Indications and Contraindications for Mechanical Ventilation

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

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

Why exactly are you doing this?

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.  

Physiological Disturbances in Respiratory Failure
Physiological disturbance Influencing factors
Alveolar hypoventilation Respiratory rate
Alveolar gas mixture
Impaired gas diffusion  
Intrapulmonary shunt  
Ventilation-perfusion mismatch  
   
  • Alveolar hypoventilation is affected by 
  • Impaired gas diffusion is affected by:
  • Intrapulmonary shunt
  • Ventilation-perfusion mismatch

References

The chapter from Tobins was actually surprisingly unenlightening. In that book, information on this topic is scattered across about 2000 pages. If you need something to-the-point, I recommend this section (5) from an online textbook of anaesthesia. It is a brief and robust introduction to the subject matter.

R. Rodriguez-Roisin, A. Ferrer "Effects of mechanical ventilation on gas exchange" - Chapter 37 (p.759) in Tobin - Principles and Practice of Mechanical Ventilation (2md ed., 2006)

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

Oakes, Dennis L. Physiological Effects of Positive Pressure Ventilation. AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH, 1992. -this is somebody's Masters of Science thesis! They received their degree in 1992, but one expects that the fundamentals of physiology have remained the same since then.

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.