We have shown that when it is used to monitor preterm infants receiving supplemental oxygen, the Masimo SET Radical pulse oximeter returns a reduced number of values of 87–90%. This is explained by the internal calibration algorithm, which changes in this region and adjusts SpO2 values above 87% upwards. A new software algorithm removed this artefact and returns a distribution of values that is similar to other current oximeters.
According to the product information, the oximeter is validated for accuracy in human blood studies on healthy adult volunteers in induced hypoxia studies in the range of 70–100% SpO2 against a laboratory CO-oximeter. This variation equals ±1 SD, which encompasses 68% of the population. The stated accuracy for neonates is ±3%, so by this standard at least 68% of the time the displayed SpO2 reading should be within ±3% of the simultaneous CO-oximeter value on an arterial blood sample. In this regard the original oximeter performs within all recommended accuracy standards.
is targeted over a wide range, including values of 87% and 95%, this artefact is not likely to be of clinical significance. However, oximeters are sometimes used to target a narrower range or to determine a threshold saturation.7–9
Under these circumstances the effect could be greater. Oximeters are used as a screening tool to identify borderline saturations in congenital heart disease.10 11
Some cases of duct dependent systemic circulation are not identified by this approach. It is possible that a slight artefactual elevation of saturation between 87% and 95% could mask some cases. It is becoming usual for bronchopulmonary dysplasia to be categorised by whether or not a saturation of 90% is maintained breathing air.12–14
An artefactual increase in SpO2
from 89% to 91% would result in fewer infants being categorised as having bronchopulmonary dysplasia and might influence the duration of supplemental oxygen exposure.
In the neonatal oxygen trials, infants have been targeted using modified Masimo Radical SET oximeters to maintain saturations either in the range 85–89% or 91–95%.15
This artefact in the algorithm would be expected to reduce the number of saturation values in the target range in the group targeted to lower saturations. This might make this lower saturation target group harder to maintain within range, leading to more oxygen adjustments and greater variability in saturation, including time spent with high or low saturations.
When these observations were explained, Masimo Corporation provided the revised software, which eliminated the artefact, removing the deficit of values in the range 87–90%, and reducing the uplift in saturations. With assistance from Masimo the new software algorithm was installed into the oximeters of the BOOST-II UK trial in December 2008 and into the Australian and Canadian oxygen trial oximeters in early 2009, so that the final results of the international collaboration will be as generalisable as possible to current oximeters from Masimo and other manufacturers.
The SUPPORT trial was the first of the oxygen trials to complete recruitment. Outcomes to hospital discharge were published recently.9
There was a large and highly significant reduction in retinopathy of prematurity in the group who had their oxygen saturation targeted lower. The same group showed a slight excess of mortality, of borderline conventional statistical significance. The data monitoring committees of the ongoing trials in the UK, Australia and New Zealand and Canada reviewed their individual trial data in light of the SUPPORT trial results and independently recommended continuing recruitment to all three trials. The SUPPORT trial was completed before the oximeter modifications. Because the ongoing trials began evaluating an intervention that results in different saturation distributions, it is critically important to the generalisability of the trial results that a pooled analysis of the data should describe outcomes using either algorithm.
In conclusion, we have shown that the algorithm used by one generation of the Masimo SET Radical oximeter returns a distribution of saturation values above 87% that is different from other oximeters. This could be important if the device is being used to target saturation values in a narrow range or to evaluate threshold saturation values. This finding has important implications for the interpretation of emerging evidence in neonatal intensive care.