Diagnostic approach to hypoxemia

It is not always essential or practical to determine the etiology for hypoxemia in the intensive care environment, where a pragmatic approach is usually employed. In

the majority of patients VlQ mismatch is the primary cause. Hypercarbic hypoxemia should be considered in the appropriate setting, such as the sedated or immediate postoperative patient with residual anesthesia and respiratory depression. Processes affecting the nerves or muscles of the respiratory tract also cause respiratory depression. These include diseases of the respiratory centers in the medulla (Ondine's curse), the spinal conducting pathways (cervical spine injury), the spinal cord (poliomyelitis), the peripheral nerves (Guillain-Barre syndrome, myasthenia gravis), the thoracic cage (crush injuries) and the airways (upper airway obstruction).

True diffusion block occurs rarely and is usually overwhelmed by other causes, so that it is ignored in diagnosing hypoxemia.

A value for the percentage of venous admixture can be calculated for both true shunt and VlQ mismatch, and it is often difficult to differentiate between the two at any

given Fi02. However, administration of 100 per cent oxygen allows the two processes to be distinguished. VlQ scatter, in the absence of true shunt, has a negligible effect on the alveolar-arterial oxygen gradient when pure oxygen is breathed. Therefore administration of higher concentrations of oxygen allows clearer

differentiation between VlQ mismatch and true shunt using the isoshunt diagram ( Fig 6). However, concentrations of oxygen close to 100 per cent induce collapse of poorly ventilated alveoli and therefore increase shunt.

Finally, mixed venous oxygen desaturation can contribute to any of the processes already mentioned and should be considered in patients with anemia, depressed cardiac output, or increased oxygen consumption. Pulmonary arterial catheterization is often employed in the treatment of severe hypoxemia in order to manage intravascular volume and the oxygen supply and demand ratio better.

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