critical care medicine
Ventilatory Failure
Pathophysiology of Ventilatory Failure
Hypercapnia occurs when alveolar ventilation either falls or fails to rise adequately in response to increased carbon dioxide production. A fall in alveolar ventilation results from a decrease in minute ventilation or an increase in dead space ventilation without appropriate compensation by increasing minute ventilation. Ventilatory failure can occur when there is excessive load on the respiratory system (eg, resistive loads or lung and chest wall elastic loads) versus neuromuscular competence for an effective inspiratory effort. When the minute ventilation load increases (eg, as occurs in sepsis), a compromised respiratory system may not be able to meet this increased demand (for causes, see figure The balance between load (resistive, elastic, and minute ventilation) and neuromuscular competence (drive, transmission, and muscle strength) determines the ability to sustain alveolar ventilation ). Physiologic dead spaceis the part of the respiratory tree that does not participate in gas exchange. It includes Anatomic dead space (oropharynx, trachea, and airways) Alveolar dead space (ie, alveoli that are ventilated but not perfused) 70% in massive Pulmonary Embolism (PE) , severe emphysema, and Increased carbon dioxide production, as occurs with fever, Sepsis and Septic Shock , trauma, Hypercapnia lowers arterial pH ( Respiratory Acidosis ). Severe acidemia (pH PEEP = positive end-expiratory pressure. 