Learning curve of fiber-optic orotracheal intubation guided by the nasopharyngeal airway in anesthetized patients

2.1 Patient enrollment

The study was conducted from October 2020 to June 2021 at the First Affiliated Hospital of Wenzhou Medical University. Adult patients of any gender, scheduled for elective non-Ear Nose and Throat (ENT) or non-airway procedures requiring endotracheal intubation for general anesthesia were screened for eligibility. The inclusion criteria comprised (1) ASA physical status I or II, (2) aged between 16 and 70 years, (3) had a body mass index (BMI) between 18 and 25 kg/m², (4) an anticipated duration of general anesthesia for 1–3 hours, and (5) no evidence of a predictable difficult airway, as assessed by two attending anesthesiologists using the recommended criteria [12]. Exclusion criteria were medical history of coagulation disorders, presense of nasal tumours, skull base fractures, previous history of airway and cervical surgery, other oral or cervical deformities and pregnancy. Sixteen anesthesiology residents in their first year of training were recruited and randomized into two groups: a guidance group (G group) and a conventional group (C group), using computer-generated numbers. Eligible participants were required to have completed at least 100 intubations using video laryngoscopy, with those having prior experience in fiber-optic orotracheal intubation being excluded. Before the start of the study, all trainees received instruction on selecting the appropriate size of NPA and on using the flexible fiber-optic bronchoscope with an intubation manikin. Additionally, they were provided with a standardized training video detailing the anatomy of the nasopharynx and demonstrating the steps for fiber-optic orotracheal intubation. Then, each trainee performed 20 consecutive fiber-optic orotracheal intubations (defined as attempts 1–20, one case per day, over 20 days), and each intubation procedure was supervised by the same experienced attending anesthesiologist, with relevant intubation data being systematically recorded.

2.2 Anesthesia induction

All patients were required to fast for a minimum of 8 hours before the procedure, with no premedication administered. Following standard monitoring, the patients were pre-oxygenated to achieve an end-tidal oxygen concentration of at least 80% before anesthesia induction, which included intravenous administration of sufentanil (0.3 μg/kg), propofol (2 mg/kg), and rocuronium (0.9 mg/kg). Manual ventilation with 100% oxygen via a mask was provided for 2 minutes preceding endotracheal intubation.

2.3 Insertion of NPA

The appropriate size of the NPA (Wellead, China) was determined by measuring the distance from the philtrum to the ear tragus, which also indicated the depth to which the NPA should be advanced [8]. An anesthetic comprising lidocaine and phenylephrine (1 mL of phenylephrine 1% in 4 mL of lidocaine 2%) was used to anesthetize the nasal mucosa in the right nasal cavity, followed by the application of a water-based lubricant to the NPA tip. Subsequently, the NPA was advanced horizontally into the more patent nostril along the septum, following the natural curvature of the nasopharyngeal cavity floor. In the guidance group (G group), trainees were required to first locate the tip of the blue NPA, which then served as a guide for directing the flexible fiber-optic bronchoscope to the glottis during fiber-optic orotracheal intubation. Conversely, in the conventional group (C group), no NPA was utilized, and trainees performed fiber-optic orotracheal intubation directly after anesthesia induction.

2.4 Intubation and anesthetic management

A flexible fiber-optic bronchoscope (Pentax FB-15X, Montvale, NJ, USA) equipped with a lubricated cuffed endotracheal tube (size: 7 or 7.5) was introduced via a trans-oral approach, and the surface of the tube was lubricated with a water-based lubricant before insertion. To ensure a clear airway and facilitate visualization of the epiglottis during intubation, a jaw thrust was routinely applied, and the force was adjusted as necessary. Adjustments to the position of the patient’s head and neck were made to assist the residents in performing the procedure. Once a clear view of the glottis was obtained, the flexible fiber-optic bronchoscope was advanced into the trachea, guiding the endotracheal tube into place.

Sevoflurane administration began after intubation, maintaining the end-tidal sevoflurane concentration between 2–2.5% to ensure adequate anesthesia. Mechanical ventilation settings were adjusted to maintain the end-tidal carbon dioxide concentration within 35–45 mmHg. After completing general anesthesia, the endotracheal tube was removed based on standard criteria for extubation. An investigator, unaware of group allocation, evaluated intubation-related airway complications, such as sore throat, hoarseness, oral mucosal injury and laryngeal edema.

2.5 Outcomes

The primary outcome evaluated was the success rate of first-attempt intubation, defined as the accurate insertion of the endotracheal tube into the trachea on the initial attempt. Confirmation of correct tube placement was based on the observation of five consecutive, normal-appearing end-tidal carbon dioxide waveforms. An attempt was considered unsuccessful if intubation could not be achieved within 150 seconds or if the endotracheal tube was incorrectly positioned. In cases where trainees from either group failed in their intubation attempts, patients were promptly administered additional intravenous propofol (0.5–1 mg/kg) followed by 2 minutes of manual ventilation with 100% oxygen. Subsequently video laryngoscopy was performed as a rescue procedure to ensure airway patency by the supervising anesthesiologist to ensure airway patency.

Secondary outcomes comprised the time to glottis exposure, intubation time, the learning curve of the resident, and airway-related complications:

(1) Time of glottis exposure: This was measured in seconds from the moment the tip of the flexible fiber-optic bronchoscope passed the patient’s teeth to when an adequate view of the glottis was obtained. Failure to visualize the glottis within 120 seconds led to exclusion from the final glottis exposure time analysis.

(2) Time of intubation: Intubation time began when the tip of the fiber-optic bronchoscope passed the patient’s teeth and ended once five consecutive end-tidal carbon dioxide waveforms were observed. Patients with unsuccessful first attempts were excluded from the intubation time analysis.

(3) Learning curve of the resident: The learning curves of the residents were determined by recording the number of successful and unsuccessful orotracheal intubations across 20 attempts. Additionally, proficiency in fiber-optic orotracheal intubation was defined as a stable 95% success rate, and the attempts required to achieve this were recorded by an investigator [13, 14].

(4) Airway-related complications: These include sore throat, hoarseness, oral mucosal injury and dysrhythmia, were documented within 24 hours after extubation.

2.6 CUSUM analysis

For this study, an acceptable failure rate for fiber-optic orotracheal intubation was established at 5% (0.05), referencing studies by Schaefer et al. [13] and Smith et al. [14]. CUSUM analysis was performed to monitor skill acquisition progression, which was defined as:

Sn = Σ (Xn − f0), where Xn = 0 for success and 1 for failure, with n representing the attempt number and f0 = representing the acceptable failure rate [15, 16].

The learning curves for anesthesiology residents from both groups were constructed by plotting the number of attempts (abscissa) against the CUSUM values (ordinate). Polynomial function curves were derived using MATLAB software (version 8.6, The MathWorks, Natick, MA, USA). The point at which the first integral of X after the curve’s peak occurs indicates the minimum number of cases required for a trainee to master this skill.

2.7 Sample size calculation

Sample size calculation was based on a precedent study [13], where each trainee underwent 20 attempts at fiber-optic orotracheal intubation to assess the learning curve. As all 16 first-year anesthesiology residents participated in the fiber-optic training course, a total of 160 patients per group was deemed to be necessary.

2.8 Statistical analysis

Data analysis was performed SPSS 20.0 software (SPSS Inc., Armonk, NY, USA). Continuous data following a normal distribution are presented as mean ± standard deviation, with differences between groups assessed using Student’s t-test. Categorical data are expressed as numbers (%) and analyzed using the chi-squared test for inter-group comparisons. The incidence of trainees from each group exceeding designated intervention thresholds in their CUSUM plots was analyzed using the χ² test. Median CUSUM scores at each attempt were compared between groups using the Mann-Whitney U test. A p value < 0.05 was considered statistically significant.

3.1 Patients clinical characteristics

Of the 360 participants screened, 320 were enrolled and evenly distributed into the G and C groups, with 160 participants in each. No significant differences were observed between the groups regarding age, body mass index, gender, Mallampati grade, or duration of anesthesia (p > 0.05). The demographic and clinical data of the enrolled patients are summarized in Table 1.

3.2 Intubation success rate

In the first 150 seconds, the G group achieved a 93.13% success rate for first-attempt orotracheal intubation using a flexible fiber-optic bronchoscope, compared to 85.63% in the C group with the conventional method (p < 0.05, Table 2). In instances where the initial attempt failed, both groups attained a 100% success rate for the second attempt using video laryngoscopy.

3.3 Time of glottis exposure and intubation

In Table 3, it can be observed that the time needed for glottis exposure was notably decreased in the G group compared to the C group (p < 0.05). Likewise, the duration of fiber-optic orotracheal intubation was significantly shorter when guided by a preplaced oropharyngeal airway.

3.4 Learning curve

The success rate for tracheal intubation approached 100% after seven attempts with the guidance of the oropharyngeal airway device. In contrast, success using the conventional method remained below 80% before the ninth attempt and only exceeded 90% only after 12 attempts. Fig. 1 illustrates that a minimum of eight attempts was necessary for the G group to master fiber-optic orotracheal intubation, while the C group required 13 attempts (Fig. 2). CUSUM analysis revealed a steeper learning curve in the G group compared to the C group.

Fig. 1.The learning curve of Group C.

Fig. 2.The learning curve of Group G.

3.5 Airway-related complications

The incidence of airway-related complications following intubation was comparable between the two groups, with no significant differences observed in the occurrence of sore throat, hoarseness, oral mucosal injury or laryngeal edema (p > 0.05, Table 4).