From the General Anesthesia Division, Children's Hospital of Philadelphia, Perelman School of Medicine/University of Pennsylvania, Philadelphia, Pennsylvania.
Anesth Analg. 2014 Apr;118(4):776-80. doi: 10.1213/ANE.0000000000000148.
Physiologic dead space is defined as the volume of the lung where gas exchange does not occur. Apparatus dead space increases dead space volume, causing either increased PaCO2 or the need to increase minute ventilation to maintain normocapnia. Children are especially vulnerable because small increases in apparatus dead space can significantly increase dead space to tidal volume ratio (Vd/Vt). The effect of changes in dead space on arterial CO2 (PaCO2) and required minute ventilation were calculated for patients weighing 2 to 17 kg that corresponds to 0 to 36 months of age. Apparatus volumes for typical devices were obtained from the manufacturer or measured by the volume of water required to fill the device. The relationship between the fraction of alveolar CO2 (FaCO2) and dead space volume (Vd) was derived from the Bohr equation, FaCO2 = VCO2/(RR*(Vt - Vd)), where VCO2 is CO2 production, RR is respiratory rate, and Vt is tidal volume. VCO2 was estimated by using Brody's equation for humans aged up to 36 months, (VCO2 = 5.56*(wt)), where weight is in kilogram. Initial conditions were Vt = 8 mL/kg, Vd/Vt = 0.3, and a RR of 20 breaths per minute. The relationship between PaCO2 and dead space was determined for increasing Vd. Rearranging the Bohr equation, the RR required to maintain PaCO2 of 40 mm·Hg was determined as dead space increased. The apparatus Vd of typical device arrangements ranged from 8 to 55 mL, and these values were used for the dead space values in the model. PaCO2 increased exponentially with increasing apparatus dead space. For smaller patients, the PaCO2 increased more rapidly for small changes in Vd than that in larger patients. Similarly, RR required to maintain PaCO2 of 40 mm·Hg increased exponentially with increasing dead space. Increasing apparatus Vd can lead to exponential increases in PaCO2 and/or RR required to maintain normal PaCO2. The effect on PaCO2 is less as patient weight increases, but these data suggest it can be significant for typical circuit components up to at least 17 kg or aged 36 months.
生理死腔是指气体交换未发生的肺容量。仪器死腔增加会增加死腔量，导致 PaCO2 升高或需要增加分钟通气量以维持正常碳酸血症。儿童尤其容易受到影响，因为仪器死腔的微小增加可显著增加死腔与潮气量的比值（Vd/Vt）。计算了体重为 2 至 17 公斤（对应 0 至 36 个月龄）的患者中死腔变化对动脉 CO2（PaCO2）和所需分钟通气量的影响。从制造商处获得典型设备的仪器体积或通过填充设备所需的水量进行测量。肺泡 CO2（FaCO2）与死腔量（Vd）之间的关系来自 Bohr 方程，FaCO2 = VCO2/（RR*（Vt-Vd）），其中 VCO2 是 CO2 产生量，RR 是呼吸频率，Vt 是潮气量。VCO2 通过用于 36 个月以下人类的 Brody 方程进行估计，（VCO2 = 5.56*（wt）），其中体重为千克。初始条件为 Vt = 8 mL/kg，Vd/Vt = 0.3，RR 为 20 次/分钟。随着 Vd 的增加，确定了 PaCO2 与死腔之间的关系。通过重新排列 Bohr 方程，确定了随着死腔增加而维持 40 mmHg·Hg 的 PaCO2 所需的 RR。典型设备布置的仪器 Vd 范围为 8 至 55 mL，这些值用于模型中的死腔值。随着仪器死腔的增加，PaCO2 呈指数增加。对于较小的患者，与较大的患者相比，Vd 较小的变化会导致 PaCO2 更快地增加。同样，维持 40 mmHg·Hg 的 PaCO2 所需的 RR 也随着死腔的增加呈指数增加。增加仪器死腔会导致 PaCO2 和/或维持正常 PaCO2 所需的 RR 呈指数增加。随着患者体重的增加，对 PaCO2 的影响会减小，但这些数据表明，对于典型的回路组件，其影响至少可达 17 公斤或 36 个月龄。
