In adult patients receiving invasive mechanical ventilation, how do plateau pressure (Pplat) and peak pressure (Ppeak) differ, and what are the recommended limits for each?

Plateau Pressure vs Peak Pressure in Mechanical Ventilation

Key Definitions and Differences

Plateau pressure (Pplat) reflects alveolar pressure and lung distension at end-inspiration, while peak pressure (Ppeak) includes both alveolar pressure and the resistive pressure from airflow through the airways. 1

Physiological Distinction

• Ppeak represents the maximum pressure during inspiration and includes:

  • Elastic recoil pressure of the lung and chest wall
  • Resistive pressure from airflow through airways and endotracheal tube
  • Measured during active gas flow 2, 3

• Pplat represents the pressure after an inspiratory hold (no flow):

  • Reflects true alveolar distending pressure when measured correctly
  • Eliminates the resistive component
  • Requires 0.5-5 seconds of end-inspiratory occlusion, though measurements at 3-5 seconds are most accurate 2

• The difference between Ppeak and Pplat indicates airway resistance:

  • Large gradient suggests high airway resistance (bronchospasm, secretions, small endotracheal tube)
  • Small gradient suggests normal airways with primarily elastic problems 2

Recommended Pressure Limits

Plateau Pressure Limits

Pplat should be maintained ≤30 cmH₂O to prevent ventilator-induced lung injury (VILI) in ARDS patients. 1, 4

• The landmark ARDSNet trial established this threshold by demonstrating reduced mortality with Pplat ≤30 cmH₂O combined with tidal volumes of 6 ml/kg predicted body weight 1
• For surgical patients and those without ARDS, the same ≤30 cmH₂O limit applies 1
• During recruitment maneuvers, transient pressures of 30-40 cmH₂O (non-obese) or 40-50 cmH₂O (obese) may be used briefly 1

Peak Pressure Considerations

There is no specific upper limit for Ppeak alone, as it does not directly reflect lung distension. 1, 2

• Ppeak can be elevated due to high airway resistance without causing VILI if Pplat remains ≤30 cmH₂O 2, 5
• The ARDSNet control group used Pplat ≤50 cmH₂O (not Ppeak) as their upper limit, which was associated with higher mortality 1

Driving Pressure: The Superior Metric

Driving pressure (ΔP = Pplat - PEEP) is a better predictor of outcomes than either Pplat or tidal volume alone. 1, 4, 6

• Driving pressure reflects the ratio of tidal volume to respiratory system compliance 1
• Values >15 cmH₂O are associated with worse outcomes 1
• This metric accounts for the "functional" size of the lung available for ventilation 1
• Should be monitored continuously alongside Pplat in all mechanically ventilated patients 1, 6

Critical Measurement Considerations

Timing of Plateau Pressure Measurement

• Pplat measured at 0.5 seconds overestimates static elastance by 14-29% and underestimates resistance by 24-34% compared to measurements at 3-5 seconds 2
• For accurate assessment, perform inspiratory hold for at least 3 seconds 2
• Very early measurements (0.5s) may show Pplat values 1-2 cmH₂O higher than at 5 seconds 2

Special Populations Requiring Adjustment

In patients with elevated intra-abdominal pressure or chest wall stiffness, Pplat may exceed 30 cmH₂O without excessive lung distension. 1, 5

• Transpulmonary pressure (Pplat - esophageal pressure) more accurately reflects true lung distension 5
• Elevated IAP increases Pplat through increased pleural pressure, not lung overdistension 5
• In these cases, relying solely on Pplat ≤30 cmH₂O may lead to under-ventilation 5
• Conversely, atelectasis can cause normal Pplat despite excessive distension of remaining aerated lung 5

Spontaneous Breathing Efforts

During pressure support ventilation, the direction of change from Ppeak to Pplat during inspiratory hold indicates the degree of patient effort. 7

• If Pplat < Ppeak by >1 cmH₂O ("negative"), significant spontaneous effort is present 7
• If Pplat > Ppeak by >1 cmH₂O ("positive"), high elastic work is present 7
• When Pplat - Ppeak ≥7 cmH₂O, esophageal pressure typically exceeds -10 cmH₂O, indicating excessive effort 7

Practical Clinical Algorithm

1. Set initial ventilator parameters:

   • Tidal volume: 6-8 ml/kg predicted body weight 1, 6
   • PEEP: Start at 5 cmH₂O (never use zero PEEP) 1, 6

2. Measure Pplat with 3-5 second inspiratory hold:

   • Ensure Pplat ≤30 cmH₂O 1, 4
   • Calculate driving pressure (Pplat - PEEP), target <15 cmH₂O 1, 4, 6

3. Assess Ppeak - Pplat gradient:

   • Large gradient (>10 cmH₂O): Address airway resistance (suction, bronchodilators, larger ETT) 2
   • Small gradient (<5 cmH₂O): Problem is primarily elastic (lung or chest wall) 2

4. If Pplat >30 cmH₂O, determine cause:

   • Check for elevated IAP, obesity, or chest wall restriction 1, 5
   • Consider esophageal manometry to measure transpulmonary pressure 5
   • If transpulmonary pressure is acceptable, Pplat >30 cmH₂O may be tolerated 1
   • If true lung overdistension, reduce tidal volume further (toward 4 ml/kg) 1, 6

5. Monitor continuously:

   • Dynamic compliance, driving pressure, and Pplat 1, 6
   • Reassess after position changes, circuit disconnections, or clinical deterioration 1

Common Pitfalls to Avoid

• Do not use Ppeak alone to assess risk of VILI - it includes resistive pressure and does not reflect alveolar distension 1, 2
• Do not measure Pplat too early (<3 seconds) as this overestimates true alveolar pressure 2
• Do not rigidly apply Pplat ≤30 cmH₂O in patients with elevated chest wall elastance without considering transpulmonary pressure 1, 5
• Do not ignore driving pressure - it may be more important than Pplat or tidal volume alone 1, 4
• Do not assume Pplat accurately reflects lung distension in patients with atelectasis - remaining aerated lung may be overdistended despite normal Pplat 5