In the present study assessing the utility of oxygen insufflation during FOBI, VPaO2 was significantly reduced when oxygen was insufflated through the working channel. However, patients with and without oxygen insufflation showed no significant differences in the intubation success rate, time to intubation, visual field, and occurrence of complications.
Despite recent advances in airway management devices such as videolaryngoscopes and supraglottic airways, FOBI is still considered the gold standard for difficult airway management(14, 15) However, it requires a longer time than does intubation with a direct laryngoscope. In addition, secretions and bleeding in the airway obscure the airway anatomy and complicate intubation.(3) Therefore, methods to improve patient oxygenation after the induction of anesthesia can facilitate faster and safer FOBI.
Several studies have attempted to overcome the limitations of FOBI. A direct laryngoscope or GlideScope can assist the placement of FOB near the glottis and facilitate easy intubation.(16–18) In addition, supraglottic airways such as I-gel or LMA can be used as conduits for FOBI, providing a better route to the glottic inlet and, at the same time, ventilating the lungs during the procedure.(19, 20) However, these techniques require the use of additional devices and result in an increased time to intubation. Especially, they are inappropriate in patients with limited mouth opening.(21) Oxygen insufflation through the working channel during FOBI does not require extra time and devices. Oxygen insufflation through the working channel of FOB could alleviate both hypoxia and visual field impairment in children.(5) In our study, VPaO2 was 1.01 ± 0.39 mmHg/s in the N group, which was more than twice the value for the O group (0.42 ± 0.42 mmHg/s). If the hypoxia-free apnea time during FOBI is defined as the time from the discontinuation of mask ventilation at a PaO2 of 500 mmHg to the achievement of 90% SpO2 at a PaO2 of 60 mmHg, under the assumption that PaO2 exhibits a linear decrease during apnea, the hypoxia-free apnea duration could be approximately 10 min longer in the O group than in the N group (1047.62 s vs. 435.64 s, respectively).
This calculation is based on a ventilatory mass flow, also known as apneic oxygenation. During regular breathing in an adult, oxygen and carbon dioxide are exchanged between the lungs and blood at a flow rate of 250 ml/min. During apnea, the carbon dioxide flow returning to the lungs is significantly reduced to 8–20 ml/min while the oxygen flow to the blood is maintained. Therefore, negative pressure is generated in the lungs according to the volume difference during oxygen and carbon dioxide exchange; this facilitates the movement of oxygen from the pharynx to the lungs.(8, 22)
Apneic oxygenation could be applied in various clinical situations involving different types of devices, including nasal prongs, nasopharyngeal catheters, and tracheal or bronchial catheters. The first two are commonly used because they are practical.(8) Oxygenation with nasal prongs at 5 l/min during FOBI could lower the decrease in PaO2 at 3 min during apnea.(23) In Teller’s crossover study evaluating the influence of oxygen delivery via a nasopharyngeal catheter at 3 l/min, apnea was continued for 10 min or until SpO2 decreased to 92%. None of the patients in the apneic oxygenation group showed an SpO2 of < 97% until 10 min. On the other hand, the mean apnea time in the control group was 6.8 min and the lowest SpO2 value was 91%.(24) In present study, we used the working channel of FOB to continuously deliver oxygen as FOB moved toward the trachea. This increases the efficiency of oxygen delivery to the lungs and could be particularly useful for patients prone to desaturation during apnea, such as obese patients.(25)
One limitation of apneic oxygenation is that it cannot efficiently remove carbon dioxide from the blood, resulting in an increase in the carbon dioxide level at a rate of 1.1–3.4 mmHg/min and, eventually, hypercarbia and acidosis.(8) Therefore, it should not be used in patients with a risk of hypercarbia-related complications. Of late, the use of a high flow nasal cannula has garnered attention in various clinical situations, and it can be effectively used for apneic oxygenation during intubation as well. Studies found that it could deliver a high concentration of oxygen, generate a positive airway pressure of approximately 7 cmH2O, and slow down the increase in the carbon dioxide level during apneic oxygenation.(22, 26) In the present study, the increase in PaCO2 during the apneic period was similar in the O and N groups.
With regard to the visual field, the number of patients with an excellent visual field was not significantly different between groups, although the number was higher in the O group. This result was inconsistent with those of previous reports, and there are a few possible explanations for the discrepancy. Unlike Rosen’s study, which included pediatric patients with difficult airways and involved the use of smaller FOBs,(5) the present study included adult patients without anticipated difficult airways and involved the use of FOBs with a larger diameter. Moreover, all patients were premedicated with glycopyrrolate in order to minimize secretions.(3) The visual field can be easily disrupted during awake FOBI because of patient movement and lens fogginess caused by spontaneous breathing.(7) However, we performed the study under anesthesia with complete muscle relaxation, so there were no disturbances during FOBI. Therefore, the intubation conditions were quite good, and the actual effect of oxygen insufflation may not have been as significant as expected.
The two groups in our study showed a comparable intubation success rate and time to intubation; this could be attributed to the lack of differences in the visual field quality. The optimal intubation conditions may be another factor that repressed the influence of oxygen on the intubation-related parameters. We believe that different results may be derived if the measurements were recorded in emergency situations involving patients with unanticipated difficult airways. Further studies should take this aspect into consideration and assess the usefulness of oxygen insufflation during FOBI in different clinical scenarios.
From our results, it is evident that oxygen insufflation through the working channel of FOB can reduce VPaO2 during FOBI. Oxygen insufflation through the working channel of FOB can cause rare but serious complications such as gastric rupture and pneumothorax.(10, 11, 27) Oxygen can enter the trachea or esophagus during the procedure. If oxygen is delivered to the esophagus and stomach, it could cause nausea, vomiting.(10, 12) If FOB with oxygen insufflation enters the stomach, it could cause significant distension of the stomach and even rupture, because the maximal capacity of the stomach is approximately 1 l, and FOB could deliver approximately 2.5 l of oxygen in just 30 s, theoretically.(10, 28) However, according to Wong’s review of several studies on nasal or nasopharyngeal apneic oxygenation, no complications related to pressure effects have been reported till date.(8) The esophagus is closed by a sphincter, and approximately 20 cmH2O of pressure is required to open it.(29) According to a previous report using nasal high flow oxygen insufflation, the mean airway pressure was approximately 7 cmH2O,(26) which is considerably lower than the pressure required to open the esophageal sphincter. In the present study, none of the patients complained of vomiting or abdominal distension, and only one patient in group O developed postoperative nausea. Furthermore, there was no case of mucosal injury or desaturation during the procedure. This was probably due to the optimal intubation conditions and the completion of intubation within 5 min, which is considered safe if the patient is preoxygenated.(26)
This study has some limitations. First, FOBI was conducted under optimal conditions as described earlier. Therefore, the results cannot be applied to other clinical situations such as emergency difficult airway management. Second, our sample size was enough for VPaO2 measurement but not adequate to assess the occurrence of complications, which are anyways rare.