In patients with chest trauma, impaired ventilation, in spite of measures to ensure an open airway, relief of chest wall pain, and drainage of hemopneumothorax, is an indication for ventilatory support. Respiratory failure associated with a flail chest is best treated by early endotracheal intubation and ventilatory assistance, particularly if there are associated injuries, and even if the patient's breathing intially seems adequate. Ventilatory assistance should also be strongly considered if the patient is in shock, has had multiple injuries, is comatose, requires multiple transfusions, is elderly, or has preexisting pulmonary disease. A respiratory rate greater than 30 to 35 breaths per minute, a vital capacity less than 10 to 15 mL/kg, and/or a negative inspiratory force (NIF) less than 25 to 30 cmH 2O can also be considered early indications for ventilatory support.
All trauma patients should be monitored by pulse oximetry. In patients with severe chest trauma, an arterial blood gas should be drawn soon after admission and at frequent intervals thereafter. If arterial P o2 is less than 50 mmHg while the patient is breathing room air or less than 80 mmHg while the patient is breathing supplemental oxygen (equivalent to an Fio2 of 0.4 or more), the patient should generally be given ventilatory assistance. Metabolic acidosis with an arterial P co2 insufficiently decreased to compensate for the decrease in [HCO 3-] is another indication for ventilatory support. Although there are several formulas relating the expected change in Pco2 to the magnitude of the decrease in [HCO3-], the simplest approach is to expect a 1:1 relationship. That is, for each milliequivalent per liter decrease in [HCO3-], ventilatory response should result in a decrease of P co2 of 1 mmHg. Thus, in the face of metabolic acidosis, Pco2 less than 40 mmHg may still be inappropriately high. (See Chap, 21 for a detailed discussion.)
When used properly, pulse oximetry can also reduce the need for arterial blood gas (ABG) determinations; however, since the Sa o2 indicated by the pulse oximeter is often 2 to 3 percent higher than that seen with ABG, one should try to keep the saturation more than 92 to 93 percent.
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