危重病学(上海交通大学)机械通气的临床应用.ppt

* * * * /RITCHISO/301notes6.htm * * As outlined in the preceding slides, hypoxemia (not hypoxia) can be the result of hypoventilation (not enough delivered) or not matching the delivery to the loading sites (V/Q mismatch). Shunt, whether intracardiac or intrapulmonary, is the ultimate form of V/Q mismatch (V/Q =? ). Diffusion impairments must be significant to result in hypoxemia and are rarely of clinical relevance in pediatrics. Hypoventilation is the primary cause of hypercarbia. V/Q mismatch must be profound before hypercarbia results for the reasons discussed in the previous slide. * * 呼吸衰竭 大手术后呼吸支持 麻醉和术中应用 心肺复苏 * * The partial pressure of carbon dioxide in the arterial blood is directly related to metabolic production and indirectly related to minute ventilation. To be accurate, it is alveolar minute ventilation that matters. When a child is tachypneic, the minute ventilation may not change since the increase in rate is balanced by a decrease in tidal volume. However, the amount of dead space has not changed so the effective tidal volume will decrease and hence the effective minute ventilation and thus PaCO2 will increase despite the increased respiratory rate. Likewise, any process that increases dead space without changing minute ventilation will result in an increase in PaCO2 . * * To be simplistic, oxygenation involves getting enough oxygen to an alveolus that is perfused. The more volume of gas that can be delivered for exchange, the better. The higher the driving pressure for that gas exchange, the better. Ideally, ventilation (V) and perfusion (Q) are matched so that oxygen is where the blood is (V/Q = 1). When gas exchange does take place, it is so rapid that a hemoglobin molecule passing through an alveolar capillary is fully saturated before it is one third of the way across. The rest of the capillary represents a reserve for when transit time is increased (e.g., tachycardia) or for when diffusion is slowed (e.g., pulmonary edema, fibrosis