Continuing Education Activity

The procedure of endotracheal intubation plays an important role in maintaining a patient's airway in various clinical scenarios, such as surgeries and critical care. However, this process can be challenging in certain patients due to anatomical variations, commonly referred to as 'difficult airways.' Clinicians can utilize the 3-3-2 rule as a predictive tool to identify and prepare for these challenging scenarios in advance to minimize complications. The 3-3-2 rule involves measuring 3 different distances in the patient's neck using the clinician's fingers. These measurements aid in predicting the ease or difficulty of intubation. Additional tools such as the LEMON scale and the Mallampati scoring system also play a valuable role in the evaluation of the airway. This activity provides an overview of the procedures and techniques involved in endotracheal intubation, with a particular emphasis on the application of the 3-3-2 rule. It also highlights the role of the interprofessional team in efficiently managing patients who require intubation.

Objectives:

• Screen patients with difficult airways using the 3-3-2 rule to promptly recognize individuals who may require special consideration and preparation for airway management.
• Differentiate airway classes using the Mallampati scoring system to assess the visibility of specific anatomical structures during a mouth-wide-open examination.
• Evaluate for airway obstruction as part of the LEMON scale to stratify patients based on their risk of difficult airway.
• Collaborate with anesthesiologists, surgeons, and other relevant healthcare professionals to develop comprehensive airway management plans based on the results of the 3-3-2 rule assessment.

Introduction

The airway is an essential component of the body that requires protection, regardless of the reason for a patient's presence in a hospital, whether it's for outpatient surgery or admission to the intensive care unit (ICU) for observation and therapy. Hence, when contemplating intubation, physicians must carefully assess the potential for intubation failure and strategically optimize various factors to enhance the likelihood of success. Approximately 1%-3% of patients who require endotracheal intubation face challenges due to difficult airways (DAs). Identifying such patients is of utmost importance as it enables clinicians to prepare and mitigate potential complications adequately. The 3-3-2 rule is an assessment tool for predicting difficult intubations (DIs) in the cases of unexpected DAs.[1][2][3]

According to the American Society of Anesthesiologists, intubation is determined to be difficult to secure when a proficient and skilled anesthesiologist requires more than 3 attempts or exceeds a duration of 10 minutes for successful endotracheal intubation. Likewise, ventilation is considered challenging when a skilled clinician is unable to maintain an oxygen saturation level of above 90% while utilizing a facemask for ventilation, even with a 100% fraction of inspired oxygen (FIO₂) used for oxygenation.

It is essential to manage the airway very promptly, in a very time-sensitive manner, as any delay in adequate oxygenation or ventilation can result in the development of hypoxia and hypercapnic abnormalities, which can be detrimental at the cellular level. Hypoxic brain injury can result in permanent neuronal damage and acidosis due to hypoxia and hypercapnia, which can escalate to cardiac arrest or fatality.

Research investigations have examined the correlation between the palpability of the cricothyroid membrane (CTM) and the prediction of DAs based on the 3-3-2 rule. An observational study involved 60 female patients undergoing non-neck surgery, with exclusions made for individuals with neck pathology or a history of neck surgery. The 3-3-2 rule evaluates 3 specific measurements, including the interincisor distance, hyoid-to-mental distance, and thyroid-to-hyoid distance. The study participants were categorized into 2 groups: the non-DA (NDA) and the DA groups. Ultrasonography was used to confirm the accuracy of CTM palpation. The study's findings indicated that the rate of successful CTM palpation was higher in the NDA group than in the DA group. Although there was no significant difference in age between the 2 groups, the DA group had a higher body mass index (BMI). The successful palpation of the CTM was more challenging in patients who exhibited a positive 3-3-2 rule. This result suggests that airway prediction tools, such as the 3-3-2 rule, could play a crucial role in identifying the CTM, ultimately enhancing safety measures for surgical patients.[4]

Research studies have examined the relationship between specific patient characteristics and the success rate of CTM palpation, an essential procedure in emergency airway management. It has been observed that patients with shorter interincisor, hyoid-to-mental, or thyroid-to-hyoid distances are at a higher risk of CTM palpation failure. In addition, it was observed that individuals with challenging airway variables often possess a higher BMI. The traditional approach of identifying CTM through palpation between the cricoid and thyroid cartilage has demonstrated imprecise results. Furthermore, factors such as gender, obesity, and neck pathologies can impact the accuracy of CTM palpation. However, despite these complications, it is suggested that weight, height, BMI, neck circumference, and CTM dimensions may not significantly affect the precision of CTM palpation.

Ultrasound is emerging as a more accurate diagnostic technique for locating the CTM, particularly in patients with complicated airway anatomy. The 3-3-2 rule, which relies on anatomical information to predict potential challenges in endotracheal intubation, could prove advantageous in identifying the CTM. The findings show promising reproducibility of CTM palpation using the quick, direct palpation method along with the 3-3-2 rule and portable bedside ultrasound equipment.[4]

Function

The 3-3-2 rule is an estimation tool to assess whether the anatomical structure of the neck will allow for the appropriate alignment and opening of the throat and larynx. Based on the given anatomical findings, this tool provides a rough estimate regarding the feasibility of achieving proper alignment for direct visualization of the larynx.[3]

3: The measurement of 3 fingers between the upper and lower teeth of a patient's open mouth indicates the ease of accessing the airway through the oral opening. In a typical patient, the ability to open their mouth wide enough to accommodate 3 fingers placed between the incisors indicates adequate mouth opening. This facilitates the insertion of the laryngoscope and allows for a direct view of the glottis.

3: A measurement of 3 fingers from the anterior tip of the mandible to the anterior neck estimates the volume of the submandibular space. In a typical patient, it is possible to place 3 fingers on the floor of the mandible between the mental angle and the neck in the vicinity of the hyoid bone. Under normal circumstances, this distance should measure approximately 7 cm. If the measurement is less than 3 finger widths, it suggests that the laryngeal axis is more acute than the pharyngeal axis. This indicates that aligning the oral opening to the pharyngeal opening will be challenging. Furthermore, it suggests that there will be limited space available for the displacement of the tongue within the throat. The 3-finger rule has certain limitations, as the distance can vary based on factors such as height and ethnicity. To address these limitations, an alternative method known as the ratio of height-to-thyromental distance (RHTMD) has been proposed.

2: A measurement of 2 fingers between the floor of the mandible and the thyroid notch on the anterior neck determines the location of the larynx relative to the base of the tongue. In a typical patient, 2 fingers can fit into the superior laryngeal notch. However, direct laryngoscopy becomes challenging and potentially impossible if the larynx is positioned too high in the neck and the distance measures less than 2 fingers. This is due to the acute angle between the base of the tongue and the larynx, making it difficult to visualize the larynx directly with ease.

An indication of potential DI is observed when the interincisor or hyoid-mental distance measures less than 3 fingers or when the hyoid-thyroid cartilage distance measures less than 2 fingers. Based on reports, the occurrence of DI varies between patient populations, ranging from 1.5% to 13%. Combining these measurements with the Mallampati score when assessing an airway enhances the positive predictive value in determining a challenging airway.

Additional factors to be considered in preparing for the intubation of a patient should include the following:

Assessment of Atlantooccipital Extension: This assessment involves instructing the patient to fully flex and stabilize their neck while looking at the floor and the wall. To assess the flexion movement of the cervical spine, the patient is asked to touch the manubrium sterni with the chin. Successful completion of this maneuver indicates an adequate range of motion in flexion and extension, which helps align the oral pharyngeal and laryngeal axis in a straight line. This alignment indicates a potentially easier intubation process.

A Combined Assessment of the Mandibular Space: This assessment, along with the 3-3-2 rule and atlantooccipital extension, will help determine the alignment of the laryngeal and pharyngeal axis with the atlantoaxial joint when the neck is extended. This evaluation provides further insight into how easily the laryngeal and pharyngeal axes will align with the atlantoaxial joint during the neck extension.

The Warning Sign of Delilkan: To perform this assessment, the clinician places an index finger of each hand in 2 specific locations. One finger is placed submentally, beneath the chin, whereas the other finger is positioned under the inferior occipital prominence, with the head in a neutral position. The patient is then instructed to extend their head and neck fully. If the submental finger is observed to be higher than the finger on the inferior occipital prominence, intubation should proceed without difficulty. However, the clinician should anticipate a DA if the finger on the inferior occipital prominence remains higher than the submental finger.

Prayer Sign: It is considered a positive sign when the patient is unable to approximate the palmar surfaces of the phalangeal joints while pressing their hands together. This sign is commonly observed in patients with advanced diabetes and carries a significantly high positive predictive value for cervical spine immobility. Consequently, it serves as an indicator of potential difficulty during endotracheal intubation.

Predictive parameters indicating DI include a Mallampati score above 3, thyromental distance (TMD) less than 6.5 cm, interincisor gap below 4.4 cm, a BMI above 30 kg/m², and tracheal deviation. In addition, factors such as limited movement in the temporomandibular joint (TMJ) and reduced larynx exposure also contribute to DI. However, these parameters exhibit a high heterogeneity level, limiting their diagnostic value. For instance, the definitions of mandibular protrusion and neck circumference vary among studies, thereby preventing a meta-analysis due to inconsistencies. Furthermore, factors such as obesity and gender, especially males, increase the odds of DI. Interestingly, patients who have undergone thyroid surgery are often considered to be at a higher risk of DI, but a study found a lower incidence of DI (7.21%) compared to the general population (10%). The tracheal deviation was the only significant predictive parameter specifically linked to thyroid surgery.[5]

Ultrasound presents potential advantages over traditional methods for measuring condylar mobility and TMD (tongue thickness, TT) for airway assessment. The undiagnosed cases of temporomandibular disorders during surgery are mentioned, highlighting the limitations of the maximal mouth-opening method for assessing TMJ mobility.

The study highlights the impact of increased TT and tongue size on laryngoscopy and the occurrence of DI, emphasizing the significance of predicting DI accurately. To minimize age-related variations in airway measurements, the age range of study participants of 18 to 60 was specified. Furthermore, the observation of a male predominance among patients and previous research, suggesting a correlation between higher BMI in males and DI, are acknowledged. The Mallampati and Cormack-Lehane grading systems are used to assess tongue size and glottic view, respectively, with correlations to DI.

The modified Mallampati scale (MMS) and Wilson score are identified as predictors of DI, although with limited sensitivity. In addition, ultrasound measurements of TT and condylar mobility also demonstrate correlations with DI. The role of condylar translation during laryngoscopy and the implications of reduced condylar mobility on the procedure are discussed.

BMI is identified as a predictive factor for DI, as individuals with higher BMI often exhibit a thicker tongue and increased DI. The study assesses the correlations between condylar mobility and various parameters, including age, Mallampati score, BMI, and Wilson score, to investigate their potential impact on DIs.[6]

Clinical Significance

Recognizing the likelihood of a difficult airway in a patient enables clinicians to plan and mitigate the risks associated with airway-related complications proactively. A prospective observational study involving 156 patients undergoing intubation in the emergency department demonstrated that the LEMON scale evaluation effectively stratified patients based on their risk of DI. The 3-3-2 rule plays a crucial role in the planning process as an essential component of the LEMON scale.[7]

LEMON Stands For

L: Look Externally

Evaluate the patient for external indicators that may suggest the presence of difficult endotracheal intubation. These indicators can include the abnormal shape of the face, extreme cachexia, poor dentition, edentulous mouth, morbid obesity, high arching palate, short neck, large front teeth, surgical scar indicating previous tracheostomy, which may suggest the presence of tracheomalacia, narrow mouth, face, or neck pathology in patients.

E: Evaluate

During the evaluation process, the 3-3-2 rule plays a crucial role in assessing specific measurements (3 separate distances) on the patient using the examiner's fingers, which are as follows:

3: Measurement of the interincisor space: The distance between the upper and lower teeth of the open mouth of a patient should ideally be greater than 3 fingers to facilitate easier intubation.

3: Measurement of the hyoid-mental distance: This measurement involves assessing the distance from the anterior tip of the mandible to the anterior neck on the hyoid bone. It should be greater than 3 fingers to indicate a more favorable airway for intubation.

2: Measurement of the hyoid-thyroid cartilage distance: This measurement evaluates the distance between the floor of the mandible at the hyoid bone to the thyroid notch on the anterior neck. It should be greater than 2 fingers, suggesting a more favorable airway condition for intubation.

M: Mallampati Scoring

Mallampati scoring is a system that assesses the anatomy of the mouth and the visibility of different anatomical structures in a patient when they open their mouth as wide as possible. The scoring is typically performed in a sitting position and may not be possible in an emergency situation. The scoring ranges from classes 1-4, with class 1 indicating easy intubation and class 4 suggesting the most challenging intubation scenario.[8][9][10]

• Class 1 Mallampati score: Visualized structures include the soft palate, uvula, fauces, and anterior and posterior pillars.
• Class 2 Mallampati score: Visualized structures include the soft palate, fauces, and uvula.
• Class 3 Mallampati score: Visualized structures include the soft palate and the base of the uvula.
• Class 4 Mallampati score: Here, the soft palate is not visible.

O: Obstruction

It is essential to evaluate whether the airway may be obstructed by various factors such as a foreign body, abscess, tumor, or swelling of the soft tissues. This assessment becomes particularly crucial in cases such as burn victims or trauma patients with an expanding hematoma, where the risk of airway compromise is heightened.

N: Neck Mobility

In alert and awake patients, it is essential to assess their neck mobility by observing if they can place their chin on their chest and the extent to which they can tilt their head backward. Decreased neck mobility serves as a negative predictor for complications during intubation.

Other Issues

Additional considerations are as follows:

• It is essential to anticipate a DA situation in patients as some of them, initially appearing normal or easy to intubate, may unexpectedly present challenges. The 3-3-2 rule serves as a valuable tool in anticipating airway complications.
• The 3-3-2 rule is useful, but its significance is enhanced with the Mallampati score.
• The 3-3-2 rule and other estimations do not play a significant role in emergent intubations.

Enhancing Healthcare Team Outcomes

The airway is an exceptionally vital body component that must be protected regardless of whether a patient is in a hospital setting, undergoing outpatient surgery, or being admitted to the ICU for observation and treatment. While many intubations are uncomplicated, certain cases may present DAs, and mishandling them can have fatal consequences for the patient.

For this reason, when clinicians consider intubation, they must thoroughly assess the risk of potential intubation failure and take measures to optimize the variables for successful airway management. Apart from anesthesiologists, physicians from various specialties, nurse anesthetists, and clinical pharmacists are often enlisted to assist in the preparation and intubation of a patient. Nevertheless, all personnel involved must be fully aware of the 3-3-2 rule. A difficult airway cart should be readily available at the patient's bedside to ensure preparedness for DIs. In addition, all physicians within the hospital possessing intubation skills should be informed and available as backups in case of a DI scenario. Furthermore, having the reversal agents for sedation and paralysis readily accessible at the bedside is essential in case the physician cannot successfully intubate the patient.

Nurses in the emergency departments are frequently involved in securing and assisting in the placement of tubes, as well as monitoring patient vital signs following intubation, and communicating any concerns to the attending physician. Failure to intubate a patient on time can lead to critical situations, including cardiac arrest. Therefore, when anticipating a DA, it is advisable to consult an anesthesiologist to ensure appropriate expertise and resources are available. In certain scenarios, oral intubation may not be feasible, necessitating an emergent tracheostomy as an alternative airway management approach. The nursing staff should be prepared to assist the clinician during airway procedures. They are often involved in ensuring proper patient alignment and controlling the positioning of the patient's head. Following intubation, nurses should closely monitor the patient's condition, promptly reporting any changes in oxygen saturation or signs of respiratory distress to the attending clinician. It is recommended for nurses to place signs outside the patient's room indicating a "difficult intubation" status. This keeps the medical team informed, particularly in situations where there may be a risk of self-extubation or the need for immediate intervention.[11][12][13]

The utilization of the 3-3-2 rule is not limited to a specific healthcare discipline. It is necessary for all members of the interprofessional team who are involved in the intubation process to be familiar with the rule, understand its application, and maintain open communication with other team members if issues arise. This collaborative approach is essential for advancing patient outcomes in cases of intubation. [Level 5]

Skills

A thorough understanding of the 3-3-2 rule is a valuable skill for healthcare providers involved in airway management. 

Strategy

It is most appropriate and ideal to maintain continuous closed-loop communication among all members of the perioperative care team regarding the necessity of using the 3-3-2 rule, the appropriate technique to be used, and any potential management issues that may arise during the process.

Ethics

It is essential to prioritize obtaining thorough informed consent from patients who can consent and those who lack the capacity, ensuring the involvement of their designated and authorized decision-maker before implementing the 3-3-2 rule. It is most appropriate to create an environment where all team members feel empowered to openly express any concerns regarding the process to the team or the patient. This approach ensures buy-in from all stakeholders and provides additional layers of review and insight into potential issues as soon as possible, allowing for prompt resolution and optimization of the process.

Responsibilities

All team members must effectively communicate their concerns, responsibilities, and activities with all other team members, both contemporaneously and as indicated, throughout the perioperative period. This communication should occur promptly and based on each team member's professional discretion.

Interprofessional Communication

All team members need to promote the free flow of information and concerns while maintaining a culture of respect and professionalism without allowing or producing an environment of hostility.

Care Coordination

All interprofessional team members should consider it their responsibility to neither disrupt the work done by other team members nor to, through their actions or inaction, create additional issues or increase the workload for other team members.