A prospective observational study to evaluate a simple airway management algorithm in the austere environment using the McCoy laryngoscope

Background Problems related to inadequate oxygen or ventilation remain an important issue regarding anesthesia care, representing more than 30% of primary airway complications in both North America and United Kingdom. Several guidelines have been proposed since the early 1990's. Although they have had important effects on patient's outcome and survival, it is dicult to identify the optimal guideline. This project prospectively evaluated the success rate of a simple intubation management algorithm with costless and low learning curve equipment. Methods this study included all adult patients (18 years or older) who underwent induction of elective general anesthesia performed by the researchers. A total of 293 patients were included. Their ASA physical status ranged from 1 to 4. After induction of general anesthesia and conrmation of adequate ventilation the algorithm was followed. If ventilation with face mask was impossible or oxygen saturation reached below 90%, the protocol was interrupted. The algorithm was centered on the use of McCoy laryngoscope, "backward, upward and right upward and rightward pressure" BURP maneuver and bougie. All patients were evaluated before the induction of anesthesia. The following characteristics were noted: age, sex, dental status (good, regular and poor, dened by the patients themselves), Mallampati index, personal history of apnea or snoring, mouth opening (mm), body mass index (m 2 /kg), cervical circumference (cm), previous history of dicult airway, thyromental distance and previous treatment with cervical radiation. Results all patients were successfully intubated following the algorithm. Conclusions this algorithm, centered on the use of McCoy laryngoscope, BURP maneuver and bougie, was able to be used successfully in patients of daily practice with a high success rate.


Background
Most problems in airway management can be solved by proper adherence to prede ned algorithms [1].
Several national anesthesiology societies and experts have proposed strategies using different techniques for "cannot intubate, cannot oxygenate" (CICO) events [2,3,4]. Although these protocols contributed to decrease the occurrence of respiratory adverse events mainly in the induction of anesthesia [5], claim reports demonstrate that airway management di culties are still associated with brain damage or death, representing a considerable fraction of anesthesia cases on litigation-based studies [6,7]. Considering that, additional educational support and management strategies seems relevant to improve patient's safety.
In these circumstances, most algorithms for airway management incorporate devices conceived to facilitate tracheal intubation or to create a patent airway, but they are most based on expert opinion and lack prospectively validations [1,2]. Besides, the applicability of strategies may be more or less suitable depending on the location and on the familiarity to the equipment described [1]. Low income countries can bene t with simple and costless algorithms.
Our aim was to evaluate prospectively the success rate of a simple intubation management algorithm with costless and low learning curve equipment [3,8]. The devices used in the study were: McCoy articulated blade, gum elastic bougie (GEB) and LMA Fastrach™ and the protocol was followed by two anesthesiology residents in the operating room (OR).

Ethics
The study was approved by the Ethics Committee (Plataforma Brasil) at University of Sao Paulo Clinics Hospital (HCFMUSP) (license number: 68742617.3.0000.0068). All patients were submitted to and signed written informed consent.

Study Design and Setting
This was a prospective observational study conducted by two second year and third year anesthesia residents from August 2017 to January 2019. They were given a theoretical-practical training with the devices on standard intubation mannequins before the beginning of the research.
All patients were evaluated by the trainees before the induction of anesthesia and the following characteristics of the patient's physical and historical examination were noted: age, sex, dental status (good, regular and poor, de ned by the patients themselves), Mallampati index, personal history of apnea or snoring, mouth opening (mm), body mass index (m 2 /kg), cervical circumference (cm), previous history of di cult airway, thyromental distance and previous treatment with cervical radiation.
HCFMUSP is a tertiary teaching hospital that includes a central surgical unit made of 30 ORs. Since there are no anesthetist nurses in our institution and in our country, all ORs must have an anesthesiologist or a senior anesthesia resident taking care for the patients. On a daily basis, each OR has two to three surgeries including all surgical specialties. In cases where an anesthesia resident is present, four-hands induction of anesthesia is systematically performed and the trainee usually initiate standard airway management.

Patients
The study included all adult patients (18 years or older) who underwent induction of elective general anesthesia performed by the researchers. Subjects who underwent orotracheal intubation with topical anesthesia and/or conscious sedation, obstetric patients and patients submitted to general anesthesia in emergency situations were excluded.

Algorithm Description
All patients were put in sni ng position and pre-oxygenation was performed for 3 minutes with 100% oxygen ow on a face mask and general anesthesia with neuromuscular block was performed. The anesthesia provider assessed the ease of face mask ventilation and graded it using the HAN scale [9].
The rst tracheal intubation attempt was with the McCoy laryngoscope without articulating the lever, like a conventional laryngoscope. At that time, the modi ed Cormack-Lehane classi cation (CLm) was observed. In case of failure and/or CLm of 3A or higher, the articulated blade was used in conjunction with "backward, upward and rightward pressure" (BURP) maneuver. In case of a new failure, the bougie would be used for CLm 3A or lower. And if intubation was not achieved, the Fastrach™ was used and one blind attempt of tracheal intubation was done through the device (Fig. 1). Tracheal intubation was con rmed after observation of 3 capnography curves and adequate capnometry.
If face mask ventilation was impossible or if arterial oxygen saturation (SpO2) decreased to less than 93%, Fastrach™ would be used to rescue oxygenation and the study would be interrupted. From this moment, the attending physician responsible for the case would be able to use other methods to oxygenate the patient or to discontinue intubation attempts and allow the patient to recover.

Outcome variables
The primary outcome was the success rate for tracheal intubation using the algorithm. The secondary outcome was the evaluation of clinical characteristics correlated to di cult direct laryngoscopy intubation.

Statistical Analyses
Descriptive statistics, including frequency counts, proportion, mean, and standard deviation (SD) calculation, were computed using Chi-squared, ANOVA and likelihood ratio tests with IBM-SPSS software, version 20.0 (Chicago, IL).

Results
Two residents performed tracheal intubations during 18 months of the research. A total of 293 patients were included in the study and all patients were successfully intubated following the algorithm.
Intubation through laryngeal mask Fastrach ™ was not necessary because all cases ended on the third step of the algorithm (articulated McCoy blade + BURP + Bougie). Two subjects were excluded from the study because their surgeries were previously canceled. No patients presented impossible ventilation by face mask and there was no hypoxemia during intubation attempts in the study cases.
Patients characteristics are given in table 1. Airway management of all patients are shown in the owchart of gure 2. The algorithm was effective in solving all unanticipated airway problems.  As secondary outcomes, we noticed that HAN scale, Mallampati grades and BMI statistically in uenced for di culty in intubations (table 3). Table 3. Secondary outcome analysis.

Discussion
The use of the algorithm was effective in managing the airways in this protocol. This research showed that most airways in elective surgeries scenarios can be managed with a simple algorithm limited to a small number and low-cost airway devices. It was also demonstrated that this sequence can be applied in a major teaching hospital where physicians under training perform intubations. Although guidelines and recommendations for the management of unexpected di cult airway already exist [2,4,10,11], they are not wholly applicable in particular places or conditions. This algorithm was followed aiming to adopt few mandatory devices which can be easily introduced into an OR daily routine.  [10,11,12,13]. Literature also demonstrated that the McCoy blade is less useful in easy laryngoscopies. In practice, however, we observed that it is not a problem, since a good view of the larynx can be achieved with the levering in neutral position [10,13]. External laryngeal pressure, used as BURP maneuver in our study seemed to be effective because it moved the glottic opening into the laryngoscopic line of vision. Thus, articulated McCoy blade and BURP maneuver had an additive effect in most of our patients.
There were no modi cations in only 8 patients with CLm grade 3A. Previous studies also had similar problems when they analyzed the applicability of McCoy blade. The poor e cacy of McCoy blade was associated to patients' diseases [14] and in some cases it was not enough to lift the base of the tongue and vallecula [14,15]. In this research, three of these patients were submitted to total thyroidectomy and ve presented Mallampati grade 3 or 4. It is possible that the underlying pathology and the soft tissue oropharyngeal disproportion may contribute to decrease visualization or mobility of the epiglottis in these cases.
The bougie was used the third step because of its e cacy, simplicity of use and low cost. Reports have demonstrated a high success rate of tracheal intubation associated with the use of bougie in case of unpredicted di cult laryngoscopy [1,3,16]. In this study, bougie was used to aid intubation in patients with CLm grade 2B and 3A, being effective in all cases.
In comparison with other studies, it was found a similar proportion of grade 3 views (4.8%) at laryngoscopy with the blade in neutral position. The incidence of grades 3 or 4 laryngeal views varies between 0.3% and 13.3% in unselected populations [17,18]. This led us believe that the McCoy blade in neutral position functions similarly to the Macintosh blade for glottic view.
This study has some limitations. Reports of CICO cases varies from 1 in 5000 [19] to 1 in 32000 [20]. The patient population is small and can miss the most di cult cases. During elective general anesthesia, situations such as CICO or CLm 3B and 4 are rare events. Incidence of Cormack-Lehane grade 3 and 4 is around 4.9% [21] and CLm grade 3B is close to 1.8% [17]. Because of that, a higher number of subjects is important for a more accurate analysis of our algorithm. The study did not reach the nal step of the algorithm using LMA Fastrach™. Another limitation is that successful use of the algorithm was based on practical experience of only two researchers who were second-and third-year residents. Besides that, during the study they may have improved their skills with the algorithm devices. Thus, their nal cases could be biased by acquired experience.

Conclusions
In conclusion, we demonstrated that this algorithm, centered on the use of McCoy laryngoscope, BURP maneuver and bougie, was able to be used successfully in patients of daily practice with a high success rate. As the levering laryngoscope reduces the incidence of di cult laryngoscopy, we would recommend it to be added to the tools available in the operating rooms especially in hospitals where video laryngoscopes are not available.

Consent for publication
All patients were submitted to and signed written informed consent.

Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Figure 1
Intubation management algorithm.