What does a D-shaped canine heart indicate?

D-Shaped Heart in Dogs: Indicator of Right Ventricular Pressure Overload

A D-shaped (or "reverse D") left ventricle on imaging indicates right ventricular pressure overload from pulmonary hypertension, causing the interventricular septum to flatten or bow leftward into the left ventricular chamber. This geometric distortion occurs when right ventricular systolic pressure approaches or exceeds left ventricular pressure, fundamentally altering normal cardiac geometry 1.

Pathophysiological Mechanism

The D-shaped configuration represents a critical hemodynamic abnormality:

• Septal flattening occurs when elevated right ventricular systolic pressure causes the interventricular septum to lose its normal convex curvature toward the right ventricle and instead flattens or bows into the left ventricular cavity 2, 3
• This geometric change is most pronounced during systole when right ventricular pressure peaks, though it may persist into diastole with severe pulmonary hypertension 3
• The finding indicates that right ventricular afterload has increased substantially, typically from pulmonary arterial hypertension 1

Clinical Significance and Associated Findings

When a D-shaped heart is identified, expect concurrent abnormalities:

• Right ventricular hypertrophy develops as an adaptive response to chronic pressure overload, observed in dogs with severe pulmonary hypertension 2
• Right ventricular dilation progresses as the chamber decompensates, with the right ventricular end-diastolic area index significantly elevated in moderate to severe pulmonary hypertension 3
• Pulmonary artery enlargement accompanies the pressure overload, with main pulmonary artery to aorta ratios exceeding 1.0 and right pulmonary artery to aorta ratios exceeding 0.5 2
• Right-sided congestive heart failure becomes more likely, with significantly higher right ventricular end-diastolic area indices in dogs with R-CHF (13.7 cm²/m²) versus those without (9.4 cm²/m²) 3

Common Underlying Etiologies in Dogs

The D-shaped configuration most frequently results from:

• Heartworm disease (Dirofilaria immitis) causing pulmonary endarteritis and secondary pulmonary hypertension, with over 70% of infected dogs showing at least one relevant finding suggestive of pulmonary hypertension 2
• Chronic pulmonary disease leading to pulmonary arterial hypertension 1
• Congenital heart disease with left-to-right shunting causing pulmonary overcirculation 1

Diagnostic Approach

Echocardiography is essential for confirming the D-shaped configuration and quantifying severity:

• Measure Right Pulmonary Artery Distensibility (RPAD) Index: values <29.5% indicate moderate to severe pulmonary hypertension, with RPAD <23% defining severe pulmonary hypertension 2, 4
• Assess tricuspid regurgitation pressure gradient to grade pulmonary hypertension severity (mild, moderate, severe) 3, 1
• Calculate right ventricular end-diastolic area index (RVEDA divided by body surface area) to quantify right ventricular enlargement 3
• Evaluate right ventricular systolic time intervals, with acceleration time <80 ms and AT/ET <0.31 indicating severe pulmonary hypertension 2

Thoracic radiographs provide supportive evidence but cannot replace echocardiography:

• The "reverse D" cardiac silhouette on radiographs correlates with increasing pulmonary hypertension severity 1
• Pulmonary artery enlargement ratios (CrPA/R4 ≥1.08 and CdPA/R9 ≥1.10) show high sensitivity (87.5% and 96.88% respectively) for detecting pulmonary hypertension 4
• However, radiographic findings alone perform poorly in differentiating severity grades and should be used in conjunction with Doppler echocardiography 1

Prognostic Implications

The presence of a D-shaped heart carries significant prognostic weight:

• Severity of tricuspid regurgitation is the main independent predictor of right ventricular enlargement in dogs with pulmonary hypertension 3
• Dogs with persistent moderate to severe pulmonary hypertension after treatment (such as heartworm adulticide therapy) may benefit from long-term monitoring and medical management 2
• Right-sided congestive heart failure development is predicted by elevated right ventricular end-diastolic area index 3

Critical Clinical Pitfall

Do not rely solely on radiographic assessment of the D-shaped configuration. While the "reverse D" sign on thoracic radiographs is significantly associated with pulmonary hypertension severity, it has poor discriminatory ability between different severity grades 1. Always confirm with echocardiography to quantify pulmonary pressures, assess right ventricular function, and guide therapeutic decisions 2, 1.