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DRUG INTERACTIONS
Drug Interactions
If other antiarrhythmic drugs are being used, additive effects on the heart may occur with PA administration, and dosage reduction may be necessary (see
WARNINGS
).
Anticholinergic drugs administered concurrently with PA may produce additive antivagal effects on A-V nodal conduction, although this is not as well documented for PA as for quinidine.
Patients taking PA who require neuromuscular blocking agents such as succinylcholine may require less than usual doses of the latter, due to PA effects on reducing acetylcholine release.
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OVERDOSAGE
Progressive widening of the QRS complex, prolonged
Q-T and P-R intervals, lowering of the R and T waves, as well as increasing
A-V block, may be seen with doses which are excessive for a given
patient. Increased ventricular extrasystoles, or even ventricular
tachycardia or fibrillation may occur. After intravenous administration
but seldom after oral therapy, transient high plasma levels of PA
may induce hypotension, affecting systolic more than diastolic pressures,
especially in hypertensive patients. Such high levels may also produce
central nervous depression, tremor, and even respiratory depression.
Plasma levels above 10 mcg/mL are increasingly associated
with toxic findings, which are seen occasionally in the 10 to 12 mcg/mL
range, more often in the 12 to 15 mcg/mL range, and commonly in patients
with plasma levels greater than 15 mcg/mL.
Treatment
of overdosage or toxic manifestations includes general supportive
measures, close observation, monitoring of vital signs and possibly
intravenous pressor agents and mechanical cardiorespiratory support.
If available, PA and NAPA plasma levels may be helpful in assessing
the potential degree of toxicity and response to therapy. Both PA
and NAPA are removed from the circulation by hemodialysis but not
peritoneal dialysis. No specific antidote for PA is known.
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CONTRAINDICATIONS
Complete Heart Block:
Procainamide should not be administered to patients with complete
heart block because of its effects in suppressing nodal or ventricular
pacemakers and the hazard of asystole. It may be difficult to recognize
complete heart block in patients with ventricular tachycardia, but
if significant slowing of ventricular rate occurs during PA treatment
without evidence of A-V conduction appearing, PA should be stopped.
In cases of second degree A-V block or various types of hemiblock,
PA should be avoided or discontinued because of the possibility of
increased severity of block, unless the ventricular rate is controlled
by an electrical pacemaker.
Idiosyncratic Hypersensitivity:
In patients
sensitive to procaine or other ester-type local anesthetics, cross
sensitivity to PA is unlikely. However, it should be borne in mind,
and PA should not be used if it produces acute allergic dermatitis,
asthma, or anaphylactic symptoms.
Lupus Erythematosus:
An established diagnosis
of systemic lupus erythematosus is a contraindication to PA therapy,
since aggravation of symptoms is highly likely.
Torsades de Pointes:
In the unusual
ventricular arrhythmia called “les torsades de pointes”
(twistings of the points), characterized by alternation of one or
more ventricular premature beats in the directions of the QRS complexes
on ECG in persons with prolonged Q-T and often enhanced U waves, Group
1A antiarrhythmic drugs are contraindicated. Administration of PA
in such cases may aggravate this special type of ventricular extrasystole
or tachycardia instead of suppressing it.
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