Vital signs

A little while back, MedGadget reports a new device for noninvasively measuring carbon monoxide levels in blood, the Masimo Rainbow SET Pulse CO-Oximeter. This is potentially great news, as carbon monoxide poisoning symptoms are vague and often attributed to other causes, especially when a clear history of exposure isn't available.

Also, in the past, to confirm carbon monoxide, arterial or venous blood had to be drawn and sent to the lab, costing valuable time and delaying therapy. Worse, the standard noninvasive test of blood oxygenation, the pulse oximeter, can't distinguish between oxygenated hemoglobin and CO-hemoglobin -- sometimes fooling docs (or, let's say, med students) into thinking oxygenation was adequate (ie, the pulseox could read 95% saturation, even if it was 80% HbO2 and 15% Hb-CO.

So, this new device should save lives and improve outcomes. But don't take my word for it, read the breathless press release:

Now, with Masimo Rainbow SET, Masimo ushers a new era where carbon monoxide and potentially other vital parameters can be safely and accurately monitored continuously and noninvasively... Rainbow technology is based on system theory and adaptive signal processing algorithms first introduced to the medical community with Masimo SET in 1996. Rainbow technology goes further with the ability to analyze information from a sophisticated new sensor technology that employs eight wavelengths. The Rainbow sensor generates an extraordinarily rich physiological data stream, which opens the door to monitoring additional parameters that presently require costly, inconvenient blood gas measurements that are often unavailable when the information is needed most.

Eight wavelengths! Current pulse oximeters employ two wavelengths to detect blood oxygenation, as discussed in this report:

The pulse oximeter calculates the absorption based on two physical principles: the two wavelengths of light are absorbed differently by oxygenated hemoglobin and deoxygenated hemoglobin; and tissues such as bone and fat and venous blood absorb a relatively constant amount of light, producing a relatively constant rate of light absorption.

I wish there was more information available about how the new CO-oximeters work, particularly, how they overcome the limitations of the O2 meters. Masimo's first of the line, the Rad-57, debuted at EMS Today in Philadelphia last weekend, so materials developed for the demo may be forthcoming.

Still, it's important to remember that, whatever readout these CO-oximeters show, the level of carboxyhemoglobin does not always correlate with severity of symptoms or neuropsychiatric sequelae.

And, just like the pulse-ox's SpO2 readout is just a proxy for oxyhemoglobin content, an output of carboxyhemoglobin in grams per dL is more reproducible and medically significant than a percentage (full disclosure: I've worked with one of these authors, he's the same one who punctured the myth that nail polish tarnishes pulse oximetry readings).

Particularly interesting will be how many cases of chronic, low-level carbon monoxide exposure this new detector picks up. This device could be a big step forward for emergency care and public health.