Chapter 13: Fio₂, Positive End-Expiratory Pressure, and Mean Airway Pressure

Introduction

The principles associated with management of oxygenation are more complex than those associated with ventilation. Provided that cardiovascular function and V̇co₂ are constant, increases in alveolar ventilation generally result in decreases in Paco₂ and vice versa. Oxygenation status, although dependent on Fio₂, is also affected by cardiopulmonary disease, positive end-expiratory pressure (PEEP), and mean airway pressure (P̄aw). In this chapter, the aspects of mechanical ventilation that affect oxygenation are discussed, as well as approaches to these techniques during patient management.

Pathophysiology of Hypoxemia

Normal Pao₂ is 80 to 100 mm Hg when breathing room air at sea level, with hypoxemia defined as a Pao₂ of < 80 mm Hg. To maintain normal tissue oxygenation it is necessary to provide an adequate Fio₂, appropriate matching of ventilation and perfusion (V̇/Q̇), sufficient hemoglobin, adequate cardiac output, and appropriate O₂ unloading to the tissue. A breakdown at any stage in this process may result in tissue hypoxia. At sea level, hypoxemia results from one of a number of alterations in cardiopulmonary function. Specifically, hypoxemia is caused by shunt, V̇/Q̇ mismatch, diffusion defect, and hypoventilation. Hypoxemia is also worsened by cardiovascular compromise. A reasonable target Pao₂ in mechanically ventilated patients is 55 to 80 mm Hg (Spo₂ 88%-95%).

Shunt

Shunt is perfusion without ventilation. When present, venous blood (shunted blood) mixes with arterialized blood in the pulmonary veins or left heart causing a decrease in Pao₂ of blood leaving the left heart. Because the majority of O₂ is carried by hemoglobin, even a small shunt (Figure 13-1) can result in significant hypoxemia. Increasing Fio₂ improves oxygenation only in the settings of small shunt. A large shunt is unresponsive to an Fio₂ increase. Improvement in oxygenation in the setting of a large shunt is usually focused on resolution of the shunt (eg, decompression of a pneumothorax, resolution of a pneumonia, re-expansion of atelectasis, diuresis). The use of PEEP, recruitment maneuvers, and maneuvers to elevate P̄aw might improve oxygenation in this setting. A common, but often unrecognized, cause of shunt in mechanically ventilated patients is a patent foramen ovale. A functionally closed foramen ovale may open during mechanical ventilation and acute respiratory failure.