Airway Monitoring

Anatomy and Physiology

Monitoring a patient in respiratory distress involves gathering both subjective and objective information.[1] A focused history and physical examination offer valuable insights into the patient's ability to protect their airway. A thorough understanding of airway anatomy aids in conducting a thorough examination. The airway can be categorized into 2 parts: the upper airway, encompassing the nasal cavity, oropharynx, and larynx, and the lower airway, starting with the trachea and extending through the bronchi, bronchioles, and alveoli (see Image. Airway and Lung Anatomy).

An airway exam should include an assessment of mouth opening, dentition, thyromental distance, neck circumference, Mallampati score (see Image. Mallampati Score Visual), and cervical spine mobility. The mnemonic LEMON is often used to evaluate the airway.

• L: Look
• E: Evaluate
• M: Mallampati
• O: Obstruction
• N: Neck mobility

Indicators such as mouth opening of fewer than 3 fingers, a large neck, a short thyromental distance of fewer than 3 fingerbreadths, Mallampati 3 or 4, or limited neck extension should alert the clinician to a possible difficult airway and prompt appropriate preparation.[2] Additional signs indicating a patient may be difficult to intubate may include prominent upper teeth or a receding lower jaw. Congenital malformations, prior surgical intervention, or history of radiation, and trauma to the airway may also be a strong indicator of a difficult airway. Different airway examination tests can be used; however, their accuracy may vary.[3][4][5] Some of the commonly used airway examination tests and their accuracy include the following:

• Mallampati test
  • This test is a simple, noninvasive method for predicting difficult intubation by assessing the visibility of oropharyngeal structures. However, the Mallampati test has limited accuracy, with sensitivity ranging from 24% to 62% and specificity from 78% to 82%.
• Thyromental distance
  • The thyromental distance measures the space between the thyroid notch and the mentum (lower jaw). A shorter distance is associated with a difficult airway. The test has a sensitivity of 44% to 55% and a specificity of 85% to 89%.
• Sternomental distance
  • This test measures the distance between the sternal notch and the mentum. A shorter distance suggests a difficult airway. The sensitivity of the sternomental distance test ranges from 50% to 62%, with a specificity of 88% to 89%.
• Interincisor gap
  • The interincisor gap is the distance between the upper and lower incisors with the mouth fully open. A smaller gap indicates a difficult airway. The test has a sensitivity of 29% to 61% and a specificity of 83% to 85%.
• Neck mobility
  • Limited neck mobility can make airway management more challenging. The test has a sensitivity of 29% to 54% and a specificity of 85% to 89%.
• Upper lip bite test
  • The upper lip bite test involves asking the patient to bite their upper lip with their lower incisors. The test has a sensitivity of 68% to 93% and a specificity of 82% to 97%.

Airway ultrasound also has a role in predicting difficult intubations.[6][7] The most accurate predictor of difficult intubation is an interincisor distance of less than 3 cm. Intubation should always be confirmed with a colorimetric carbon dioxide (CO2) indicator or a continuous end-tidal CO2 (ETCO2) monitor.

Physical exam signs, including fogging/mist in the endotracheal tube lumen, chest rise with each breath, and bilateral breath sounds, can help confirm endotracheal intubation; however, objective parameters should take priority. Although a chest x-ray can help visualize placement, it is not mandatory. Clinical correlation is recommended instead.

The most reliable measures of endotracheal intubation are direct visualization during laryngoscopy and a persistently elevated ETCO2 level.[8] In addition to the LEMON mnemonic for airway assessment, other factors such as the patient's level of consciousness, respiratory rate, oxygen saturation, and use of accessory muscles can provide valuable information about the patient's airway status. Auscultation of breath sounds can also help identify abnormalities, such as wheezing, stridor, crackles, or diminished breath sounds.[9][10]

Indications

Airway monitors are crucial for evaluating a patient’s physical condition and clinical status. Signs that a patient may necessitate airway intervention, such as endotracheal intubation or ventilation support using noninvasive modalities like bilevel positive airway pressure, include symptoms such as dysarthria, tripod posture, drooling, increased respiratory effort, accessory muscle use, nasal flaring, tachypnea, declining mental status, cyanosis, and escalating FiO2 demand. Obtaining a comprehensive medical history and physical examination can be arduous when managing patients in respiratory distress. In such instances, healthcare professionals must employ alternative methods to assess the patient’s airway.

Monitoring vital signs and conducting laboratory tests, such as blood gas analysis, can provide valuable insights and help identify impending respiratory failure. Airway monitoring is indicated in patients undergoing sedation for procedures or those receiving narcotics and/or benzodiazepines. These patients necessitate special attention as they pose a potential risk for respiratory distress, particularly in the obese and elderly populations. These patients should be assessed and reassessed frequently, and appropriate monitors, such as continuous pulse oximetry, should be used to help identify oversedation or apnea. 

Contraindications

Contraindications to airway monitoring are confined to patients in hospice or undergoing palliative care during the final stages of life. 

Equipment

Capnography is an airway monitoring method applicable to controlled and spontaneously ventilating patients. The amount of infrared light absorbed by carbon dioxide in exhaled gas is measured and reported as a concentration value. Waveform capnography can provide qualitative data via waveform analysis and quantitative data via alveolar ETCO2.[11] Other forms of capnography, such as colorimetric capnography, can confirm endotracheal intubation in remote, out-of-hospital settings, emergency departments, or intensive care units. Capnography can provide reliable information confirming adequate ventilation and cellular gas exchange. 

The waveform shows 4 phases of the respiratory cycle:

• Phase 1: Inspiratory baseline
• Phase 2: Expiratory upstroke
• Phase 3: Expiratory plateau
• Phase 4: Inspiratory downstroke

Predicting catastrophic events is possible by analyzing changes in waveform morphology and amplitude. Elevated levels of ETCO2 may indicate various medical conditions, including rebreathing, hypoventilation, malignant hyperthermia, sepsis, insufflation during laparoscopic surgery, or bicarbonate administration. Decreases in ETCO2 are found during hyperventilation, hypothermia, low cardiac output states (eg, pulmonary embolism or cardiac arrest), disconnection of the circuit, or accidental extubation.

Morphological perturbations such as up-slanting of the expiratory upstroke or prolongation of the plateau phase can be evidence of airway obstruction, as seen in bronchospasm, asthma, pregnancy, chronic obstructive pulmonary disease, or a kinked endotracheal tube. Waveform capnography is a crucial monitoring tool for intubated patients, and its interpretation can aid in managing their ventilation. Capnography has limitations, especially when a patient is not intubated or doesn't have a secure airway.

Simply observing a positive return of ETCO2 may or may not confirm the endotracheal placement of the endotracheal tube (ETT). However, a sustained ETCO2 level above 30 mm Hg for at least 3 breaths can confirm ETT placement. Notably, if a patient consumed a bicarbonate solution or carbonated beverage before intubation, this could alter ETCO2 readings. Furthermore, capnography is limited in pulseless patients because it requires cardiac output. However, it can guide chest compressions, where a sudden increase in ETCO2 may suggest a return of spontaneous circulation.

Capnography has recently been further studied and developed for various clinical applications. The usefulness of capnography as a noninvasive method for monitoring ventilation has been demonstrated in various settings, including procedural sedation, the management of acute respiratory distress syndrome, and emergency medicine for the early detection of respiratory failure.[12][13][14] Using capnography has also improved patient safety and outcomes by reducing the incidence of adverse events related to hypoventilation and hypoxia.[15] Recent technological advances have led to new capnography devices that are more portable, user-friendly, and versatile, enabling broader use across diverse clinical scenarios.[13] Despite these advances, it is essential to understand the limitations of capnography and pulse oximetry, as well as the factors that influence their accuracy.

The pulse oximeter is a noninvasive monitor that detects changes in hemoglobin oxygenation saturation and heart rate. Pulse oximetry functions on spectrophotometry and photoplethysmography. Oxyhemoglobin absorbs infrared light at a wavelength of 940 nm, and deoxyhemoglobin absorbs red light at 660 nm. When oxyhemoglobin absorbs emitted infrared light, it allows red light to be transmitted and received by the photodetector. The red-to-infrared light ratio determines the percentage of hemoglobin saturation with oxygen. The pulsations occurring with each heartbeat are recorded as arterial blood and thus provide arterial blood oxygenation.[16]

Although a pulse oximeter can help determine blood oxygen levels, it has some limitations. For example, carboxyhemoglobin resulting from carbon monoxide poisoning can cause falsely elevated oxygen saturation readings because it absorbs light at the same wavelength as oxyhemoglobin. Methemoglobinemia, which has the same absorption coefficient for red and infrared light (resulting in a 1:1 red/infrared ratio), depicts a saturation of approximately 85%.

This phenomenon results in a falsely low oxygen saturation when saturation is greater than 85% and a falsely elevated saturation when it is less than 85%. Other factors that can affect the accuracy of oxygen saturation readings include acid-base disorders, severe hypoxemia, dark nail polish, excessive motion, ambient light, low perfusion, hypothermia, increased systemic vascular resistance, profound anemia, methylene blue dye, and malpositioning. These limitations must be considered when interpreting pulse oximeter readings.

Examples of adjunct airway monitors that further guide clinical decision-making include chest x-ray, ultrasound, and bronchoscopy in the intubated patient. These modalities can help identify underlying lung pathologies, airway obstruction, and foreign bodies and provide visual guidance during endotracheal intubation.[17] In particular, point-of-care lung ultrasound has gained popularity recently due to its noninvasive nature, rapid results, and portability, making it an ideal tool for use in emergency settings and intensive care units.[18]

In conclusion, capnography is a valuable method for monitoring airway and ventilation in spontaneously breathing and ventilated patients. With its ability to provide real-time information on the patient's ventilation status, capnography is an essential tool in various clinical settings. While it has limitations, recent technological developments have expanded its applications and improved its usability. Capnography and pulse oximetry can help guide patient management and improve patient safety.

Personnel

All individuals involved in patient care must be able to identify the signs of respiratory distress. These include personnel in the prehospital setting, emergency room staff, nurses and medical assistants, and clinicians. In such cases, activating a rapid response and initiation of oxygen supplementation may assist in providing immediate care and preventing patient decompensation.

Qualified clinicians experienced in airway management should be consulted, and an objective assessment conducted. Additionally, when a challenging airway is anticipated, it is recommended to have a supraglottic airway device and a video-assisted laryngoscope available to facilitate intubation. Should the patient present as an unanticipated difficult airway, a surgical front-of-neck airway should be considered if the patient cannot be ventilated or intubated by airway experts. 

The Difficult Airway Society (DAS) 2025 guidelines highlight improvements in managing unanticipated difficult tracheal intubation and should be reviewed by clinicians involved in airway management. Airway monitors recommended by the DAS include waveform capnography (checked before induction of anesthesia, if possible) and continuous throughout airway management, continuous and audible pulse oximetry, and quantitative neuromuscular monitoring. Additionally, vital signs, including a 3- to 5-lead electrocardiogram and blood pressure monitoring, are crucial for assessing the patient's clinical stability.[19]

Technique or Treatment

Airway assessment and planning must be performed before intervention. As stated by the DAS and the American Society of Anesthesiologists, continuous airway supplementation throughout airway management is essential to optimize the chances of first-attempt success during tracheal intubation. A thorough understanding of the Difficult Airway Algorithm is required for clinicians involved in airway management (see Image. The American Society of Anesthesiologists [ASA] Difficult Airway Algorithm: Adult Patients 2022).[19][20]

In difficult airways, the effectiveness of airway nerve blocks versus airway anesthesia without nerve blocks for awake tracheal intubation has been compared. Airway nerve blocks significantly reduced intubation time, improved the quality of anesthesia, and decreased overall complications. In conclusion, airway nerve blocks provide superior airway anesthesia for awake tracheal intubation, improving intubation conditions and patient satisfaction while reducing complications.[21]

In ventilated individuals, understanding transpulmonary pressure is essential for assessing lung stress. This accurately reflects alveolar pressure and serves as a crucial physiological marker. Esophageal manometry, which utilizes esophageal balloon catheters, is used to measure pleural pressure and make necessary ventilator adjustments.

However, calibration methods are necessary to account for the balloon's air volume. Clinical applications of esophageal manometry include assessing lung and chest wall compliance, estimating breathing effort for ventilator weaning, detecting upper airway obstruction postextubation, and identifying patient-ventilator asynchrony.[22] Results from clinical trials and meta-analyses suggest that intraoperative quantitative monitoring significantly reduces the risk of residual neuromuscular blockade and improves postoperative respiratory outcomes by minimizing adverse events. Consequently, numerous international anesthesia societies endorse the use of quantitative monitoring when administering neuromuscular blocking agents in perioperative settings.[23]

Capnography, or real-time monitoring of exhaled CO2, is a valuable tool for assessing patient health during anesthesia, in emergency departments, and in critical care units, with ETCO2 concentration as the fundamental measurement. The applications of this technology are varied, including verifying ETT placement, assessing disease severity and treatment response, and monitoring sleep apnea in outpatient settings. With advances in technology, smaller sensors have been deployed in various settings, including prehospital environments and high-performance military aircraft. These offer possibilities for new applications that can be integrated into breathing masks.[24]

Different surgical conditions require specific ventilation strategies, each with its own challenges and considerations. For instance, laparoscopic surgery involving creating a pneumoperitoneum can lead to reduced lung compliance and increased ventilatory pressures due to the diaphragm's cephalad displacement. While prone positioning can benefit the respiratory system, it requires careful airway management to prevent ETT disconnection.

One-lung ventilation during thoracic surgery affects ventilation-perfusion matching and requires lung-protective strategies. Jet ventilation is a specialized technique requiring a shared airway or a still chest. This type of ventilation can be performed at low frequencies for laryngeal procedures or at high frequencies for specialized interventions, with considerations for airway pressures and gas exchange. Knowing these ventilation strategies and their implications in various surgical scenarios can enhance patient care and outcomes.[25]

Clinical Significance

Identifying impending respiratory distress in a patient can lead to early intervention and greatly reduce morbidity and mortality. Personnel in the prehospital setting, emergency room, and throughout the hospital should be briefed on methods to activate a rapid response in a deteriorating patient and be aware of who the airway experts are and how to reach them at any given time. Untreated hypoxia can result in tissue and organ injury, and even death. 

Properly monitoring an intubated individual's airway involves considering several variables. The ventilator visually represents a patient's ventilatory mechanics and can detect acute changes. Radiographic imaging can help identify developing lung pathology or diaphragmatic abnormalities. The patient's airway must be monitored appropriately when weaning them from a ventilator. Following specific criteria is essential for safe extubation and preventing the need for reintubation.

Enhancing Healthcare Team Outcomes

Effective airway monitoring requires a structured, anticipatory approach grounded in early recognition, technical proficiency, and coordinated team dynamics. Patient-centered care is best achieved through an interprofessional model in which nursing staff and emergency medical services, often the first to assess the airway, initiate basic airway management in alignment with American College of Surgeons Advanced Trauma Life Support and American Heart Association guidelines, prioritizing airway stabilization to reduce morbidity and mortality.[26] Physicians, advanced practitioners, nurse anesthetists, and respiratory therapists must demonstrate expert-level skill in advanced airway techniques, recognizing that improper placement can lead to hypoxia, gastric insufflation, acidosis, organ ischemia, and death. Continuous monitoring, including clinical assessment and adjuncts such as pulse oximetry and capnography, combined with vigilance from all team members, enables early detection of deterioration and timely escalation.

Optimal outcomes depend on disciplined interprofessional communication, ethical practice, and seamless care coordination. Closed-loop communication ensures that concerns about airway status are conveyed and addressed promptly, while standardized protocols and shared mental models reduce variability and errors. Pharmacists guide medication selection for rapid-sequence intubation and minimize adverse drug interactions; radiologists confirm endotracheal tube placement and evaluate pulmonary pathology; and respiratory therapists provide critical expertise in ventilatory support and early identification of respiratory decline. Ethical considerations, including informed consent when feasible, reinforce patient autonomy and transparency. All team members share responsibility for maintaining situational awareness, respecting open communication, and coordinating care without introducing inefficiencies or risk, thereby enhancing patient safety, team performance, and overall clinical outcomes.