Walking the tightrope: Mixed respiratory failure exacerbated by heart rate control and concurrent pneumonia in an elderly patient with new-onset atrial fibrillation

RATIONALE: The interplay between new-onset atrial fibrillation (AF), heart failure (HF) with preserved ejection fraction (HFpEF), and acute infection in the elderly presents a complex clinical challenge. Rate control is a cornerstone of AF management, but its application in frail patients with limited cardiac reserve and concurrent infection remains uncertain. This case illustrates the diagnostic and therapeutic difficulties encountered when these conditions converge, highlighting the need for dynamic assessment and individualized treatment strategies. PATIENT CONCERNS: An 86-year-old woman with a history of hypertension, HFpEF, and cerebral infarction presented with a 3-day history of worsening dizziness and dyspnea. On admission, she was afebrile with clear lung auscultation but had a rapid, irregular heart rate of 152 bpm. DIAGNOSES: Electrocardiogram confirmed new-onset AF with rapid ventricular response. Elevated N-terminal pro-B-type natriuretic peptide confirmed acute HF exacerbation. Although an initial chest computed tomography showed only minor chronic changes, the patient subsequently developed fever, productive cough, and hypoxemia. A repeat chest computed tomography revealed new multifocal pulmonary infiltrates and bilateral pleural effusions. Sputum culture later confirmed Klebsiella pneumoniae infection. The final diagnosis was mixed respiratory failure exacerbated by rapid AF in a patient with underlying HFpEF. INTERVENTIONS: Initial treatment included diuresis, anticoagulation, and rate control with metoprolol tartrate. Upon clinical deterioration, noninvasive ventilation was initiated. Anti-infective therapy was intensified empirically with moxifloxacin and ceftriaxone, later adjusted to ceftriaxone monotherapy based on culture results and an adverse reaction to moxifloxacin. Diuretic therapy was continued, and the heart rate target was relaxed to 80 to 100 bpm. Electrolyte and acid-base status were closely monitored and corrected. OUTCOMES: The patient's hemodynamics stabilized, respiratory status improved, and N-terminal pro-B-type natriuretic peptide declined, with pleural effusions largely resolved on ultrasound. She was discharged home in stable condition after a 14-day hospital stay on guideline-directed medical therapy. LESSONS: In geriatric patients, infection can present atypically and act as a silent trigger for HF decompensation, leading to mixed respiratory failure. Rate control targets should be individualized and dynamically adjusted, particularly during acute illness, to avoid potential compromise of cardiac output. This case underscores the importance of diagnostic vigilance, serial imaging, electrolyte monitoring, and flexible, pathophysiology-guided management in optimizing outcomes in this vulnerable population.