PEEP(Positive-end expiratory pressure)

PEEP is an adjuvunt to the mode ventilation used in cases where the FRC is reduced. At the end of expiration, the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure. The presence of PEEP opens up collapse or unstable alveoli and increases the FRC and surface area for gas exchange, thus reducing the size of the shunt. Thus, if a large shunt is found to exist based on the estimation from 100% FiO₂ (see above), then PEEP can be considered and the FiO₂ can be lowered (<60%), to maintain adequate PaO₂, thus reducing the risk of oxygen toxicity.

In addition to treating a shunt, PEEP is also therapeutic in decreasing the work of breathing. In pulmonary physiology, compliance is a measure of the "stiffness" of the lung and chest wall. The mathematical formula for compliance (C) = change in volume / change in pressure. Therefore, a higher compliance means that only small increases in pressure can lead to large increases in volume, which means the work of breathing is reduced. As the FRC increases with PEEP, the compliance also increases, since the partially inflated lung takes less energy to inflate further.

Indications

PEEP is a cardiodepressant and can cause severe hemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular. As such, it should be judiciously used and is indicated in two circumstances.

• If a PaO₂ of 60 mmHg cannot be achieved with a FiO₂ of 60%
• If the initial shunt estimation is greater than 25%

If used, PEEP is usually set with the minimal positive pressure to maintain an adequate PaO₂ with a safe FiO₂. As PEEP increase intrathoracic pressure, there can be a resulting decrease in venous return and decrease in cardiac output. A PEEP of less than 10 cmH₂O is usually safe if intravascular volume depletion is absent. Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is > 10 cmH₂ for hemodynamic monitoring. More recent literature has failed to find outcome benefits with routine PA catheterization when compaired to simple central venous pressure monitoring.^[2] If cardiac output measurement is required, minimally invasive techniques, such as esophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives.^[3][4] PEEP should be withdrawn from a patient until adequate PaO₂ can be maintained with a FiO₂ < 40%. When withdrawing, it is decreased through 1-2 cmH₂O decrements while monitoring hemoglobin-oxygen saturations. Any unacceptable hemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation.