1.5 Conclusion

How natural and beautiful is the flow of inhaling and exhaling—like waves upon the shore. It is a deceptively simple process because it is so fundamental to life, but when we look deeper, we learn that each step is so important to the overall flow of air.

But what happens when you change the smallest little aspect of the process? For instance, what if you sedated the brain and stopped the contraction of the diaphragm? Suddenly, the passive flow of air no longer has a trigger that causes the negative pressure in the lungs—therefore no air flowing into the lungs. Or, what if the positive pressure applied overcomes the negative pleural pressure that facilitates the air being left in the alveoli? The alveoli would attempt to equilibrate this pressure differential and tend to collapse fully. Finally, instead of air flowing easily into the lungs via a pressure gradient, what if the air was forced into the lungs with a forceful push? If you think of letting a bowling ball gently roll down the alley versus hurling it down the lane as hard as you can, in the first case, the ball most likely won’t knock down any bowling pins, but a forceful throw has enough force to send them all flying. The same trauma can be seen in your lungs.

Mechanical ventilation occurs by forcefully pushing air into the lungs. It occurs with a closed system by means of an endotracheal tube, and it forces the alveoli to inflate based on the settings that the ventilator is programmed to deliver. As you can already tell from just this simple description, mechanical ventilation is much different from the natural process of the body.

Like the majority of medical treatments and interventions, there is a downside to mechanical ventilation and this is it: if not done very carefully, the forced air of mechanical ventilation will cause trauma to all those delicate alveoli in the lungs. When we understand that this trauma can occur if we are not careful, it allows us to approach ventilation with the respect needed, to provide safe care for critically ill patients. Eliminating or interrupting spontaneous respiration introduces a large number of problems, as well as the concern of causing damage if ventilation is not done safely. These concerns can be easily mitigated by setting the ventilator appropriately based on the patient and situation you are dealing with. One size does not fit all when it comes to mechanical ventilation. Now that we understand the natural state of breathing, we should also understand how important it is to match the natural breathing process as much as possible with our ventilation settings.

Review

The cascade of breath, in and out, is all about the flow of air from high to low pressures. It starts in inspiration, with the lungs having a negative (lower) pressure from the diaphragm contracting. Air flows in until the diaphragm stops contracting, which stops that negative pressure. Inspiration stops. At this point, the lungs are full of air and are experiencing a higher pressure than the outside world. Exhalation starts as air flows from high to low pressure once again. This continues until the pressure in the lungs sufficiently drops—yet the lungs always maintain FRC, thereby decreasing the pressure needed to open the lungs again.

For a helpful video-based overview of the respiratory process, please watch the Crash Course video on the Respiratory System:

“Chapter 1 Conclusion” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.