THE NORMAL AND ABNORMAL P WAVE
December 17, 2012 Leave a comment
THE GENESIS OF THE NORMAL P WAVE
Complete atrial activation takes 0.099 sec +- 0.012 sec, the max duration of normal atrial activation is thus 0.11 sec
Since SA node is situated in the RA, so Right atrial activation begins first. It is reflected by the proximal or ascending limb of the P wave in the frontal plane leads, most commonly lead II and ends at the apex of P wave.
The duration of RA activation ranges from 0.02 – 0.04 sec. Since, SA and AV node are located in RA, the sinus impulse reaches the AV node in 0.03 sec, i.e, before atrial activation as a whole has completed.
LA activation begins 0.03 sec after RA activation and constitutes the distal half of the P wave in lead II. The duration of the LA activation ranges from 0.05-0.06 sec.
P wave is thus a composite deflexion of RA and LA activation. The P wave is inscribed at a constant speed so that the limbs are smooth with no irregularities.
THE MEAN FRONTAL PLANE DIRECTION OF ATRIAL ACTIVATION IS INFERIORLY AND TO THE LEFT.
The normal P wave is best evaluated in terms of the following parameters:
1. The P wave form in lead II
2. The P wave form in lead V1
3. The frontal plane P wave axis
THE P WAVE FORM IN STANDARD LEAD II
The normal P wave is best seen and studied in lead II because frontal plane P wave axis is usually directed to the positive pole of this lead. The P wave in II is pyramidal in shape with somewhat rounded apex. Its limbs are smooth with no irregularities.
The duration of P wave is 0.08-0.10 sec, but is no greater than 0.11sec
The maximal normal amplitude is 2.5mm, but the normal P wave is usually no greater than 2 mm.
THE P WAVE FORM IN LEAD V1
The P wave is usually studied in V1 since the initial and terminal components of the P wave are clearly identified and easily separated in this lead.
The P wave in V1 is normally BIPHASIC, having an initial positivity and terminal negativity.
The reason for biphasic p wave is : SA node is situated in the RA and is thus activated first and the vector of RA activation is directed anteriorly and slightly to left. i.e, towards lead V1. This lead will consequently record an initial positive deflexion, which is normally less than 1.5 mm in amplitude.
The LA activation begins slightly later than RA and overlaps with the terminal activation of the RA. Since, the LA s situated posteriorly, the vector is directed slightly away from V1. This results in the terminal shallow negative deflexion in V1. The depth is less than 1 mm and 0.03 sec duration. The P wave is thus a composite deflexion of RA and LA.
THE FRONTAL PLANE P WAVE AXIS
The normal P wave axis is +45 to +65*.
P wave axis >+70* is right axis deviation
P wave axis < +45* is left axis deviation
Since most P wave axis is in the region of +50*, it is aligned in the positive pole of II.
Will result in 3 basic ECG changes
1. Prolongation and delay of the LA component of the atrial activation:
The P wave is prolonged due to delay of the LA activation. The caharecteristic features will manifest in II, I or AVL when there is left axis deviation. The P wave will show
a) Double peak, notch or camel hump
b) Increased duration of P wave to >0.11 sec
The duration of notch >0.04 sec ( see first fig above)
*** The frontal plane P wave axis will determine the best lead to examine P wave. If axis is 50*- best lead is II, if axis is 0* than lead I
2. Increased posterior deviation of LA vector:
With LA enlargement, the LA component of atrial activation is prolonged, increased in magnitude and directed further posteriorly. The LA vector is consequently oriented more directly away from V1, and V1 will reflect a relatively deep , delayed and widened terminal negative component.
The P Terminal force or Morris Index: In lead V1, Depth of terminal P wave (mm) multiply by duration of terminal P wave (sec). Units mm.sec
If terminal P force >0.03 mm.sec –> LA enlargement
3. Left axis deviation of the men manifest frontal plane P wave axis:
LA atrial enlargement is usually associated with left axis deviation of P wave axis. The p wave axis is directed to the region of +45 to -30* on the frontal plane.
clinical significance: LA enlargement occurs in systemic HTN, increased LA pressure.
RIGHT ATRIAL ENLARGEMENT
The diagnosis of RA enlargement depends upon one or both of the following ECG manifestations
1. Abnormalities of the P wave
2 Abnormalities of QRS complex
ABNORMALITIES OF P WAVE IN RA ENLARGEMENT
1. Increased Amplitude of P wave in certain Limb leads
With RA enlargement the initial or RA component of P wave is increased both in amplitude and duration. The P wave amplitude > 2.5 mm
Best seen in II, as the P wave axis tends to be 60*
But if the P wave has right axis deviation- 80-90*, than tall p wave amplitude will be reflected in II, III, AVF
** Though with RA enlargement the duration of RA component is increased but it will encroach the terminal left atrial component of P wave, therefore the total duration of P wave will be in normal limits.
** Even if P wave in II is 2 mm- should raise suspicion of RA enlargement if pointed.
** In COPD pts- downdisplacement of heart can cause negative or dominant negative P waves in V1.
** P tricusidale: When P wave in frontal plane leads is notched, and the first component is increased in amplitude and taller than second component. Reflects biatrial enlargement and is frequently seen with TV disease, as well as with MV disease with Pulmonary HTN.
2. Increased amplitude of the initial P wave deflexion in lead V1
With RA enlargement, the initial deflexion of P wave in V1 will become taller, more pointed and symmetrical. The amplitude > 1.5 mm. The P wave usually dominantly positive with relatively small negative component. P wave may be entirely positive with no negative component.
The duration of positive component in V1 > 0.04 sec
The above manifestations are due to greater and more direct alignment of right atrial vector with lead V1.
3. A potential tendency of right P wave axis deviation in acquired heart disease:
Like in emphysema, p wave axis will be +60 to +90
** When the tall peak P wave of right atrial enlargement is associated with right P wave axis deviation in acquired heart disease, it is called “P Pulmonale”.
4. Normal P wave axis or a potential left P wave axis deviation in congenital heart disease
Like in TOF or pulmonary stenosis, the mean P wave axis is directed normally +40- +70. Sometimes slightly left +45 to 0.
** When tall peak P wave is associated with left axis deviation of P wave in congenital heart disease, it is referred to as ” P congenital”.
5. Rapid inscription of negative component of the P wave in lead V1: early terminal P wave negativity in lead V1:
Uncommonly RA enlargement may manifest with terminal negativity in lead V1. The intriscoid deflexion will not exceed 0.03 sec, in contrast to LA enlargement. In such cases, lead V2 ill show tall and peak P wave.
ABNORMALITIES OF QRS WHICH REFLECT RA ENLARGEMENT
It is becoming more evident that RA enlargement diagnosis can be made more confidently from changes of QRS than from P wave abnormalities. The 2 changes that suggest RA enlargement are
1. A qR complex in lead V1:
– Frequenty an indirect sign for RA enlargement and is usually due to tricuspid insufficiency.
– Tall R wave is an expression of RV hypertrophy.
– Initial small q wave is result of anatomic shift of heart from RA enlargement.
** Note that, in nearly all cases of RVH, the tall R wave in lead V1 will reflect an initial slur/notch/small q wave.
2. Diminution in the size of QRS deflexion in lead V1 with a marked increase of QRS amplitude in lead V2
– QRS complex is suggestive of RA enlargement if whole QRS magnitude is small in V1 and whole QRS magnitude in V2 is three times greater.
Manifests as follows:
1. Frontal plane leads and left precordial leads will show
– Wide and notched P wave
– Plus inceased amplitude of P wave
- When such a P wave has initial component taller than terminal, it is called “P Tricuspidale”, because it is frequently associated with TV disease, or can occur with MV ds with pulm HTN.
2. Lead V1: initial component of P wave is taller than normal + peaked, and associated with terminal deep, wide and delayed component.
THE P’ WAVE OF RETROGRADE ATRIAL ACTIVATION
With retrograde activation with impulse arising from AV node or passing through it, than the P’ wave axis is directed in the region of -80 to -90*. It is directed to negative poles of II, I, AVF and will result in negative deflexion in these leads.
P’ wave is represented in V1 by tall, totally positive, narrow and peaked deflexion. It is narrower, more sharply pointed than the P wave of RA enlargement