Intubation & Ventilation How do I decide?

      
       

 

          
       

In general, patients require mechanical ventilation due to airway problems, failure to ventilate or failure to oxygenate. Often all three problems exist simultaneously: a patient who has taken an overdose of opioids, will have diminished level of consciousness, will hypoventilate, leading to increased PaCO2, and will become hypoxemic, due to alveolar CO2 displacing alveolar O2.

Indications for intubation:

The main indication for intubation is airway protection / control of airway: when in doubt intubate! Such circumstances may be:

1.   Loss of gag/cough reflex e.g. head injury with GCS <8 (to prevent massive aspiration).

2.  Airway obstruction: acute laryngeal edema – e.g. inhalation burn, Ludwig’s angina, epiglottitis.

3.  Anticipated loss of control of the airway: anticipated laryngeal edema– e.g. neck trauma, acute stridor etc.

Patients are usually intubated for controlled mechanical ventilation as an endotracheal tube or tracheostomy will provide a good seal for controlled ventilation: inspired volumes and pressures are consistent; compared with non invasive methods. Finally, the presence of an artificial airway facilitates removal of obstructive material from the airway (airway toilet – suctioning of secretions).

Indications for mechanical ventilation:

  • Ventilation Failure
  • Oxygenation Failure

Failure to Ventilate

Characterized by reduced alveolar ventilation which manifests as an increase in the PaCO2 > 50 mmHg. The best method of classifying .this is to follow the respiratory  pathways from the brainstem to the alveoli, and then ask whether a pathology exists at each particular site. Often patients have multiple problems: e.g. narcosis, pulmonary edema, pleural effusion, obesity

Neurological Problems

Central: loss of ventilatory drive due to sedation, narcosis, stroke or brain injury.

Spinal: spinal cord injury, cervical – loss of diaphragmatic function, thoracic – loss of intercostals.

Peripheral: nerve injury (e.g. phrenic nerve in surgery), Guillain-Barre syndrome (demyelination), poliomyelitis, motor neurone disease.

Muscular Problems

Myopathic disorders – myasthenia gravis, steroid induced myopathy, protein malnutrition.

Anatomical Problems

Chest Wall – rib fractures or flail chest, obesity, abdominal hypertension, restrictive dressings

Pleura – pleural effusions, pneumothorax, hemothorax.

Airways – airway obstruction (in lumen, in wall, outside wall), laryngeal edema, inhalation of a foreign object, bronchospasm.

Gas Exchange Problems

Ventilation perfusion mismatch, particularly increased alveolar deadspace (often due to hyperventilation), acute lung injury (ALI), lung contusion.

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Failure to Oxygenate

Oxygenation failure (leading to hypoxemia) most often occurs at a microscopic level at pulmonary capillary-alveolar interface. Classically injuries are divide up into diffusion defects and ventilation perfusion mismatch, of which pure dead space ventilation is at one extreme (alveoli are perfused but not ventilated) and shunt is at the other (alveoli are ventilated but not perfused). Often, in acute lung injury, a variety or abnormalities are present in the same lung. Oxygenation failure may also occur at the cellular level, due to poisoning (cellular hypoxia)

Diffusion abnormality – this is caused by thickening of the alveoli (pulmonary fibrosis) or increased extracellular fluid – pulmonary edema. This obstructs gas exchange. As the passage of oxygen from alveolus to artery is more difficult, hypoxemia may result, particularly with rapid heart rates - there is insufficient time for blood to oxygenate.

Ventilation/Perfusion Mismatch:

Dead Space Ventilation  (or high V/Q)– Alveoli are ventilated but not perfused. An extreme example of this is a pulmonary embolus. More frequently, rapid shallow breathing (tidal volumes of <250ml) leads to increased alveolar dead space, and hypercarbia (wasted ventilation).
Dead space may be anatomical - the conducting airways (approximately 150ml -wasted ventilation is an example of ventilating predominantly the airways) or physiological, for example in hemorrhage or hypotension, perfusion pressure to apical lung units may fall, leading to alveolar dead space.

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Shunt (or low V/Q)– where alveoli are perfused but not ventilated: well oxygenated blood becomes mixed with deoxygenated blood. This occurs in airway collapse, pneumonia, pulmonary hemorrhage (contusion), ARDS/ALI. In acute lung injury (ALI), intermittent alveolar collapse in expiration often leads to hypoxemia, due to shunt:

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Click here for more information on ventilation-perfusion mismatch

In addition, there may be a problem with oxygen delivery and utilization: if the cardiac output is low, if the patient is edematous of if a specific pathology interferes with the normal processes.

Inability to extract at cellular level – sepsis, cyanide or carbon monoxide poisoning

It is important to remember that not all gas exchange abnormalities occur due to lung pathology:

  • Cardiac disease – poor LV function – pulmonary edema, decreased lung compliance
  • Upper abdominal/thoracic surgery/abdominal hypertension– splinting, hypoventilation and airway collapse.
  • Gastric distension and aspiration
  • Metabolic issues – muscle wasting, hypo- kalemia, hypo- magnesemia, hypo-phosphatemia, hypothyroidism, Cushing’s syndrome.

Obviously, none of these indications is absolute, there may be reversible causes, and each patient must be evaluated individually. Many patients will respond to CPAP or other forms of non-invasive ventilation. Nonetheless, if there is any doubt, intubation is the safest course, the complications are considerably less than those of hypoxic insults.

Copyright Patrick Neligan 2001-2002

          
                   
       

         
     

       
       

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