Key Calculations and Considerations for Cardiac ASD Catheterization Study
Cardiac catheterization for atrial septal defect (ASD) assessment should focus on quantifying shunt magnitude, pulmonary vascular resistance, and hemodynamic impact to determine intervention necessity. 1
Essential Hemodynamic Calculations
1. Shunt Quantification
- Pulmonary-to-Systemic Flow Ratio (Qp:Qs)
- Most critical calculation for ASD assessment
- Formula: Qp:Qs = (SaO₂ - MvO₂) / (PvO₂ - PaO₂)
- SaO₂ = Systemic arterial oxygen saturation
- MvO₂ = Mixed venous oxygen saturation
- PvO₂ = Pulmonary venous oxygen saturation
- PaO₂ = Pulmonary arterial oxygen saturation
- Significant shunt: Qp:Qs > 1.5:1 1
- Hemodynamically significant ASD typically has Qp:Qs of 2:1 or greater 2
2. Pressure Measurements
- Chamber Pressures
- Right atrium (RA)
- Left atrium (LA)
- Right ventricle (RV)
- Left ventricle (LV)
- Pulmonary artery (PA)
- Aorta
- Pressure Gradients
- Across the ASD (RA to LA)
- Across pulmonary and systemic circulations
3. Pulmonary Vascular Resistance (PVR)
- Formula: PVR = (mPAP - PCWP) / Qp × 80 [Wood units]
- mPAP = Mean pulmonary artery pressure
- PCWP = Pulmonary capillary wedge pressure
- Qp = Pulmonary blood flow
- PVR assessment critical for determining operability in adults with longstanding ASD 1
- Elevated PVR (>5 Wood units) may contraindicate ASD closure 1
4. Systemic Vascular Resistance (SVR)
- Formula: SVR = (MAP - RAP) / Qs × 80 [Wood units]
- MAP = Mean arterial pressure
- RAP = Right atrial pressure
- Qs = Systemic blood flow
Anatomical Assessment
1. ASD Characterization
- Location classification 1, 3
- Secundum (mid-septum) - most common, amenable to device closure
- Primum (low septum) - associated with AV valve abnormalities
- Sinus venosus (high septum) - often with partial anomalous pulmonary venous return
- Coronary sinus defect (rare)
- Size measurement
- Direct measurement with sizing balloon
- Multiple projections to capture true dimensions
2. Associated Anomalies
- Partial anomalous pulmonary venous return
- Mitral valve abnormalities (especially with primum ASD)
- Pulmonary valve stenosis
- Coronary sinus defects
Clinical Decision-Making Parameters
1. Indications for Intervention 1
- Qp:Qs > 1.5:1
- Evidence of RV volume overload
- History of paradoxical embolism
- Exercise intolerance attributable to ASD
2. Contraindications for Closure 1
- Advanced pulmonary vascular obstructive disease
- Severe pulmonary hypertension with right-to-left shunting
Presentation Format for Case Discussion
1. Patient Demographics and History
- Age, gender, presenting symptoms
- Previous cardiac evaluations
- Exercise capacity and functional status
2. Non-invasive Imaging Review
- ECG findings (right axis deviation, incomplete RBBB in secundum ASD; superior left axis deviation in primum ASD) 1, 4
- Chest X-ray findings (RV enlargement, increased pulmonary vascularity) 1
- Echocardiographic data (defect size, location, RV size, estimated PA pressures) 1
3. Catheterization Data Presentation
- Oxygen saturations at key locations
- Calculated Qp:Qs ratio
- Pressure measurements in all chambers
- PVR and SVR calculations
- Angiographic findings
4. Management Recommendations
- Closure indication based on hemodynamic data
- Method of closure (device vs. surgical)
- Timing considerations
- Follow-up recommendations
Common Pitfalls to Avoid
- Oxygen step-up misinterpretation: Ensure proper sampling locations for accurate Qp:Qs calculation
- Inadequate assessment of pulmonary veins: Always rule out partial anomalous pulmonary venous return
- Overlooking multiple defects: Thorough angiographic assessment needed
- Misclassification of ASD type: Impacts closure approach (device vs. surgical)
- Underestimating pulmonary hypertension: Critical for determining closure safety 1
Remember that cardiac catheterization is not indicated in younger patients with uncomplicated ASD when imaging results are adequate 1, but remains valuable for assessing complex anatomy, pulmonary vascular resistance, and coronary artery disease risk in older patients.