Chapter 27: Blood Gases

Introduction

Blood gas and pH measurements allow evaluation of oxygenation, ventilation, and acid-base balance. Either arterial or mixed venous blood gases can be assessed. This chapter covers important aspects of blood gas assessment as it relates to the mechanically ventilated patient.

Oxygenation

Partial Pressure of Oxygen

The normal range of Pao₂ is 80 to 100 mm Hg in healthy young persons breathing room air at sea level. Pao₂ decreases with age, altitude, and lung disease. Hypoxemia occurs when the lungs fail to adequately oxygenate arterial blood. Pao₂ is a reflection of lung function and not hypoxia per se. Hypoxia can occur without hypoxemia and vice versa. Causes of hypoxemia and hypoxia are listed in Table 27-1. The best Pao₂ in critically ill mechanically ventilated patients is unknown, but a target Pao₂ of 55 to 80 mm Hg (at sea level) is usually acceptable. Pao₂ must be balanced against the potentially toxic effects of Fio₂ and alveolar distending pressure. For mechanically ventilated patients with severe lung disease, permissive hypoxemia may be a desirable alternative to applying potentially injurious ventilator setting to normalize the Pao₂.

Oxygen Saturation

The relationship between Pao₂ and oxygen saturation of hemoglobin (Sao₂) is described by the oxyhemoglobin dissociation curve (Figure 27-1). This is a sigmoid relationship, with hemoglobin having a greater affinity for oxygen at a high Po₂ (eg, in the lungs, where the Po₂ is high) and a lower affinity ...