Understanding Vena Cava to Right Pulmonary Artery Notation on Heart Catheterization Reports
The notation "Vd to Rpda" on a heart catheterization report most likely represents oxygen saturation measurements or pressure readings obtained sequentially from the vena cava (Vd) to the right pulmonary artery (Rpda), which are essential components of comprehensive hemodynamic assessment during right heart catheterization.
Standard Right Heart Catheterization Protocol
During right heart catheterization, the catheter is advanced sequentially through the venous system, and measurements are obtained at each anatomic location 1:
- Oxygen saturations are measured in the superior vena cava (SVC), inferior vena cava (IVC), right ventricle (RV), and pulmonary artery (PA) to detect intracardiac shunts 1
- Pressure measurements are recorded at each location including right atrial pressure, right ventricular pressure, and pulmonary artery pressures (systolic, diastolic, and mean) 1
- The catheter measures pressures as it advances from the vena cava through the right atrium, right ventricle, and into the pulmonary artery 1
Clinical Significance of Sequential Measurements
Detection of Intracardiac Shunts
Oxygen saturation "step-ups" between the vena cava and pulmonary artery indicate left-to-right shunting from conditions such as atrial septal defect (ASD), ventricular septal defect (VSD), or patent ductus arteriosus 1. A significant increase in oxygen saturation (typically >7-10%) between sequential chambers confirms the presence and location of a shunt 1.
Hemodynamic Assessment
The pressure gradient from the vena cava through the right heart to the pulmonary artery helps identify 1:
- Right ventricular dysfunction: Elevated right atrial pressure with normal or low pulmonary artery wedge pressure suggests primary right heart failure 2
- Pulmonary hypertension: Mean pulmonary artery pressure >20 mmHg defines pulmonary hypertension 3
- Tricuspid valve disease: Pressure differences between right atrium and right ventricle indicate stenosis or regurgitation 1
Interpreting Abnormal Findings
Pre-capillary vs. Post-capillary Pulmonary Hypertension
The relationship between pulmonary artery pressure and pulmonary capillary wedge pressure (PCWP) distinguishes the etiology of pulmonary hypertension 2:
- PCWP ≤15 mmHg indicates pre-capillary pulmonary hypertension (pulmonary arterial hypertension, chronic thromboembolic disease) 2, 3
- PCWP >15 mmHg indicates post-capillary pulmonary hypertension from left heart disease 2, 3
- Transpulmonary gradient (mean PA pressure minus PCWP) >12 mmHg suggests intrinsic pulmonary vascular changes beyond passive elevation 2
Vena Cava Backflow
Abnormal flow patterns in the vena cava indicate right ventricular dysfunction 4:
- Backflow during atrial contraction (late diastole) results from increased right ventricular end-diastolic stiffness and impaired filling 4
- Backflow during systole indicates significant tricuspid regurgitation 4
- Backward fraction >20% correlates with elevated right atrial pressure (r=0.77) and right ventricular stiffness (r=0.65) 4
Common Pitfalls and Caveats
Measurement Accuracy
Pressure measurements must be obtained at end-expiration during normal breathing to minimize respiratory artifact 1. Inspiration decreases intrathoracic pressure and artificially lowers pulmonary artery pressure relative to atmospheric pressure 1.
Conditions Affecting Interpretation
Several conditions complicate the relationship between vena cava and pulmonary artery measurements 1, 2:
- Severe tricuspid regurgitation causes overestimation or underestimation of cardiac output by thermodilution and affects right atrial pressure interpretation 1
- Positive end-expiratory pressure (PEEP) ventilation elevates measured pressures without reflecting true intravascular volume 1, 2
- Right atrial pressure rarely correlates with left atrial pressure in acute heart failure, so avoid using central venous pressure as a surrogate for left ventricular filling pressure 1, 2
Volume vs. Pressure Assessment
Inferior vena cava diameter provides complementary information to right atrial pressure and may better reflect intravascular volume changes 5. The correlation between IVC diameter and right atrial pressure is only modest (r=0.41), and hemoconcentration correlates with IVC diameter changes (r=0.35) but not with right atrial pressure changes 5.
Safety Considerations
Right heart catheterization performed in experienced centers carries low risk 6: