What are
the anesthetic implications of intracardiac lesions with left-to-right
shunting?
Lesions with left-to-right shunting include all
forms of atrial septal defect (ASD) and ventricular septal defect (VSD), patent
ductus arteriosus (PDA), and other large aorto-pulmonary connections. The
magnitude and direc-tion of the shunt depends on the difference between the
outflow resistances of the two connections and the size of the defect. The
exceptions are communications between the left ventricle and right atrium where
obligatory shunting of blood occurs due to the large pressure difference
between these two cardiac chambers.
A left-to-right shunt only minimally influences
uptake of inhalation anesthetic agents, unless cardiac output is depressed, in
which case induction is accelerated. Induction with intravenous agents is
delayed, since much of the injected drug is recirculated into the lung. Agents
that cause myocardial depression are poorly tolerated in infants with limited
cardiac reserve. Ketamine, opioid/relaxant tech-niques, or low-dose inhalation
anesthesia with isoflurane or sevoflurane are usually well tolerated.
Infants with large left-to-right shunts have
chronically congested lungs with decreased compliance, increased clos-ing
volume, and increased airway resistance. Therefore, they should have their
airway secured and ventilation controlled during anesthesia. However, close
attention needs to be paid to maintenance of a relatively high pul-monary
vascular resistance (PVR) to avoid excessive perfu-sion of the lungs. This can
be accomplished by using the minimal oxygen concentration necessary to maintain
ade-quate oxygen saturation and by maintaining normocarbia to mild hypercarbia.
Although mild afterload reduction leads to
increased systemic output and a reduction in left-to-right shunting, excessive
systemic vasodilatation can lead to right-to-left shunting and cyanosis if the
PVR is high.
Endocarditis prophylaxis is required for
nonrepaired VSD and PDA, the first 6 months following repair, or after 6 months
if there is a residual lesion present.
Prevention of paradoxical air embolism from IV
lines is mandatory.
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