In another study, that was carried out in mechanically ventilated patients with multisystem trauma, trends in the arterial to end-tidal carbon dioxide gradient magnitude were not reliable, and concordant direction changes in ETCO 2 and PaCO 2 are not assured ( 12). One study reported that, PaCO 2 gives a poor estimate of PaCO 2 in patients with respiratory failure ( 11). However, the monitor may prevent morbidity in patients requiring tight control of PaCO 2 ( 10). These data do not support routine monitoring of end-tidal CO 2 during short transportation times in adult patients requiring mechanical ventilation. There is wide variation in the gradient between PaCO 2 and ETCO 2 depending on the patient’s condition, and this relationship does not remain constant over time, thus it is not useful in pre-hospital ventilation management. PaCO 2 cannot be estimated by the ETCO 2 method in a pre-hospital setting ( 9). ETCO 2 is a less accurate measure of PaCO 2 with tidal volume breathing and in patients with pulmonary disease. Noninvasive end-tidal carbon dioxide pressure (ETCO 2) monitoring may adequately predict PaCO 2 in non-intubated emergency department patients with respiratory distress, who are able to produce a forced expiration ( 8). However, its clinical validity is questionable in patients who have the greatest need for end-tidal PaCO 2 monitoring (i.e., patients who have respiratory distress or who are breathing spontaneously and overriding the ventilator). In a study that was carried out in ventilated head trauma patients, end-tidal PaCO 2 monitoring correlated well with PaCO 2 in patients without respiratory complications or without spontaneous breathing ( 7). End-tidal carbon dioxide measurements may be sufficient measures of PaCO 2 in selected patients and obviate the need for repeat arterial blood gas determination. One study indicated that measurements of end-tidal carbon dioxide concentrations correlated well with PaCO 2 values in non-intubated patients presenting with a variety of conditions to emergency departments ( 6). In a study that was conducted by Flanagan et al., end-tidal CO 2 measurement provided an accurate estimation of PaCO 2, even during episodes of severe hypocarbia ( 5). It has been used extensively in operating rooms, intensive care units, emergency departments and in pre-hospital setting ( 1- 4). End-tidal CO 2 monitors are used to estimate arterial CO 2 pressure (PaCO 2), but appropriate use of this noninvasive method of assessing blood gases in ventilated patients remains unclear.
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