Topic Progress:

Airway Pressure

PAW = PResist + Pelast + PEEP

PAW = (Flow x Resistance) + (Volume/Compliance) + PEEP

  • Alterations in flow or resistance will therefore not reflect alterations in the alveolar pressure. For example, high airway pressure with bronchospasm is due to resistance in the airways (flow/resistance) not due to increased alveolar pressure.

PAW is measured routinely on the ventilator as Peak Inspiratory Pressure (PIP) 

  • The first curve displayed on your ventilator screen is PAW over time:


Maximum acceptable PIP usually < 35cmH2O



Alveolar Pressure

PALV = (Volume/Compliance) + PEEP


This is measured by determining the pressure across the whole of the respiratory cycle and on the ventilator, is measured as inspiratory pause pressure or plateau pressure

  • Inspiratory hold means the flow is stopped but the expiratory valve is maintained closed, the flow reduces to zero allowing airway and alveolar pressures to equalize
  • Since there is no flow, the pressure measured is only affected by lung elastance

PTOTPResist + Pelast

(Flow x Resistance) + (Volume/Compliance)

  • An inspiratory hold requires the patient to be paralyzed and to be on VC-AC (see next chapter for details)
  • The resulting measure is called a plateau pressure (Pplat) and is in fact the alveolar pressure

PALV should be kept < 30cmH2O to minimize lung injury


[az_toggle_section] [toggle title=”You are curious to see what is the Pplat of your patient so you proceed to an inspiratory hold. The results are: PIP 23cmH2O, Pplat 19cmH2O. You had set a tidal volume (TV) of 500mL, flow at 20L/min and PEEP at 5cmH2O. What are the compliance and resistance of your patient?” id=”tgl-1″] Compliance = ∆V/∆P (ml/cmH2O)

Compliance = ∆V / (Pplat-PEEP) = 500mL / (19 – 5 cmH2O) = 35-36 mL/cmH2O


Resistance = ∆P/flow (cmH2O/L/s)

Resistance = (PIP – Pplat) / flow = (24 – 19 cmH2O) / 20L/min = 5cmH2O / 0.33L/s = 15 cmH2O/L/s

You might have noticed that these are the same answers as before, and they are! While we had given you the corresponding pressures required to accommodate the TV and to generate flow in the preceding question stems, in a real clinical setting, the inspiratory hold is the only way for you to figure them out at your patient’s bedside. [/toggle] [/az_toggle_section]

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