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Emergency evaluation of the respiratory system should begin with an initial visual assessment of respiratory rate and effort to collect early information on patient condition, including observations of breathing with a restrictive pattern, abdominal effort, or paradoxical breathing. This, in conjunction with thorough auscultation to identify abnormal lung sounds indicative of pulmonary parenchymal disease or pleural space disease, is critical. Because vomiting is a common finding in acute abdomen patients, they may be at higher risk for aspiration pneumonitis and pneumonia, and may have increased ventral lung sounds. Pulmonary manifestations of sepsis, including septic pneumonitis, acute lung injury, and acute respiratory distress syndrome, may also be present. In addition, patients who have endured abdominal trauma may also have concurrent thoracic trauma, including rib fractures, pleural space disease, and pulmonary contusions. Even though physical examination is usually very effective in identifying clinically significant thoracic injuries, thoracic focused assessment with sonography for trauma ((FAST) and thoracic radiography are useful to confirm and/or differentiate between these causes.

Ancillary diagnostics to evaluate the respiratory system include pulse oximetry and arterial blood gas analysis. Pulse oximetry is a rapid, noninvasive way to measure oxygen saturation of the blood, with normal values ranging from 95% to 100%, and with severe hypoxemia indicated by levels less than 90%. Arterial blood gas analysis can be used to help distinguish the cause of severe hypoxemia, which occurs when the partial pressure of oxygen in arterial blood (PaO₂) is less than 60 mmHg. Evaluation of ventilation status is necessary to differentiate between hypoventilation (hypoxemia combined with hypercapnia at a partial pressure of carbon dioxide in arterial blood, PaCO₂, > 45 mmHg) and diffusion impairment, ventilation/perfusion mismatch or a shunt, all of which should have normal PaCO₂. Oxygen responsiveness as indicated by improvement of PaO₂ with supplementary oxygen challenge is supportive of ventilation/perfusion mismatch, decreased levels of inspired oxygen, or hypoventilation.

Oxygen therapy is provided by flow‐by supplementation, nasal cannulae (to include high‐flow oxygen therapy), or placement of the patient in an oxygen chamber. These methods all provide a fraction of inspired oxygen (FiO₂) of approximately 30–100% and are often sufficient to help a patient maintain adequate oxygen saturation and normal arterial blood oxygen levels (PaO₂ 80–120 mmHg). In patients with pleural space disease, thoracocentesis is performed for both diagnostic and therapeutic purposes. Response to treatment is determined largely by serial pulse oximetry measurements, blood gas analysis, and repeated clinical assessment of patient respiratory effort and comfort. In patients who are persistently hypoxemic despite oxygen supplementation, or those with severe hypoventilation (PaCO₂ > 60 mmHg) and/or respiratory fatigue, mechanical ventilation is indicated to provide ventilatory support to the patient while the underlying disease is addressed.