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Objectives
List causes of hypoxemia and hypoxia.
Describe the oxyhemoglobin dissociation curve.
Calculate alveolar Po2.
Calculate the various indices of oxygenation.
Describe the relationship between Paco2, alveolar ventilation, and carbon dioxide production.
Calculate dead space and alveolar ventilation.
List causes of respiratory and metabolic acid-base disturbances.
Use the anion gap (AG) to differentiate causes of metabolic acidosis.
Use the strong ion difference (SID) to differentiate acid-base disturbances.
Discuss the controversy related to temperature adjustment of blood gases and pH.
Discuss the physiologic variables affecting venous blood gases.
Discuss brain tissue oxygen monitoring.
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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.
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Partial Pressure of Oxygen
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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.
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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 ...