What parameters can be monitored with a Pulmonary Artery Catheter (PAC) and how are they interpreted?

Pulmonary Artery Catheter Monitoring Parameters and Interpretation

When you insert a PAC, you can directly measure right atrial pressure, right ventricular pressure, pulmonary artery pressure (systolic, diastolic, and mean), pulmonary capillary wedge pressure (PCWP), cardiac output via thermodilution, and mixed venous oxygen saturation (SvO₂), with modern catheters also providing continuous cardiac output and right ventricular end-diastolic volume. 1

Direct Measured Parameters

Pressure Measurements

• Right Atrial Pressure (RAP): Normal 2-8 mmHg; reflects right ventricular preload but rarely correlates with left atrial pressures in acute heart failure patients 1
• Right Ventricular Pressure: Systolic normally 15-30 mmHg, end-diastolic 2-8 mmHg 1
• Pulmonary Artery Pressure: Systolic 15-30 mmHg, diastolic 4-12 mmHg, mean 9-18 mmHg 1
• Pulmonary Capillary Wedge Pressure (PCWP): Normal <12 mmHg; estimates left ventricular filling pressure 1, 2

Cardiac Output and Oxygen Parameters

• Cardiac Output (CO): Measured via thermodilution; modern catheters provide semi-continuous measurements 1, 2
• Mixed Venous Oxygen Saturation (SvO₂): Measured in pulmonary artery; normal 65-75%; provides global index of oxygen delivery/consumption balance 1, 3
• Venous Oxygen Saturation: Can be measured in superior vena cava or right atrium as estimate of oxygen transport 1

Advanced Parameters

• Right Ventricular End-Diastolic Volume (RVEDV): Available with modern catheters 1, 3
• Right Ventricular Ejection Fraction: Measured by some advanced PACs 1

Calculated/Derived Parameters

Resistance Calculations

• Pulmonary Vascular Resistance (PVR): Calculated as (mean PAP - PCWP) / CO; normal <2-3 Wood units 2, 4
• Systemic Vascular Resistance (SVR): Calculated as (MAP - RAP) / CO; normal 800-1200 dynes·s·cm⁻² 2, 4
• Pulmonary Vascular Resistance Index (PVRI): PVR indexed to body surface area (Wood units·m²) 4

Cardiac Performance Indices

• Cardiac Index (CI): CO divided by body surface area; normal >2.5 L/min/m²; decreased if <2.2 L/min/m² 1, 4
• Cardiac Power Output: Calculated as (CO × MAP) ÷ 451 2
• Right Ventricular Stroke Work Index: Derived parameter assessing RV contractility 2
• Pulmonary Artery Pulsatility Index: Calculated parameter for RV function assessment 2

Clinical Interpretation Algorithm

Step 1: Assess Cardiac Output and Perfusion Status

• Low CI (<2.2 L/min/m²) with Low PCWP (<14 mmHg): Indicates hypovolemia; therapy is fluid loading 1
• Low CI with High PCWP (18-20 mmHg) and SBP >85 mmHg: Indicates cardiogenic shock with adequate pressure; use vasodilators (nitroprusside, nitroglycerin) and IV diuretics 1
• Low CI with High PCWP and Low SBP: Indicates severe cardiogenic shock; consider inotropic agents (dobutamine, dopamine) and vasoconstrictive inotropes if SBP critically low 1

Step 2: Evaluate Mixed Venous Oxygen Saturation

• SvO₂ <65%: Indicates inadequate oxygen delivery relative to consumption; requires intervention to improve CO, hemoglobin, or oxygen saturation 3
• Decreasing SvO₂ trend: Global index of deteriorating DO₂/VO₂ relationship; use all PAC parameters to guide therapeutic decisions 3

Step 3: Calculate and Interpret Vascular Resistances

• PVR >3 Wood units: Defines precapillary pulmonary hypertension 2, 4
• PVR >2.5 Wood units or PVRI >4 Wood units·m²: Contraindication for congenital heart disease shunt closure 4
• PVR/SVR ratio: In pulmonary hypertension, SVR must exceed PVR to prevent right ventricular ischemia 4

Step 4: Assess Right Ventricular Function

• RVEDV and ejection fraction: Evaluate RV performance, particularly important in right heart failure 1
• RAP elevation with low CO: Suggests RV failure or significant tricuspid regurgitation 1

Critical Pitfalls and Limitations

Measurement Accuracy Issues

• PCWP is NOT accurate for LVEDP in mitral stenosis, aortic regurgitation, ventricular interdependence, high airway pressure, or stiff LV (from LVH, diabetes, fibrosis, inotropes, obesity, ischemia) 1
• Severe tricuspid regurgitation can overestimate or underestimate cardiac output by thermodilution 1
• PEEP ventilation affects CVP and RAP measurements 1
• Catheter position and balloon inflation: Avoid repeated wedge measurements; withdraw catheter to main PA before CPB; ensure proper waveform to avoid pulmonary artery rupture (50% fatal) 1

Technical Considerations

• Measure pressures at end-expiration during spontaneous breathing when intrathoracic pressure approaches atmospheric 1
• Zero transducer at mid-thoracic line for accurate pressure readings 4
• Verify waveform quality: Damped tracings suggest catheter malposition or thrombus 1

Clinical Context Limitations

• Single measurements represent only a snapshot; continuous monitoring or serial measurements provide better assessment 1, 3
• General anesthesia can lower systemic arterial blood pressure and affect resistance calculations 4
• Intraindividual variability: Measurements can vary significantly even at rest 2

When to Remove the PAC

Remove the catheter as soon as specific hemodynamic data are no longer needed (typically when diuretic and vasodilating therapy have been optimized), as complications increase with duration of use 1. The PAC is indicated for haemodynamically unstable patients not responding predictably to traditional treatments, particularly those with combined congestion and hypoperfusion 1.