运用容量·CO2曲线测量无效腔的原理

无效腔是指潮气量中因未与肺毛细血管内血流接触而不参与气体交换的部分。无效腔通常采用容量-CO2曲线即呼出气CO2／潮气量曲线计算，此法可在床边简便评估某通气模式下的无效通气部分。现在Bohr原始无效腔量可无创即时获得，就像平均肺泡C仍分压（PACO2）可通过CO2直接曲线获得。Enghoff修正的Bohr公式（以PaCO2代替PACD2）所得出的数值代表整体无效气体交换指数，而不是真正“无效腔”，因为该值受各种导致通气／灌注比失调因素影响，包括真正无效腔及分流。因而，从Bohr与Enghoff公式获得的结果具有不同的生理学意义，临床医生们在解读患者数据时必须知晓这种差异。本文中我们将描述使用容量CO2曲线测量无效腔的原理，讨论其主要临床应用及相关的错误概念。

Dead space is the portion of a tidal volume that does not participate in gas exchange because it does not get in contact with blood flowing through the pulmonary capillaries. It is commonly calculated using volumetric capnography, the plot of expired carbon dioxide （CO2） versus tidal volume, which is an easy bedside assessment of the inefficiency of a particular ventilatory setting. Today, Bohr＇s original dead space can be calculated in an entirely noninvasive and breath-by-breath manner as the mean alveolar partial pressure of CO2 （PaCO2） which can now be determined directly from the capnogram. The value derived from Enghoff＇s modification of Bohr＇s formula （using PaCO2 instead of PaCO2） is a global index of the inefficiency of gas exchange rather than a true ＂dead space＂ because it is influenced by all causes of ventilation/perfusion mismatching, from real dead space to shunt. Therefore, the results obtained by Bohr＇s and Enghoff＇s formulas have different physiological meanings and clinicians must be conscious of such differences when interpreting patient data. In this article, we describe the rationale of dead space measurements by volumetric capnography and discuss its main clinical implications and the misconceptions surrounding it.