Download Heart Block Review Mechanism of First Degree Heart Block First

Mechanism of First Degree
Heart Block
Heart Block Review
First Degree Block
• Not a “stand alone” rhythm
• Must interpret underlying rhythm
• Only one P-wave per QRS and the PRI is
>0.20 seconds and CONSTANT
• QRS are almost always narrow
AV node holds each impulse longer than normal
before conducting it to the ventricles. Each impulse
is eventually conducted.
First Degree Heart Block
(SR with 1st Degree Block)
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Mechanism of Wenckebach (Type I
Second-Degree Heart Block)
As the sinus node initiates impulses, each one is delayed in
the AV node a little longer than the preceding one, until one
impulse is eventually blocked completely. Those impulses
that are conducted travel normally through the ventricles.
Conduction Patterns in Wenckebach
Type I Second-Degree Heart Block
(Wenckebach)
• More P-waves than QRS, Every QRS is
caused by a P-wave, and PRI
INCREASES until a QRS drops
• QRS are most always narrow
Mechanism of Type II Second
Degree Heart Block
The AV node selectively conducts some beats while
blocking others. Those that are not blocked are
conducted through to the ventricles, although they
may encounter a slight delay in the node. Once in the
ventricles, conduction proceeds normally.
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Type II Second Degree Block
Conduction Ratios in Type II
Second Degree Heart Block
• More P-waves than QRS, But every QRS
is caused by a P-wave and the PRI is
CONSTANT
• PRI may be normal or prolonged
• Conduction may be constant (2:1, 3:1, 4:1,
etc); OR may be variable
• QRS normally narrow, but may be broad
Conduction Ratios in Type II
Second Degree Heart Block
Type II Second Degree Heart Block
with a Prolonged PRI
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Mechanism of 3rd Degree Heart Block
The block at the AV node is complete. The sinus
beats cannot penetrate the node and thus are not
conducted through to the ventricles. An escape
mechanism from either the junction or the ventricles
will take over to pace the ventricles. The atria and
ventricles function in a totally dissociated fashion.
A–V Dissociation in CHB
3rd Degree Heart Block
• More P-waves than QRS, QRS NOT
caused by P-waves, and there is no
correlation between P-waves and QRS
• P-P interval is constant and the R-R
interval is constant
• QRS may be broad or narrow
– Narrow = junctional escape
– Broad = ventricular escape
A–V Dissociation in CHB
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A–V Dissociation in CHB
A–V Dissociation in CHB
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