Measurement of Pulmonary Capillary Wedge Pressure and Mean Pulmonary Artery Pressure

General Principles

The American College of Cardiology recommends measuring 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 minimize respiratory artifacts and obtain accurate intravascular pressures 1, 2
During spontaneous inspiration, intrathoracic pressure decreases, which slightly reduces pulmonary artery pressure (PAP) relative to atmospheric pressure and increases venous return, according to the American College of Cardiology 2
At end-expiration, intrathoracic pressure approaches atmospheric pressure, providing the most accurate reflection of true intravascular pressures, as stated by the American College of Cardiology 2
Measurements should be taken over 2-3 respiratory cycles at end-expiration to reduce variability, as recommended by the American College of Cardiology 2
The American Heart Association suggests measuring pressures at end-inspiration when intrathoracic pressure is closest to atmospheric during positive pressure ventilation 3
Positive pressure ventilation creates larger, partly artifactual swings in pressure that become accentuated compared to spontaneous breathing, according to the American College of Cardiology 2
The effects of mechanical ventilation on hemodynamics are opposite to spontaneous breathing, making end-inspiration the appropriate measurement point, as stated by the American Heart Association 3

Patients must be in passive mechanical ventilation with no spontaneous breathing efforts for reliable interpretation, as recommended by clinical guidelines 4
Regular cardiac rhythm is essential—atrial fibrillation invalidates pressure measurements, according to clinical guidelines 4
Avoid measurements during exercise, hyperventilation, or Valsalva maneuvers, as these create exaggerated pressure swings, as stated by the American College of Cardiology 2
If optimal wedge pressure tracing cannot be obtained, directly measure left ventricular end-diastolic pressure (LVEDP), as recommended by the American College of Cardiology 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, according to the American College of Cardiology 1
The wedge pressure should be measured in several segments of the pulmonary vasculature to exclude segmental variations, as stated by the American College of Cardiology 1

Blood pH and gas status critically affect pulmonary vascular tone in children—ensure awareness of arterial blood gases during measurement, as recommended by the American Heart Association 3
Acidosis from hypercarbia or hypoperfusion causes pulmonary vasoconstriction, while alkalosis causes vasodilation, according to the American Heart Association 3
Sedation and anesthetic drugs may alter pulmonary pressures, affecting interpretation, as stated by the American Heart Association 3

Never measure during the first minute of spontaneous breathing when respiratory drive may still be suppressed, as recommended by the American Thoracic Society 5
Do not rely on PCWP alone in patients with suspected pulmonary veno-occlusive disease, as wedge pressure may be normal in some segments, according to the American College of Cardiology 1
Avoid measurements during active expiratory muscle recruitment, which makes interpretation extremely difficult, as stated by the American Thoracic Society 6, 7, 8
In patients with severe tricuspid regurgitation, use Fick method rather than thermodilution for cardiac output to ensure accurate pulmonary vascular resistance calculations, as recommended by the American College of Cardiology 1

The European Respiratory Society recommends that proper wedge position is critical for accurate LVEDP assessment, and the pulmonary artery catheter must be advanced until the balloon occludes a pulmonary artery branch, creating a static column of blood that reflects left atrial pressure, which approximates LVEDP 9
A true wedge tracing should demonstrate characteristic a and v waves with mean pressure measured at end-expiration, according to the European Respiratory Society 9
Blood sampling from the wedged catheter should show oxygen saturation equal to systemic arterial blood, confirming true wedge position, as recommended by the European Respiratory Society 9
Measurements should be averaged over three readings from different pulmonary segments to exclude segmental variations, according to the European Respiratory Society 9
The external pressure transducer must be zeroed at the mid-thoracic line (halfway between anterior sternum and bed surface) in a supine patient, as recommended by the European Respiratory Society 9

An overwedged catheter occurs when the balloon is overinflated or advanced too far distally, causing dampened pressure waveforms with loss of characteristic a and v waves, according to the European Respiratory Society 9
An overwedged catheter can also cause artificially elevated pressure readings that do not reflect true left atrial pressure, as reported by the European Respiratory Society 9
An underwedged catheter fails to achieve complete occlusion, resulting in persistent pulmonary artery pulsations in the tracing rather than the characteristic wedge waveform, according to the European Respiratory Society 9
Underestimation of true LVEDP can occur with an underwedged catheter, as reported by the American Heart Association 10

PCWP serves as a surrogate for left atrial pressure and generally correlates with LVEDP, but important discrepancies can occur, according to the European Respiratory Society 9
Normal PCWP values are ≤12 mmHg; values >15 mmHg indicate elevated left ventricular filling pressures, as recommended by the European Respiratory Society 9

When uncertainty exists about wedge position or LVEDP estimation, verify proper wedge position by confirming characteristic a and v waves, obtaining blood gas from wedged position showing arterial-level oxygen saturation, and measuring from multiple pulmonary segments, according to the European Respiratory Society 9
If wedge tracing quality is suboptimal, consider direct left ventricular catheterization to measure LVEDP, particularly when left heart disease is suspected based on clinical risk factors, as recommended by the European Respiratory Society and the American Heart Association 9, 11
Interpret PCWP in clinical context, recognizing that PCWP >15 mmHg indicates post-capillary pulmonary hypertension from left heart disease, while PCWP ≤15 mmHg with elevated pulmonary artery pressure suggests pre-capillary pulmonary hypertension, according to the European Respiratory Society and the American College of Chest Physicians 9, 12

Avoid repeated balloon inflations/deflations in distal pulmonary arteries, which increases risk of vessel rupture, as warned by the European Respiratory Society 9
Account for measurement timing: pressures should be recorded at end-expiration in spontaneously breathing patients, according to the European Respiratory Society 9