According to arterial blood gas results, his pH should be 7.25 or greater,2 and the PaO2 (partial pressure of arterial oxygen) should be 60 mm Hg or greater. Your patient ought to be on no more than 50% FiO2 (fraction of inspired oxygen), and 8 cm H2O or less of positive end-expiratory pressure (PEEP).2 The PaO2/FiO2 (P/F) ratio, an excellent predictor of oxygenation status and readiness to wean, should be greater than 150 – 300. This figure is eas ily calculated by dividing the PaO2 by the FiO2. For example, if your patient's PaO2 is 80 mm Hg and his FiO2 is 0.4 (40%), his P/F ratio is 200.
In addition, assess hemoglobin level, which should be at least 8 gm/dL. Also be sure magnesium, potassium, and phosphorus are within normal range, since low levels of electrolytes impair respiratory muscle function. Once your patient meets all of the criteria listed in the box on page 37, he's ready to try breathing on his own.