Calculation of Pulmonary and Systemic Vascular Resistance

Introduction to Vascular Resistance Calculation

• The American College of Cardiology states that both pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) are calculated using the same fundamental principle: the pressure gradient across the vascular bed divided by cardiac output, with PVR using transpulmonary gradient and SVR using the systemic arterial-to-venous gradient 1

PVR Calculation Formula

• The American College of Cardiology recommends that PVR is calculated as (mean pulmonary artery pressure minus mean pulmonary capillary wedge pressure) divided by cardiac output 1
• The formula is: PVR = (mPAP - PCWP) / CO, with units expressed as Wood units or dynes·s·cm⁻⁵ 1, 2, 3
• To convert Wood units to dynes·s·cm⁻⁵, multiply by 80, according to the American Journal of Respiratory and Critical Care Medicine 2
• Normal PVR values are typically <2-3 Wood units, as stated by the American Journal of Respiratory and Critical Care Medicine and Circulation 2, 3

Required Measurements for PVR

• Mean pulmonary artery pressure (mPAP) should be measured via right heart catheterization, as recommended by the American Journal of Respiratory and Critical Care Medicine and the American College of Cardiology 1, 2
• Pulmonary capillary wedge pressure (PCWP) or pulmonary artery wedge pressure (PAWP) should be measured by wedging a pulmonary catheter with an inflated balloon into a small pulmonary arterial branch, according to the American Journal of Respiratory and Critical Care Medicine and the American College of Cardiology 1, 2
• Cardiac output (CO) should be measured in L/min, typically via thermodilution or Fick method, as stated by the American Journal of Respiratory and Critical Care Medicine and the American College of Cardiology 1, 2

SVR Calculation Formula

• The American College of Cardiology states that SVR is calculated as the systemic mean arterial blood pressure minus right atrial pressure divided by cardiac output 1
• The formula is: SVR = (MAP - RAP) / CO, with units expressed as dynes·s·cm⁻², according to the American College of Cardiology 1
• Normal SVR values are approximately 800-1200 dynes·s·cm⁻², as stated by Praxis Medical Insights 4

Required Measurements for SVR

• Mean arterial pressure (MAP) should be measured from systemic arterial line or calculated as (systolic BP + 2 × diastolic BP) / 3, according to the American College of Cardiology and Praxis Medical Insights 1, 4
• Right atrial pressure (RAP) or central venous pressure (CVP) should be measured via central venous catheter, as recommended by the American Journal of Respiratory and Critical Care Medicine and the American College of Cardiology 1, 2
• Cardiac output (CO) should be measured in L/min, as stated by the American College of Cardiology 1

Critical Factors to Consider

• Direct measurement of oxygen consumption (VO₂) is preferable to estimation, particularly in children <3 years of age, as the LaFarge equation can overestimate VO₂ and lead to underestimation of PVR, according to Circulation 5
• In patients with tricuspid regurgitation and right ventricular dilatation, cardiac output measurements by thermodilution can be erroneous, affecting all derived resistance calculations, as stated by Praxis Medical Insights 6
• Cardiac index (CI) should be calculated by dividing CO by body surface area (L/min/m²) for size-adjusted comparisons, according to the American Journal of Respiratory and Critical Care Medicine and the American College of Cardiology 1, 2

Clinical Context for PVR Interpretation

• In congenital heart disease, surgical repair is recommended when PVR is less than one-third of SVR, as stated by Praxis Medical Insights 4
• A PVR >6 Wood units·m² predicts poor outcomes in single ventricle patients undergoing cavopulmonary surgery, according to Circulation 5
• A transpulmonary gradient >6 mm Hg (mPAP - PCWP) suggests high risk for poor outcomes in cavopulmonary anastomosis, as stated by Circulation 5
• PVR >8 Wood units·m² in children with ventricular septal defects has been associated with poor surgical outcomes, though positive acute vasodilator testing may predict benefit, according to Circulation 5

Critical Hemodynamic Relationship

• In pulmonary arterial hypertension patients, SVR must be maintained greater than PVR to prevent right ventricular ischemia, as right ventricular coronary perfusion occurs during both systole and diastole, according to Circulation Research and Praxis Medical Insights 4, 7
• If systolic pulmonary arterial pressure exceeds systolic systemic arterial pressure (PVR > SVR during systole), right ventricular ischemia results, as stated by Circulation Research and Praxis Medical Insights 4, 7

Measurement Considerations

• Volume status assessment in pulmonary hypertension patients is notoriously difficult, and non-invasive estimates of central venous pressures may be misleading, requiring direct central line measurement, according to Circulation Research and Praxis Medical Insights 4, 7
• During mechanical ventilation with high PEEP, calculating the transmural value of PCWP allows estimation of true left ventricular filling pressure, as stated by Praxis Medical Insights 6
• PCWP may be recorded with or without V-wave; this should be documented, according to the American College of Cardiology 1

Common Pitfalls and Caveats

• Measurements should be obtained under standardized conditions, as general anesthesia can lower systemic arterial blood pressure and affect resistance calculations, according to Circulation 5
• Acute hemodynamic evaluation provides only a snapshot and may not represent overall clinical status, as stated by Circulation 5
• Echocardiographic estimation of pulmonary artery systolic pressure has limited positive predictive value (25-64%) for confirming pulmonary hypertension, according to the American Journal of Respiratory and Critical Care Medicine 2

Vascular Resistance Units and Clinical Applications

Introduction to Vascular Resistance Units

• The European Society of Cardiology and European Respiratory Society recommend using Wood units to express vascular resistance, as they simplify calculations and reduce mathematical errors 8
• The American College of Cardiology guidelines state that Wood units are preferred over dynes·s·cm⁻⁵ for clinical practice 9

Clinical Applications of Wood Units

• The American College of Cardiology recommends using Wood units for decision-making in congenital heart disease repair, with a contraindication for shunt closure if PVR >2.5 WU 10
• The European Association for the Study of the Liver recommends using Wood units to assess liver transplant candidacy in portopulmonary hypertension, with a requirement of PVR <3 WU 11
• The American College of Cardiology guidelines define pre-capillary pulmonary hypertension as PVR >3 WU in 2015 guidelines, and >2 WU in 2024 guidelines 8

Critical Clinical Thresholds

• The American College of Cardiology guidelines state that a PVR >2.5 WU or >4 WU·m² indexed is a contraindication for congenital heart disease repair 10
• The European Association for the Study of the Liver recommends that portopulmonary hypertension transplant eligibility be assessed using PVR <5 WU with mPAP <35 mmHg, or PVR <3 WU with mPAP 35-45 mmHg 11

Practical Recommendations

• The European Society of Cardiology and European Respiratory Society recommend using Wood units for all clinical decision-making and documentation, as this aligns with current international guidelines and minimizes calculation errors 8

Calculating Pulmonary Vascular Resistance

Terminology and Measurement

• The European Society of Cardiology guidelines use the term PAWP (Pulmonary Artery Wedge Pressure) interchangeably with PCWP, particularly in European guidelines 12, 13, 14
• The measurement of PCWP/PAWP/PAOP is obtained by advancing a balloon-tipped catheter into a small pulmonary arterial branch until the inflated balloon "wedges" or occludes flow, allowing measurement of downstream left atrial pressure 13, 14
• Proper technique is essential for accurate PVR calculation, including zeroing the external pressure transducer at the mid-thoracic line and avoiding repeated balloon deflations and inflations in distal pulmonary arteries 13, 14

Clinical Decision Thresholds

• The European Heart Journal recommends a PVR threshold of >3 Wood units for the diagnosis of pulmonary arterial hypertension (precapillary pulmonary hypertension) 12, 13, 14
• A PVR >2.5 Wood units or >4 Wood units·m² indexed is considered a contraindication for congenital heart disease shunt closure, according to the European guidelines 13, 14

Alternative Approach

• The transpulmonary pressure gradient (TPG = mPAP - PCWP) provides valuable information about pulmonary vascular abnormality independent of calculated PVR, and a TPG >6 mmHg suggests high risk for poor outcomes in cavopulmonary anastomosis 15

Calculating Pulmonary Vascular Resistance

Measurement Techniques and Terminology

• The American College of Cardiology and European Heart Society use the term PCWP, while the European Respiratory Society uses PAWP, and both terms refer to the same measurement used in the PVR calculation formula, with the measurement technique being identical for all terms 16, 17, 18
• Accurate PCWP/PAWP measurement is essential for correct PVR calculation, and must be obtained with proper technique, including zeroing the external pressure transducer at the mid-thoracic line and avoiding repeated balloon deflations and inflations in distal pulmonary arteries, as recommended by the European Heart Society 17
• A PCWP/PAWP >15 mmHg excludes the diagnosis of pre-capillary pulmonary arterial hypertension, according to the European Heart Society guidelines 17

Clinical Application Thresholds

• The European Respiratory Society defines pre-capillary pulmonary hypertension by PVR >3 Wood units, and the 2024 guidelines define it as PVR >2 Wood units, regardless of which terminology is used for the wedge pressure measurement, with a threshold of PVR >2.5 Wood units for shunt closure in congenital heart disease 18

Pulmonary Vascular Resistance Index Measurement and Interpretation

Definition and Units

• The Pulmonary Vascular Resistance Index (PVRI) is measured in Wood units·m² (WU·m²), which represents the indexed value of pulmonary vascular resistance adjusted for body surface area 19, 20
• PVRI is expressed as Wood units multiplied by meters squared (WU·m²), which represents the indexed value of pulmonary vascular resistance adjusted for body surface area 19, 20

Clinical Thresholds and Prognosis

• PVRI >6 WU·m² predicts poor prognosis in children with congenital heart disease, regardless of lung morphology 19, 20
• PVRI <7 to 8 WU·m² in response to vasodilator challenge predicts good surgical outcomes in patients with simple shunts 19, 20
• The indexed value (PVRI) is particularly important in pediatric populations where body size varies significantly and affects hemodynamic measurements 19, 20

Distinction from Non-Indexed PVR

• Non-indexed pulmonary vascular resistance (PVR) is measured in Wood units (WU) alone, without the m² designation 21

Calculating Pulmonary Vascular Resistance Index

Clinical Applications

• The American Heart Association recommends that PVRI <6 WU·m² is an indicator for repair in children with structural heart disease (ASD, VSD, PDA) 22
• The American College of Cardiology suggests that PVRI ≥6 WU·m² indicates that repair is not indicated unless acute vasodilator testing demonstrates reversibility (absolute PVRI <6 WU·m² and PVR/SVR <0.3) 22

Measurement Considerations

• The American Heart Association states that mean pulmonary artery pressure (mPAP) must be measured via right heart catheterization, with measurements taken at end expiration if breathing spontaneously, or at end inspiration if mechanically ventilated 23
• The American College of Cardiology recommends that pulmonary capillary wedge pressure (PCWP) be obtained by wedging a balloon-tipped catheter into a small pulmonary arterial branch with the balloon inflated, and that the external pressure transducer must be zeroed at the mid-thoracic line 23
• The American Heart Association notes that blood pH has a potent effect on pulmonary vascular tone—acidosis causes vasoconstriction while alkalosis causes vasodilation, and that awareness of arterial blood gas measurements during catheterization is critical for accurate interpretation of baseline hemodynamics 23

Avoiding Common Pitfalls

• The American College of Cardiology advises that inaccurate wedge pressure can be avoided by verifying PCWP accuracy, especially if values seem unusual; measuring LV end-diastolic pressure simultaneously if needed 23
• The American Heart Association recommends that timing of measurements be ensured, with pressures recorded at appropriate points in the respiratory cycle (end expiration for spontaneous breathing, end inspiration for mechanical ventilation) 23

Calculating Pulmonary Vascular Resistance

Introduction to PVR Calculation

• The American College of Cardiology recommends using Pulmonary Capillary Wedge Pressure (PCWP) as a validated surrogate for left atrial pressure in the absence of pulmonary vein obstruction, with measurement obtained by wedging a balloon-tipped catheter into a small pulmonary arterial branch with the balloon inflated, and PCWP should be measured at end-expiration during spontaneous breathing 24

Technical Considerations for PVR Calculation

• Proper PCWP measurement technique is essential for accurate PVR calculation, including measuring at end-expiration during spontaneous breathing 24

Clinical Scenarios for Direct Left Atrial Pressure Measurement

• Direct left atrial pressure or LVEDP measurement is recommended in patients where left heart disease is the likely etiology, such as those with orthopnea or associated risk factors 24
• Direct left atrial pressure measurement is indicated when there is doubt about the accuracy of PCWP, elevation of pulmonary artery pressure is out of proportion to mean gradient and valve area, or clinical and hemodynamic findings are discrepant 24, 25, 26

Limitations of PCWP

• PCWP may overestimate the true transmitral gradient due to phase delay when compared to direct left ventricular pressure 25
• PCWP can be inaccurate in certain clinical scenarios, such as in patients with pulmonary hypertension, and may underestimate true PVR in single-ventricle patients after bidirectional Glenn operation 25

Measurement of Mean Pulmonary Artery Pressure for PVR Calculation

Direct Invasive Measurement via Right Heart Catheterization

• Mean pulmonary artery pressure (mPAP) must be measured directly via right heart catheterization (RHC) to calculate pulmonary vascular resistance (PVR), as this is the gold standard and only validated method for accurate PVR determination, according to the American College of Radiology 27

Noninvasive Estimation Methods

• Phase-contrast imaging techniques can measure average blood flow velocity of the main pulmonary artery, which correlates with mean pulmonary arterial pressure, as suggested by the American College of Radiology 28, 29
• Decreased pulmonary artery blood flow velocity correlates to increased vascular resistance, as reported by the American College of Radiology 28, 29
• A small study suggested that estimation of mPAP from high-temporal-resolution phase-contrast MRI is possible, but further investigation in a larger population is still needed, according to the American College of Radiology 28

Clinical Thresholds for PVR

• PVR >3 Wood units defines precapillary pulmonary hypertension (2018 guidelines), and PAWP ≤15 mm Hg must accompany elevated PVR for precapillary PH diagnosis, as stated by the American College of Radiology 27
• Doppler echocardiography can estimate systolic pulmonary artery pressure using tricuspid regurgitation velocity, but has limited positive predictive value (25-64%) for confirming pulmonary hypertension, as reported by the American College of Chest Physicians 30
• Echocardiographic estimation of pulmonary artery systolic pressure has limited accuracy and should not replace RHC for PVR calculation in clinical decision-making, according to the American College of Chest Physicians 30

Pulmonary Vascular Resistance Index in Clinical Practice

Preoperative Thresholds for Surgical Outcomes

• Many centers use a preoperative PVR less than 10 to 14 Wood units and a pulmonary/systemic resistance ratio less than or equal to two-thirds as thresholds associated with better surgical outcomes, as recommended by the American Heart Association 31, 32

Diagnostic Criteria for Pulmonary Arterial Hypertension

• The diagnostic criteria for PAH requires both a mean PAP greater than 25 mmHg and a PVR greater than 3 Wood units, according to the American College of Cardiology 33, 34
• The 2015 ESC/ERS guidelines included PVR >3 Wood units in the hemodynamic definition of PAH, as stated by the European Society of Cardiology 35

Hemodynamic Measurements

• Measurements should be obtained under standardized conditions, as general anesthesia can lower systemic arterial blood pressure and affect resistance calculations, as noted by the American Heart Association 31, 32