# The hypoxemic patient

When a patient becomes hypoxemic, we first apply supplemental oxygen.

## The hypoxemic patient

When a patient becomes hypoxemic, we first apply supplemental oxygen. Won-derfully, the patient’s saturation usually responds. Should it fall again, we can simply increase the inspired oxygen concentration once again – but we are not solving the problem. The adequate saturation may lull us into an inappropriate sense of security regarding the well-being of the patient.

Assume this patient requires 50% inspired oxygen to maintain a SpO2 of 90%, much less than we would expect with that FiO2. We estimate the degree of the oxygenation problem by looking at the difference between the alveolar and arterial oxygen concentrations. With a SpO2 of 90%, we can assume the PaO2 to be around 60 mmHg (from Fig. 10.3).

Next, we need to know what the alveolar concentration of oxygen would be in the patient breathing 50% oxygen. The alveolar air equation comes to our aid. We estimate PACO2 and R to be 40 mmHg and 0.8, respectively.

Therefore, we would expect the PaO2 to be close to 300 mmHg instead of the observed 60 mmHg. This “A-a difference” (often mislabeled an A – a gradient) may be due to a problem with oxygen diffusion and/or matching of ventilation and perfusion (V/Q). In healthy patients some 4% of the venous blood will manage to make it through a right (venous blood in the pulmonary artery) to left (arterial blood in the pulmonary vein) shunt. Thus, normally we expect to see a slightly lower partial pressure of oxygen in arterial blood than in alveolar gas. However, a patient requiring 50% inspired oxygen to barely maintain a SpO2 of 90% should worry us greatly.

If a patient is hypoxemic on room air, giving supplemental oxygen is a great first step, but the source of the hypoxemia should be sought and appropriately treated.

## The patient receiving intravenous sedation presents another situation in which the alveolar air equation can help. Some physicians routinely place these patients on supplemental oxygen by nasal cannula, resulting in a PaO2 of 150 mmHg or more (well into the flat part of the oxyhemoglobin dissociation curve: Fig. 10.3). If the patient now hypoventilates, his PaCO2 will rise and PaO2 will fall, but his SpO2 can stay deceptively normal. Thus, not giving supplemental oxygen (to a patient with normal oxygenation) will make the SpO2 a sensitive indicator of respiratory depression. Once a drop in saturation occurs, we need to treat the patient’s hypoventilation.

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