Due to their relatively high oxygen consumption, smaller children, and especially infants, undergo rapid desaturation with cessation of ventilation, even with normal lungs. Once drugs have been administered in RSI, patients rapidly become apneic. Positive-pressure mask ventilation is not performed at this point due to the risk of gastric insufflation and resultant regurgitation. Therefore, it is important to maximize the length of time that adequate saturation is maintained by maximizing the reservoir of oxygen in the lungs. The lung volume remaining at end-expiration, or functional residual capacity (FRC), contains less than 20% oxygen in a patient breathing room air, with nearly 80% occupied by nitrogen. Preoxygenation is effectively denitrogenation of FRC, and it increases by roughly fivefold the oxygen reservoir in the lungs once apnea occurs. Effective denitrogenation is ideally accomplished by having a patient breathe 100% oxygen from a tight-fitting mask for 2 min or for four vital capacity breaths. This is often not possible with a critically ill prediatric patient, but as long a period of preoxygenation as circumstances permit should be provided prior to RSI.
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