Activation begins predominantly in the middle third of the left side of Interventricular Septum.From there , the initial wave of Depolarization spreads toward the right side of the septum.
A small resultant vector which is directed rightward, anteriorly or superiorly Produces the initial QRS deflection of the ECG. This is displayed as a small positive wave in V1, V2 and aVR and a small negative wave (septal q) in lead l, aVL, V5 and V6.
After that, the impulse spreads throughout the apical and free walls of both ventricules in an endocardial to epicardial direction. Because of the much larger mass of the left ventricule. Now the resultant vector is directed leftward and inferiorly. It produces the major the major deflection of the QRS complex.
It produces large positive deflection in lead l, aVL, V5 and V6 and large negative deflection in lead V1, V2 and aVR because of direction and location of the Conduction and leads respectively. Example:01
In the precordial leads, the QRS complex is usually characterized by progression from an rS to a qR pattern in the right and left precordial leads respectively.
The point at which the pattern changes from rS to Rs configuration, that is the lead in which an isoelectric RS pattern is recorded is known as transitional zone and normally occurs in V3 and V4.
Transitional zone shift to the right to lead V2 is called early transition and transitional zone shift to the left to V5 or V6 is called delayed transition.
Normal QRS Complex has the following characteristics:
- Duration: 0.06 to 0.10 second
- Axis: -30 to +90 degrees and
- Transitional zone: in V3 or V4.
The QRS Axis:
The normal QRS Axis is in the frontal plane is -30 to +90 degrees. An axis between -30 to -90 degrees indicates left axis deviation and that between +90 to ±180 degrees indicates right axis deviation. When the QRS Axis falls between -90 and ±180 degrees or when R and S Waves in all limb leads are equal, the axis is considered indeterminate.
If the mean QRS Axis is near 90 degrees,it is referred to as a vertical heart pattern, and if it is near 0 degrees, it is termed horizontal Heart pattern.
The commonest causes of right axis deviation are :
- Right ventricular hypertrophy
- Lateral Myocardial Infarction
- Ventricular pre-excitation( with Left free wall accessory pathway), and
- Left posterior hemiblock.
The commonest causes of the left axis deviation are:
• Left anterior fascicular block
• Inferior Myocardial wall Infarction
• Ventricular pre-excitation ( with posteroseptal accessory pathway ), and
Amplitude is displayed as vertical deflection of 10 mm for 1 mV.
Time is displayed on the horizontal scale as 10 mm for 400 msec (paper speed 2.5cm /second).
Low Voltage :
Low Voltage ECG is difined as an amplitude of whole QRS Complex (R plus S) as the following:
- 5 mm or less in each limb leads
- 10 mm or less in each precordial leads
Common causes of Low Voltage are
- Ischemic or no ischemic cardiomyopathy and
- Pericardial or plural effusion.
ECG – Congestive heart failure triad:
- Relatively low voltage (QRS voltage <8mm) in each of limb leads
- Relatively Prominent QRS voltage in each chest leads ( SV1 or SV2+RV5 or RV6 > 35mm) and
- Slow R wave progression; R less than
Heart rate measurement:
At a standard paper speed of 2.5 cm/sec, estimation of atrial ( p waves) or ventricular ( QRS waves) rates utilizes two rules:
A.When the rhythm is regular:
Rate = 1500/ distance between Two consecutive identical waves ( p or QRS) in mm.
B. When rhythm is irregular:
Rate= number of identical complexes between 3 second marks ( 7.3 cm apart) x 20.
If rate is regular :
Rate = 300 / number of large box in two consecutive R waves and
Rate= 1500/ number of Small box between two successive R waves.
If the rate is irregular:
Rate = number of QRS complex on ECG multiply by 6 seconds.