From the Guidelines
To calculate pulmonary vascular resistance (PVR) in a patient with a right-to-left heart shunt during right heart catheterization, you must use the modified Fick principle to account for the shunt, as stated in the 2019 AHA/ACC guideline for the management of adults with congenital heart disease 1.
Key Steps for Calculation
- Obtain the mean pulmonary artery pressure (mPAP) and left atrial pressure (LAP) or pulmonary capillary wedge pressure (PCWP) as a surrogate.
- Calculate the effective pulmonary blood flow (Qp) using oxygen consumption and the difference between pulmonary venous and pulmonary arterial oxygen content.
- The formula is PVR = (mPAP - LAP)/Qp × 80, expressed in Wood units.
Importance of Accurate Measurement
This modified approach is necessary because the shunt causes some blood to bypass the lungs, making conventional cardiac output measurements unreliable for PVR calculations.
Additional Considerations
- Oxygen saturation samples must be taken from multiple locations including the pulmonary artery, pulmonary vein (or left atrium if accessible), and systemic arterial circulation, as emphasized in the pediatric pulmonary hypertension guidelines from the American Heart Association and American Thoracic Society 1.
- Accurate hemoglobin measurement is also essential.
Clinical Context
Invasive hemodynamic assessment still has an important role as a confirmatory tool and for the evaluation of pulmonary vasoreactivity, which carries prognostic significance 1.
Decision Making
Pulmonary blood flow and thus Qp:Qs can be difficult to calculate accurately because of differences in right/left PA blood flow caused by the flow from the PDA, making invasive hemodynamics including pulmonary vascular resistance generally relied on for decision-making 1.
From the Research
Calculating PVR during Right Heart Catheterization
To calculate the Pulmonary Vascular Resistance (PVR) during a right heart catheterization in a patient with a right to left heart shunt, the following steps can be considered:
- Measure the cardiac output (CO) using either the thermodilution method or the indirect Fick method 2.
- Measure the mean pulmonary artery pressure (mPAP) and the pulmonary capillary wedge pressure (PCWP) or left atrial pressure (LAP) 3.
- Calculate the PVR using the formula: PVR = (mPAP - PCWP or LAP) / CO 3.
- Note that the choice of method for measuring CO may affect the calculated PVR, with the thermodilution method potentially being more accurate in patients with elevated mPAP 2.
- In patients with single-ventricle physiology, the use of pulmonary artery wedge pressure (PAWp) as a surrogate for LAP may underestimate the true PVR 4.
Considerations for PVR Calculation
- The accuracy of PVR calculation is crucial in patients with right to left heart shunt, as it can impact therapeutic management strategies 3.
- The choice of method for measuring CO and the interpretation of hemodynamic measurements can be challenging due to differences in techniques 3.
- In patients with univentricular physiology, a PVR measurement of greater than 2.3 W.U. by the PAWp method should prompt a direct LA pressure measurement to determine true surgical risk 4.
- CMR fluoroscopy guided cardiac catheterization can provide accurate measurements of cardiac output and PVR using CMR flow and the Fick principle 5.