What is the difference between Pulmonary Artery Pressure (PAP) and Pulmonary Artery Wedge Pressure (PAWP)?

Difference Between Pulmonary Artery Pressure (PAP) and Pulmonary Artery Wedge Pressure (PAWP)

Pulmonary Artery Pressure (PAP) measures the pressure within the pulmonary arteries, while Pulmonary Artery Wedge Pressure (PAWP) reflects left atrial pressure and is used to distinguish between pre-capillary and post-capillary causes of pulmonary hypertension.

Key Differences

Measurement Technique

• PAP: Measured directly in the pulmonary artery with the catheter tip in the flowing blood
• PAWP: Measured by advancing a balloon-tipped catheter until it "wedges" in a small pulmonary artery branch, occluding forward flow 1

What They Reflect

• PAP: Reflects the pressure in the pulmonary arterial system
• PAWP: Reflects left atrial pressure and, by extension, left ventricular filling pressure 1

Components of PAP

• Systolic PAP: Highest pressure during right ventricular contraction
• Diastolic PAP: Lowest pressure during right ventricular relaxation
• Mean PAP (mPAP): Average pressure throughout the cardiac cycle

Clinical Significance

• PAP: Elevated in all forms of pulmonary hypertension
• PAWP: Helps distinguish between:
  • Pre-capillary PH (PAWP ≤15 mmHg) - pulmonary arterial disease
  • Post-capillary PH (PAWP >15 mmHg) - left heart disease 1

Diagnostic Importance

In Pulmonary Hypertension Classification

The relationship between PAP and PAWP is crucial for classifying pulmonary hypertension:

1. Pre-capillary PH (Groups 1,3,4,5):

   • mPAP >20 mmHg
   • PAWP ≤15 mmHg
   • PVR ≥3 Wood units 1

2. Isolated post-capillary PH (Groups 2,5):

   • mPAP >20 mmHg
   • PAWP >15 mmHg
   • PVR <3 Wood units 1

3. Combined pre- and post-capillary PH (Groups 2,5):

   • mPAP >20 mmHg
   • PAWP >15 mmHg
   • PVR ≥3 Wood units 1

Derived Hemodynamic Parameters

The relationship between PAP and PAWP allows calculation of important parameters:

• Transpulmonary Gradient (TPG): mPAP - PAWP

  • Used to distinguish "passive" PH (TPG <12 mmHg) from "reactive" PH (TPG ≥12 mmHg) 1

• Diastolic Pressure Gradient (DPG): Diastolic PAP - PAWP

  • Better reflects pulmonary vascular disease as it's less influenced by PAWP changes 1

Clinical Pitfalls

Potential Measurement Errors

• When PAWP exceeds PADP (pulmonary artery diastolic pressure), the PAWP reading is likely erroneous and catheter repositioning is needed 2
• PAWP may underestimate left ventricular end-diastolic pressure (LVEDP) in older patients 3
• PAWP may overestimate LVEDP in patients with atrial fibrillation, rheumatic valve disease, or enlarged left atrium 3

Technical Considerations

• PAWP should be measured at end-expiration to minimize respiratory variation
• The external pressure transducer should be zeroed at the mid-thoracic line in a supine patient 1
• Repeated balloon inflations in end pulmonary arteries should be avoided due to risk of pulmonary artery rupture 1

Summary

Understanding the difference between PAP and PAWP is fundamental for:

• Accurate diagnosis of pulmonary hypertension
• Proper classification of PH etiology
• Selection of appropriate treatment strategies
• Monitoring disease progression and treatment response

This distinction is critical because treatment approaches differ significantly between pre-capillary and post-capillary pulmonary hypertension, directly impacting patient morbidity and mortality.