Division of Pulmonary Medicine, Faculty of Medicine and Dentistry, University of Alberta, AB, Canada.
G.F. MacDonald Centre for Lung Health, Covenant Health, Edmonton, AB, Canada.
J Physiol. 2019 Nov;597(22):5385-5397. doi: 10.1113/JP277793. Epub 2019 Aug 25.
Precapillary gas exchange for oxygen has been documented in both humans and animals. It has been suggested that, if precapillary gas exchange occurs to a greater extent for inert gases than for oxygen, shunt and its effects on arterial oxygenation may be underestimated by the multiple inert gas elimination technique (MIGET). We evaluated fractional precapillary gas exchange in canines for O and two inert gases, sulphur hexafluoride and ethane, by measuring these gases in the proximal pulmonary artery, distal pulmonary artery (1 cm proximal to the wedge position) and systemic artery. Some 12-19% of pulmonary gas exchange occurred within small (1.7 mm in diameter or larger) pulmonary arteries and this was quantitatively similar for oxygen, sulphur hexafluoride and ethane. Under these experimental conditions, this suggests only minor effects of precapillary gas exchange on the magnitude of calculated shunt and the associated effect on pulmonary gas exchange estimated by MIGET.
Some pulmonary gas exchange is known to occur proximal to the pulmonary capillary, although the magnitude of this gas exchange is uncertain, and it is unclear whether oxygen and inert gases are similarly affected. This has implications for measuring shunt and associated gas exchange consequences. By measuring respiratory and inert gas levels in the proximal pulmonary artery (P), a distal pulmonary artery 1 cm proximal to the wedge position (using a 5-F catheter) (D) and a systemic artery (A), we evaluated precapillary gas exchange in 27 paired samples from seven anaesthetized, ventilated canines. Fractional precapillary gas exchange (F) was quantified for each gas as F = (P - D)/(P - A). The lowest solubility inert gases, sulphur hexafluoride (SF ) and ethane were used because, with higher solubility gases, the P-A difference is sufficiently small that experimental error prevents accurate assessment of F. Distal samples (n = 12) with oxygen (O ) saturation values that were (within experimental error) equal to or above systemic arterial values, suggestive of retrograde capillary blood aspiration, were discarded, leaving 15 for analysis. D was significantly lower than P for SF (D/P = 88.6 ± 18.1%; P = 0.03) and ethane (D/P = 90.6 ± 16.0%; P = 0.04), indicating partial excretion of inert gas across small pulmonary arteries. Distal pulmonary arterial O saturation was significantly higher than proximal (74.1 ± 6.8% vs. 69.0 ± 4.9%; P = 0.03). Fractional precapillary gas exchange was similar for SF , ethane and O (0.12 ± 0.19, 0.12 ± 0.20 and 0.19 ± 0.26, respectively; P = 0.54). Under these experimental conditions, 12-19% of pulmonary gas exchange occurs within the small pulmonary arteries and the extent is similar between oxygen and inert gases.
在人和动物中都记录到了毛细血管前的氧气交换。有人提出，如果毛细血管前的气体交换对于惰性气体比对氧气更广泛，那么多重惰性气体消除技术（MIGET）可能会低估分流血量及其对动脉氧合的影响。我们通过测量近端肺动脉、远端肺动脉（楔形位置近端 1 厘米处）和体动脉中的 O 和两种惰性气体（六氟化硫和乙烷）来评估犬类的肺前毛细血管气体交换分数。大约 12-19%的肺气体交换发生在小（直径 1.7 毫米或更大）肺动脉中，氧气、六氟化硫和乙烷的气体交换分数相似。在这些实验条件下，这表明仅存在小的毛细血管前气体交换对计算分流的幅度以及 MIGET 估计的相关肺气体交换的影响。
尽管这种气体交换的程度不确定，但已知一些肺气体交换发生在肺毛细血管前，氧气和惰性气体是否受到类似影响尚不清楚。这对测量分流血量和相关气体交换后果有影响。我们通过测量 7 只麻醉、通气犬的 27 对近端肺动脉（P）、楔形位置近端 1 厘米处的远端肺动脉（D）（使用 5-F 导管）和体动脉（A）中的呼吸和惰性气体水平，评估了这两种气体的肺前毛细血管气体交换。用 F=（P-D）/（P-A）定量表示每个气体的前毛细血管气体交换分数（F）。选择最低溶解度的惰性气体六氟化硫（SF ）和乙烷，因为在高溶解度气体中，P-A 差异足够小，以至于实验误差会阻止对 F 的准确评估。由于远端样本（n=12）的氧饱和度值（在实验误差范围内）与或高于体动脉值，提示有逆行毛细血管血液抽吸，因此丢弃了这些样本，留下 15 个用于分析。SF 的 D 值明显低于 P（D/P=88.6±18.1%；P=0.03）和乙烷（D/P=90.6±16.0%；P=0.04），表明部分惰性气体通过小肺动脉排出。远端肺动脉的氧饱和度明显高于近端（74.1±6.8% vs. 69.0±4.9%；P=0.03）。SF 、乙烷和 O 的前毛细血管气体交换分数相似（0.12±0.19、0.12±0.20 和 0.19±0.26，分别；P=0.54）。在这些实验条件下，12-19%的肺气体交换发生在小肺动脉中，氧气和惰性气体之间的程度相似。
