At what phase of the respiratory cycle should mean pulmonary artery (PA) pressure be measured for accuracy in mechanically ventilated patients?

Measuring Mean Pulmonary Artery Pressure in Mechanically Ventilated Patients

Mean pulmonary artery pressure should be measured at end-expiration in mechanically ventilated patients to obtain the most accurate and clinically reliable values. 1

Respiratory Cycle Timing for Measurement

Standard Recommendation

• The American Heart Association recommends measuring mean pulmonary artery pressure at end-expiration during spontaneous breathing, or at end-inspiration if the patient is mechanically ventilated. 1
• However, this guideline requires clarification based on the specific clinical context and degree of respiratory effort. 2, 3

Critical Distinction Based on Respiratory Effort

For passively ventilated patients (no spontaneous breathing):

• Measure at end-expiration when intrathoracic pressure returns to baseline between positive pressure breaths. 2
• End-expiratory measurements in assist/control mode without active breathing efforts produce the most accurate automated pressure readings with minimal error. 2

For patients with active respiratory effort despite mechanical ventilation:

• The end-expiratory measurement often overestimates the true transmural pressure because active inspiratory muscle contraction creates large negative intrathoracic pressure swings. 3
• In patients with respiratory excursions ≥15 mm Hg in their pressure tracings, the mid-point value (measured halfway between end-expiration and the nadir during inspiratory triggering) more closely approximates the true filling pressure than the end-expiratory value. 3
• The difference between end-expiratory and relaxed measurements increases proportionally with the magnitude of respiratory excursions (r=0.51, p<0.01). 3

Common Pitfalls and How to Avoid Them

Automated vs. Manual Measurement Errors

• Automated monitoring systems systematically produce errors that vary by ventilatory mode: 2
  • In spontaneous breathing: underestimate mean PA pressure (p<0.01) and wedge pressure (p<0.001), with 42% having clinically important wedge pressure errors. 2
  • In IMV mode: underestimate mean PA pressure (p<0.05) and wedge pressure (p<0.001). 2
  • In assist/control: overestimate wedge pressure (p<0.001) but do not significantly affect mean PA pressure. 2

Active Respiratory Muscle Effort

• When large respiratory excursions are present (≥15 mm Hg swing in pressure tracing), the end-expiratory value overestimates true transmural pressure by an average of 11±5 mm Hg. 3
• In 88% of cases with large respiratory excursions, the mid-point measurement more accurately reflects the relaxed pressure than the end-expiratory measurement. 3
• The most reliable approach is to measure after administering a non-depolarizing muscle relaxant to eliminate respiratory muscle activity, though this is not always clinically feasible. 3

Practical Algorithm for Clinical Use

Step 1: Assess respiratory effort

• Examine the pressure waveform for respiratory excursions
• If excursions are <15 mm Hg → use end-expiratory measurement 3
• If excursions are ≥15 mm Hg → proceed to Step 2 3

Step 2: For large respiratory excursions

• Record the mid-point value (halfway between end-expiration and inspiratory nadir) 3
• If clinical decision-making requires high accuracy, consider brief muscle relaxation to obtain a true baseline measurement 3
• Document which measurement technique was used for serial comparisons 1

Step 3: Ensure proper technique

• Zero the external pressure transducer at the mid-thoracic line 1
• Avoid repeated balloon inflations/deflations in distal pulmonary arteries 1
• Use manual measurement at end-expiration rather than relying on automated systems, particularly in spontaneously breathing or IMV patients 2

Additional Technical Considerations

• Respiratory rate, thoracic compliance, and PEEP level do not significantly affect the magnitude of measurement error between automated and manual techniques. 2
• PA diastolic pressure has the largest automated measurement error across all ventilatory modes (99% of spontaneously breathing patients had clinically important errors). 2
• When calculating pulmonary vascular resistance, inaccurate pressure measurements directly compromise the validity of derived hemodynamic parameters. 1