Page 178 - Problem-Based Feline Medicine

170 PART 3 CAT WITH SIGNS OF HEART DISEASE

RHYTHM STRIP: II
50 mm/sec; I cm/mV P P P (blocked)

Figure 10.7. Second-degree AV block Mobitz type 1. Note progressive lengthening of the P–R interval.

● The heart rate may be normal or lower than normal. ● The response to atropine should be determined. With
● There is progressive prolongation of the P–R normal AV nodal function, blocked P waves should
interval. disappear and the rhythm becomes regular.
● The P–R interval prolongation is seen at progres-
Second-degree AV block Mobitz type II:
sively decreasing increments.
● The treatment depends on the severity of the block.
● There is progressive shortening of the R–R interval.
● If the ventricular rate is adequate and the patient
● One or more of the P waves are blocked.
does not have symptoms that are explained by the
● The P–R interval of the beat immediately after a
cardiac rhythm, then treat the underlying disease.
blocked P wave is shorter than the beat immedi-
● If the patient is symptomatic for the bradyarrhythmia:
ately before the blocked beat.
– Medical therapy can be tried:
Second-degree AV block Mobitz type II. – Theophylline 20 mg/kg PO every 24 h.
● The basic rhythm is a sinus rhythm. – Propantheline bromide 7.5 mg/cat PO every
● The P–R interval and the R–R interval are 8–12 h.
constant. – Terbutaline 0.625 mg/cat PO every 8–12 h.
● There is unexpected failure of a P wave to conduct – Responses to medical therapy are variable and
to the ventricle. usually temporary, and a pacemaker is usu-
● The ventricular rate is slower than the atrial rate. ally indicated.
● The degree of block, i.e. the relative number of
P waves and QRS complex is variable. Prognosis
● The QRS complex morphology may be abnormal.
Second-degree AV block Mobitz type I:
● The prognosis is good.
Differential diagnosis
Second degree AV block Mobitz type II:
When the atrial and ventricular rates are multiples of ● If the patient is asymptomatic and there is no pro-
each other and variable in rate, third-degree AV block gression of the severity of the AV block, then the
may mimic second-degree AV block Mobitz type II. prognosis is favorable.
Usually the QRS complexes are wide and bizarre with ● This form of AV block may progress to third-
third-degree AV block, but look normal with second- degree AV block.
degree AV block. ● Restoration of normal AV conduction is not expected.
Artifact mimicking a blocked P wave. Premature beats
can be distinguished from motion artifact because they THIRD-DEGREE AV BLOCK
occur simultaneously in all leads.
Classical signs

Treatment ● Weakness.
● Lethargy.
Second-degree AV block Mobitz type I: ● Collapse.
● Usually does not require therapy.