Inspiratory Pressures

Pressure Controlled Ventilation (PCV)

End-inspiratory pressure= alveolar pressure. The pressure is essentially constant during PCV- high flow at the beginning to get to target pressure, then flow tapers until it ends (no airflow at end inspiration). Can't measure resistance because flow rate is dynamic.

Volume Controlled Ventilation (VCV)

Peak inspiratory pressure (PIP)- maximal pressure with inspiration. Sum of plateau pressure and pressure required to overcome airway resistance. Keep <40 cm H2O, SCCM recommends below 30 for ARDS. Abnormalities: elevated PIP indicates high resistance (secretions, bronchospasm, biting tube).

Plateau pressure= alveolar pressure. Mean pressure during end-inspiratory pause, basically when there is no air movement. Not affected by resistance. Goal ≤30 cm H2O. Abnormalities: elevated plateau pressure indicates poor compliance.

Driving pressure= plateau - PEEP. Goal ≤15 cm H2O (>15 is associated with ↑mortality). PEEP can either improve or worsen driving pressure.

• If the set PEEP promotes recruitment→ ↓driving pressure.

• If the set PEEP creates overdistension of the alveoli→ ↑driving pressure.

End Expiratory Pressure

Positive end expiratory pressure (PEEP)- lowest pressure that avoids alveolar collapse, which occurs when intrapleural pressure is higher than intra-alveolar pressure. This is indicated by the lower bend on the pressure/ volume curve, known as the lower inflection point.

Mean Airway Pressure

Mean airway pressure (MAP)- average pressure the lungs are exposed to during the breathing cycle. One of the two parameters that determine oxygenation.

     - How to increase MAP: ↑PEEP. If using IRV, ↑inspiratory time (Thigh) and ↑inspiratory pressure (Phigh).

Parameters that Impact Airway Pressures

Resistance- change in pressure relative to flow (PIP - plateau/ peak inspiratory flow).

• Relationship between PIP and plateau is directly related to airway resistance.

• ↑PIP and [PIP - plateau >5 cmH2O]= ↑resistance (bronchospasm, ETT obstruction/ kink).

• ↑PIP and ↑plateau [PIP - plateau <5 cmH2O]= ↓compliance (PTX, ARDS, pneumonia, edema, auto-PEEP).

Compliance- change in volume per change in pressure. Normal- 50-100 mL/ cm H2O