Heart rate (bpm)
<70
71~90
91~110
111~130
>130
Age (years) ≥18
0.20
0.19
0.18
0.17
0.16
Age (years) 14~17
0.19
0.18
0.17
0.16
0.15
Age (years) 7~13
0.18
0.17
0.16
0.15
0.14
Age (years) 1.5~6
0.17
0.165
0.155
0.145
0.135
Age (years) 0~1.5
0.16
0.15
0.145
0.135
0.125
3.2 Abnormal P-R Interval
Generally, a prolonged P-R interval longer than 0.20 s is an indication of delayed conduction from the atria to the ventricles, and the patient is said to have atrioventricular block with different causes; shortened P-R interval less than 0.12 s on the other hand is an indication of enhanced conduction from the atria to the ventricles, which is often seen in preexcitation syndromes.
3.2.1 Prolonged P-R Interval (Atrioventricular Block)
Atrioventricular block (AV block, AVB) is the impaired impulse conduction from the atria to the ventricles due to pathologically prolonged refractory period of some parts in the atrioventricular conduction pathways. AV block can mean delayed, incompletely, or completely blocked impulse conduction.
The ECG tracing of atrial depolarization is the P wave, while the ventricular depolarization, the QRS complexes. Normally, every P wave is followed by a corresponding QRS complex, and the time duration of the P-R interval will not exceed a certain range. When there is an AV block, the ECG shows the association between P wave and the corresponding QRS complexes which is abnormal: the P-R interval may prolong, or the corresponding QRS is absent after the P wave.
AV block can be divided into first degree, second degree, high degree, and third degree according to the severity. First degree, second degree, and high degree are also known as incomplete AV block, while third degree is also known as complete AV block.
3.2.1.1 First-Degree AV Block
First-degree AV block is a delay of conduction from the atria to the ventricles, characterized by the P-R interval prolonged over the normal range of the electrocardiogram. However, every supraventricular impulse is able to pass to the ventricles without any dropped beats no matter how long the P-R interval is.
[ECG Recognition]
1.
The P-R interval is more than 0.20 s (>0.22 s in the elderly, >0.18 s in children under the age of 14). The P-R intervals are mostly between 0.21 and 0.35 s.
2.
The P-R interval is greatly affected by age and heart rate of the patient. Additionally, in patients with first-degree AV block, the P-R interval is longer than the upper limit normal range corresponding to the patient’s age group (see Table 3.1).
3.
On two continuous electrocardiogram examinations of a patient, the P-R interval is shown to be more than 0.04 s longer than that of the previous one without obvious change in the heart rate.
3.2.1.2 Second-Degree AV Block
In second-degree AV block, the impulses from the atria to the ventricles are partly interrupted, but not every atrial impulse is able to pass through the AV node to the ventricles, which is defined as dropped beat. As shown in electrocardiogram, not every P wave is followed by a corresponding QRS complex. Second-degree AV block is first described by Wenckebach and MorbitzMorbitz, and therefore it is called Wenckebach and MorbitzMorbitz AV block (type I and type II).
3.2.1.3 Second-Degree AV Block Type I (Mobitz Type I AV Block, Wenckebach Block)
Type I second-degree AV block, also known as Wenckebach block or Mobitz type I AV block, is the most common type in second-degree AV block. Wenckebach block, which is always due to a block within the AV node or in the proximal bundle of His, is mostly a functional block with good prognosis.
[ECG Recognition]
1.
The P-R interval is progressively prolonged with each beat until one QRS complex dropped.
2.
The P wave is regular sinus P wave.
3.
After the dropped QRS complex, item 1 repeats.
4.
The ratio of conduction can be fixed or varied; the latter one is more common in clinical practice.
3.2.1.4 Second-Degree AV Block Type II (Mobitz Type II Block)
Second-degree AV block type II, also known as the Mobitz type II block, is relatively rare in second-degree AV block. Second-degree AV block type II is mostly an organic disease, or due to a block below the AV node in the distal or branches of bundle of His. Patients with second-degree AV block type II usually have poorer prognosis.
[ECG Recognition]
1.
The P-R interval is constant.
2.
Regular P wave with abrupt QRS complex drops.
3.
The QRS complexes can be normal (if the block happens in distal bundle of His) or resemble the ECG variant of the bundle branch block or fascicular block in morphology (if the block happens in bundle branch).
4.
The conduction ratio can be constant or varied.
3.2.1.5 High-Degree AV Block
Atrioventricular conduction ratio, which means the ratio of P waves to QRS complexes, is often used to measure the severity of AV block. When a tracing shows 4:3 block, it means only three out of four atrial impulses are able to pass to the ventricles with one impulse blocked; similarly, 4:1 block means only one out of four atrial impulses is able to pass to the ventricles with three impulses blocked. High-degree AV block is identified when two or more successive P wave impulses are not able to reach the ventricles.
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