Shepherd A P, Steinke J M, McMahan C A
Department of Physiology, University of Texas Health Science Center, San Antonio 78284-7756, USA.
Int J Cardiol. 1997 Oct 10;61(3):247-59. doi: 10.1016/s0167-5273(97)00167-8.
As a quantitative assessment of the magnitude of shunting, the ratio of pulmonary to systemic blood flow (Qp/Qs) plays an important role not only in the oximetric diagnosis of intracardiac and great-vessel shunts but also in the treatment of the patient. However, the oxygen saturation measurements used to compute the Qp/Qs ratio contain errors due to physiological variability and measurement error of the oximeter used to analyze the blood samples. We have developed a mathematical model to describe the variability that oximetry errors contribute to the uncertainty in the Qp/Qs ratio. Using this model, we compute the probability of making an inappropriate recommendation regarding corrective surgery when a particular value of the ratio is the criterion for surgery, e.g. a Qp/Qs ratio >2. This report also contains a spreadsheet that readers can use to analyze their own oximetry data by computing confidence intervals for the Qp/Qs ratio. The results presented here support the following conclusions. First, because the Qp/Qs ratio is calculated from saturation measurements at four different sites, oximetry errors make the Qp/Qs ratio less effective at detecting the presence of a shunt than the conventional step-up method that depends on samples from only two sites. Second, although oximetry errors are equally likely to cause the calculated Qp/Qs ratio to overestimate the true Qp/Qs ratio as to underestimate it, the overestimations on average have greater magnitudes than the underestimations. Third, in comparison with an oximeter that has 2.5% measurement error, using an oximeter with 1% or less error greatly reduces the uncertainty in the Qp/Qs ratio and thus increases the probability of reaching the right decision regarding corrective surgery. Fourth, the variability in apparent Qp/Qs ratios is also greatly diminished by taking multiple blood samples from each of the four requisite sites and averaging them before calculating the Qp/Qs ratio. Although increasing the number of blood samples from each site can compensate for the error of an oximeter, this approach can be impractical, particularly if the oximeter error is 2.5% or greater.
作为对分流程度的定量评估，肺循环与体循环血流量之比（Qp/Qs）不仅在心脏内和大血管分流的血氧定量诊断中起着重要作用，而且在患者治疗中也起着重要作用。然而，用于计算Qp/Qs比值的血氧饱和度测量值由于生理变异性和用于分析血样的血氧计的测量误差而存在误差。我们开发了一个数学模型来描述血氧测定误差对Qp/Qs比值不确定性的影响。使用这个模型，当特定的比值作为手术标准时，例如Qp/Qs比值>2，我们计算出关于矫正手术做出不适当建议的概率。本报告还包含一个电子表格，读者可以通过计算Qp/Qs比值的置信区间来分析自己的血氧测定数据。这里给出的结果支持以下结论。首先，由于Qp/Qs比值是根据四个不同部位的饱和度测量值计算得出的，血氧测定误差使得Qp/Qs比值在检测分流存在方面不如传统的仅依赖两个部位样本的逐步升高法有效。其次，尽管血氧测定误差导致计算出的Qp/Qs比值高估或低估真实Qp/Qs比值的可能性相同，但平均而言，高估的幅度大于低估的幅度。第三，与测量误差为2.5%的血氧计相比，使用误差为1%或更小的血氧计可大大降低Qp/Qs比值的不确定性，从而增加做出关于矫正手术正确决策的概率。第四，通过从四个必要部位中的每个部位采集多个血样并在计算Qp/Qs比值之前求平均值，表观Qp/Qs比值的变异性也会大大降低。尽管增加每个部位的血样数量可以补偿血氧计的误差，但这种方法可能不切实际，特别是当血氧计误差为2.5%或更大时。
