Currently, the standard for measuring Carbon Monoxide (CO)
is CO-oximetry spectrophotometry via blood gas analysis. However the RAD-57
from Masimo, claims the ability to detect CO concentrations using a non-invasive
instrument based on light spectrophotometry – a device similar to a pulse
oximeter that measures CO. In fact, the device manufacturer claims the RAD-57
has the ability to measure functional O2-Hb as well as CO-Hb (SpCO). The
question we ask is whether there is a role in the use of RAD-57 in the
detection of CO in a prehospital setting or Emergency Department, and how
reliable are these measurements? Can the RAD-57 readings be used clinical to
truly risk stratify patients with CO poisoning?
Four prospective studies currently exist and the rest are
case studies. These studies compared two variables; blood CO-Oximetry vs RAD-57
(SpCO) values, and were used to asses for % difference (bias), accuracy, and
precision.
The first study, Barker et al, investigated the device in 10
healthy volunteers who were exposed to CO in a gas mixture until their
CarboxyHb level reached 15%. The comparison revealed an uncertainty of +/- 2%
from CO-Ox readings.
The prospective observational study by Suner et al used
RAD-57 to measure CO levels for 10,856 patients from the ED. In the study 28
patients read positive with the RAD-57 of which 11 had no apparent clinical
suspicion. Even though the study was able to show cases of occult CO poisoning,
the data on the accuracy of those devices were not reported.
The third study Touger et al enrolled 120 patients with
suspected CO toxicity based on history and was found to have an accuracy of
1.4% but had confidence intervals from -11.6 to 14.4 which demonstrates very
poor accuracy of the device. Moreover the study revealed the RAD-57 only
detected 11 out of the 23 patients that had COHb > 15%. Which means the
preciseness of readings were also questionable due to the high false negative
rate. This Annals of Emergency Paper concluded that Rad57 should not be used
interchangeably with blood readings.
Roth et al. a prospective study actually found a positive
outcome. This study measured the RAD-57 SpCO in 1278 ED pts in which 17 were
positive, with a relatively low bias of 2.32% and precision of 4.01%. This
study also revealed there as an increase in erroneous readings with increasing
CO-Hb concentrations. The study might have suffered from selection bias, as not
all patients that had SpCO readings had comparison CO-Hb results.
Nilson et al. demonstrated instances of elevated SpCO in the
pre-hospital setting (n=1700). This initial elevated non-invasive reading did
lead to faster blood CO-Hb measurement and time to hyperbaric therapy. It
should be noted that that clinical outcomes were not measured. Case reports
revealed when SpCO was used as a screening measure, all 5 cases that had a
positive reading had a lower clinically acceptably level on CO-Ox (15% vs 10%).
False positive results in such scenarios especially in the prehospital setting
could lead to mismanagement of resources as well as add additional costs to the
healthcare system.
Now how does SpCO stand up to concomitant
hemoglobinopathies. Feiner et al. assessed the accuracy of SpCO reading with
concomitant Methemglobenima, which revealed a linear increase in error with
increasing MetHb levels, also questioning the reliability of SpCO readings.
From these limited studies we can infer that the use of the
RAD-57 has significant limitations. Poor precision and sensitivity issues
render that clinician unable to rule out CO poisoning in the field as well as
the Emergency Department. Positive readings may guide triage however with the
devices’ poor accuracy; the benefits might be overshadowed by the harm. Until
improvements in device accuracy, we cannot recommend routine use of this
device.
References:
Barker SJ, Curry J, Redford D. Measurement of
carboxyhemo- globin and methemoglobin by pulseoximetry: a human vol- unteer
study. Anesthesiology 2006;105:892–7
Suner S, Partridge R, Sucov A, Valente J, Chee K, Hughes A,
Jay G. Non-invasive pulse CO-oximetry screening in the emergency department
identifies occult carbon monoxide toxicity. J Emerg Med 2008;34:441–50
Touger M, Birnbaum A, Wang J, Chou K, Pearson D, Bijur P.
Performance of the Rad-57 pulse COoximeter compared with standard laboratory
carboxyhemoglobin measurement. Ann Emerg Med 2010;56:382– 8
Feiner JR, Bickler PE, Mannheimer PD. Accuracy of
methemoglobin detection by pulse CO-oximetry during hypoxia. Anesth Analg
2010;111:143– 8
Roth D, Herkner H, Schreiber W, Hubmann N, Gamper G, Laggner
AN, Havel C. Accuracy of noninvasive multiwave pulse oximetry compared with
carboxyhemoglobin from blood gas analysis in unselected emergency department
patients. Ann Emerg Med 2011;58:74 –9
Nilson D, Partridge R, Suner S, Jay G. Non-invasive carboxy-
hemoglobin monitoring: screening emergency medical services patients for carbon
monoxide exposure. Prehosp Disaster Med 2010;25:253– 6
O’Malley GF. Non-invasive carbon monoxide measurement is not
accurate. Ann Emerg Med 2006;48:477– 8
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