Non-Invasive Mechanical Ventilation in Children: An Overview

Abstract

Ventilatory control is a precisely tuned physiologic process that maintains systemic oxygen and carbon dioxide levels within very narrow ranges required for appropri-ate cellular function. This delicate balance has resulted in a complex system that senses changes in oxygen (O 2), carbon dioxide (CO 2), and pH in arterial blood and in the brain and responds to these changes through compensatory responses in the upper airway and respiratory pump muscles to maintain ventilation (see Fig. 2.1). Oxygen levels are primarily sensed by the peripheral chemoreceptors, the carotid bodies, and the aortic bodies. CO 2 is sensed mainly by central chemoreceptors that are widely distributed in the brainstem and, along with the peripheral chemorecep-tors, input to the medullary respiratory centers in the brainstem. Perturbations in O 2 , CO 2 , and pH will prompt precise changes in the depth of respiration (tidal volume), respiratory rate, and/or the breathing pattern via diaphragmatic, accessory chest wall and abdominal muscular contraction. Upper airway musculature also receives input from central and peripheral chemoreceptors resulting in contraction to main-tain patency. When alveolar ventilation is insufficient and oxygen levels become too low and/ or carbon dioxide levels too high, cellular function is impaired. The alveolar venti-lation equation is useful in understanding the physiologic processes leading to hypoventilation and indicates that systemic carbon dioxide levels (pCO 2) are pro-portional to the ratio between systemic carbon dioxide production (VCO 2) and alve-olar ventilation (VA) as follows: