AIRWAY GLOSSARY

Airway

A generic term covering anything from the lips to the alveoli, although often divided into the upper airway (oral cavity, glottis, trachea) and the lower airway (bronchi down to alveoli). The airway is the top priority in all clinical emergencies, as in the following generic priority sequence:

(1) Airway
(2) Breathing
(3) Circulating
(4) Drugs

Airway Burns

Singed nose hairs or nasal soot in patients who have suffered burns elsewhere makes the wise clinician suspicious for airway burns. Such patients may develop airway edema that could be life-threatening and that requires intubation to ensure a secure airway. The airway damage can best be assessed by fiberoptic bronchoscopy (principally looking for airway edema). Carbon monoxide levels should be measured in all such patients and 100% oxygen should be given to drive off the carboxy molecules from the hemoglobin binding sites.

Airway Obstruction

Airway obstruction frequently occurs following the induction of general anaesthesia. Traditionally this has been attributed to the tongue falling back against the posterior pharyngeal wall (and is remedied by placing an appropriately sized oropharyngeal airway). Recent studies suggest that obstruction by the soft palate or epiglottis from reduced airway tone may also be responsible. Hypoventilation may result from airway obstruction, leading to hypercarbia and then hypoxemia.

Airway obstruction can also occur from airway edema (e.g., following extensive head and neck surgery or anaphylaxis), from airway infections (e.g., epiglottitis), from airway tumors( laryngeal papillamotosis), or from foreign bodies (e.g., aspirated hunk of steak).

Alveolar Ventilation ()

This is the volume of gas per minute that takes part in gas exchange

= (VT - VD) x f

where VT is the tidal volume, VD is the respiratory deadspace, and f is the breathing rate (respiratory frequency). Alveolar ventilation may be reduced with airway obstruction or low respiratory rates (such as are encountered with excessive doses of narcotic analgesics such as morphine). Reduced alveolar ventilation leads to hypercarbia, and possible hypoxemia. Hypoxemia due to hypoventilation is far less likely when the patient is receiving oxygen.

Apnea

Complete cessation of breathing. Obviously apnea must be managed promptly to avoid hypercarbia and hypoxemia. Ultimately, apnea leads to cardiac arrest preceeded by bradycardia (usually).

ARDS (Adult Respiratory Distress Syndrome)

ARDS is a sometimes deadly lung problem following trauma, aspiration pneumonitis and countless other clinical insults, resulting in small, stiff, hard-to-ventilate lungs. This deadly respiratory condition is often frequented in the ICU, with ventilators straining away to push gas into small stiff lungs.

The syndrome histologically is similar to respiratory disease of the premature newborn (RDS), hence its name, but rather that resulting from pulmonary immaturity, ARDS is the consequence of many clinical insults, aspiration pneumonitis, massive trauma, and septicemia being common examples.

An important part of ARDS therapy is to distend the lung to make it bigger so that more alveoli are open. This is done using PEEP (Positive End Expiratory Pressure), a technique where the airway pressure waveform is maintained positive (e.g., 10 cm H2O) even during expiration. PEEP therapy impairs venous return to the heart, making it problematic in hypovolemic patients.Another problem with ARDS is leaking pulmonary capillaries that may lead to pulmonary edema. This necessitates careful fluid management and sometimes requires that PA line monitoring (Swan-Ganz catheter) be used.

ARDS carries with it three management issues:

  1. oxygenation
  2. ventilation
  3. fluid management
  4. intubation

The high degree of pulmonary shunt present is ARDS necessitates that two ventilator parameters (FIO2 and PEEP) be adjusted upward jointly to achieve adequate oxygenation (eg. SaO2 > 0.9 of PaO2 > 60 mmHg). Fio2 is the fraction of inspired oxygen (eg. 0.5 for 50% oxygen). PEEP is the Positive End Expiratory Pressure of minimal lung distending pressure (eg. 5 cm H2O). FIO2 and PEEP are not adjusted independently but rather jointly, even sometimes by quantitative linking (eg. PEEP = 15 FIO2). Patients that are hypovolemic may tolerate PEEP less well and require less strong linkage to FIO2.

Ventilation may be difficult in ARDS patients since high ventilating pressures are required to achieve normal tidal volumes. The high ventilating pressures required are suspected to be a cause of more lung injury, leading some researchers to advocate the use of "permissive hypercapnia" a ventilating protocol allowing higher PCO2 levels ad more acidotic pH levels but with much smaller tidal volumes and airway pressures, and less potential for barotrauma. Special ventilation modes such as inverse ratio ventilation (EI ratio < I), airway pressure release ventilation, and high frequency jet ventilation.

ARDS has associated with it increases in vascular permeability (leaky capillaries) that mandates special care regarding fluid management. Hypovolemia can be avoided by monitoring urine output and cardiac filling pressures (CVP, PCWP).

Patients with ARDS require intubation. High cuff pressures, required when high ventilating pressures are used, may damage tracheal mucosa and cause long term injury (eg. Tracheal stenosis from scarring). After 10 to 14 days of intubation, consideration is given to a formal tracheostomy. Tracheal suction is also needed to avoid buildup of secretions. Reintubation of patients with ARDS may be necessary because of cuff leaks or because of the buildup of hardened secretions within the lumen of the ETT.

Aspiration

Refers to getting harmful debris [usually gastric in origin] into the lung's trachea and bronchi with the considerable potential to cause aspiration pneumonitis, a often deadly inflammatory reaction leading to ARDS. Aspiration pneumonitis is said to be more likely when the volume of aspirate exceeds 25 ml and when it has a pH under 2.5. Techniques to reduce the likelihood of aspiration with ETT insertion include (1) pharmacological adjuncts such as gastric motility agents, (2) use of awake intubation (airway reflexes remain intact), and (3) use of a rapid sequence induction technique employing preoxygenation, predetermined drug doses and cricoid pressure.

Asthma Treatment

Medications used in asthma management include:

  1. Beta-agonists
  2. Theophylline products
  3. Corticosteroids
  4. Anti-cholinergic medications

In addition, number of anaesthetic agents (eg. halothane, ketamine) have a salutory effect in asthma.

Inhaled Beta-adrenergic agonists such as albuterol (salbutamol), terbutaline, fenoterol, pirbuterol or salmeterol remain the mainstay of treatment; although, their role in the chronic treatment of asthma is in a flux, with many clinicians preferring the use of inhaled corticosteroids as first-line therapy and with beta-agonists reserved for "prn" use.

Intravenously administered theophylline products such as aminophylline are waning in clinical popularity since they have a very poor toxic/therapeutic index and do not add significant clinical benefit in patients already receiving beta-agonists for an acute exacerbation of asthma. Its main role is not in preventing acute attacks in the chronic asthmatic, especially at night. In patients with COPD, improvements in diaphragmatic function and in muco-ciliary clearance make Theophylline a useful adjunct as well.

Corticosteroids are now taking on a new importance in treating patients with reactive airways disease ( RAD) as the inflammatory element in RAD is more appreciated. Inhaled corticosteroids are now often used as first-line asthma therapy while intravenous steroids are often used preoperatively in patients in with moderately severe asthma or in those previously requiring steroid therapy. However, in contrast to beta-agonists, whose clinical effect is almost immediate, IV steroids take several hours to work.

Awake Intubation

The process of inserting an endotracheal tube (ETT) through a patient's vocal cords into his or her trachea while the patient is conscious. Patient cooperation is far more easily achieved by using local anaesthesia or nerve blocks (for example, using lidocaine in a dose not exceeding 5 mg/kg). Awake intubation has the advantage that the anesthetist can always "back off" if intubation becomes difficult, and no bridges have been burned by giving potent, potentially dangerous drugs to the patient (e.g. succinylcholine). Awake intubation is often carried out without drugs in newborns with aspirated meconium (fetal feces) or in adults who are moribund. Otherwise local airway anesthesia should be used.

"Bail-Out" Algorithm (to awaken patient after failed intubation)

A strategy used when mask ventilation is becoming difficult following the induction of general anesthesia complicated by unsuccessful intubation. This is a setting where you want the patient to wake up and breath spontaneously.

1. Ensure that the patient is not in laryngospasm and that the patient's head and jaw are positioned properly. Call for help.

2. Insert an airway of some kind

A - oral airway

B - nasopharyngeal airway

C - LMA (Laryngeal Mask Airway)

WARNING: Airway insertion may lead to laryngospasm in lightly anesthetized patients

3. Utilize a two-person technique whereby one person manages the mask and holds the jaw in position using both hands, while the other ventilates the patient by hand using the rebreathing bag.

4. As a last resort, a surgical airway ( TTJV, cricothyroidotomy) is sometimes necessary.

Bradypnea

Decreased breathing rate, usually under 10 breaths per minute. This is often due to the administration of narcotic analgesics such as morphine, meperidine or fentanyl. Other causes may include increased intracranial pressure (eg from brain tumor). You should know the differential diagnosis of bradynea.

Can't Intubate Algorithm

This clinical algorithm applies when the the patient cannot be intubated (but can be ventilated adequately with bag/mask/valve resuscitator apparatus)

1. Wake the patient up and proceed with awake intubation

OR

  1. Insert ILM and then an ETT via the ILM (keep patient asleep)
    OR
  2. Use Syracuse-Patil face mask to facilitate fiberoptic intubation (keep patient asleep)

  1. REMEMBER TO CALL FOR HELP

Can't Ventilate Algorithm (patient intubated)

  1. Remember differential diagnosis:

2. Administer 100% O2; Check airway pressure and listen for wheezes and equal air entry bilaterally.

3. Rule out an ETT problem by passing an ETT suction catheter through the ETT after disconnecting from the patient breathing circuit.

  1. Rule out a ventilator problem by hooking up the ETT to a manual resuscitator (Ambu, Laerdal) instead of the automatic ventilator. Ventilation should be easy.
  2. If it is a patient problem, then consider such possibilities as bronchospasm or airway collapse according to the available clinical data. Collect information about ventilator settings, airway pressure parameters ( peak airway pressure, PEEP), the equality of air entry on both sides, the presence of crackles or wheezes on auscultation or factors such the use of a bronchial blocker (for one-lung ventilation) or the presence of an aspirated foreign body (e.g. aspirated peanut) or a pneumothorax (e.g. following CVP line insertion)

REMEMBER TO CALL FOR HELP

Cormack and Lehane Grading of View at Laryngoscopy

Grades III and IV are termed "difficult intubation"

Dead Space (VD)

Volume of gas delivered to the patient in inspiration that does not participate in gas exchange. VD may be estimated using the Bohr equation


where VT is the tidal volume, PECO2 is the mixed expiratory carbon dioxide tension, and PaCO2 is the arterial carbon dioxide tension.

Difficult Intubation (DI)

A situation where the patient is known or expected to be difficult to intubate using standard laryngoscopes. Such cases are often managed using a fiberoptic bronchoscope or Bullard laryngoscope to facilitate ETT placement, often with the patient awake.

Dyspnea

Shortness of breath.

Early Morning Sniffing Position

A patient head position that facilitates intubation by lining up the oral, pharyngeal and laryngeal axes. In the "sniffing position" the neck is flexed while the atlanto-occipital joint is extended. Placing a pillow under the shoulders and head to maintain this position is often helpful.

Endotracheal Tube (ETT)

A "breathing tube", often with a cuff at the distal end, allowing a sealed leak-free connection between a ventilator and a patient's trachea. Such an arrangement allows both positive pressure ventilation and spontaneous breathing. Typically, a size 7.5 (7.5 mm inner diameter) is used for adult women, with a size 8.5 for men. In some centers it is customary to cut the ETT prior to use (e.g. oral ETTs: women - cut at 23 cm; men - cut at 25 cm). The ETT cuff pressure should be kept below 25 cm H2O to prevent injury to the tracheal mucosa.

Specialized ETTs are available where ETT kinking is a concern, for special kinds of surgery (ENT surgery, laser surgery) or where one-lung ventilation is needed. Uncuffed tubes offer reduced protection against aspiration and are used in preadolescent children according to the following approximate guidelines:



ETVC Endotracheal Ventilation Catheter (Tube Exchanger)

A device used to facilitate reintubation following a trial of extubation. Prior to extubation the ETVC is placed into the ETT and the ETT withdrawn over it holding the ETVC in place. If reintubation becomes necessary the ETVC can be used as a guide to direct the new ETT through the cords. The ETVC can also be used to administer low flow oxygen deep into the lungs (eg. 2 l/min flow rate) as well as for capnography or even emergency jet ventilation in a manner similar to TTJV.

Extubation

The process of removing an ETT from the patient's trachea. This should ordinarily only be done with the patient awake and obeying verbal commands. Even so, catastrophies on extubation can occur, such as total collapse of the airway in a patient with tracheomalacia. Sometimes it is wise to extubate over an ETT exchange catheter.

Extubation Guidelines

1. Patient should be fully awake

2. Airway tone should be recovered with cough and gag reflexes intact

3. An ETVC should be employed if appropriate (difficult intubation patients)

4. Technical criteria should be satisfied in patients with poor respiratory function.

(a) Patient can maintain adequate oxygenation

(b) VC > 15 ml/kg

(c) NIF > 20 cm H2O

Facial Trauma

Victims of facial trauma may succumb from airway edema or loss of airway structural support. Intubation may be difficult in patients with Lefort I or II fractures (which puts bony fragments in the nasal airway). Oral intubation may be difficult because of trismus or even because of bizarre things such as a knife impaled into the neck. Furthermore, intubation may be complicated by distortions in the anatomy due to trauma or hematoma formation. Sometimes fiberoptic intubation works with such patients, but not if the airway is very bloody (poor view). Sometimes a tracheostomy under local anaesthesia is needed.

Head Trauma

Blunt and penetrating head trauma victims from motor vehicle accidents often require intubation to allow therapeutic hyperventilation to reduce cerebral edema. A PaCO2 level between 28 and 32 mmHg is often sought. In addition, these patients may need intubation simply to protect the airway against aspiration, as the patient's gag reflex may be obtunded from the head injury.

Hypoventilation

Ventilation inadequate to the body's metabolic needs, so that hypercarbia results.

Laryngeal Mask Airway (LMA)

A relatively new method of airway management that has become extremely popular in Europe (and to a lesser degree in North America). The LMA is a device that is seated over the glottis with the epiglottis often sitting in its bowl. Advantages of the LMA over the ETT include: ease of insertion, less stimulating to the airway and reusability. The main disadvantage of LMA is that it does not protect against aspiration or laryngospasm.

Laryngoscope

An instrument to provide illumination to the glottis so as to facilitate passing an ETT through the patient's vocal cords. Of course, laryngoscopes are also used to examine for any pathology (edema, bleeding, polyps, fibrosis). The most popular laryngoscope, the Macintosh design, is curved so that the end fits into the vallecula, lifting the epiglottis out of the way to expose the vocal cords. Special laryngoscopes also exist, such as the straight blade (Miller) design (passed posterior to the epiglottis, avoiding the vallecula, and the Bullard laryngoscope, often very helpful when mouth opening is quite limited.

Laryngoscopy

The art and science of viewing the larynx. First achieved indirectly using mirrors in the middle 1800s, direct laryngoscopy followed in the late 1800s / early 1900s to allow tracheal intubation. When laryngoscopy is performed for diagnostic or therapeutic purposes under general anaesthesia (eg. by propofol infusion) one's goals are to provide for relaxation of the jaw muscles and vocal cords during the procedure, with subsequent recovery of the laryngeal reflexes without incurring the wrath of laryngospasm. More commonly, however, laryngoscopy is carried out to allow intubation.

Management of Laryngospasm

Laryngospasm, the reflex closing of the glottis by the glottic musculature, is a protective mechanism provided by evolution that sometimes makes airway management difficult. Laryngospasm may occur from airway irritation such as might occur following excessive instrumentation of the airway or with secretions and blood irritating the vocal cords at light planes of anesthesia. Full laryngospasm may make ventilation impossible, at least until the muscles relax from the resulting severe hypoxia. While applying sustained positive pressure or deepening the anaesthetic with IV lidocaine or propofol, are sometimes effective in breaking laryngospasm, I prefer to use small doses of succinylcholine (as little as 10 mg will often do) to break laryngospasm when necessary. Always inspect inside the mask and mouth if laryngospasm occurs - sometimes laryngospasm is the first sign that the patient has aspirated some gastric contents.

Obesity

Obese patients are at increased risk of obstructive sleep apnea. Very obese patients may be difficult to intubate and even more difficult to ventilate after induction of anaesthesia, since anesthesia often leads to decreased muscle tone in the upper airway. This, in conjunction with redundant folds of oropharyngeal tissue, often leads to the tongue, soft palate and/or epiglottis obstructing the airway. Obese patients are also more prone to hypoxemia because of their small FRC and heavy chest wall.

Positive Pressure Ventilation (PPV)

The process of forcing gases down a patient's trachea using either a manual control technique or using an automatic ventilator. PPV can be done using a manual resuscitator or the rebreathing bag on the anaesthesia machine. But for long cases it makes more sense to use an automatic ventilator.

Pulse Oximetry

Since the commercialization of pulse oximetry two decades ago, thousands of lives have been saved by early detection of patient hypoxemia. Using the technologies of infrared spectroscopy and microprocessor-based signal processing, the pulse oximeter provides an indication of tissue oxygenation on an ongoing basis with a simple little finger probe not much bigger than a clothespin. Although pulse oximeters suffer form many pitfalls (eg. poor signals when patients are cold or vasoconstricted) they are an essential component to patient monitoring. No elective case should be started without a pulse oximeter. Pulse oximeters indicate the arterial blood saturation ie, the degree to which arterial blood hemoglobin binding sites are occupied with oxygen molecules. Patients are hypoxemic when arterial saturation falls under 90%.

Respiratory Compliance

A measure of the distensibility of the lung and chest wall, expressed as volume change per unit pressure change (ml/cm H2O)

Respiratory Failure

In its most basic form, PaCO2 too high or PaO2 too low.

Resuscitation

Resuscitation from cardiac arrest and other deadly situations often requires intubation both to supply high concentrations of oxygen and ventilate off carbon dioxide, as well as to protect the airway from being soiled by gastric contents.

Retrograde Intubation

Retrograde intubation involves passing a guidewire out the mouth via a puncture through the cricothyroid membrane. The guidewire is then strengthened by loading a sheath over it and passing an ETT into the trachea using the sheath as a guide. It offers special potential as a means of awake intubation in locations where fiberoptic intubation is unavailable (eg. third world countries).

Spontaneous Ventilation (SV)

Breathing using diaphragmatic +/- intercostal muscles. The diaphragm is innervated by C3 C4 C5 , so high cervical injuries are sometimes incompatible with SV and these patients may need diaphragmatic pacemakers for survival. SV can be disturbed in many ways: obstructive sleep apnea, airway infections (eg. epiglottitis), trauma to the airway etc. In such cases the use of positive pressure ventilation (PPV) may sometimes help.

Syracuse-Patil Face Mask

An anesthesia face mask with a side port for the introducion of a fiberoptic bronchoscope.

Tachypnea

Increased breathing rate. As lungs become stiff the patient finds it easier to take in smaller breaths and make up the difference with an increase in respiratory rate. Other causes may apply too, such as breathing in cardon dioxide in a bad rebreathing system, or sepsis syndrome. You should know the differential diagnosis of tachypnea.

Tidal Volume (VT)

Volume of gas delivered to the lungs during inspiration.

Transtracheal Jet Ventilation (TTJV - Needle Cricothyroidotomy)

In desperate circumstances injection of oxygen under high pressure directly into the trachea can be life-saving. This is done by inserting a #14 gauge IV catheter through the cricothyroid membrane and applying intermittent bursts of high-pressure oxygen through this catheter. A special nonkinkable needle for TTJV is available (Cook).

The original description of this technique, known as transtracheal jet ventilation (TTJV) suggested a 50 PSI pressure head, but clinical experience at this pressure shows that barotrauma (eg. pneumothoraces) are common at this pressure. A more reasonable amount might by 10 PSI; no one knows what the "best" choice is yet. Because of these concerns, many experts advocate the use of an emergency cricothyroidotomy kit. [Complications of TTJV include pneumothorax, pneumomediastinum, pneumopericardium, subcutaneous emphysema, esophageal perforation and infection.

Ventilators

Ventilators are used in operating rooms and intensive care units (ICU) for respiratory support of patients who cannot breathe on their own. ICU ventilators are more complicated and more flexible than OR ventilators. There are 5 main ventilator parameters.

Main Ventilator Parameters

1. Tidal Volume (eg. 700 ml) [Volume of gas injected into trachea with each breath]

2. Respiratory Rate (eg. 12 breaths/minute)

3. FIO2 (Fraction of Inspired Oxygen) (eg. 0.6 or 60% oxygen)

4. PEEP (Positive End Expiratory Pressure) (eg. 5 cm H2O)

5. I:E ratio (eg. 1:3)

Time for inspiration in relation to time for expiration


AIRWAY-RELATED ABBREVIATIONS

ABGArterial Blood Gases
CVPCentral Venous Pressure
DLTDouble Lumen ETT
ETTEndotracheal Tube
ICUIntensive Care Unit
ILMIntubating Laryngeal Mask
LMALaryngeal Mask Airway
PaCO2Arterial carbon dioxide level
PaO2Arterial oxygen level
PEEPPositive End Expiratory Pressure
PPVPositive Pressure Ventilation
SaO2Arterial oxygen saturation
TTJVTransTracheal Jet Ventilation