4. What is acute hypoxemic respiratory failure?

Hypoxemic respiratory failure is defined as acute respiratory distress resulting in a PaO2 of less than 60 mmHg, despite addition of supplemental oxygen of at least 60%. Typically, this involves the pulmonary alveoli component of the pulmonary system. Hypoxemic respiratory failure is also called "lung failure" or "oxygenation failure." Causes include 1) decreased inspired oxygen content [FIO2] (e.g., high altitude ascent or reduction in the FIO2 setting on a mechanical ventilator), 2) hypoventilation (e.g., respiratory paralysis, airway obstruction, or atelectasis), 3) diffusion impairment (e.g., severe pneumonia, interstitial fibrosis, or interstitial pulmonary edema), 4) ventilation-perfusion (V/Q) mismatch (e.g., emphysema, alveolar pulmonary edema, pneumothorax, atelectasis), and 5) intra- and extrapulmonary shunting (technically the most severe form of V/Q mismatch; e.g., lung consolidation, atelectasis).

5. What are the fundamental initial treatment priorities in any patient with respiratory distress?

It is crucial to always remember that re-establishment of adequate arterial oxygen tension and removal of excessive CO2 are the overriding aims of the immediate treatment of patients with severe respiratory distress. The major ways in which to achieve this aim, regardless of the underlying cause of distress, are establishing a patent airway, instituting or assisting ventilation, and maintaining an adequate oxygen tension, by administration of supplemental oxygen, to maximize oxygen delivery.

6. What are the most useful diagnostic "tools" for use in evaluation patients with respiratory distress?

The simplest and often most useful tools are a good history, detailed physical exam, and careful chest auscultation. Other tools useful in diagnosing causes of respiratory distress include arterial blood gas analysis, pulse oximetry, capnography, thoracic radiography, and lung perfusion scans (not usually in the emergent patient, however).

7. Describe measures that allow differentiation of the various causes of hypoxemia in emergency patients with respiratory distress.

Hypoxemia is diagnosed by the presence of an SpO2 of less than 90% or an arterial blood gas (ABG) analysis that reveals a PaO2 of less than 60 mmHg. ABG analysis is essential for proper interpretation of causes of hypoxemia. Alternatively, use of pulse oximetry and capnography can be useful in diagnosis. Hypoxemia with hypercapnia defines hypoventilation as the underlying cause. Hypoxemia with normocapnia implies diffusion impairment, ventilation/perfusion imbalance ("V/Q mismatch"), or shunt as underlying causes. In veterinary patients, diffusion impairment is rarely severe enough to cause hypoxemia in and of itself. Response to oxygen supplementation usually allows differentiation between V/Q mismatch and shunting. Typically, the patient with a V/Q mismatch will demonstrate marked response (i.e., improved PaO2) with supplemental oxygen; whereas, the patient with shunt only minimally shows improvement in PaO2, if at all (i.e., by definition, refractory hypoxemia with < 10 mmHg increase with at least 40% oxygen administration).

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