Believe it or not we don't intubate people to put them on a ventilator. Rather we put them on the ventilator because we have had to intubate them.
So why do we need to intubate them?
Let's work our way through using an ABCDE approach.
Airway
The patients airway may be compromised. This could be initially supported with other airway adjuncts such as a guedel airway, but it might need more than that if the situation demands it.
There could be TRAUMA which has deformed or obstructed the airway. The endotracheal tube will then enable us to bypass that obstruction or deformity.
There may be FOREIGN BODIES in the airway which are causing an issue. I extend this to mean substances such as vomit and blood in the airway which normally should not be there. If a patient vomits and aspirates that vomit, for example, they can become very unwell.
The TONGUE is one of the major reasons the airway can become obstructed. This could be due to some swelling of the tongue- ANAPHYLAXIS for example- or due to the patients lowered level of consciousness causing their muscle tone to be reduced enabling the tongue to fall backwards.
Breathing
We might need to intubate the patient because they are no longer able to breathe effectively.
Remember here that we are talking about ventilation and oxygenation when discussing the patients ability to breathe.
An inability to oxygenate, resulting in low oxygen saturations or low PaO2 is known as a TYPE I failure. The oxygen is not getting into the patients circulation in sufficient quantity. This could be due to a PNEUMONIA, or due to some PULMONARY OEDEMA which is compromising the ability of the lung to absorb that oxygen and diffuse it into the blood.
An inability to ventilate- and in this context we mean the patients ability to move the gases in and out of the lung itself- will result in a TYPE II failure. In this situation they are not able to remove the waste gas CO2 sufficently with each breath, resulting in a gradual rise in the levels. This situation can often arise due to chronic conditions such as COPD. For many reasons we try not to ventilate those with this condition, but if the CO2 rises high enough they will drop their level of consciousness resulting in them breathing even less effectively and not protecting their airway.
Intubation then becomes the only option.
Circulation
Perhaps the oxygen can no longer get in to the patients blood, not because there is a problem with the lung, but because there is one with their circulation.
A good example of this would be the PULMONARY EMBOLISM where there is an obstruction in one of the vessels within the lung circulation. This means that blood will not flow past some of the alveoli so no gas exchange will take place. In most cases this does not require intubation, but if very large then the patient will need very high concentrations of oxygen, and eventually this may be best delivered by a ventilator whilst we support the patient.
It may also be that the patient is suffering from a degree of SHOCK. This will result in inflammation and vasodilation which can eventually result in end organ damage. The main organs involved will be the lungs (ARDS), heart, kidneys, gut and liver and given time this will overwhelm the patient. They will tire, may become less responsive and will then require intubation and ventilation to support them whilst we treat the cause of their shock.
Remember the different types of shock can all lead to this situation- distributive shock and obstructive shock.
Disability
The patient may no longer be able to breathe for themselves because of a problem with their brain. Their LEVEL OF CONSCIOUSNESS has fallen.
Why might this have happened though?
It may be due to a head injury or some kind of intracerebral event such as a bleed or ischaemic event. This is then compromising the parts of the brain that control respiration.
It could be due to the deliberate or accidental ingestion of drugs which sedate the patient such as in a drug overdose. Whilst we wait for the effects of these drugs to wear off we need to support the patients airway and supplement their respiratory function.
Nuero muscular disorders can sometimes compromise the patients ability to breathe effectively. Conditions such as guillain barre syndrome and myasthenia gravis can have this effect and the patient will require respiratory support.