Positive-pressure ventilation often requires an artificialairway, though it can be provided noninvasively via tight-fitting nasal or full face masks. Noninvasive ventilation is particularly useful in patients with obstructive sleep apneaand neuromuscular disease who require support only part ofthe day, but it can also be used to assist ventilation continuously for patients in acute respiratory failure from a varietyof causes.
No method of mechanical ventilation truly simulates natural breathing. All methods have their drawbacks and complications. Positive pressure is transmitted to the entire thoraxand may impede venous return to the heart during inspiration. The airways and lung parenchyma may be damaged byinflation pressures and high concentrations of inspired O2. In general, inflation pressures should be limited to those necessary to provide sufficient lung expansion for adequate ventilation and the prevention of atelectasis. Pressure-cycled and volume-cycled ventilators (conventional ventilation) are themost widely used modalities in pediatrics; however, high-frequency jet ventilation and high-frequency oscillatory ventilation are used often in neonatology and may be used inpatients with severe lung disease who are failing conventionalmechanical ventilation.
Chapter 134 u Control of Breathing 461
Ventilation is controlled primarily by central chemoreceptors located in the medulla that respond to intracellular pH and Pco2 levels (Fig. 134-1). To a lesser extent, ventilation is modulated by peripheral receptors located in the carotid and aortic bodies, which respond predominantly to Po2. The central receptors are quite sensitive. Small acute changes in Paco2 normally result in significant changes in minute ventilation. When the Pco2 is chronically elevated, the intracellular pH returns to normal levels because of compensatory increases in the bicarbonate level, and ventilatory drive is not increased. The peripheral receptors do not stimulate ventilation until the Pao2 decreases to approximately 60 mm Hg. These receptors become important in patients with chronic Paco2 elevation who may have a blunted ventilatory response to CO2.
The output of the central respiratory center also is modulated by reflex mechanisms. Full lung inflation temporarily inhibits inspiratory effort in infants (Hering-Breuer reflex)through vagal afferent fibers. Other reflexes from the airways and intercostal muscles may influence the depth and frequency of respiratory efforts (see Fig. 134-1).
An acute life-threatening event (ALTE) is defined as any unexpected and frightening change in condition characterized by
PCO2, chemoreceptors sensory receptors (mechanoreceptors, +
PO2 stretch, irritant and +
PaO2, PaCO2