Fig. 2.1
Cause of the morphology of sinus P wave
[Characteristics of Electrocardiogram of the Normal Sinus P Wave]
1.
P wave in sinus rhythm appears small, rounded, and upright in leads I, II, aVF, and V4 to V6.
2.
Sinus P wave is inverted in lead aVR.
3.
In other leads, it can be upright, inverted, or biphasic (half upright, half inverted).
4.
Normal duration of sinus P wave should be less than 0.12 s and amplitude of P wave less than 0.25 mV in limb leads or amplitude less than 0.2 mV in chest leads.
[ECG Tracing]
2.1.1 Abnormality in Frequency of Sinus P Wave
The frequency of normal P wave is 60–100 bpm. When the frequency is less than 60 bpm, it is sinus bradycardia (Fig. 2.3); when it is more than 100 bpm, it is sinus tachycardia (Fig. 2.4).
2.1.1.1 Sinus Bradycardia
[ECG Recognition]
1.
Sinus P wave is present.
2.
The frequency of P wave is less than 60 bpm.
3.
It may be accompanied by sinus arrhythmia.
2.1.1.2 Sinus Tachycardia
[ECG Recognition]
1.
Sinus rhythm.
2.
The frequency of P wave is more than 100 bpm and maybe higher in children.
2.1.2 Abnormality in Voltage or Duration of Sinus P Wave (Atrial Enlargement)
Since the sinus node is located at the junction of the superior vena cava and the right atrium, the electrical impulses discharged by the excited sinus node first activate the right atrium and then the left atrium. The depolarization of all atria is reflected by P wave on ECG; therefore, the right atrial depolarization takes the first 2/3 of P wave, and the depolarization of the left atrium takes the last 2/3, which means the middle 1/3 of the P wave is the sum of synchronal depolarization of the right and left atria (Fig. 2.5). Normal P wave appears small and rounded in lead II and biphasic in lead V1. Lead II and lead V1 are regarded as the best leads to analyze the electrical activities of both atria on ECG.
Fig. 2.5
Compositions of sinus P wave
2.1.2.1 Abnormality in Voltage of Sinus P Wave (Right Atrial Enlargement)
If right atrial enlargement exists, the right atrial depolarization vector which directs anteriorly to the bottom right will increase. The increased depolarization vector is closer to the positive poles of leads II, III, and aVF, causing P waves to be tall and peaked and obviously increased amplitude in leads II, III, and aVF.
[ECG Recognition]
1.
In leads II, III, and aVF, P wave is abnormally tall and peaked, and the voltage exceeds 0.25 mV (P pulmonale).
2.
Electrical axis of P wave often exceeds 70°.
3.
The duration of the P wave is still within normal range.
2.1.2.2 Abnormality in Duration of Sinus P Wave (Left Atrial Enlargement)
If left atrial enlargement exists, the prolonged depolarization time will broaden the P wave on ECG. Besides, as a result of the increased left atrial depolarization vector, the resultant atrial depolarization vector points left posteriorly (much closer to the direction of leads I, II, aVR, and aVL) and opposite to the direction of lead V1 on the horizontal plane. This leads to obvious widening of P wave in leads I, II, aVR, and aVL and the widening of negative portion of the P wave in lead V1 (Fig. 2.7).
[ECG Recognition]
1.
P wave in leads I, II, aVR, and aVL is widened to over 0.12 s (P mitrale).
2.
P waves are mostly double-peaked. The second peak is often bigger than that of the first one, and the interspike interval often exceeds 0.04 s.
3.
In lead V1 the voltage of the P wave increases to over 2 mV and appears biphasic. The terminal negative portion is apparently widened (>40 ms) and deepened (>1 mm), which makes the Ptf-V1 ≤ −0.04 mm · s (Ptf-V1 is the terminal vector in lead V1, the cross product of the negative depth (mm), and duration (s) of the P wave in lead V1).
2.2 Non-sinus P Wave
After the discussion, readers are supposed to understand the characteristics of waveform of the sinus P wave and be able to conduct rough ECG diagnosis using the morphological features of the sinus P wave. Now that sinus P wave exists, non-sinus P wave exists correspondingly. First, let’s draw an overview of the morphological features of non-sinus P wave according to those of sinus P wave:
1.
Though the P wave is upright in lead II and inverted in lead aVR, its morphology is different from normal sinus P wave.
2.
Manifestation of non-sinus P wave: The P wave is inverted in lead II and upright in lead aVR. It is generated by the impulse conducted retrogradely from the atrioventricular node, and then the atria are excited, and the P wave (retrograde P wave) is produced.
2.2.1 P Wave Morphologically Different from Normal Sinus P Wave (Atrial P Wave)
2.2.1.1 Premature Atrial Contraction
The contraction from the atrial ectopic pacemaker that occurs earlier than expectation is called premature atrial contraction. Since it originates in the atrial ectopic pacemaker, different atrial depolarization sequence is produced comparing with sinus pacemaker. As a consequence, P wave generated under this circumstance is different from sinus one, and in order to distinguish them, we use P′ instead.
[ECG Recognition]
1.
P′ wave that occurs prematurely is different from sinus P wave morphologically.
2.
P′ waves are usually followed by QRS complexes with normal morphology and duration (normal anterograde conduction, Fig. 2.8a); a few P′ waves are followed by wide, bizarre QRS complexes (aberrant conduction, Fig. 2.8b); a few other P′ waves are followed by no QRS complexes (nonconduction, Fig. 2.8c).
3.
The P′-R interval is no less than 0.12 s.
4.
In most cases the compensatory pause is incomplete.
[ECG Tracing] (Fig. 2.8)
Fig. 2.8
Premature atrial contraction. (a) Normal anterograde conduction; (b) aberrant conduction; (c) non-conduction
2.2.1.2 Atrial Escape Beat
Escape beat and premature beat are a pair of opposite concepts; the former means the impulses are discharged later than expectation, and the latter comes earlier than expectation. The occurrence of atrial escape beat mainly because: ① the sinus node, for some reason, cannot discharge impulses normally (including the rate of impulses slowing down or asystole); ② the impulses cannot conduct anterogradely due to conduction disturbance; ③ other reasons that can cause a long pause. In this case, the downstream ectopic pacemaker will be released from the suppression of normal rates and discharge impulses in its natural cycles. When only 1 or 2 impulses are discharged by them, it is called escape beat, and if 3 or more are discharged consecutively, it is called escape rhythm.
According to different sites of origin, there are atrial escape beat (rare, Fig. 2.9), junctional escape beat (the most common type, discussed in detail in Sect. 2.3 of this chapter, Fig. 2.15), and ventricular escape beat (common, discussed in detail in Chap. 4).
[ECG Recognition]
1.
P′ appears after a long pause and is different from the sinus P wave morphologically.
2.
The P′-R interval is no less than 0.12 s.
3.
P′ is followed by QRS complexes of normal morphology and duration; wide, bizarre QRS complexes are rare.
4.
It is called atrial escape beat if there are 1–2 abnormal beats discussed above; it is called atrial escape rhythm if 3 or more such beats are seen in a row, and the rate normally ranges between 50 and 60 bpm.
2.2.2 Non-sinus P Wave (Retrograde P′ wave)
2.2.2.1 Premature Atrial Contraction
Ectopic pacemaker located in lower part of the atrium discharges impulses ahead of time and produces retrograde P′ waves (Fig. 2.10).
[ECG Recognition]
1.
Retrograde P′ wave which occurs ahead of time and precedes QRS complexes.
2.
Usually P′ waves are followed by QRS complexes of normal morphology and duration (normal anterograde conduction); a few P′ waves are followed by wide, bizarre QRS complexes (aberrant conduction); a few other P′ waves are followed by no QRS complexes (nonconduction).
3.
The P′-R interval is no less than 0.12 s.
4.
Usually the compensatory pause is incomplete.
2.2.2.2 Premature Junctional Contraction
The impulse discharged ahead of time by the ectopic pacemaker at the atrioventricular junction is called premature junctional contraction. The junctional impulse which appears early is able to conduct anterogradely and retrogradely at the same time. Retrograde conduction activates the atria and subsequently produces retrograde P′ waves, while anterograde conduction activates the ventricles and produces QRS complexes. Due to different velocities of retrograde and anterograde conduction, the retrograde P′ wave may present before (Fig. 2.11a) or after the QRS complexes (Fig. 2.11b). Based mainly on the P′-R interval and whether the compensatory pause is complete, we can distinguish the premature junctional contraction from the premature atrial contraction when retrograde P′ wave precedes the QRS complex. If the P′-R interval is no less than 0.12 s and the compensatory pause is not complete, it is premature atrial contraction; if P′-R interval less than 0.12 s with a complete compensatory pause, it is premature junctional contraction.
Fig. 2.11
Premature junctional contraction. (a) Retrograde P′ waves appear before the QRS complexes; (b) retrograde P′ waves appear after the QRS complexes
[ECG Recognition]
1.
QRS complexes that occur ahead of time usually have normal morphology or sometimes become bizarre as a result of aberrant intraventricular conduction.
2.
The retrograde P′ wave may appear before the QRS complex (the P′-R interval is less than 0.12 s in adults, no more than 0.10 s in children, or differs largely from the sinus P-R interval), after the QRS complex (the P′-R interval <0.20 s), or be buried in the QRS complex and difficult to distinguish (absent P wave).
3.
Usually the compensatory pause is complete.
2.2.2.3 Weird Retrograde P′ Wave (LA/RA Reversal)
When you are reading an ECG and find P′ wave (i.e., inverted P wave in lead II and upright P wave in lead aVR), you are supposed to consider that misplacement of left arm and right arm leads could possibly happen, besides junctional premature contraction.
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