in the previous issue of critical care , rumpf and colleagues   evaluated the potential contribution of measuring end - tidal carbon dioxide ( co2 ) for suspected pulmonary embolism ( pe ) in the prehospital setting .
capnography has been studied for decades as a potential diagnostic tool for patients with suspected pe .
indeed , pe is expected to create areas of reduced arterial flow with normal or increased alveolar ventilation , resulting in increased alveolar dead space volume and reduced global expired co2 .
this should create a difference between arterial and end - tidal co2 values , as first demonstrated by robin and colleagues   in 1959 .
however , during the two following decades , several authors pointed out the numerous pitfalls and sources of errors in assessing the arterial to end - tidal co2 difference in the clinical suspicion of pe , and this test was finally abandoned until the nineties [ 3 - 5 ] .
three elements explain the current resurgence of expired co2 measurement in the suspicion of pe .
first , technical improvements now allow measuring co2 not only for monitoring purposes in intubated patients in operating rooms but also as a diagnostic tool in spontaneously breathing patients in the emergency department or even in the field .
second , volumetric capnography , which displays expired co2 as a function of the expired volume of the patient , did much to renew interest in capnography because of its potential for better performance in diagnosing pe than the arterial to endtidal co2 difference , even though that expectation could not be confirmed by recent results .
finally , in the era of non - invasive strategies for pe combining several tests of various types , such as clinical evaluation , biological tests , and imaging , the evaluation of a potential role for co2 measurement in combination with those other instruments made sense .
numerous studies are available , and although none to date has been able to prove the safety of such a non - invasive strategy incorporating capnography with a high enough level of evidence to allow its recommendation in daily clinical practice , the venue remains interesting [ 7 - 11 ] . where then can we place the endeavor of rumpf and colleagues ?
they included 131 consecutive patients suspected of pe who had an abnormal rapid point - of - care d - dimer result in a prehospital setting and evaluated them with a combination of clinical probability of pe ( two - level wells score ) and measurement of the end - tidal partial pressure of co2 ( pco2 ) .
pe was diagnosed in the emergency department by a positive spiral computed tomography , a high - probability v / q scan , or a positive pulmonary angiogram .
the combination of a normal end - tidal co2 value ( defined as higher than 28 mm hg based on a receiver operating characteristic analysis ) and an unlikely probability of pe had a 100% sensitivity and 100% negative predictive value ( 95% confidence interval [ ci ] 90% to 100% ) for ruling out pe .
in contrast , the association of a low end - tidal co2 value ( less than 28 mm hg ) and a high clinical probability had only an 86% positive predictive value for pe , and further tests would certainly be required in such patients . clearly , those results are preliminary .
this is a small series and it was designed to set the cutoff value for this particular capnography technique and assess its feasibility in the field .
moreover , as acknowledged by the authors themselves , the clinicians who established the diagnosis were not blinded to either clinical assessment or capnography results . finally , the prevalence of pe is unusually high , although this would tend to bias the results toward lower , not higher , sensitivity .
but the sheer simplicity of the technique used by rumpf and colleagues   is appealing and certainly deserves validation in a large - scale prospective study .
indeed , it emphasizes the use of expired co2 alone without associated arterial pco2 , and this is a pragmatic issue in modern emergency medicine .
also , the use of capnography in the prehospital setting is interesting : there might be situations in which a rapid and rough evaluation of the patient 's expired co2 status would help emergency physicians in making vital decisions , such as starting thrombolysis for a suspected fulminant pe , as well as in monitoring the hemodynamic effect of thrombolysis in such patients . finally , the merit of the article by rumpf and colleagues   is to remind us that clinical applications of capnography are still growing , especially amongst spontaneously breathing patients .
physicians dealing with acute medicine should make every effort to become familiar with expired co2 measurement .
inconclusive capnographic results related to tachypneic or apprehensive patients do not overcome the potential for expired co2 to be placed inside the diagnostic algorithm of a challenging disease like pe .
co2 : carbon dioxide ; pco2 : partial pressure of carbon dioxide ; pe : pulmonary embolism .
 capnography has been studied for decades as a potential diagnostic tool for suspected pulmonary embolism . despite technological refinements and its combination with other non - invasive instruments , 
 no evidence to date allows recommending the use of expired carbon dioxide measurement as a rule - out test for pulmonary embolism without additional radiological testing . 
 further investigations are , however , still warranted . 
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