Let us start by saying that mechanical ventilation is not a benign therapy. It is not a beautiful universally tolerated life-saving solution to any sort of respiratory failure. It is a crude and ugly mechanical torture. Imagine being captured by a stranger, drugged, tied to a bed and forcefully insufflated with pressurised gas. Needless to say, we reserve such treatment only for situations where it can be reasonably expected to have some positive effect, and where the alternative is death or severe disability. This chapter discusses the indications for mechanical ventilation, as well as situations where it may be inappropriate. It is important for the intensivist to be intimately familiar with the latter subject area; one needs to be able to justify the reason for withholding lifesaving treatment beyond "they're old and frail" or "the COPD is too severe". 

Indications for intubation and ventilation

When thinking out loud about the physiological effects of any therapy, at the forefront of discussion one should keep the question, what precisely is the physiological problem? In respiratory failure, only a few things could go wrong. In fact, only two. Lets face it, all of respiratory medicine boils down to two problems: either there is not enough oxygen, or there is too much carbon dioxide. Well, perhaps rather than say "too much" or "not enough", we could expand the definitions by saying "the wrong amount" but otherwise this  reductio  is pretty solid. Thus, one might ask: how did this happen? Several discrete disturbances exist.  These disturbances are influenced by a variety of factors, and it possible to influence those factors with positive pressure.  

Indications for intubation:

  • To overcome an airway obstruction and to protect the airway
  • To allow access to the lower airway for suctioning of secretions
  • To allow mechanical ventilation in a patient in whom non-invasive ventilation is contraindicated.

Indications for mechanical ventilation:

  • To manipulate PaO2 and PaCO2
  • To decrease the work of breathing (whether to reduce respiratory distress or to decrease total body oxygen demand)
  • To increase the functional residual capacity (FRC)
  • To stabilize the chest wall in serious chest injuries

Complications of mechanical ventilation

Equipment failure

You are dependent on a machine for this basic need of breathing. This machine, if improperly functioning, will likely kill you. The same goes for the connection tubing, the ventilator settings, and the nurse who suctions you. All of these must be perfectly aligned to generate a good outcome.

Reduced lung defence

Yes, we say that we intubate and ventilate people because we would like to get at the secretions in their lower airway.

However, the actual huge plastic tube in their trachea also contributes negatively to the defence of the lungs from secretions.

Not only is there a mechanical disturbance of the normal cough process- there is also the heavy sedation and/or neuromuscular blockade, all of which contribute to ones' inability to cough normally.

Furthermore, you have just put in a slippery tube to act as a conduit for upper airway secretions, allowing them to slide down into the defenceless trachea. Oral organisms are gross, and they will reproduce out of control in the lung, leading to ventilator-associated pneumonia.

Assault by ventilation

The act of ventilating somebody is a perverse thing to do, as it is a positive pressure process (in contrast to the normally negative pressure process of breathing). The process of distending somebody's lungs with positive pressure may also give rise to ventilator-associated lung injury. Pneumothorax may result from an unnaturally high set pressure levels, or if the patient coughs while on the ventilator (generating high pressure of their own accord). In truth, the process of being ventilated is amazingly uncomfortable and people may find themselves extremely unhappy to be in that position.

In short, though blood gas parameters may be improved, the process actually INCREASES their distress.

The unwanted effects of positive pressure

  • Increased intracranial pressure
  • Decreased preload; thus decreased cardiac output
  • Increased right ventricular afterload, and thus worsening of right heart failure
  • Decreased splanchnic and renal blood flow
  • Mucosal ulceration due to decreased gastric blood flow

Other disadvantages of mechanical ventilation

  • Gut distension (with NIV and air swallowing)
  • Sleep disturbance and neuropsychiatric complications thereof
  • Increased sedation requirements and neuropsychiatric complications thereof
  • Increased demands on nursing staff, and the need for 1:1 nursing

References

The National Heart, Lung, and Blood Institute ARDS Clinical Trials Network. Higher versus lower positive end-expiratory pressure in patients with the acute respiratory distress syndrome. NEnglJMed 2004; 351: 327–36.

Soni, N., and P. Williams. "Positive pressure ventilation: what is the real cost?." British journal of anaesthesia 101.4 (2008): 446-457.

Kumar, Anil, et al. "Continuous positive-pressure ventilation in acute respiratory failure: effects on hemodynamics and lung function." New England Journal of Medicine 283.26 (1970): 1430-1436.