During general anesthesia, what is the clinical significance of peak inspiratory pressure versus plateau pressure and what are typical examples of causes for elevated values?

Peak vs Plateau Pressure During General Anesthesia

Fundamental Distinction

Plateau pressure (Pplat) reflects true alveolar distending pressure and is the critical parameter for preventing ventilator-induced lung injury, while peak pressure (Ppeak) includes both alveolar pressure and the resistive component from airway flow and should not be used alone to assess lung injury risk. 1

The gradient between Ppeak and Pplat indicates airway resistance—a large gradient suggests bronchospasm, secretions, kinked endotracheal tube, or a small-diameter tube 1. During controlled ventilation, monitoring both pressures as displayed waveforms provides breath-by-breath information about chest-lung mechanics 2.

Clinical Significance and Target Values

Plateau Pressure Limits

• Maintain Pplat ≤30 cmH₂O in all mechanically ventilated patients during surgery 2, 1
• This threshold applies to both patients with and without acute respiratory distress syndrome 1
• Pplat >30 cmH₂O is associated with increased mortality and postoperative pulmonary complications 2
• Measure Pplat using a 3-5 second inspiratory hold maneuver 1

Peak Pressure Considerations

• No specific upper limit exists for Ppeak because it does not directly reflect lung distension 1
• Ppeak alone should never be used to assess ventilator-induced lung injury risk 1
• High Ppeak with normal Pplat indicates increased airway resistance, not alveolar overdistension 1

Driving Pressure: The Superior Metric

Driving pressure (ΔP = Pplat - PEEP) is a better predictor of clinical outcomes than either Pplat or tidal volume alone 2, 1. This metric reflects the ratio of tidal volume to respiratory system compliance 2.

• Target ΔP <15 cmH₂O 1
• Driving pressures >15 cmH₂O correlate with increased mortality and prolonged ventilation 1
• Lower intraoperative ΔP values reduce postoperative pulmonary complications 2

Practical Examples of Elevated Pressures

High Ppeak with Normal Pplat (Increased Airway Resistance)

• Bronchospasm: Ppeak 35 cmH₂O, Pplat 22 cmH₂O, gradient 13 cmH₂O 1
• Secretions/mucus plugging: Similar pattern with large Ppeak-Pplat gradient 1
• Small endotracheal tube (e.g., 6.0 mm in adult): Increased flow resistance 1
• Kinked or partially obstructed tube: Acute rise in Ppeak without Pplat change 1

High Pplat with Proportional Ppeak (Decreased Compliance)

• Pneumothorax: Both pressures elevated, reduced compliance 3
• Pulmonary edema/ARDS: Pplat 28-32 cmH₂O, reduced functional lung units 2
• Mainstem intubation: Unilateral ventilation reduces total compliance 3
• Abdominal insufflation during laparoscopy: Cephalad diaphragm displacement 2

Special Populations Requiring Adjusted Interpretation

In obese patients or those with elevated intra-abdominal pressure, Pplat may exceed 30 cmH₂O without harmful lung overdistension 1, 4. The chest wall stiffness increases transrespiratory pressure while transpulmonary pressure (Pplat minus esophageal pressure) remains acceptable 1, 4.

• Consider measuring esophageal pressure to calculate transpulmonary pressure in these patients 1, 4
• If transpulmonary pressure is safe, Pplat up to 29-32 cmH₂O may be tolerated 4
• Otherwise, reduce tidal volume toward 4 ml/kg predicted body weight 1

Recommended Intraoperative Ventilator Settings

Initial Settings

• Tidal volume: 6-8 ml/kg predicted body weight (not actual body weight) 2
• PEEP: Start at 5 cmH₂O; avoid zero PEEP 2
• Inspiratory flow: ≤12 L/min minimizes resistive pressure during pressure-volume measurements 5
• Respiratory rate: Adjust to maintain normocapnia 2

Monitoring Protocol

• Measure Pplat with 3-5 second inspiratory hold every 30-60 minutes 1
• Continuously monitor Ppeak, Pplat, PEEP, and calculate ΔP 2, 1
• Display pressure-time waveforms for real-time assessment of respiratory mechanics 2
• Set audible alarms for high Ppeak and Pplat specific to each patient 2

Common Pitfalls and How to Avoid Them

Critical Errors to Avoid

• Do not rely on Ppeak alone to assess lung injury risk—always measure Pplat 1
• Do not measure Pplat too early (<3 seconds)—this overestimates true alveolar pressure 1
• Do not rigidly enforce Pplat ≤30 cmH₂O in obese patients without considering transpulmonary pressure 1, 4
• Do not ignore driving pressure—it may be more prognostically important than Pplat or tidal volume 1
• Do not use actual body weight for tidal volume calculations in obese patients—use predicted body weight based on height and sex 2

Algorithmic Approach to Elevated Pressures

When Ppeak is elevated:

1. Perform inspiratory hold to measure Pplat 1
2. Calculate Ppeak-Pplat gradient 1
3. If gradient >10 cmH₂O: investigate airway resistance (suction secretions, check tube position, treat bronchospasm) 1
4. If gradient <10 cmH₂O: proceed to evaluate Pplat 1

When Pplat >30 cmH₂O:

1. Assess for elevated intra-abdominal pressure, obesity, or chest wall restriction 1
2. If present, consider measuring transpulmonary pressure 1, 4
3. If transpulmonary pressure acceptable, current Pplat may be tolerated 1, 4
4. If transpulmonary pressure elevated or unavailable, reduce tidal volume incrementally toward 4 ml/kg 1
5. Reassess ΔP—if >15 cmH₂O, further reduce tidal volume or adjust PEEP 1

During recruitment maneuvers:

• Transient pressures of 30-40 cmH₂O (non-obese) or 40-50 cmH₂O (obese) may be applied safely for brief periods 1
• Use the lowest effective pressure for the shortest duration 2