NONPULMONARY FACTORS



Abnormalities other than alterations in lung function may influence the Pao2 through their ef­fect on the mixed venous Po2(Py02). The Pv02 is decreased when cardiac output is inappropriately low, when 02 consumption (Vo2) is increased (as with exercise or fever), or when the hemoglobin concentration or 02 saturation is low. For any lung unit, the resultant end-capillary Po2 is influ­enced by the Pvo2, although the magnitude of this effect on the arterial 02 content will be greatest in lungs with Va/Q inequality or shunt (Fig. 17-9). The importance of this phenomenon is the rec­ognition that a fall in Pao2 in a patient with lung disease may be due to one of these nonpulmonary factors rather than a deterioration in lung func­tion, thus requiring a very different intervention.