A client admitted with acute decompensated heart failure has SpO₂ of 88% and audible crackles throughout both lung fields. What is the PRIORITY nursing action?

ACUTE DECOMPENSATED HEART FAILURE with pulmonary oedema and SpO₂ of 88% is a respiratory emergency requiring immediate intervention. HIGH FOWLER'S POSITION (90 degrees or sitting bolt upright): MECHANISM — gravity pulls fluid from the upper lung fields downward; reduces venous return to the heart (decreasing preload); reduces the work of breathing; allows the diaphragm to descend more fully (abdominal organs don't compress the diaphragm as much when upright); EFFECT — can improve SpO₂ and reduce dyspnoea within minutes; OXYGEN: Supplemental oxygen increases the partial pressure of oxygen in the alveoli; in severe cases, non-invasive positive pressure ventilation (CPAP, BiPAP) may be needed if SpO₂ doesn't improve with supplemental oxygen; HIGH-FLOW OXYGEN target: SpO₂ above 94% in most patients; SUBSEQUENT INTERVENTIONS (ordered by priority after positioning and O₂): IV diuresis (furosemide IV — onset 15-30 min, faster than oral); nitrates (IV nitroglycerin for preload and afterload reduction); morphine (reduces preload and anxiety — use cautiously); ACEI/ARB/neprilysin inhibitor for longer-term management; URINARY CATHETER: Important for accurate output monitoring but not before addressing the immediate respiratory compromise; OUTPUT ASSESSMENT follows stabilisation of acute respiratory status; ECOG: Important but not before respiratory stabilisation.