Effect of oximetry error on the diagnostic value of the Q(p)/Q(s) ratio

A. P. Shepherd, J. M. Steinke, C. A. McMahan

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


As a quantitative assessment of the magnitude of shunting, the ratio of pulmonary to systemic blood flow (Q(p)/Q(s)) 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 Q(p)/Q(s) 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 Q(p)/Q(s) 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 Q(p)/Q(s) ratio >2. This report also contains a spreadsheet that readers can use to analyze their own oximetry data by computing confidence intervals for the Q(p)/Q(s) ratio. The results presented here support the following conclusions. First, because the Q(p)/Q(s) ratio is calculated from saturation measurements at four different sites, oximetry errors make the Q(p)/Q(s) 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 Q(p)/Q(s) ratio to overestimate the true Q(p)/Q(s) 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 Q(p)/Q(s) ratio and thus increases the probability of reaching the right decision regarding corrective surgery. Fourth, the variability in apparent Q(p)/Q(s) ratios is also greatly diminished by taking multiple blood samples from each of the four requisite sites and averaging them before calculating the Q(p)/Q(s) 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.

Original languageEnglish (US)
Pages (from-to)247-259
Number of pages13
JournalInternational Journal of Cardiology
Issue number3
StatePublished - Oct 10 1997


  • Atrial septal defect
  • Blood flow
  • Cardiac output
  • Circulation
  • Hemoglobin
  • Hypoxia
  • Mathematical model
  • Oxygen consumption
  • Oxygen transport
  • Pulmonary blood flow
  • Systemic blood flow

ASJC Scopus subject areas

  • Cardiology and Cardiovascular Medicine


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