The Art of Medical Ventilation

We visited grandma on the weekend. There was a new nurse there. She looked at the ventilator and exclaimed that the reading was 55, so grandma was doing good. I quizzed her on what 55 meant. Did not get much info. I talked to the respiratory therapist. She had a lot to say. But I still could not grasp what 55 meant in the big scheme of things.


So I decided to do some research. The ventilator we are talking about is a medical ventilator. In the old days, it was called a respirator. It blows air with extra oxygen into the patient. The air is heated and humidified as well. The ventilator also monitors things such as the air pressure, volume, and rate of flow into the patient.


An invasive use of the ventilator requires a breathing tube to be inserted in the patient. The tube inserted into the patient is called a trach tube during a process called intubation. The tube goes ito the patient's windpipe. The tube can have a cuff, making it a cuffed tube. The cuff holds the tube in place and prevent leaks. Non-invasive use of a ventilator means air is blown into the nose/mouth.


The ventilator is a life critical system. The goal is to wean the patient off the ventilator. A respiratory therapist tunes the settings of the ventilator. There are some risks to using a ventilator such as pneumonia and/or damage to the vocal chords. The ventilator also makes it hard to cough.


There are different techniques a ventilator can control the flow of air into a patient. It can keep up a certain pressure of air. It can also ensure a specific volume of air enters the patient. It could also concentrate on delivering air at certain time periods. Or it can use a combination of pressure, volume, and timing.


There are different modes that a ventilator can be set to run under. AC mode is when the ventilator kicks in to assist each time a patient breathes. SIMV will also kick in when the patient breathes, but only do so for a certain amount of breathes per minutes (the extra breaths are unassisted). The PSV mode is when the ventilator ensures a certain amount of pressure is present when the patient breathes. And finally the ARPV mode provides a constant pressure of air.


There are many readings on the ventilator to control its usage. The respirator rate is how many breathes per minute are delivered. The tidal volume is how much air is sent in per breath. The flow is how fast the air gets delivered to the lungs. The waveform is how the amount of pressure varies over the duration of one breath. And finally there is the oxygen concentration, which I discuss below.


Oxygen concentration is also known as FiO2, or fraction of inspired oxygen. It is expressed as a percentage. It tells you how much oxygen is in the air delivered to the patient. Oxygen in the tank is 100% oxygen. Air is 21% oxygen, but mostly consists of nitrogen. The ventilator artificially adds more oxygen to the air given to the patient.


Patients might initially be given 100% oxygen for a very short amount of time. They will be getting more than 21% oxygen if they are using a ventilator. The percentage could be lower than 40% for some patients. Above 60% oxygen for sustained periods is considered toxic. If you are on 60+% oxygen for more than 12 to 24 hours, you risk damage to the patient. Most of the time, the oxygen concentration will be set to below 50% to prevent toxicity/damage to the patient.