NBRC CRT Prep Online

Use the alveolar gas equation: PAO2 = FiO2(Pb − PH2O) − (PaCO2/R). First compute the inspired oxygen term: (745 − 47) = 698 mm Hg, so FiO2(Pb − PH2O) = 0.90 × 698 = 628.2 mm Hg. Next compute the CO2 correction: PaCO2/R = 38/0.8 = 47.5 mm Hg. Thus, PAO2 = 628.2 − 47.5 = 580.7 mm Hg. The A–a gradient is P(A–a)O2 = PAO2 − PaO2 = 580.7 − 72 = 508.7 mm Hg, which rounds to 509 mm Hg. The choice with a value near 518 mm Hg can result from using an incorrect respiratory quotient (such as R = 1). The larger value near 551 mm Hg can result from failing to subtract water vapor pressure. The low value near 110 mm Hg reflects an incorrect application of the alveolar gas equation or mixing up PaO2 and PAO2.

This patient has acute hypercapnic respiratory failure with respiratory acidosis (low pH with elevated PaCO2 and no metabolic compensation), which indicates inadequate ventilation during a COPD exacerbation. Because he is alert, cooperative, and can protect his airway, the best next step is the choice describing non-invasive positive pressure ventilation, which reduces work of breathing, improves alveolar ventilation, lowers PaCO2, and often prevents the need for intubation in COPD exacerbations with acidosis. The choice stating “bronchodilator therapy only” is insufficient when acidosis from ventilatory failure is already present. The choice describing “100% oxygen via non-rebreather mask” is not appropriate initial management in COPD because oxygen should be titrated (commonly to SpO2 88%–92%) to avoid worsening hypercapnia and V/Q mismatch. The choice stating “immediate intubation” is generally reserved for inability to protect the airway, severe encephalopathy, hemodynamic instability, copious secretions, or failure of noninvasive ventilation.

As altitude increases, ambient (outside) pressure decreases. Gas in an air-filled ET tube cuff can expand as ambient pressure drops (Boyle’s law), which can increase cuff volume and/or the pressure the cuff exerts on the tracheal wall. For this reason, cuff pressure should be monitored and adjusted during ascent. An existing pneumothorax tends to expand during ascent (not descent) because trapped intrathoracic gas expands as ambient pressure decreases. In unpressurized aircraft, the partial pressure of oxygen decreases with altitude, so patients often require more supplemental oxygen rather than less. Equipment interference is not inherently more likely in helicopters than fixed-wing aircraft.