Schober Andrew, Feiner John R, Bickler Philip E, Rollins Mark D
From the Departments of Anesthesia and Perioperative Care (A.S., J.R.F, P.E.B., M.D.R.), Obstetrics and Gynecology, and Surgery (M.D.R.), University of California, San Francisco, California.
Anesthesiology. 2018 Jan;128(1):97-108. doi: 10.1097/ALN.0000000000001898.
Cerebral oximetry (cerebral oxygen saturation; ScO2) is used to noninvasively monitor cerebral oxygenation. ScO2 readings are based on the fraction of reduced and oxidized hemoglobin as an indirect estimate of brain tissue oxygenation and assume a static ratio of arterial to venous intracranial blood. Conditions that alter cerebral blood flow, such as acute changes in PaCO2, may decrease accuracy. We assessed the performance of two commercial cerebral oximeters across a range of oxygen concentrations during normocapnia and hypocapnia.
Casmed FORE-SIGHT Elite (CAS Medical Systems, Inc., USA) and Covidien INVOS 5100C (Covidien, USA) oximeter sensors were placed on 12 healthy volunteers. The fractional inspired oxygen tension was varied to achieve seven steady-state levels including hypoxic and hyperoxic PaO2 values. ScO2 and simultaneous arterial and jugular venous blood gas measurements were obtained with both normocapnia and hypocapnia. Oximeter bias was calculated as the difference between the ScO2 and reference saturation using manufacturer-specified weighting ratios from the arterial and venous samples.
FORE-SIGHT Elite bias was greater during hypocapnia as compared with normocapnia (4 ± 9% vs. 0 ± 6%; P < 0.001). The INVOS 5100C bias was also lower during normocapnia (5 ± 15% vs. 3 ± 12%; P = 0.01). Hypocapnia resulted in a significant decrease in mixed venous oxygen saturation and mixed venous oxygen tension, as well as increased oxygen extraction across fractional inspired oxygen tension levels (P < 0.0001). Bias increased significantly with increasing oxygen extraction (P < 0.0001).
Changes in PaCO2 affect cerebral oximeter accuracy, and increased bias occurs with hypocapnia. Decreased accuracy may represent an incorrect assumption of a static arterial-venous blood fraction. Understanding cerebral oximetry limitations is especially important in patients at risk for hypoxia-induced brain injury, where PaCO2 may be purposefully altered.
脑氧饱和度测定法（脑氧饱和度；ScO2）用于无创监测脑氧合情况。ScO2读数基于还原血红蛋白和氧化血红蛋白的比例，作为脑组织氧合的间接估计值，并假定动脉血与静脉血在颅内的比例是固定的。改变脑血流量的情况，如PaCO2的急性变化，可能会降低准确性。我们评估了两种商用脑氧饱和度测定仪在正常碳酸血症和低碳酸血症期间一系列氧浓度下的性能。
将Casmed FORE-SIGHT Elite（美国CAS医疗系统公司）和Covidien INVOS 5100C（美国Covidien公司）氧饱和度测定仪传感器放置在12名健康志愿者身上。改变吸入氧分数张力以达到七个稳态水平，包括低氧和高氧的PaO2值。在正常碳酸血症和低碳酸血症情况下均获取ScO2以及同时进行的动脉血和颈静脉血气测量。使用制造商指定的动脉血和静脉血样本加权比率，将氧饱和度测定仪偏差计算为ScO2与参考饱和度之间的差值。
与正常碳酸血症相比，低碳酸血症期间FORE-SIGHT Elite偏差更大（4±9%对0±6%；P<0.001）。INVOS 5100C在正常碳酸血症期间的偏差也较低（5±15%对3±12%；P = 0.01）。低碳酸血症导致混合静脉血氧饱和度和混合静脉血氧张力显著降低，以及在不同吸入氧分数张力水平下氧摄取增加（P<0.0001）。偏差随氧摄取增加而显著增加（P<0.0001）。
PaCO2的变化会影响脑氧饱和度测定仪的准确性，低碳酸血症时偏差会增加。准确性降低可能代表对固定动静脉血比例的错误假设。了解脑氧饱和度测定法的局限性在有缺氧性脑损伤风险的患者中尤为重要，因为在这些患者中PaCO2可能会被有意改变。
