Mean airway pressure typically refers to the mean pressure applied during positive-pressure mechanical ventilation. Mean airway pressure correlates with alveolar ventilation, arterial oxygenation, ^[1] hemodynamic performance, and barotrauma. ^[2] It can also match the alveolar pressure if there is no difference between inspiratory and expiratory resistance. ^[3]

Equations

There are several equations aimed at determining the real mean airway pressure.

Volume control ventilation

In ventilation with a square flow waveform this equation can be used:

${\displaystyle {\bar {P}}_{aw}=0.5\times (PIP-PEEP)\times (T_{I}/T_{tot})+PEEP}$

where:

• ${\displaystyle {\bar {P}}_{aw}}$ = mean airway pressure
• ${\displaystyle PIP}$= peak inspiratory pressure
• ${\displaystyle PEEP}$= peak end expiratory pressure
• ${\displaystyle T_{I}}$= inspiratory time
• ${\displaystyle T_{tot}}$= cycle time

Pressure control ventilation

During pressure control ventilation this variant of the equation can be used:

${\displaystyle {\bar {P}}_{aw}=(PIP-PEEP)\times (T_{I}/T_{tot})+PEEP}$ where:

• ${\displaystyle {\bar {P}}_{aw}}$ = mean airway pressure
• ${\displaystyle PIP}$= peak inspiratory pressure
• ${\displaystyle PEEP}$= peak end expiratory pressure
• ${\displaystyle T_{I}}$= inspiratory time
• ${\displaystyle T_{tot}}$= cycle time ^[3]

Airway pressure release ventilation

In airway pressure release ventilation (APRV) a variation of the previous equation must be used for the variables:

${\displaystyle {\bar {P}}_{aw}={\frac {(P_{high}\times T_{high})\,+(P_{low}\times T_{low})}{T_{high}+T_{low}}}}$

where:

• ${\displaystyle {\bar {P}}_{aw}}$= mean airway pressure
• ${\displaystyle {P}_{high}}$= peak inspiratory pressure (PIP)
• ${\displaystyle {P}_{low}}$= peak end expiratory pressure
• ${\displaystyle {T}_{high}}$= time spent at ${\displaystyle {P}_{high}}$
• ${\displaystyle {T}_{low}}$= time spent at ${\displaystyle {P}_{low}}$ ^[4]

Other equations

${\displaystyle M_{PAW}={\frac {f\times T_{i}}{60}}\times (P_{IP}-PEEP)+PEEP}$

${\displaystyle M_{PAW}={\frac {F_{1}}{F_{1}+F_{E}}}\times P_{IP}+\left(1-{\frac {F_{1}}{F_{1}+F_{E}}}\right)\times PEEP}$

${\displaystyle M_{PAW}={\frac {(R)(T_{i})(P_{I})+[60-(R)(T_{i})](PEEP)}{60}}}$

${\displaystyle M_{PAW}={\frac {f\times T_{i}}{60}}\times (P_{IP}-PEEP)+PEEP}$ ^[5]

${\displaystyle M_{PAW}={\frac {(T_{i}\times P_{IP})+(T_{e}\times PEEP)}{T_{i}+T_{e}}}}$

Clinical significance

Mean airway pressure has been shown to have a similar correlation as plateau pressure to mortality. ^[6]

MAP is closely associated with mean alveolar pressure and shows the stresses exerted on the lung parenchyma on mechanical ventilation. ^[7]

In high frequency oscillatory ventilation, it has been suggested to set the mean airway pressure six above the lower inflection point on the lungs P-V curve. ^[8]

See also

• Ventilators
• Mechanical ventilation
• Modes of ventilation
• Mean systemic pressure

References