• Clinical science

Airway management and ventilation

Summary

Airway management is the evaluation, planning, and use of medical procedures and devices for the purpose of maintaining or restoring ventilation in a patient. These procedures are indicated in patients undergoing general anesthesia and in patients with respiratory failure or acute airway obstruction.

In endotracheal intubation, a tube is inserted orally into the trachea to provide oxygen via mechanical ventilation during general anesthesia. The tube should be placed under direct visualization with the help of a laryngoscope. Correct placement is established based on measurement of exhaled CO2 and evidence of bilateral breath sounds on auscultation. Complications of endotracheal intubation include dental damage, esophageal misplacement of the tube, infections, and tracheal stenosis. If long-term intubation is expected, a tracheostomy is usually performed. This procedure involves the creation of a reversible or permanent stoma below the cricoid cartilage. Cricothyrotomy is an airway of last resort and performed in emergency situations, such as acute obstruction of the airway by blood, edema or a foreign body. An incision is made in the membrane between the thyroid and cricoid cartilage to obtain airway access.

Once access to an airway has been established by any of the above mentioned routes, mechanical ventilation is used to assist or replace spontaneous breathing. The breathing rate, amount of oxygen per breath, applied pressure, and other parameters can be adjusted to achieve best possible ventilation and oxygenation. Complications of mechanical ventilation include barotrauma (e.g., pneumothorax, pneumomediastinum) and ventilator-induced lung injury, which clinically resembles ARDS. Once patients show sufficient spontaneous breathing, they are weaned off ventilation support.

Intubation

Endotracheal intubation

  • Definition: orotracheally or nasotracheally placement of a cuffed tube below the vocal cords
  • Indications
    • General anesthesia
    • Failure (or pending failure) of ventilation or oxygenation
    • Glasgow Coma Score ≤ 8
  • Procedure
    1. Preoxygenation: administration of 100% oxygen via face mask prior to intubation to ensure sufficient time to perform intubation.
    2. Sedation (e.g., propofol)
    3. Muscle relaxation (e.g., succinylcholine)
    4. Positioning of patient with mild cervical flexion (sniffing position)
    5. Placement of the endotracheal tube with the help of a laryngoscope: observe the tip of the tube passing into the larynx through the vocal cords (direct visualization).
    6. Signs of proper tube placement
      • Auscultation of bilateral breath sounds over lungs
      • Measurement of CO2 in the exhaled air (end-tidal CO2)
      • Distal tip should be 2–6 cm above carina (check with chest x-ray and reposition if necessary)
    7. Inflate the cuff to secure the tube if proper placement is ensured.

Complications of intubation

Extubation

References:[1][2][2][2][3][4][5]

Surgical airway management

Tracheostomy

Cricothyrotomy

References:[3][6][7][8]

Mechanical ventilation

Basics

  • Definition: use of a respirator to assist or completely replace spontaneous breathing
  • Indications
    • General anesthesia
    • Breathing support
      • Respiratory distress or arrest
      • Hypoxemia (PaO2 < 70 mm Hg) or hypercapnia (PaCO2 > 50 mm Hg)
      • Metabolic acidosis
      • Respiratory muscle fatigue
      • Impaired consciousness and inability to protect airways
  • Technique: : positive-pressure ventilation
  • Ventilation parameters
    • Tidal volume: amount of air passed through an endotracheal or tracheostomy tube per breath
      • Healthy lung: 8–12 mL/kg body weight
      • Lung-protective: 6–8 mL/kg body weight
    • Respiratory rate: approx. 10–15/min
    • Inspiratory oxygen concentration (FiO2)
      • Anesthetic induction: 100% (short-term)
      • During surgery: 30%–100%
    • Oxygenation improves with PEEP, FiO2 (additional oxygen supply)
    • Ventilation improves with respiratory rate, tidal volume
  • Capnometry/capnography: assessment of CO2 concentration in expired air to determine whether oxygenation is adequate
  • Weaning: Process of easing a patient off mechanical ventilatory support.
    • Spontaneous breathing trial
      • A test that is used to determine whether a mechanically ventilated patient is ready to breathe without a ventilator.
      • Performed once the patient has:
        • A stable/resolving lung disease
        • Normal PaO2 and O2 saturation on FiO2 < 0.4–0.5 and PEEP < 5–8 mm H2O,
        • A pH ≥ 7.35
        • Hemodynamic stability with little or no vasopressor therapy
        • An ability to spontaneously initiate an inspiratory effort (i.e., good neuromuscular function).

Ventilator settings

  • Assist control (AC) ventilation: guarantees delivery of a preset tidal volume
    • In this mode, breathing can be triggered by the patient or the ventilator.
  • Pressure-support ventilation: patients initiate breathing themselves and the ventilator delivers support with a preset pressure value ; used primarily for weaning purposes
  • Continuous positive airway pressure: (CPAP): positive pressure ventilation with a mask for patients with obstructive sleep apnea
  • Positive end-expiratory pressure (PEEP): ventilator setting that can be chosen to increase functional residual capacity (FRC) by reducing alveolar collapse during mechanical ventilation
    • Definition: Positive pressure (∼ 5 cm H2O) in the lung is maintained over the entire inhalation and expiration phases.
    • Mechanism of action
      • PEEP ↑ alveolar pressure and alveolar volume → collapsed or unstable alveoli reopen → improves ventilation/perfusion relation
      • Provides an adequate arterial PaO2 at a low and safe concentration of oxygen (< 60%) → reduces the risk of oxygen toxicity
    • Advantages
    • Side effects

Mechanical ventilation with a very high PEEP may result in pneumothorax!

Special scenarios

  • Suspected tube displacement: examination via auscultation of the thorax (lung fields in a side comparison) and abdomen (epigastrium)
  • Insufficient oxygenation: decrease in the peripherally measurable oxygen saturationFiO2
  • Alkalosis in BGA (hypocapnia): tidal volume, respiratory rate
  • Acidosis in BGA (hypercapnia)tidal volume, respiratory rate
  • Patients with COPD or ARDS: reduce tidal volume to prevent ventilator-induced lung injury (lung-protective ventilation), a condition clinically resembling ARDS
  • ↑ Intracranial pressure: raise head of bed and initiate hyperventilation < 30 minutes after onset

Bronchospasm

  • Definition: bronchoconstriction during anesthesia; causes severe hypoxia and hypotension, if left untreated
  • Epidemiology: incidence ∼ 0.2%
  • Risk factors
    • Smoking
    • Reactive airway disease (e.g., asthma, COPD)
    • Viral upper respiratory tract infection
  • Clinical features
  • Differential diagnosis: mechanical obstruction (e.g., blocked or misplaced tracheal tube), laryngospasm, anaphylaxis (e.g., caused by medication, blood products), pneumothorax, pulmonary embolism, or aspiration
  • Treatment
    • Basic measures
      • Discontinuation manipulating measures/surgery
      • Manual ventilation with a FiO2 of 100%
      • Deepen anesthesia
      • Exclude differential diagnosis
    • Pharmacotherapy in severe bronchospasm

References:[2][2][2][3][5][9][10][11]