This prospective observational study showed that orally placed AEC tended to have a higher dislocation rate compared to nasally placed AEC (odds ratio 2.86). However, and contrary to our expectations, this finding was not statistically significant (95% confidence interval of the odds ratio was 0.66–12.39).
The non-invasive capnography proofed to be a double-edged sword and not highly reliable to verify the position of the AEC. On the one hand, presence of CO2 was highly suggestive of correct intratracheal position, as was reflected by the high positive predictive value of 99%. On the other hand, obstruction of the AEC lumen was frequent, which led to loss of CO2 reading and required additional care. Apparently, the only reliable option is to check AEC position via (nasal) flexible scope.
Based on our results of a relatively high overall dislocation rate of 4.4%, and our findings that dislocation cannot be ruled out by non-invasive means, we argue that the insertion of an AEC is not a reliable back-up for re-intubation in case of a known difficult airway. Furthermore, the chance of an esophageal dislocation should be a strong argument against the application of oxygen via the AEC, as has been pointed out by others [6,7,8]. Our dislocation rate of 4.4% was even smaller than reported before: A small audit in 18 patients revealed a dislocation rate of 11%, a further 11% did not tolerate the AEC .
Of note, the catheters were often inserted too deeply, compared to current guidelines: the mean insertion depth of oral catheters was 26.6 cm compared to a recommended maximum of 25 cm [10, 11]. Inserting AEC beyond recommended limits may lead to airway trauma and potentially death because of a (tension) pneumothorax, especially when additional oxygen is applied via the AEC . Fortunately, we never encountered this complication in our study, but we also did not apply additional oxygen via the AEC. Catheters that feature a soft tip may have the potential to reduce the incidence of barotrauma and airway injury, but this has not been studied so far . Either way, it is crucial to carefully avoid deep insertion of the AEC, and given the fact that others have reported complications from too deeply inserted AECs one must assume that this remains one of the main complications of AECs. A safer way may be to provide nasal oxygen during the re-intubation attempt, either low-flow  or high-flow nasal oxygen .
Oral catheters were removed earlier compared to nasal catheters, although this was frequently due to planned removal. However, the drop in AEC numbers over the first hour was more dominant in the oral group, perhaps reflecting frequent patient discomfort caused by coughing and retching with an oral AEC in place. In fact, the only statistically significant difference we could find was a higher incidence of postoperative retching in the oral group compared to nasally placed catheters (p < 0.01).
When looking at the re-intubation rate, a surprisingly high number of patients was re-intubated after removal of the AEC (4 out of 9). The fact that only 5 of 198 AECs were used for re-intubation also means that the use of 97.4% of the AECs (193 of 198) was unnecessary, which led to unnecessary patient discomfort, costs and potential adverse events. However, it is extremely difficult to predict which patients will require re-intubation, and for those patients who do require re-intubation a (correctly positioned) AEC can potentially be life-saving or at least avoid an emergency front of neck access. The overall re-intubation rate (9 of 198, 5%) was lower than the 8% reported earlier , although our data regarding re-intubation comprise only patients who were re-intubated in the PACU, not patients who were re-intubated in the operating room or patients who did not receive an AEC at all.
The use of an AEC for re-intubation in expected difficult extubation is recommended by many experts and guidelines [2, 6]. In our study, the success rate of re-intubation via AEC was 100% (5 out of 5), similar to the overall success rate of 92% (47 of 51) reported by Mort . In that study, in addition to the benefit of high re-intubation success rates, the use of an AEC was associated with fewer episodes of severe hypoxemia (6% vs. 19%), of multiple intubation attempts (10% vs. 77%) and of esophageal intubation (0% vs. 18%), as pointed out in the accompanying editorial by Biro and Priebe . To further increase the re-intubation success rate there is also the possibility to use an Aintree Intubation Catheter (Cook Medical Inc., Bloomington, IN, USA) in order to reduce the gap between a small AEC and the tracheal tube .
Our study also confirms the necessity of the presence of an adequate anesthesia service for high risk patients, even many hours postoperatively. As was described before, re-intubation can become necessary many hours postoperatively [5, 16]. Almost half of all re-intubations in our study occurred after removal of the AEC, between 1 and over 12 h after removal. To further improve the tolerance to the AEC, a wire-based AEC is available (Cook Staged Extubation Catheter™, Cook Medical Inc., Bloomington, IN, USA). A small preliminary study suggested high tolerance, as 17 of 23 patients (73%) tolerated the wire for 4 h, although “tolerated” was not further quantified . Success rate and dislocation rate have not been proven to be different from the conventional AEC: Nasal endoscopy was performed in 11 of these patients and revealed one wire dislocated to the esophagus, which would correspond to a dislocation rate of 9%. In another recent small study, Furyk et al. reported an 8% failure rate in 23 low-risk patients when oral intubation was performed via the wire-based catheter .
Limitations of the study
Several limitations need to be mentioned. Foremost, several patient data sets were tainted with missing data. For example, 16 (8%, Table 2) of all catheters were removed by the patients themselves without giving us the possibility to check fiberoptically for correct position. Finally, the power of the study was too low, as the null hypothesis could not be rejected. To find a difference between the dislocation rates of 7.2% vs. 2.6%, inclusion of over 600 patients would have been necessary, almost four times more than was anticipated. Furthermore, patients were not randomized, instead the attending anesthesiologist or the requirements of the surgical procedure decided about the placement of either an oral or a nasal AEC.
On the other hand, the combined assessment of waveform capnography and fiberoptic visualization of the correct or incorrect tube position in the PACU was never reported before. This allowed to calculate positive and negative predicted values for the use of capnography to verify correct tracheal position of the exchange catheters.