When should pulmonary capillary wedge pressure (PCWP) and mean pulmonary artery pressure (mPAP) be measured in spontaneously breathing and mechanically ventilated patients?

Timing of PCWP and mPAP Measurements

Measure pulmonary capillary wedge pressure (PCWP) and mean pulmonary artery pressure (mPAP) at end-expiration in spontaneously breathing patients and at end-inspiration in mechanically ventilated patients to obtain the most accurate intravascular pressures. 1

Spontaneously Breathing Patients

Measurement Timing

• Take measurements at end-expiration when intrathoracic pressure approaches atmospheric pressure, providing the most accurate reflection of true intravascular pressures 1
• During spontaneous inspiration, intrathoracic pressure decreases, which artificially reduces pulmonary artery pressure relative to atmospheric pressure and increases venous return 1
• Measure over 2-3 respiratory cycles at end-expiration to reduce variability 1

Critical Prerequisites

• Ensure regular cardiac rhythm—atrial fibrillation invalidates pressure measurements 1
• Avoid measurements during exercise, hyperventilation, or Valsalva maneuvers, as these create exaggerated pressure swings 1
• Never measure during the first minute of spontaneous breathing when respiratory drive may still be suppressed 1
• Do not measure during active expiratory muscle recruitment, which makes interpretation extremely difficult 1

Mechanically Ventilated Patients

Measurement Timing

• Measure pressures at end-inspiration when intrathoracic pressure is closest to atmospheric during positive pressure ventilation 1
• Positive pressure ventilation creates larger, partly artifactual swings in pressure that become accentuated compared to spontaneous breathing, with effects opposite to spontaneous breathing 1
• Research in COPD patients confirms that measuring at end-expiration during mechanical ventilation leads to overestimation of intravascular pressures; averaging over the respiratory cycle or using the RAP waveform to correct for intrathoracic pressure provides better accuracy 2

Essential Requirements

• Patients must be in passive mechanical ventilation with no spontaneous breathing efforts for reliable interpretation 1
• Regular cardiac rhythm is essential—atrial fibrillation invalidates measurements 1
• Normal chest wall compliance is required for valid assessment 3

Technical Considerations for All Patients

Measurement Technique

• Measure wedge pressure in several segments of the pulmonary vasculature to exclude segmental variations 1
• If optimal wedge pressure tracing cannot be obtained, directly measure left ventricular end-diastolic pressure (LVEDP) 1
• Direct LVEDP measurement is particularly recommended when left heart disease is the likely etiology, such as in patients with orthopnea or associated risk factors 1

Common Pitfalls

• A PCWP ≤15 mmHg reliably indicates normal left ventricular filling pressure in pre-capillary pulmonary hypertension patients 4
• However, 39% of patients with PCWP >15 mmHg may have LVEDP ≤15 mmHg, meaning they would be erroneously diagnosed with pulmonary venous hypertension based on PCWP alone 4
• Do not rely on PCWP alone in patients with suspected pulmonary veno-occlusive disease, as wedge pressure may be normal in some segments 1
• In patients with severe tricuspid regurgitation, use Fick method rather than thermodilution for cardiac output to ensure accurate pulmonary vascular resistance calculations 1

Special Populations

Pediatric Patients

• Blood pH and gas status critically affect pulmonary vascular tone in children—ensure awareness of arterial blood gases during measurement 1
• Acidosis from hypercarbia or hypoperfusion causes pulmonary vasoconstriction, while alkalosis causes vasodilation 1
• Sedation and anesthetic drugs may alter pulmonary pressures, affecting interpretation 1