The aim of this study was to compare the efficacy of a new portable device, EMMA™, for measuring carbon dioxide in expired air compared to carbon dioxide levels in venous blood. The point was to see whether this device could be used as an auxiliary tool for evaluating the accuracy of bag-valve mask ventilation. The main conclusion is that when patients are well under anaesthesia, are hemodynamically stable and adequately ventilated by a trained provider, the device gives acceptable values for exhaled carbon dioxide as compared to venous blood gases. However, our results may not necessarily be transferable to less experienced BVM provider and patients in the prehospital settings. Further studies should include patients and health care providers from the prehospital setting. In an emergency setting, patients are not normally well monitored. Furthermore, many untrained personnel are involved and adequate airway management is sometimes difficult to evaluate. Conventionally, for unconscious patients, ETI is regarded as the gold standard for airway management in ALS, even if the airway management can be easily maintained [
1]. However, several studies point to difficulties in using ETI in prehospital settings [
3,
4,
6]. Furthermore, prehospital ETI does not appear to have benefits over BVM ventilation and it does not seem to improve neither survival nor neurologic outcome [
5]. Particularly, there are disadvantages using ETI in prehospital settings when the procedure is performed by less experienced paramedics or when the tube cannot be inserted due to the lack of experience from necessary anaesthetic drugs. BVM ventilation is the basic technique for all health care providers [
1] and guidelines from ERC states that all health care providers should be familiar with the BVM for ventilation during cardiopulmonary resuscitation [
8].
There is an increasing interest for the use of end-tidal carbon dioxide measurement in the emergency care and previous studies have for i.e. described how nasal entidal carbon dioxide measurement could assess patients' acute respiratory problems in prehospital settings [
9,
10]. In this study we evaluated the EMMA™ device during BVM ventilation under ideal conditions with a trained provider and healthy patients were included.
Capnography is a non-invasive infrared spectroscopy technology for continuous measurement of carbon dioxide (CO
2) content throughout the respiratory cycle. When capnograms are used to evaluate the end-tidal concentration of carbon dioxide it must be interpreted in conjunction with other clinical findings such as the work of breathing, CO
2 transport and elimination as well as changes in cardiac output during volume resuscitation [
11]. Normally, when the partial pressure of carbon dioxide is measured invasively there is a slight discrepancy between blood values and expired carbon dioxide due to dead space of the lung and bronchial tree. This gradient is low, usually around 0.66 kPa at a lower ETCO
2 level. This gradient, however, could increase due to patient aging [
12]. This was not adjusted for in this study. The results in this study underlines that when the patients are comfortably anaesthetized there is an acceptable agreement between ETCO
2 values by the device and simultaneously collected PvCO
2 blood samples. The Bland-Altman plots (Fig
2) show agreement between ETCO
2 and PvCO
2 within 2 SD. The limits of agreement are wide, reflecting the large variation, but considered clinically acceptable in view of the normal difficulties of providing an adequate airway by using BVM and also the spread of different ages of the patients. The strength in the study is that the same experienced anaesthesiologist was the sole provider of ventilation for all the patients. This can also be a limitation as he is able to influence the measurement from the device during BVM. The study did not start until the patients were fully anaesthetized and hemodynamically stable. The patients chosen were all ASA I and II and therefore easily maintained. A weakness could be the difficulty of keeping an adequate airway by BVM. This is highly dependable on the provider skill and technique. Furthermore, we used venous blood gases for simplicity and the lack of an arterial line. Mixed venous blood gases reflect desaturated blood which should more easily attract CO
2 due to the Haldane effect [
11,
13]. However, a recent study illuminates that peripheral venous blood correlates reasonably well with arterial values, at least for ph, bicarbonate and PCO
2[
14].