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Acute respiratory failure in children is the inability of the respiratory system to support oxygenation, ventilation, or both. Hypoxic respiratory failure is defined by an arterial partial pressure of oxygen (PaO2) below 60 mm Hg, which typically produces an arterial oxygen saturation of 90%. Ventilation is the elimination of CO2 and is measured by the arterial partial pressure of CO2 (PaCO2). Acute hypercarbic respiratory failure is defined by an acute increase in PaCO2 greater than 50 mm Hg. It is typically associated with a respiratory acidosis pH of <7.35. Venous blood may be sampled in lieu of arterial blood to obtain the venous partial pressure of CO2 (PvCO2); however, it can only be accurately stated that the PaCO2 is no higher than the PvCO2. Therefore, when PvCO2 is <50 mm Hg, acute hypercarbic respiratory failure can be ruled out but a PvCO2 of 55 mm Hg does not guarantee a diagnosis of hypercarbic respiratory failure. PvCO2 is a test that has high sensitivity but poor specificity for diagnosing hypercarbic respiratory failure. PvCO2 should be interpreted carefully based on location of sampling, manner of sampling, and cardiac output.
Epidemiology
Acute respiratory failure is a common reason for admission to the pediatric intensive care unit (PICU). The epidemiology is not well described due to inconsistent and heterogeneous diagnostic criteria. In patients with respiratory failure who have underlying pediatric acute respiratory distress syndrome (ARDS), epidemiologic data reveal an annual incidence of 2.3% of PICU admissions, and a mortality rate of 24% to 34%.1,2
Physiology and Pathophysiology
Normal control of breathing is a complex interaction between the vasculature, brain, lungs, and respiratory apparatus. Peripheral chemoreceptors, located in the aortic and carotid bodies, are sensitive to PaO2, PaCO2, and pH. A decrease in PaO2, a decrease in pH or an increase in CO2 results in signaling to increase ventilation. Central chemoreceptors in the brain are sensitive to cerebral spinal fluid (CSF) pH. The blood-brain barrier allows CO2, but not hydrogen ions, to pass freely so the CSF pH is determined by PaCO2. Therefore, the central chemoreceptors can detect small changes in CO