Gowda M S, Klocke R A
Department of Medicine, State University of New York at Buffalo, USA.
Crit Care Med. 1997 Jan;25(1):41-5. doi: 10.1097/00003246-199701000-00010.
To determine the usefulness of indices of hypoxemia in assessing patients with the adult respiratory distress syndrome (ARDS).
Retrospective analysis of previously published data that describe the distributions of ventilation and pulmonary blood flow in ARDS.
University research laboratory.
Sixteen patients with ARDS.
The FIO2 was varied between 0.21 and 1.0 in a computer model of gas exchange, based on a 50-compartment model of ventilation/perfusion inhomogeneity plus true shunt and deadspace. The indices of hypoxemia that were calculated as a function of inspired oxygen concentration included PaO2/FIO2, arterial/alveolar ratio (PaO2/alveolar PO2), the alveolar-arterial PO2 difference (P[A-a]O2), respiratory index (P[A-a]O2/PaO2), and venous admixture.
The PaO2/FIO2 ratio in patients with moderate shunts (< 30%) varied considerably with alteration in FIO2. At both extremes of FIO2, the PaO2/FIO2 in these patients was substantially greater than at intermediate FIO2. Patients with larger shunts (> 30%) had greater PaO2/FIO2 ratios at low FIO2, but the PaO2/FIO2 ratios decreased to relatively stable values at FIO2 values of > 0.5. In all patients, PaO2/FIO2 remained relatively stable at FIO2 values of > or = 0.5 and PaO2 values of < or = 100 torr (< or = 13.3 kPa). Other PO2-based indices exhibited less stability as FIO2 was varied. If hypoxemia resulted from true shunting, venous admixture was found to be stable at all FIO2 values. However, approximately one half of patients had clinically important hypoxemia resulting from mismatching of ventilation and blood flow. In these patients, venous admixture varied substantially with change in FIO2, and the degree of variation was proportional to the fraction of cardiac output perfusing gas exchange units with ventilation/perfusion ratios of < 0.1.
All indices of hypoxemia are affected by changes in FIO2 in patients with ARDS. PaO2/FIO2 ratio exhibits the most stability at FIO2 values of > or = 0.5 and PaO2 values of < or = 100 torr (< or = 13.3 kPa), and is a useful estimation of the degree of gas exchange abnormality under usual clinical conditions. Venous admixture varies substantially with alteration of FIO2 in patients who have clinically important ventilation/perfusion abnormalities. Under these circumstances, venous admixture is a poor indicator of the efficiency of pulmonary oxygen exchange, even if venous admixture is calculated from measured arterial and venous oxygen content values. Estimated venous admixture, based on an assumed arterial-venous oxygen content difference, is even more unreliable.
确定低氧血症指标在评估成人呼吸窘迫综合征（ARDS）患者中的实用性。
对先前发表的描述ARDS通气和肺血流分布的数据进行回顾性分析。
大学研究实验室。
16例ARDS患者。
在气体交换计算机模型中，基于通气/灌注不均一性加真性分流和死腔的50房室模型，将吸入氧分数（FIO2）在0.21至1.0之间变化。作为吸入氧浓度函数计算的低氧血症指标包括动脉血氧分压/吸入氧分数（PaO2/FIO2）、动脉/肺泡比（PaO2/肺泡氧分压）、肺泡-动脉氧分压差（P[A-a]O2）、呼吸指数（P[A-a]O2/PaO2）和静脉血掺杂。
中度分流（<30%）患者的PaO2/FIO2比值随FIO2的改变有很大变化。在FIO2的两个极端值时，这些患者的PaO2/FIO2明显高于中间FIO2值时。分流较大（>30%）的患者在低FIO2时具有较高的PaO2/FIO2比值，但在FIO2>0.5时，PaO2/FIO2比值降至相对稳定的值。在所有患者中，当FIO2≥0.5且PaO2≤100托（≤13.3千帕）时，PaO2/FIO2保持相对稳定。随着FIO2的变化，其他基于氧分压的指标表现出较小的稳定性。如果低氧血症是由真性分流引起的，发现静脉血掺杂在所有FIO2值时均稳定。然而，约一半患者的临床重要低氧血症是由通气和血流不匹配所致。在这些患者中，静脉血掺杂随FIO2的变化有很大差异，且变化程度与灌注通气/灌注比<0.1的气体交换单位的心输出量分数成正比。
ARDS患者的所有低氧血症指标均受FIO2变化的影响。当FIO2≥0.5且PaO2≤100托（≤13.3千帕）时，PaO2/FIO2比值表现出最大稳定性，是通常临床条件下气体交换异常程度的有用估计指标。在存在临床重要通气/灌注异常的患者中，静脉血掺杂随FIO2的改变有很大变化。在这种情况下，即使静脉血掺杂是根据测量的动脉和静脉氧含量值计算得出的，静脉血掺杂也不是肺氧交换效率的良好指标。基于假定的动脉-静脉氧含量差估计的静脉血掺杂甚至更不可靠。
