave height → <25 mm in limb leads and <10 mm in chest leads.
PR Interval
Between P waves and before QRS.
Includes P waves + PR segment.
Here P waves denote atrial depolarization.
PR segment denotes AV nodal delay.
Normal PR interval = 1 large square interval = 0.20 sec.
(Normal range: 0.12–0.20 sec)
QT Interval
Between QRS & end of T wave.
QRS = Ventricular depolarization
End of T = Ventricular repolarization
So it includes the whole ventricular activity. Physiologically QT interval is TVA = Total Ventricular Activity.
QT normal duration:
2 large squares = 0.40 sec.
Usually >12 small boxes is called prolonged.
(QT is double of PR interval → just to remember)
QT interval always changes according to the HR of an individual. So we now use:
QTc = Corrected QT Interval
QTc = Normal QT interval / √RR interval
This formula is called Bazett’s Formula.
QRS interval:
0.1–0.11 sec
(Less than 3 small boxes)
P Wave Abnormalities
P wave:
<3 small squares (<0.12 sec)
Vertical height:
≤2.5 mm in limb lead
≤1.5 mm in chest lead
Abnormalities:
P wave is seen in Lead II.
1. Absent P wave
Seen in:
Atrial fibrillation
Sick sinus syndrome
Hyperkalemia
(Atrial fibrillation → R-R interval is irregular)
2. Wide & Notched P wave (P mitrale)
Seen in:
Left atrial enlargement
e.g., Mitral stenosis
As left atrium is dilated, current will travel more slowly through the atria.
3. Tall & Peaked P wave (P pulmonale)
Seen in:
Right atrial enlargement
Right atrial overload
Pulmonary artery hypertension
(If lungs = pulmonary → think P pulmonale)
Short PR Interval (<0.12)
Why shortened? Because normal AV nodal delay is bypassed.
Ventricles are prematurely activated by an accessory bundle of Kent, so there is no normal AV nodal delay.
Condition called:
Wolff-Parkinson-White Syndrome (WPW)
(Delta wave seen in ECG)
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