i. Findings. PaCO2 > 50 mm Hg indicates ventilatory failure; however, many patients have chronic ventilatory failure with renal compensation (retaining HCO3-). Clinical exam and absolute pH is a better guide than PaCO2 to determine need for ventilatory assistance. Significant acidemia suggests acute respiratory acidosis or acute decompensation of chronic respiratory acidosis. Respiratory acidosis with rapidly falling pH or absolute pH < 7.24 is an indication for ventilatory support.
ii. Causes. Prototype of pure ventilatory failure is patient with a drug overdose in whom there is sudden loss of central respiratory drive with uncontrolled hypercarbia. Patients with central hypoventilation, Werdnig-Hoffmann disease (anterior horn cell disease), Guillain-Barré syndrome (peripheral motor nerve disease), botulism (disease of neuromuscular junction), obstructive pulmonary disease, and sepsis may have hypercarbic ventilatory failure. Neuromuscular failure is a category of ventilatory failure that deserves special mention. A progressive rise in respiratory rate, increased work of breathing, and abdominal paradox (inward movement of abdominal wall during inspiration rather than normal outward motion), decreased vital capacity, or decreased negative inspiratory force (> -25 cm H2O) implies impending respiratory failure.
c. Mixed respiratory failure. Many patients have failure of both ventilation and oxygenation. An example is patient with neuromuscular disease and pneumonia. Indications for ventilatory support remain the same. Threshold for initiating ventilatory support is even lower for patients with ventilation and oxygenation failure, because oxygen management may further compound hypercarbia.
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