A102-15 Ventilation-acidosis Paradox in Necrotizing Pneumonia: Managing Auto-Peep-Driven Shock

Introduction Severe necrotizing pneumonia complicated by obstructive lung mechanics can produce a dangerous intersection of hypoxemia, hypercapnia, and lactic acidosis. Attempts to increase minute ventilation often aggravate dynamic hyperinflation and produce hemodynamic collapse. This case illustrates the “ventilation-acidosis paradox” and practical ventilator strategies to restore physiologic balance. Case Description A 58-year-old man with chronic tobacco use and presumed COPD was transferred from a community hospital for escalating hypoxemic and hypercarbic respiratory failure. He arrived intubated AC-VC, FiO2 100%, RR 18, TV 400, Peep 5 , hypotensive on triple vasopressor therapy with Norepinephrine, Phenylephrine, Vasopressin, and profoundly acidemic (pH 7.08, PaCO2 86 mm Hg, PO2 131.3 , HCO3 25.6 with lactate 8 mmol/L. CT chest demonstrated a right-upper-lobe cavitary necrotizing pneumonia with multifocal consolidations. Point-of-care ultrasound showed a hyperdynamic left ventricle and low systemic vascular resistance, indicating distributive shock, but ventilator flow curves revealed marked dynamic hyperinflation with measured auto-PEEP 21 cm H2O, producing an additional obstructive component. Ventilation was adjusted to prolong expiratory time (reduced respiratory rate, increased inspiratory flow), and lower tidal volumes to minimize air trapping. Continuous nebulized albuterol, systemic corticosteroids, and neuromuscular blockade were initiated. Auto-PEEP decreased to 12 cm H2O with improved gas exchange, but persistent acidemia limited catecholamine effectiveness. A strategy of permissive hypercapnia with cautious intravenous bicarbonate was adopted to raise systemic pH and support vascular tone. Subsequently, over the next several hours, arterial pH improved, vasopressor requirements decreased, and oxygenation gradually improved. Bronchial cultures remained negative; MRSA pneumonia was presumed and treated empirically with Vancomycin and Meropenem. Blood Cultures grew Staph Aureus MRSA. Discussion This case highlights how severe obstructive mechanics in necrotizing pneumonia can generate mixed distributive-obstructive shock. Dynamic hyperinflation elevates intrathoracic pressure, decreases venous return, and worsens hypotension. In profound acidemia, α-adrenergic responsiveness to catecholamines diminishes, creating pressor-refractory shock. Management requires recognizing intrinsic PEEP at the bedside and prioritizing lung-protective, hemodynamically permissive ventilation via reduced respiratory rate, increased inspiratory flow, limiting tidal volume, and moderate permissive hypercapnia. When metabolic acidosis coexists, cautious buffering with sodium bicarbonate can transiently improve perfusion, until ventilation has been optimized to counterbalance. Early bedside measurement of auto-PEEP and individualized ventilator adjustment can be lifesaving in similar scenarios.