Thoracic radiography is not specific for pulmonary hypertension but may demonstrate findings supportive of it. Depending on
the underlying cause of pulmonary hypertension, cardiomegaly, right-sided heart enlargement, or pulmonary artery dilation
may be evident. In cases of congestive heart failure, pneumonia, or neoplasia, pulmonary infiltrates may be present. Pulmonary
arteries may be tortuous in patients with heartworm disease (Figures 1A & 1B).
1A. (Click photo for caption)
The electrocardiogram (ECG) is a nonspecific test for pulmonary hypertension with low sensitivity. The ECG may be normal or
show right axis deviation or a right-sided heart enlargement pattern.
1B. (Click photo for caption)
The echocardiogram is the gold standard noninvasive diagnostic tool used in veterinary medicine to diagnose pulmonary hypertension.
Although tricuspid valve regurgitation maximum velocity measurement is the most common echocardiographic method used to diagnose
systolic pulmonary hypertension in a clinical setting, other subjective and objective assessments are also possible. Subjective
assessment of the cardiac structures and pulmonary artery velocity profiles can aid in diagnosing pulmonary hypertension,
and objective assessment of pulmonary valve insufficiency can help diagnose and quantify diastolic pulmonary hypertension.
Subjective echocardiographic findings. Patients with pulmonary hypertension have increased right ventricular afterload and may develop right-sided heart changes
associated with this pressure increase. Right ventricular and atrial chamber sizes may be normal or mildly to severely dilated.
Right wall thicknesses may be normal, or some degree of right ventricular hypertrophy may be evident (Figure 2). Paradoxical septal motion and septal flattening support right ventricular pressure overload. Pulmonary artery dilation
is often seen in patients with pulmonary hypertension, and in cases of heartworm infection, heartworms may be visualized in
the right heart or right pulmonary artery.
2. (Click photo for caption)
Pulmonary artery velocity profiles may also aid in the echocardiographic diagnosis of pulmonary hypertension. Spectral Doppler
echocardiography is used to measure the outflow velocity across the pulmonary valve by generating a pulmonary artery velocity
profile.10 In patients with normal pulmonary arterial pressure, a type I profile is recorded with equal acceleration and deceleration
times producing a symmetrical appearance. As pulmonary arterial pressures increase, an asymmetrical profile is recorded because
of early rapid acceleration and an earlier peak velocity. This asymmetrical profile occurs as a result of increased rate of
rise in pulmonary vascular pressures and is referred to as a type II profile. Type II profiles are associated with mild to moderate pulmonary hypertension. With severe pulmonary hypertension, a rapid
acceleration with an earlier peak velocity occurs, and a midsystolic notch is also present. This is referred to as a type III profile. The midsystolic notch indicates blood flow reversal secondary to high pulmonary vascular pressures (Figure 3).
3. (Click photo for caption)
Doppler assessment of systolic pulmonary arterial pressure. Tricuspid valve regurgitation occurs during ventricular systole and can be viewed during an echocardiogram in right parasternal
and left apical views (Figures 4 & 5). In these views, the tricuspid regurgitation jet is assessed by using color-flow Doppler, and spectral Doppler is placed
over the tricuspid regurgitation jet to obtain tricuspid regurgitation velocity profiles. The peak of the profile indicates
maximum tricuspid regurgitation velocity.
4. (Click photo for caption)
The maximum tricuspid regurgitation velocity estimates the pressure gradient across the tricuspid valve, which approximates
the pressure difference between the right ventricle and right atrium. The pressure gradient can be calculated by applying
the measured maximum tricuspid regurgitation velocity to the modified Bernoulli equation3:
5. (Click photo for caption)
pressure gradient = maximum tricuspid regurgitation velocity
When an estimated right atrial pressure is added to the calculated pressure gradient, an estimate of right ventricular pressure
can be obtained. Right atrial pressures are estimated based on subjective assessment of the right atrium's size on an echocardiogram.
A normal right atrium is assigned a pressure of 0 mm Hg, a mildly dilated right atrium has an estimated pressure of 5 mm Hg,
a moderately dilated right atrium has an estimated pressure of 10 mm Hg, and a severely dilated right atrium has an estimated
pressure of 15 mm Hg.11,12