By Mike McEvoy
EMS Editor
Firefighters know that conventional pulse oximetry, using red and infrared light to measure blood oxygenation, can’t differentiate between oxyhemoglobin and carboxyhemoglobin. This yields inaccurate readings in carbon monoxide (CO) poisoned patients and can fool the uneducated health care provider. In 2006 the RAD-57, a pulse CO-Oximeter (Masimo Corporation, Irvine, CA) hit the market. A multiwave pulse oximeter, the RAD-57 is capable of measuring not only oxyhemoglobin, but carboxyhemoglobin, methemoglobin, and most recently, hemoglobin, all with an FDA (Food and Drug Administration) approved precision. Presto: the ability to non-invasively assess civilians when responding to CO alarms, firefighters during rehab, and virtually any patient (heck, CO poisoning has never been reliably detected by symptoms alone).
With any new technology, questions arise and suspicions abound. Touger et al, in an Annals of Emergency Medicine paper published in October 2010, suggested the RAD-57 was not accurate. In his New York City (Jacobi Medical Center) study of CO poisoned patients, the RAD-57 had a sensitivity of only 48%, meaning that only 48% of actually CO poisoned patients were detected by the device. As expected, the manufacturer vigorously defended their device, suggesting that the Touger study was yet one negative appraisal of their technology amidst a plethora of more favorable evaluations. Many EMS systems, in apparent deference to skepticism, put the brakes in widespread use of RAD-57 technology. Even the Eagles, a cutting edge (and perhaps the premier annual) EMS scientific conference offered an analysis of Touger and its implications for EMS.
In July 2011 came Roth et al, also in Annals of Emergency Medicine. In this (massively larger) study done at AKH, one of the biggest hospitals in Vienna, Austria, the RAD-57 had a sensitivity of 94%. The authors concluded that the RAD-57 most certainly could reliably screen large numbers of potentially CO poisoned patients. Roth evaluated 1,578 patients compared to 120 patients in the Touger study.
The Touger study prompted Masimo to take a serious look at their product. They assigned their top engineers to study and tighten the accuracy of the SpCO technology. In May of 2011, they released a new sensor for the RAD-57, significantly improving SpCO accuracy in the lower (90-95%) oxygen saturation range. The new sensor also will not report a CO at oxygen saturations less than 90% or when methemoglobin levels are above 2%. Both these conditions create environments where the accuracy of CO measurement become less than optimal (and likewise often affect hospital blood gas machines as well). While this may befuddle Fire and EMS providers, one thing is quite certain: when a SpCO is reported, the accuracy is likely even more accurate than reported by Roth. Future iterations of the Masimo technology will likely offer an explanation to the provider in any circumstance where SpCO cannot be displayed. That promises to further increase confidence in the technology by informing the provider why an accurate SpCO cannot be obtained.
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