A client with a large pleural effusion undergoes a thoracentesis. Following the procedure, which observation warrants immediate intervention by the nurse? Reference Range: Arterial Blood Gas (ABG) pH [7.35 to 7.45] PaCO, [35 to 45 mm Hg] HCO 21 to 28 mEq/L (21 to 28 mmol/L)] PaO2 [80 to 100 mm Hg]