Biomedical Engineering Reference
In-Depth Information
Fig. 4.5 Conduction
disturbances [ 24 ]
(Used with permission
of C. Cihalik Palacky
University Olomouc,
Czech Republic)
(slowed or blocked). As in the case of SA block, described
above, there are three degrees of AV block that can combine
with each other. The particular degrees differ in the propor-
tion of conducted and unconducted impulses. Third-degree
AV block, in which no impulse is conducted from the atria to
the ventricles, is the most serious form.
difference between second-degree SA block and second-
degree AV block is that, in second-degree SA block, the P
wave as well as the QRS complex are dropped when there is
a missed beat, whereas in second-degree AV block, only the
QRS complex is dropped.
4.1.4.5 Type II Second-Degree AV Block (Mobitz II)
This block is characterized by a constant PQ interval and a
subsequent sudden dropping of a QRS complex with a pre-
served P wave. The ratio of P waves to QRS complexes is
generally n:1, or the QRS complex may be completely absent
after several normal impulse conductions to the ventricles. It
is a more serious block than Mobitz I because it may readily
change into third-degree AV block.
4.1.4.3 First-Degree AV Block
Impulse conduction from the atria to the ventricles is slowed
in first-degree AV block, which is reflected on the ECG as a
prolongation of the PQ interval more than 0.2 s, with the
length of the PQ interval being constant. It usually ranges
from 0.2 to 0.4 s, and in rare cases up to 1 s. Isolated first-
degree AV block is not treated.
4.1.4.4 Type I Second-Degree AV Block
(Mobitz I, Wenckebach)
In this type of block, there is progressive lengthening of the
PQ interval, ultimately leading to a dropped QRS complex.
The progressive prolongation of the PQ interval ultimately
results in a P wave that is not followed by a QRS complex.
After the dropped QRS complex, AV conduction recovers;
thus the whole process is repeated regularly. The ratio of P
waves to QRS complexes generally is n:(n - 1). The ECG
4.1.4.6 Third-Degree AV Block
In third-degree AV block, conduction of impulses from the
atria to the ventricles is blocked completely. No relationship
exists between the atrial and ventricular activity. The atria
are activated by the sinus node and the ventricles from a sec-
ondary focus. When the block is more proximal, it is a junc-
tional rhythm with narrow QRS complexes; when the block
is more distal, it presents as a slower idioventricular rhythm
with wide QRS complexes.
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