MEDICAL GAS THERAPY
The therapeutic medical gases include oxygen at ambient or hyperbaric pressure, helium–oxygen mixtures (heliox), and nitric oxide. Oxygen is made available in high-pressure cylinders, via pipeline sys-tems, from oxygen concentrators, as well as in liquid form. Heliox is occasionally used to partially relieve the increased work of breathing due to partial upper airway obstruction. Nitric oxide is administered as a direct, selective pulmonary vasodilator.
The primary goal of oxygen therapy is to prevent or correct hypoxemia or tissue hypoxia. Table 57–1 identifies classic categories of hypoxia.Oxygen therapy alone may not correct either hypoxemia or hypoxia. Continuous positive airway pressure (CPAP) or positive end-expiratory pres-sure (PEEP) may be required to recruit collapsed alveoli. Patients with profound hypercapnia may require ventilatory assistance. High concentrations of oxygen may be indicated for conditions requir-ing removal of entrapped gas (eg, nitrogen) from body cavities or vessels. The short-term inhalation of increased concentrations of oxygen is relatively free of complications.
Supplemental oxygen is indicated for adults, children, and infants (older than 1 month) when Pao2 is less than 60 mm Hg (8 kPa) or Sao2 or Spo2 is less than 90% while at rest breathing room air. In neonates, therapy is recommended if Pao2 is less than 50 mm Hg (6.7 kPa) or Sao2 is less than 88% (or capillary Po2 is less than 40 mm Hg [5.3 kPa]). Therapy may be indicated for patients when clini-cians suspect (rather than measure) hypoxemia or hypoxia based on a medical history and physical examination. Patients with myocardial infarction, cardiogenic pulmonary edema, acute lung injury, acute respiratory distress syndrome (ARDS), pul-monary fibrosis, cyanide poisoning, or carbon monoxide inhalation all require supplemental oxygen. Supplemental oxygen is given during the perioperative period because general anesthe-sia commonly causes a decrease in Pao2 second-ary to increased pulmonary ventilation/perfusion mismatching and decreased functional residual capacity (FRC). Supplemental oxygen should be provided before procedures such as tracheal suc-tioning or bronchoscopy, which commonly cause arterial desaturation. There is evidence that supple-mental oxygen is effective in prolonging survival of patients with chronic obstructive pulmonary disease (COPD) whose resting Pao2 is lower than 60 mm Hg at sea level. Supplemental oxygen ther-apy also appears to have a mild beneficial effect on the mean pulmonary arterial pressure and subjec-tive indices of patients’ dyspnea.
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