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Airway Management/Ventilator Management
Goals/Objectives
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Types of Intubation
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Surgical Airway
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Ventilator Settings/Management
Airway Management in the Trauma Patient
From Cameron JL, Cameron AM: Current Surgical Therapy, 10th edition (Mosby 2011)
Airway Intervention: Decision-Making and Physiologic Considerations
The decision to intervene to provide a definitive airway should be driven by the patient's presentation and ability to protect the airway. Box 58-1-1 lists some indications for definitive airway control in trauma patients. Although hypoventilation is somewhat detrimental in the injured patient, hypoxia has much greater consequences and is rapidly fatal if not corrected. Airway compromise can result in the inability to maintain SaO 2 greater than 90%, and it can be manifested by agitation, confusion, and combativeness. The latter should not be attributed to a patient's inappropriate affect until anatomic or physiologic causes are ruled out. Airway obstruction can be caused by facial trauma; foreign objects, including broken teeth; or vomitus. In both blunt and penetrating trauma, direct injury to the oropharynx or larynx and hematoma and soft-tissue damage with swelling can lead to airway obstruction.
Box 58-1-1
Indications for Definitive Airway Control in Trauma Patients
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Respiratory insufficiency
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Airway obstruction
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Glasgow Coma Scale score of ≤8
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Severe maxillofacial injury
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Thermal airway injury
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Persistent agitation
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Large and/or expanding neck hematoma
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Penetrating airway injury
A Glasgow Coma Scale (GCS) score of 8 or less is a well-accepted indication for obtaining a definitive airway and can be remembered by the simple rhyming mnemonic, “GCS of 8 means intubate.” Intoxicated or medicated patients, without other signs of injury (i.e., “found down”), present a unique problem, as their mental status may be chemically altered by drugs or alcohol, thus limiting their response to voice and stimuli. However, they may be oxygenating and ventilating without difficulty. If the decision is made to not obtain definitive airway control, these patients must be closely observed so that immediate intervention can be undertaken if decompensation occurs.
Severe maxillofacial trauma can lead to acute compromise of the airway. Fractures and soft-tissue swelling may cause severe respiratory distress and can make airway control difficult. Bleeding in the oropharynx and into the distal airway can lead to hypoxia and loss of airway control. Attempts to obtain a secure airway can also be made difficult by significant facial trauma, specifically midface injuries, where loss of normal anatomy can lead to loss of airway protection and can complicate intubation.
Thermal and inhalation injuries to the airway should be suspected in all burn victims, and early intubation should be considered even in patients protecting their airway on presentation. Singed facial or nasal hairs, carbonaceous sputum, and/or facial burns are clues to possible airway injury and should prompt rapid airway control. Waiting for progression of edema to result in voice changes and stridor will make intubation difficult. Patients who come in with a history of smoke inhalation or confinement in a smoke-filled space who do not have a clear indication for airway control should undergo bronchoscopy for evaluation of the airway with the potential for immediate intubation if severe findings are noted.
Agitated trauma patients present a major risk to themselves and to those providing their care. Agitation can be caused by brain injury, hypoxia, shock, and both drug – prescribed and illicit – and alcohol intoxication. Initial patient evaluation can be very difficult in these cases. The “rule of three” is an often quoted but rarely documented understanding: a patient who physically or verbally assaults the care team three times has declared their need for endotracheal intubation to allow rapid, safe, and proper evaluation and management.
Although the decision to obtain a definitive airway is straightforward in some patients, the decision of how and when to intubate other patients may not be so clear. For this reason, we have instituted a multidisciplinary emergency airway course for all residents from surgery, anesthesiology, emergency medicine, and otolaryngology at Johns Hopkins. The mechanical techniques of intubation are relatively easy to teach and master, but the complex decision-making that often precedes the actual intubation is much more difficult and is the mainstay of the course. Box 58-1-2 outlines a series of questions that we teach the residents, and these can be utilized in airway management decisions. If the need for a definitive airway is not immediately necessary, frequent reassessments should always be done. This is particularly true in the patient with a traumatic brain injury (TBI), where even one short period of hypoxia can lead to significantly worse outcomes. By frequently reassessing the patient, the deterioration of airway protection can be detected earlier, a plan can be formulated, and intubation can proceed in a timely fashion if needed. Once the decision has been made to obtain a definitive airway, an appropriately skilled member of the team should promptly initiate the planned method of airway control. This individual may be a physician – anesthesiologist, emergency medicine physician, or surgeon – a nurse anesthetist, or a respiratory therapist, depending on the local practice. At least one but preferably two or more back-up plans should be in place and explicitly stated for all members to hear, as the window of opportunity to obtain a definitive airway is short. At times a surgical airway is urgently needed. A cricothyroidotomy is the preferred approach ( Fig 58-1-1 ).
Box 58-1-2
Key Airway Decision-Making Questions from the Johns Hopkins Emergency Airway Course
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Does the patient need to be intubated?
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How rapidly does the patient need to be intubated?
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Will the intubation be difficult?
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What is the chosen method to control the airway?
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What are my back-up plans?
Mechanical Ventilation
From Vincent JL, Abraham E, Moore FA, Kochanek PM, Fink MP: Textbook of Critical Care, 6th edition (Saunders 2011)
Positive-pressure mechanical ventilatory support provides pressure and flow to the airways to effect oxygen (O 2 ) and carbon dioxide (CO 2 ) transport between the environment and the pulmonary capillary bed. The goal is to maintain appropriate levels of partial pressure of O 2 and CO 2 in arterial blood while unloading the ventilatory muscles. Conceptually, mechanical ventilatory support can be either total or partial. With total support, the mechanical device is designed to provide virtually all the work of breathing. Although patient effort may be present and may trigger ventilator breaths or even provide a small number of spontaneous breaths, total support should provide virtually all needed minute ventilation, with minimal patient contributions. In contrast, with partial support, the mechanical device is designed to only partially unload ventilatory muscles, requiring the patient to provide the remainder of the work of breathing. In general, total support is used in acute respiratory failure when the patient's muscles are overloaded or fatigued or when gas exchange is very unstable or unreliable. Partial support is generally used in less severe forms of respiratory failure (especially during the recovery or weaning phase). This chapter focuses on positive-pressure ventilation designed to provide total support.
Device Design Features for Total Ventilatory Support
Positive-Pressure Breath Controller
Most modern ventilators use piston-bellows systems or high-pressure gas sources to drive gas flow. Tidal breaths are generated by this gas flow and can be classified in terms of what initiates the breath (trigger variable), what controls gas delivery during the breath (target or limit variable), and what terminates the breath (cycle variable). During total support, breaths can be initiated (triggered) by patient effort (assisted breaths) or by the machine timer (controlled breaths). Target or limit variables are generally either a set flow or a set inspiratory pressure. With flow targeting, the ventilator adjusts pressure to maintain a clinician-determined flow pattern; with pressure targeting, the ventilator adjusts flow to maintain a clinician-determined inspiratory pressure. Cycle variables are generally a set volume or a set inspiratory time. Breaths can also be cycled if pressure limits are exceeded. The four common breath types supplied by modern mechanical ventilators to provide total support are volume control (VC), volume assist (VA), pressure control (PC), and pressure assist (PA). These breaths are classified by their trigger, target, and cycle features in Figure 58-2-1 .
Mode Controller
The availability and delivery logic of different breath types define the mode of mechanical ventilatory support. The mode controller is an electronic, pneumatic, or microprocessor-based system designed to provide the proper combination of breaths according to set algorithms and feedback data (conditional variables). For total support, the most commonly used modes are volume assist-control and pressure assist-control. Synchronized intermittent mandatory ventilation (SIMV) can provide VA and VC or PA and PC breaths interspersed with either unsupported or partially supported spontaneous breaths (volume-targeted SIMV and pressure-targeted SIMV, respectively). When the SIMV machine breath rate is set sufficiently high, the bulk of the work required for the desired delivered minute ventilation is borne by the ventilator such that these modes can be considered to provide virtual total support. A variation on the SIMV approach is to use a pressure-targeted mode with a long inspiratory time/short expiratory time pattern and allow spontaneous breaths to occur during the long inflation phase. This approach goes by a variety of proprietary names but is most commonly referred to as airway pressure release ventilation (APRV). These modes are summarized according to available breath types in Table 58-2-1 .
T able 58-2-1
NIH ARDS Network PEEP-F io 2 Tables
Data from Lellouche F, Brochard L. Advanced closed loops during mechanical ventilation (PAV, NAVA, ASV, SmartCare). Best Pract Res Clin Anaesthesiol . 2009;23(1):81–93.
| Conservative PEEP Approach | |||||||||||||||||
| F io 2 | 30 | 40 | 40 | 50 | 50 | 60 | 70 | 70 | 70 | 80 | 90 | 90 | 90 | 1.0 | 1.0 | 1.0 | 1.0 |
| PEEP | 5 | 5 | 8 | 8 | 10 | 10 | 10 | 12 | 14 | 14 | 14 | 16 | 18 | 18 | 20 | 22 | 24 |
| Liberal PEEP Approach | |||||||||||||||||
| F io 2 | 30 | 30 | 40 | 40 | 50 | 50 | 60 | 60 | 70 | 80 | 80 | 90 | 1.0 | 1.0 | |||
| PEEP | 12 | 14 | 14 | 16 | 16 | 18 | 18 | 20 | 20 | 20 | 22 | 22 | 22 | 24 | |||
Targets: P o 2 55–80, Spo 2 88%–95%. Move up one step if below target, down one step if above target. F io 2 , fraction of inspired oxygen; PEEP, positive end-expiratory pressure (cm H 2 O).
New ventilator designs incorporate advanced monitoring and feedback functions into these controllers to allow continuous adjustments in mode algorithms as the patient's condition changes. The most common of these new feedback designs is the addition of a volume target backup to pressure assist-control, termed pressure-regulated volume control (PRVC). This feature adjusts the inspiratory pressure level above or below the clinician-set target to achieve the volume target. A more sophisticated feedback system for pressure-targeted breaths calculates a frequency–tidal volume combination that requires the least ventilator work for the desired minute ventilation. Known as adaptive support ventilation (ASV), this mode also incorporates a calculation of the expiratory time constant to assure that an expiratory time to minimize air trapping is also present. Finally, two new modes that are driven entirely by patient effort can be set to provide virtually all the work of breathing and thus could be considered forms of total support. One is proportional assist ventilation (PAV), which drives ventilator gas flow as a proportion of patient flow demand; the other is neurally adjusted ventilator assistance (NAVA), which drives ventilator gas flow as a proportion of the diaphragmatic electromyogram signal.
Other Device Features Supporting Mechanical Ventilation
Effort sensors are pressure and/or flow transducers in the ventilator circuitry that detect patient breathing efforts and are characterized by their sensitivity and responsiveness. Blenders mix air and O 2 to produce a delivered inspired O 2 fraction (F io 2 ) from 0.21 to 1.0. On newer systems, blenders are also available for other gases such as heliox, nitric oxide, and anesthetic agents. Humidifiers adjust blended gas mixtures to approximate body conditions using either passive heat-moisture exchangers in the circuitry or active systems that add heat and moisture directly. Positive end-expiratory pressure (PEEP) is usually applied by regulating pressure in the expiratory valve of the ventilator system, but a continuous flow of source gas during the expiratory phase can produce a similar effect. The gas delivery circuit consists of flexible tubing that often has pressure or flow sensors and an exhalation valve. It is important to remember that this tubing has measurable compliance (generally 1–4 mL/cm H 2 O), and significant amounts of delivered gas may only distend this circuitry rather than enter the patient's lungs when high airway pressures are encountered.
Physiologic Effects of Positive-Pressure Mechanical Ventilation
Equation of Motion
Lung inflation during mechanical ventilation occurs when pressure and flow are applied at the airway opening. These applied forces interact with respiratory system compliance (both lung and chest wall components), airway resistance, and to a lesser extent, respiratory system inertance and lung tissue resistance to effect gas flow. For simplicity's sake, because inertance and tissue resistance are relatively small, they can be ignored, and the interactions of pressure, flow, and volume with respiratory system mechanics can be expressed by the simplified equation of motion:
In a mechanically ventilated patient, this relationship is expressed as:
where dPAO is the change in pressure above baseline at the airway opening; V′ is the flow into the patient's lungs; R is the resistance of the circuit, artificial airway, and natural airways; V T is the tidal volume; and CRS is the respiratory system compliance.
By performing an inspiratory hold at end-inspiration (i.e., no-flow conditions: V′ = 0), the components of dPAO required for flow and for respiratory system distention can be separated. Specifically, when V′ = 0 at end-inspiration, dPAO is referred to as a “plateau” pressure and reflects the static respiratory system compliance (CRS = V T /dPAOplateau). Adding dPAO to the baseline pressure gives the total respiratory system distending pressure at end-inspiration (dPAOplateau + baseline pressure = PAOplateau). Calculating the difference in dPAO during flow and during no-flow (the “peak to plateau difference”) allows the calculation of inspiratory airway resistance (R = dPAOpeak – dPAOplateau/V′).
Separating chest wall and lung compliance (CCW and CL, respectively) during a passive, machine-controlled positive-pressure breath requires an esophageal pressure measurement (Pes) to approximate pleural pressure. With this measurement, the inspiratory change in Pes (dPes) can be used in the following calculations: CCW = V T /dPes, and CL = V T /(dPAO − dPes). In clinical practice, because CCW is usually quite high and dPes is thus quite low, dPAOplateau and PAOplateau are often taken as an approximation of lung distending pressure. However, in situations in which CCW is reduced (e.g., obesity, anasarca, ascites, surgical dressings), the stiff chest wall can have a significant effect on dPAOplateau and PAOplateau and must therefore be considered when using these measurements to assess lung stretch.
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