NEWBORN HYPOXEMIA HYPOVENTILATION
Category: Child Health
Abstract : newborn hypoxemia is usually the result of V/Q mismatch or right-to-left shunting, although diffusion abnormalities and hypoventilation (eg, apnea) also may decrease oxygenation. V/Q mismatch is a major cause of hypoxemia in infants with respiratory distress syndrome (RDS) and other causes of respiratory failure. V/Q mismatch usually is caused by poor ventilation of alveoli relative to their perfu
newborn hypoxemia is usually the result of V/Q mismatch or right-to-left shunting, although diffusion abnormalities and hypoventilation (eg, apnea) also may decrease oxygenation. V/Q mismatch is a major cause of hypoxemia in infants with respiratory distress syndrome (RDS) and other causes of respiratory failure. V/Q mismatch usually is caused by poor ventilation of alveoli relative to their perfusion.
Shunting can be intracardiac (eg, congenital cyanotic heart disease) and/or extracardiac (eg, pulmonary).
During conventional ventilation, oxygenation is determined largely by the fraction of inspired oxygen (FiO2) and the mean airway pressure (MAP). MAP is the average airway pressure during the respiratory cycle and can be calculated by dividing the area under the airway pressure curve by the duration of the cycle as follows:
MAP = K * (PIP – PEEP) * TI / (TI + TE) + PEEP
This formula includes the constant determined by the flow rate and the rate of rise of the airway pressure curve (K), peak inspiratory pressure (PIP), positive end-expiratory pressure (PEEP), inspiratory time (TI), and expiratory time (TE). This equation indicates that MAP increases with increasing PIP, PEEP, TI to TI + TE ratio, and flow (increases K by creating a more square waveform).
The mechanism by which increases in MAP generally improve oxygenation appears to be the increased lung volume and improved V/Q matching. Although a direct relationship between MAP and oxygenation exists, some exceptions are found. For the same change in MAP, increases in PIP and PEEP enhance oxygenation more than changes in the ratio of TI to TE (I/E ratio). Increases in PEEP are not as effective once an elevated level (>5-6 cm H2O) is reached and may not improve oxygenation at all. In fact, a very high MAP may cause overdistention of alveoli, leading to right-to-left shunting of blood in the lungs. If a very high MAP is transmitted to the intrathoracic structures, which may occur with near normal lung compliance, cardiac output may decrease, and thus, even with adequate oxygenation of blood, systemic oxygen transport (arterial oxygen content X cardiac output) may decrease.
Blood oxygen content largely depends on oxygen saturation and hemoglobin level. Thus, transfusing packed red blood cells to infants with anemia (hemoglobin >7-10 mg/dL) who are receiving assisted ventilation is common practice. Oxygenation also depends on oxygen unloading at the tissue level, which is determined strongly by the oxygen dissociation curve. Acidosis, increases in 2,3-diphosphoglycerate, and adult hemoglobin levels reduce oxygen affinity to hemoglobin and thus favor oxygen delivery to the tissues.
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