Description
- In the US, 1–3 million patients annually require intubation and mechanical ventilation outside of the OR. This decision typically stems from the need to maintain airway patency, protection against aspiration, or provide oxygenation and ventilation for pathologic pulmonary processes.
- In the OR, intubation is typically dependent on the surgical procedure and patient comorbidities; apnea or respiratory depression is drug-induced.
- The decision to extubate is of considerable consequence, as delayed extubation is associated with increased duration of mechanical ventilation and increased mortality. Criteria include subjective, as well as objective parameters.
- However, because intubation is typically performed to facilitate the surgical procedure, extubation criteria centers around adequate reversal of neuromuscular blocking drugs and the return of airway reflexes from perioperative medications.
- Normal breathing is described as negative pressure ventilation. The diaphragm and chest wall generate negative intrapleural pressures by moving caudally and with chest wall expansion, respectively. Air movement, with resultant lung expansion, results from the creation of a pressure gradient from the oropharynx to the alveoli.
- Mechanical ventilation, on the other hand, is described as positive pressure ventilation (PPV), and has effects on the pulmonary and cardiovascular system.
- Pulmonary effects of mechanical ventilation may include
- Improvement in oxygenation, lung compliance, and V/Q matching
- Increased functional residual capacity (FRC) with resultant decreases in shunt and improved oxygenation.
- Decreased work of breathing (WOB)
- Improved respiratory acidosis
- Cardiovascular effects of mechanical ventilation may include
- The larynx is a cartilaginous skeleton with ligaments and muscle. There are nine cartilages including the thyroid, cricoid, epiglottis, arytenoids (paired), corniculate (paired), and cuneiform (paired).
- Sensory innervation
- Nose mucous membrane: Ophthalmic branch (V1) and the maxillary branch (V2) of the trigeminal nerve
- Hard and soft palate: Palatine nerves from the trigeminal nerve
- Anterior 2/3rd of tongue: Lingual nerve; a branch of the mandibular branch (V3) of the trigeminal
- Posterior 1/3rd of tongue: Glossopharyngeal nerve (CN IX)
- Sensation of taste: Anterior 2/3rd by the facial nerve (CN VII), posterior 1/3rd by the glossopharyngeal nerve
- Below the epiglottis to above the vocal cords: Internal branch of the superior laryngeal nerve (SLN), a branch of the vagus (CN X)
- Below the vocal cords: Recurrent laryngeal nerve (RLN), a branch of the vagus nerve
- Motor innervation
- Cricothyroid muscle (vocal cord adductors): External branch of the SLN.
- All other muscles of the larynx: RLN (posterior cricoarytenoid muscle is an abductor, the remaining muscles are all vocal cord adductors).
- Blood supply is via branches of the thyroid arteries. The external carotid artery gives rise to the superior thyroid artery that branches to the cricothyroid artery. During cricothyrotomy, it is best to stay midline to avoid these vessels.
Physiology/Pathophysiology
- Intubation complications include airway trauma, physiologic responses like hypertension, tachycardia, increased ocular pressures, arrhythmias, aspiration, and tube malposition.
- Mechanical ventilation (PPV) complications include
- Decreased cardiac output (CO) from diminished venous return; increased intrathoracic pressure decreases end-diastolic volume and stroke volume of both ventricles.
- Increased pulmonary vascular resistance (PVR) resulting in increased right ventricular load; this may limit left ventricular distensibility and CO.
- Extubation complications include hypertension, tachycardia, arrhythmias, coughing, breath holding, laryngospasm, airway obstruction, aspiration, increased ocular pressure, sore throat tracheomalacia, and laryngotracheal stenosis.
- Vocal cord dysfunction may require intubation to maintain a patent airway
- Bilateral paralysis of the SLN: Hoarseness and easy tiring of voice. No airway compromise.
- Bilateral partial transection: The abductor portion is more prone to injury and results in unopposed adduction. The vocal cords meet in the midline and cause complete airway obstruction.
- Bilateral paralysis of the vagus nerve: Affects both SLN and RLN with resultant flaccid, midpositioned vocal cords. Airway control is not a problem, but phonation is affected significantly (neuromuscular blocking drugs).
- General intubation criteria (any 1 of the following):
- Cardiac or respiratory arrest
- Loss of consciousness
- Hemodynamic instability <70 mm Hg
- PaO2 <45 mg despite supplemental oxygen
- Objective intubation indications (
2 of the following, in the context of respiratory distress): - Intubation indications in the intensive care unit (ICU)
- Airway patency
- Aspiration protection
- Tracheobronchial toilet
- Route for airway pressure therapy
- Neurological: central loss of ventilatory drive, spinal cord injury, brain injury, Guillain–Barré syndrome (GBS), phrenic nerve injury
- Chest wall: flail chest, rib fractures
- Intubation indications in the OR settings
- Airway control
- Unobstructed leak-free airway for prolonged ventilation
- Thoracoabdominal surgery
- Head and neck surgery
- Risk of aspiration
- Positioning (prone, lateral decubitus)
- Resuscitation of the moribund patient
- Preemptive utility if feared that ventilation/intubation may later become impossible.
- Weaning from extubation should be considered as early as possible in both the ICU and the OR. In the ICU, the majority are successfully weaned on first attempt (physicians frequently underestimate the ability of patients to be successfully weaned). In the ICU, the initial weaning is a two-step process and begins with assessment of readiness to wean, followed by suitability to extubate.
- Readiness to wean
- Clinical assessment for an intact cough reflex, absence of excessive tracheobronchial secretions, and resolution of acute phase of disease causing the intubation
- Objective assessment with a rapid shallow breathing index (RSBI). It is calculated as RR/VT. A value of <100–105 breaths/min/L predicts successful spontaneous breathing test (SBT) with a sensitivity of 0.97 and specificity of 0.65.
- Suitability to extubate can be assessed objectively with the SBT. The initial SBT should last for 30 minutes and consists of T-tube breathing or low levels of PS (5–8 cm H2O) with or without 5 cm H2O PEEP while making assessments of
- Adequate mentation
- Stable cardiovascular status (heart rate (HR)
140 bpm, SBP 90–160 mm Hg, minimal or no vasopressors) - Stable metabolic status
- Adequate oxygenation
- SaO2 >90% on fraction of inspired oxygen (FiO2) 40% or PaO2/FiO2150 mm,
- PEEP 8 cm H20
- SaO2 >90% on fraction of inspired oxygen (FiO2)
- Adequate ventilation
- RR 35 breath/min
- Mean inspiratory pressure (MIP) 20–25 cm H2O
- VT >5 mL/kg
- Vital capacity >10 mL/kg
- RR
- Failed SBT criteria include
- RR >35 breaths/minute for >5 minutes
- SaO2 <90% for >30 seconds or PaO250–60 mm Hg on FiO250%
- PaCO2 >50 mm Hg
- HR >140 bpm
- SBP >180 or <90 mm Hg
- pH <7.32
- Increased WOB, cyanosis
- Agitation, anxiety
- Depressed mental status
- Cardiac arrhythmias
- Causes of failed SBTs include
- Psychological dysfunction (anxiety, agitation, delirium)
- Infections
- Electrolyte abnormalities
- Respiratory load (COPD, asthma, pulmonary edema, infiltrates, secretions, edema)
- Cardiac load (ischemia, increased metabolic demand, CHF, anemia)
- Nutrition (deconditioning, malnutrition, obesity)
- Neuromuscular disease (depressed central drive, GBS, myasthenia gravis)
- Endocrine (hypothyroidism, hyper/hypoglycemia, adrenal insufficiency)
- Pressure support ventilation (PSV) as a weaning mode after a failed SBT has strong support in the literature. The literature does not support the use of synchronized intermittent mandatory ventilation (SIMV) alone for weaning. Little data exists for the use of SIMV/PSV combined.
- Noninvasive ventilation may shorten the duration of intubation in patients with hypercapnic respiratory failure. However, in hypoxic respiratory failure, it could be used with caution in the event of extubation failure.
- Cuff-leak test involves deflating the cuff or occluding the ETT, and observing if the patient can breathe around the tube.
- Failed leak test does not mean failed extubation, but that if extubation is performed, they may require close monitoring.
- A positive leak test does not guarantee successful extubation.
- Reasons not to extubate, even when the patient meets extubation criteria include unstable C-spine, inability to protect airway, likely to return to the OR, and potentially difficult reintubation.
- Tracheostomy may be required in patients who fail extubation, as prolonged intubation may cause airway edema and injury.
- Advantages include decreased WOB, comfort, patient communication, and pulmonary hygiene.
- Disadvantages include risk of tracheal stenosis, infection, and procedure related complications.
- Extubation criteria in the OR has unique challenges of its own
- Adequate oxygenation
- Adequate ventilation
- Full reversal of neuromuscular blockage (5 second head lift, train-of-four, sustained tetany)
- Hemodynamic stability
- Mental status intact
- Return of reflexes (cough, gag)
- Adequate acid–base status
- Normothermia
- Adequate pain control
- Stable metabolic status
- Adequate volume status
- Electrolytes
- Potential difficult extubations postoperatively include
- History of a difficult intubation
- Sleep apnea
- Airway edema
- Postsurgical complications from thyroid surgery (RLN injury risk), diagnostic laryngoscopy, uvulopalatoplasty, carotid endarterectomy (hematoma, nerve palsies), cervical spine procedures, and maxillofacial surgery.