Optimizing Physiology During Prehospital Airway Management: An NAEMSP Position Statement and Resource Document

Abstract

Airway management is a critical component of resuscitation but also carries the potential to disrupt perfusion, oxygenation, and ventilation as a consequence of airway insertion efforts, the use of medications, and the conversion to positive-pressure ventilation.

NAEMSP recommends:

• Airway management should be approached as an organized system of care, incorporating principles of teamwork and operational awareness.

• EMS clinicians should prevent or correct hypoxemia and hypotension prior to advanced airway insertion attempts.

• Continuous physiological monitoring must be used during airway management to guide the timing of, limit the duration of, and inform decision making during advanced airway insertion attempts.

• Initial and ongoing confirmation of advanced airway placement must be performed using waveform capnography. Airway devices must be secured using a reliable method.

• Perfusion, oxygenation, and ventilation should be optimized before, during, and after advanced airway insertion.

• To mitigate aspiration after advanced airway insertion, EMS clinicians should consider placing a patient in a semi-upright position.

• When appropriate, patients undergoing advanced airway placement should receive suitable pharmacologic anxiolysis, amnesia, and analgesia. In select cases, the use of neuromuscular blocking agents may be appropriate.

Key words:

• airway management
• guidelines
• physiology
• preoxygenation
• resuscitation

Introduction

Airway management is a critical component of prehospital resuscitation. While ample attention focuses on procedural outcomes such as intubation success, relatively little consideration is given to the process or physiological effects of airway management. Numerous studies underscore the influence of airway management attempts upon physiologic responses before, during, and after attempted airway insertion. These physiologic changes are critical to patient-centered outcomes. To optimize outcomes, EMS medical directors and clinicians must attend to the process of airway management, including the preparation for the procedure, implementation of adjuncts to assure safe execution of airway insertion, and close monitoring following advanced airway insertion. This resource document summarizes key considerations in the peri-airway management process.

Systems of Care

Airway management should be approached as an organized system of care, incorporating principles of teamwork and operational awareness.

Airway management involves a complex series of cognitive and psychomotor skills and may require hundreds of independent decisions. These elements are time-critical and intricately interconnected, requiring an organized approach. Effective airway interventions must integrate management of the physiological needs of the patient with the psychomotor skills of the procedure. Thus, clinicians should approach airway management using a system-of-care philosophy, incorporating principles of teamwork and operational awareness. While not specifically developed for prehospital airway management, teamwork principles and the “pit crew” approach have been advocated to help organize the approach to cardiac arrest resuscitation. Effective teamwork spans multiple competencies including communication, leadership, team coordination, and decision making (1,2). Examples of effective teamwork techniques include directed and closed-loop communication, verbalization of the airway management plan and of ongoing situational awareness to ensure a shared mental model, use of checklists, and post-event team debriefings (3–5).

Airway teamwork strategies should be planned in advance. Prehospital teams should have tasks and sub-tasks identified and specific roles delineated. Training and education should address these defined tasks and roles, with specific attention given to cognitive decision making, psychomotor skills competency/mastery, non-technical skills (human factors), and physiological monitoring using airway-specific technologies. Principles of teamwork and operational awareness should also be addressed, ideally through simulation.

Performance improvement data should validate the approach to training and education and identify future educational opportunities. Effective quality management should inform system level improvements and include individual performance feedback for clinicians.

Preventing Secondary Injury from Hypoxemia and Hypotension

EMS clinicians should prevent or correct hypoxemia and hypotension prior to advanced airway insertion attempts.

Perfusion and oxygenation may be disrupted because of advanced airway insertion. Advanced airway insertion attempts are often associated with apnea/hypopnea and hypoperfusion, placing the patient at risk for oxygen desaturation and hypotension that can result in brain injury or cardiopulmonary arrest, particularly with the use of drug-assisted airway management (DAAM) (6–8).

Pre-oxygenation prior to each attempt provides additional safe apnea time to minimize the occurrence and the depth of hypoxemia (9,10). The oxyhemoglobin dissociation curve becomes steeper below 94%, accelerating oxygen desaturation. This provides a pre-oxygenation target of an oxygen saturation (SpO₂) value of 94% or greater (11). This also suggests that laryngoscopy attempts be limited below this value. The risk of occurrence and the severity of peri-intubation hypoxemia decrease as the maximum attained pre-oxygenation SpO₂ increases (12).

Approaches to pre-oxygenation include maintenance of an open airway, including the use of nasopharyngeal and oropharyngeal devices; passive administration of supplemental oxygen; airway decontamination using suction devices; decompression of suspected tension pneumothorax; and the use of positive pressure via high-flow oxygen, continuous positive airway pressure or bilevel positive airway pressure, positive end-expiratory pressure (PEEP), pressure support, and bag-valve-mask techniques. Apneic oxygenation, or the passive administration of oxygen via nasal cannula in a patient with ineffective or absent respiration, may reduce the risk of desaturation (13–17). While the use of positive-pressure ventilation may increase the risk of gastric insufflation and regurgitation, it may be necessary in hypoxemic patients, particularly those with apnea/hypopnea or inadequate lung volumes (18).

Airway management may compromise perfusion (19). The transition from spontaneous ventilation to positive-pressure ventilation increases intrathoracic pressure leading to hypotension due to a decrease in venous return and cardiac output (20,21). In addition, positive-pressure ventilation may create or worsen tension physiology with pneumothorax (22). Furthermore, medications used to facilitate airway management may result in hypoperfusion through mechanisms including negative inotropy, bradycardia, or vasodilation (19). Finally, supraglottic airways (SGAs) may reduce cerebral perfusion, particularly in low-flow states (23,24). Prehospital interventions to reverse hypotension or enhance perfusion prior to advanced airway management may reduce complications related to hypoperfusion (25–29). Therapeutic interventions such as hemorrhage control, decompression of a tension pneumothorax, volume resuscitation, and administration of vasopressor agents mitigate peri-intubation hypotension. In addition, the selection of medications and management techniques should be predicated by the hemodynamic status of the individual patient. Medication selection is discussed further in the Prehospital Drug Assisted Airway Management Resource Document (30).

Decision Making, Timing, and Duration of Airway Attempts

Difficulties with non-invasive ventilation and advanced airway insertion should be anticipated whenever possible, and continuous physiological monitoring must be used during airway management to guide the timing of, limit the duration of and inform decision making during advanced airway insertion attempts.

Patient assessment tools may help EMS clinicians anticipate potential difficulties with noninvasive ventilation and advanced airway management. The information gleaned from these assessments may influence airway decision-making and the selection of specific techniques or devices. Existing difficult airway assessment tools were derived and validated in the in-hospital environment and may not be generalizable to prehospital patients but may still have utility in the prehospital setting. Elements of some tools require an awake patient who can follow commands and are performed on patients in a seated position without cervical spine precautions, and the patient population for elective in-hospital surgical cases does not reflect that of the prehospital environment, particularly the population requiring emergency airway management (31,32). However, certain predictors, such as obesity or anatomic challenges are likely relevant to any patient requiring emergent airway management. Finally, the training and assessment skills for prehospital clinicians differ from those of anesthesiologists who conducted most of the derivation and validation studies. The lack of consensus and variable penetration of prospective prehospital airway assessment tools represents a gap in prehospital medical knowledge.

EMS clinicians managing the airway must provide continuous monitoring to identify physiological derangement and to inform decision making to abort the airway procedure or modify the approach. The apnea/hypopnea associated with advanced airway insertion, particularly with use of DAAM, increases the risk of oxygen desaturation (7,33–35). Clinicians should perform continuous pulse oximetry to monitor oxygenation status with a goal of sustaining SpO2 at or above 94% throughout airway management efforts. Advanced airway insertion attempts may need to be abandoned and noninvasive techniques for re-oxygenation, such as bag-valve-mask ventilation, employed when SpO₂ falls below 94% (11). In addition, laryngoscopy and advanced airway insertion may increase sympathetic output, raise intracranial pressure, or trigger a vasovagal response, all of which may result in focal or global hypoperfusion (19). EMS clinicians must employ continuous heart rate monitoring and obtain frequent blood pressure measurements to rapidly identify hypoperfusion. Finally, clinicians should limit the duration of advanced airway insertion attempts and abandon attempts associated with physiological derangement. This underscores the importance of continuous monitoring to identify indicators to terminate an airway insertion attempt.

Confirming and Securing the Advanced Airway

Initial and ongoing confirmation of advanced airway placement must be performed using waveform capnography. Airway devices must be secured using a reliable method.

Prior to the introduction of end-tidal CO₂ monitoring, unrecognized misplacement of the advanced airway was common (36–39). In a prospective observational study, Katz and Falk found that 25% of endotracheal tubes (ETTs) were misplaced upon emergency department arrival, the majority of which were found to be in the esophagus (34). Similarly, Wang et al. documented that errors occurred in 22% of EMS intubation attempts. These included misplacement, dislodgment and failed intubation (38). Silvestri et al. observed a 23% incidence of unrecognized esophageal intubations in their system at baseline. Of note, this incidence fell to 0% once end-tidal CO₂ was introduced (40). Davis et al. observed only a single unrecognized misplaced ETT in over 700 patients following introduction of end-tidal CO₂ monitors (41). End-tidal CO₂ may also be effective for monitoring SGAs (42,43).

Waveform capnography is the most reliable method for confirming initial and ongoing placement and function of an advanced airway and is considered standard of care for in-hospital advanced airway management (44). Given the availability of waveform capnography, unrecognized misplacement of an advanced airway should never occur. Although colorimetric devices and capnometry can confirm initial advanced airway placement, they are less sensitive and specific in low-flow states such as cardiac arrest (45–47). Conversely, Silvestri et al. reported 100% sensitivity and 100% specificity of waveform capnography for proper ETT placement in a simulated low-flow state (47). The use of capnography to confirm ETT placement in cardiac arrest is now a Class I recommendation from the American Heart Association (48).

In addition to confirming initial advanced airway placement, capnography is the best method for ongoing monitoring of airway position. EMS clinicians should use continuous capnography during transport and movement of patients in order to monitor the position of the advanced airway and to rapidly detect dislodgement. While colorimetric devices can initially detect exhaled carbon dioxide, their effectiveness declines with exposure to air and liquids. Thus, these devices are not reliable for ongoing monitoring of advanced airway placement to detect dislodgement and cannot guide ventilation. Pulse oximetry is routinely used by EMS to monitor oxygenation status but is an unreliable strategy to confirm initial advanced airway placement and provides only delayed detection of airway dislodgement (49–51). Revisualization by direct or video laryngoscopy may be used for confirming ETT placement but cannot be used for continuous monitoring of advanced airway position. While some EMS systems allow SGA insertion by basic life support clinicians, the lack of capnography at this level must be recognized as a limitation; in which case colorimetric end-tidal CO₂ detectors along with clinical signs of proper ventilation (chest rise, auscultation of lungs sounds) are the best alternative to confirm SGA placement. In these cases, waveform capnography should be placed as soon as practicable by advanced life support or hospital personnel.

​​Importantly, EMS clinicians must be proficient with capnography and interpretation of both the value and the waveform. In the study by Vithalani et al., failed ventilation was detected by capnography waveforms retrospectively and not by the treating EMS clinicians in some cases, demonstrating the importance of training (43).

In order to prevent dislodgement following confirmation of proper position, advanced airway devices must be secured using a reliable method. Dislodgement can result in complications such as airway trauma and unplanned extubation. In a systematic review, da Silva et al. noted the frequent and dangerous complications that result from unplanned extubation and subsequent reintubation attempts including hypoxemia and hemodynamic compromise, which may result in poor patient outcomes or death (52). Adhesive tape, ties, and commercial airway device holders are all acceptable methods for securing the advanced airway. Gardner et al. conducted a systematic review of in-hospital studies and concluded that these methods were equally effective at minimizing endotracheal tube dislodgement (53). However, in a recent single-center randomized controlled trial, Landsperger et al. observed lower rates of ETT dislodgement with use of a commercial tube fastener as compared to tape, although this may be operator-dependent (54). None have been demonstrated as superior in the prehospital environment. It is important to follow manufacturer guidelines for securing SGAs, which typically include use of a specific securing device or taping maxilla-to-maxilla (55,56). Taping to the mandible may not be as effective. Da Silva et al. concluded that data supporting the use of physical restraints to prevent unplanned extubation was inconclusive and, thus, clinicians should focus on adequate sedation and close monitoring (52). However, physical restraint may still be reasonable for both crew and patient safety considerations, particularly given the challenges of monitoring in the out-of-hospital environment.

Optimizing Perfusion, Oxygenation and Ventilation

Perfusion, oxygenation, and ventilation should be optimized following advanced airway insertion.

Physiological derangement is common following advanced airway insertion, and efforts should be focused on maintaining optimal perfusion, oxygenation, and ventilation (57,58). Hypoxemia contributes to morbidity and mortality from multiple disease processes, particularly those involving cerebral injury, and should be prevented or reversed in all patients (8,59). Some data suggest that extreme hyperoxemia may contribute to free radical injury and hypoperfusion resulting in worsened neurological outcomes following brain injury (34). EMS clinicians should achieve systemic normoxemia for most patients, targeting SpO₂ values of 94-98%. Traumatic brain injury patients may suffer cerebral hypoxia despite systemic normoxemia. For these patients, increasing FiO₂ to 40-60% while maintaining SpO₂ values of 99-100% may be reasonable (35). Potential strategies to reverse hypoxemia include increasing FiO₂ and increasing available alveolar surface area for gas exchange, either through recruitment [tidal volume, inspiratory time] or through PEEP to prevent atelectasis and atelectrauma. Other strategies for improving oxygenation include ensuring good pulmonary toilet, treating underlying pulmonary disease, optimizing perfusion status, and positioning the patient to ensure efficient ventilation/perfusion matching.

Hyperventilation is generally harmful but unfortunately common in prehospital ventilation (33). For most patients, achieving eucapnia with a target PaCO₂ ∼40 mmHg is reasonable. End-tidal CO₂ may not accurately reflect PaCO₂ in critically ill and injured patients but can help avoid excessive ventilation rates (57). These are associated with inadvertent hypocapnia and elevated intrathoracic pressures, which may compromise cardiac output or result in lung injury via barotrauma. Patients with traumatic brain injury experience decreased cerebral blood flow, with increased ventilation rates leading to worse outcomes. Controversy exists for use of mild hyperventilation in patients with impending herniation; this is addressed elsewhere in this compendium (60). Patients with metabolic acidosis but without brain injury may require a lower PaCO₂ target to provide partial respiratory alkalosis unless this relative hyperventilation results in hypotension. Patients with acute respiratory distress syndrome (ARDS) or multi-organ dysfunction syndrome benefit from low tidal volume ventilation and permissive hypercapnia to avoid the adverse systemic and/or pulmonary effects of positive-pressure ventilation.

Several components of airway management may compromise perfusion. Positive-pressure ventilation may create or exacerbate hypotension and may contribute to tension physiology with pneumothorax. In addition, medications used to facilitate airway management during DAAM may have cardiovascular side effects, such as negative inotropy, bradycardia, or vasodilation. These effects may be mitigated by the use of volume or vasopressor infusion, reversal of obstructive shock (decompression), or the use of push dose vasopressors prior to induction, during, or after the procedure. Hypotension should be reversed prior to advanced airway management to reduce or limit complications related to hypoperfusion (25–28). In addition, modification of ventilation strategies, such as a reduction in ventilation rate, tidal volume/inspiratory pressure, or inspiratory time, may reduce negative effects on perfusion.

Strategies to Mitigate Aspiration and Ventilator-Associated Lung Injury

To mitigate aspiration after advanced airway insertion, EMS clinicians should consider placing a patient in a semi-upright position and use lung-protective ventilation strategies.

Early survivors of cardiac arrest and trauma are at high risk for in-hospital death from pulmonary complications that include ventilator-associated pneumonia, ventilator-associated lung injury (VALI), and ARDS (61,62). Strategies should be considered to reduce the prehospital contribution to those complications. Wang et al. performed a Cochrane review, concluding that elevation of the head and torso above 30 degrees is associated with a decreased incidence of ventilator-associated pneumonia in the intensive care unit (ICU) compared to the supine position but noted the conflicting data encountered in the review (63). Klompas suggested that the variability may have to do with the types of diseases studied and the definition of ventilator-associated pneumonia (64). Alexiou et al. found that protection was offered only when the angle was 45 degrees, as compared to either 15 or 30 degrees (65). In the absence of contraindications, it seems prudent to elevate the EMS stretcher to maintain the torso at 45 degrees after EMS placement of an advanced airway.

Lung-protective ventilation is standard-of-care in the ICU setting, but there is a paucity of published prehospital research on VALI and ARDS attributable to initial EMS management of an advanced airway by manual or mechanical ventilation. Although extrapolating in-hospital ICU data to care in the field requires caution, it seems reasonable that EMS clinicians should employ lung-protective ventilation strategies. Stephens et al. state that VALI and ARDS are time-sensitive disease processes that could potentially be prevented with early lung-protective ventilation (66). Lower tidal volumes (4–8 mL/kg ideal body weight) appear to reduce volutrauma, with improved neurocognitive outcomes demonstrated after out-of-hospital cardiac arrest (67). However, accurately measuring and delivering lower tidal volumes is challenging with manual ventilation, and even mechanically ventilated out-of-hospital patients may receive excessively large tidal volumes, underscoring the importance of education and training (68). Similarly, high airway pressures may result in barotrauma and increased intrathoracic pressure but may be avoidable through technology as well as education and training (68–71). Finally, the use of PEEP improves oxygenation and reduces atelectrauma but may be underutilized in the out-of-hospital environment and must be balanced against the associated increase in intrathoracic pressure that can compromise perfusion. The routine use of mechanical ventilators in the prehospital environment may be reasonable to control the above variables (72).

Anxiolysis, Amnesia, and Analgesia

When appropriate, patients undergoing advanced airway placement should receive suitable pharmacologic anxiolysis, amnesia, and analgesia. In select cases, the use of neuromuscular blocking agents may be appropriate.

Advanced airway management and insertion of an advanced airway are recognized as uncomfortable and potentially painful (73). Thus, an ethical obligation exists to address patient comfort during advanced airway management. In addition, reducing patient anxiety and pain may reduce or prevent brain injury and lower intracranial pressure, which have therapeutic benefits in patients with ischemic or traumatic brain injury (74). Furthermore, anxiolysis, analgesia, and/or paralysis may facilitate ventilation and reduce or avoid perfusion, oxygenation, and ventilation complications during positive-pressure ventilation. Thus, priority should be given to the administration of medication to reduce anxiety and pain. If paralysis is necessary for patient safety, oxygenation, or ventilator asynchrony, then adequate sedation and analgesia must be administered. Prehospital clinicians must monitor and recognize signs of alertness and treat appropriately to maintain adequate level of sedation, particularly with use of neuromuscular blocking agents. However, the adverse effects of these medications must be considered in the risk-benefit analysis. Sedatives and analgesics may reduce sympathetic tone, which may be an integral part of the compensatory response to hypovolemia/hypoperfusion. In addition, many sedative and analgesic agents reduce preload or cause systemic vasodilation, both of which may result in hypotension. Furthermore, some agents have direct cardiac suppressive effects.

Conclusion

Airway management influences physiologic responses before, during, and after attempted advanced airway insertion. These physiologic changes are associated with patient-centered outcomes. EMS medical directors and clinicians must attend to the process of airway management taking a systems-based approach to mitigate the risk of harm to the patient.