CHAPTER 7 : NORMAL WAVES "P"

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CHAPTER 7 : NORMAL WAVES "P"

Post by Admin on Thu Mar 18 2010, 00:47

Fortunately, you do not have to memorize 12 or more separate
patterns. Rather, if you understand a few basic ECG principles and the sequence
of atrial and ventricular depolarization, you can predict the normal ECG
patterns in each lead.


In some leads, for example, the P waves are positive
(upward); in others they are negative (downward). In some leads the QRS
complexes are represented by an rS wave; in other leads they are represented by
RS or qR waves. Finally, the T waves are positive in some leads and negative in
others. What determines this variety in the appearance of ECG complexes in the
different leads, and how does the repetitive cycle of cardiac electrical
activity produce such different patterns in these leads?





Three Basic "Laws" of Electrocardiography


To answer the preceding questions, you need to understand
three basic ECG "laws"





1.A positive (upward) deflection appears in any lead if the
wave of depolarization spreads toward the positive pole of that lead. Thus if
the path of atrial stimulation is directed downward and to the patient's left,
toward the positive pole of lead II, a positive (upward) P wave is seen in lead
II .Similarly, if the ventricular stimulation path is directed to the left, a
positive deflection (R wave) is seen in lead I .


2.A negative (downward) deflection appears in any lead if
the wave of depolarization spreads toward the negative pole of that lead (or
away from the positive pole). Thus if the atrial stimulation path spreads
downward and to the left, a negative P wave is seen in lead aVr .If the
ventricular stimulation path is directed entirely away from the positive pole
of any lead, a negative QRS complex (QS deflection) is seen .


3.If the mean depolarization path is directed at right
angles (perpendicular) to any lead, a small biphasic deflection (consisting of
positive and negative deflections of equal size) is usually seen. If the atrial
stimulation path spreads at right angles to any lead, a biphasic P wave is seen
in that lead. If the ventricular stimulation path spreads at right angles to
any lead, the QRS complex is biphasic. A biphasic QRS complex may consist of
either an RS pattern or a QR pattern.




Normal P Wave


The P wave, which represents atrial depolarization, is the
first waveform seen in any cycle. Atrial depolarization is initiated by the
sinus node in the right atrium . The atrial depolarization path
therefore spreads from right to left and downward toward the atrioventricular
(AV) junction. The spread of atrial depolarization can be represented by an
arrow * that points downward and to the patient's left .

















Notice that the positive pole of lead aVr points upward in
the direction of the right shoulder. The normal path of atrial depolarization
spreads downward toward the left leg (away from the positive pole of lead aVr
). Therefore with normal sinus rhythm lead aVr always shows a negative P wave.
Conversely, lead II is oriented with its positive pole pointing downward in the
direction of the left leg. Therefore the normal atrial depolarization path is
directed toward the positive pole of that lead. When normal sinus rhythm is
present, lead II always records a positive (upward) P wave.


In summary, when normal sinus rhythm is present, the P wave
is always negative in lead aVr and positive in lead II.




Using the same principles of analysis, can you predict what
the P wave looks like in leads II and aVr when the heart is being paced not by
the sinus node but by the AV junction (AV junctional rhythm)? When the AV
junction (or an ectopic pacemaker in the lower part of either atrium) is pacing
the heart, atrial depolarization must spread up the atria in a retrograde
direction, which is just the opposite of what happens with normal sinus rhythm.
Therefore an arrow representing the spread of atrial depolarization with AV
junctional rhythm points upward and to the right,, just the opposite of what
occurs with normal sinus rhythm. The spread of atrial depolarization upward and
to the right results in a positive P wave in lead aVr , since the stimulus is
spreading toward the positive pole of that lead.Conversely, lead II shows a
negative P wave.



















FINE NOW LETS APPLY THIS TO THE MOST COMMON MS THAT IS KEPT IN EXAM AS PER LAW 3 WHEN
STIMULATION SPREADS THRU PERPENDICULAR TO ONE LEAD THEN THERE WIL BE A BIPHASIC
WAVE ..


NOW COME TO MITRAL STENOSIS .. THE “P MITRALE” WAVECOMPLEX
IS JUS BCOZ OF THIS , NORMALLY THERE WIL BE BIPHASIC WITH EQUAL AMPLITUDE FIRST
HALF FOR (RA AND SECOND FOR LA). BUT WHEN LA HYPERTROPHIES IN MS THE LA PART(
ie., THE LATTER HALF INCREASES IN AMPLITUDE COMPARED TO THE FORMER)






COMPARE THE DIRECTION OF STIMULATION WITH THE HEXAXIAL DIAGRAM

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