Page 144 - Small Animal Internal Medicine, 6th Edition

116 PART I Cardiovascular System Disorders

as pulmonary trunk dilation. Doppler or echo-contrast study do well for years with medical management. Surgical closure
can confirm an intracardiac right-to-left shunt. Imaging the of a reversed shunt is generally contraindicated, because the
VetBooks.ir abdominal aorta during venous echo-contrast injection can shunt acts as a “pop-off” valve for high pulmonary and right
heart pressures. The acute increase in PA pressure following
show reversed PDA flow. Peak RV (and in the absence of PS,
pulmonary artery) pressure can be estimated by measuring
Shunt correction can only be considered if sildenafil therapy
the peak velocity of a TR jet. Pulmonary insufficiency flow shunt correction can cause RV myocardial failure and death.
can be used to estimate diastolic pulmonary artery pressure. effectively lowers PA pressure below systemic pressure such
Cardiac catheterization can confirm the diagnosis and quan- that the shunt direction becomes left to right. There are rare
tify the pulmonary hypertension and systemic hypoxemia. reports of successful correction of reversed PDAs following
sildenafil therapy.
Treatment and Prognosis
Therapy is aimed at reducing pulmonary arterial pressure, as
well as managing secondary erythrocytosis to minimize OTHER CARDIOVASCULAR ANOMALIES
signs of hyperviscosity. Exercise restriction is also advised.
Sildenafil citrate is a selective phosphodiesterase-5 inhibi- VASCULAR RING ANOMALIES
tor that reduces pulmonary resistance via nitric oxide– Various vascular malformations originating from the embry-
dependent pulmonary vasodilation. It can reduce the degree onic aortic arch system can occur. These can entrap the
of right-to-left shunting in dogs with pulmonary hyperten- esophagus and sometimes the trachea within a vascular ring
sion, leading to improved clinical signs and exercise toler- at the dorsal heart base. Persistent right aortic arch is the
ance, as well as decreased erythrocytosis. Doses of 1 to 3 mg/ most common vascular ring anomaly in the dog. This devel-
kg q12h or q8h generally are well tolerated and may produce opmental malformation surrounds the esophagus dorsally
some reduction in Doppler-estimated pulmonary artery and to the right with the aortic arch, to the left with the liga-
pressure. Adverse effects of sildenafil can include possible mentum arteriosum, and ventrally with the base of the heart.
hypotension, cutaneous flushing, nasal congestion, or sexual Different vascular ring anomalies can occur as well. In addi-
adverse effects, especially in intact animals. Other vasodila- tion, other vascular malformations such as a left cranial vena
tor drugs tend to produce systemic effects that are similar to cava or PDA may accompany a vascular ring anomaly. Vas-
or greater than those on the pulmonary vasculature; there- cular ring anomalies are rare in cats.
fore they are of little benefit and possibly detrimental. The vascular ring prevents solid food from passing nor-
Erythrocytosis can be managed by periodic phlebotomy mally through the esophagus. Clinical signs of regurgitation
or use of oral hydroxyurea. Ideally, the PCV is maintained at and stunted growth commonly develop within 6 months of
a level where the patient’s signs of hyperviscosity (e.g., rear weaning. Esophageal dilation occurs cranial to the ring; food
limb weakness, shortness of breath, lethargy) are minimal. may be retained in this area. Sometimes the esophagus
A PCV of about 62% to 65% has been recommended, but dilates caudal to the stricture as well, indicating that altered
this may not be optimal for all cases. esophageal motility coexists.
One method of performing phlebotomy is to remove 5 to The animal’s body condition score may be normal ini-
10 mL blood per kilogram of body weight. Another calcula- tially, but progressive debilitation ensues. A palpably dilated
tion involves removing blood volume in relation to actual cervical esophagus (containing food or gas) is evident at
and desired hematocrit as follows: Phlebotomy volume = the thoracic inlet in some cases. Fever and respiratory signs
body weight (kg) × 0.08 (percent blood volume) × 1000 mL/ including coughing, wheezing, and cyanosis usually signal
kg × (actual hematocrit – desired hematocrit)/(actual hema- secondary aspiration pneumonia. However, in some cases
tocrit). Regardless of which calculation is chosen, it is gener- a double aortic arch can cause stridor and other respiratory
ally advisable to replace the blood with an equal volume of signs secondary to tracheal stenosis. Vascular ring anomalies
isotonic fluid to avoid significant hemodynamic shifts. In by themselves do not result in abnormal cardiac sounds.
most patients with marked erythrocytosis, a large bore Thoracic radiographs show a leftward tracheal deviation
needle (>18G) must be used. near the cranial heart border on DV view. Other common
Hydroxyurea therapy (40-50 mg/kg by mouth q48h or signs include a widened cranial mediastinum, focal narrow-
3×/week) can be a useful alternative to periodic phlebotomy ing and/or ventral displacement of the trachea, air or food
in patients with secondary erythrocytosis. A complete blood in the cranial thoracic esophagus, and sometimes evidence
cell count and platelet count should be monitored weekly or of aspiration pneumonia. A barium swallow allows visualiza-
biweekly to start. Possible adverse effects of hydroxyurea tion of the esophageal stricture over the heart base and
include anorexia, vomiting, bone marrow hypoplasia causing cranial esophageal dilation (with or without caudal esopha-
cytopenias, alopecia, and pruritus. Depending on the geal dilation).
patient’s response, the dose can be divided q12h on treat- Surgical division of the ligamentum arteriosum (or other
ment days, administered twice weekly, or administered at vessel if the anomaly is not a persistent right aortic arch) is
less than 40 mg/kg. the recommended therapy. In some cases, a retroesophageal
The prognosis is generally poor in animals with pulmo- left subclavian artery or left aortic arch is also present and
nary hypertension and shunt reversal, although some patients must be divided to free the esophagus. Medical management