Final answer:
A PA catheter is usually needed when the PEEP level is high, typically more than 10 cmH₂O. It is used to monitor hemodynamic parameters in the pulmonary artery.
Step-by-step explanation:
A PA (Pulmonary Artery) catheter is usually needed when the PEEP (Positive End-Expiratory Pressure) level is high. A high PEEP level is typically considered to be more than 10 cmH₂O.
A PA catheter measures the pressure inside the pulmonary artery and is used to monitor hemodynamic parameters such as cardiac output, pulmonary artery pressure, and pulmonary wedge pressure.
Therefore, considering the options given, a PA catheter would likely be needed at a PEEP level of 15 cmH₂O or 20 cmH₂O.