So, what is a mechanical ventilator, and when is it needed?

19th Dec 2020

A mechanical ventilator is a life support machine that breathes for the patient. It can assume all of the work of breathing or some of the work of breathing. In other words, it can provide full or partial respiratory support.

Man on mechanical ventilator. Illustration.

Figure 1. A mechanical ventilator is a life support machine that breathes for the patient, providing full or partial respiratory support.Man on mechanical ventilator. Illustration.

Mechanical ventilation is used for those patients who cannot breathe adequately. Clinically, mechanical ventilation is indicated when a person cannot achieve an appropriate level of ventilation to maintain adequate gas exchange and acid-base balance, which is usually obtained from an arterial puncture or arterial blood gas (ABG).

Lungs with blocked trachea and a needle representing arterial blood gas. Illustration.

Figure 2. Abnormal respiration is indicated on an arterial blood gas (ABG) by abnormal arterial carbon dioxide (PaCO2) or arterial oxygen (PaO2) levels. 

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How does an arterial blood gas indicate abnormal respiration?

Abnormal respiration is usually manifested in an arterial blood gas in one of two ways or both.

Increased PaCO2

First, the patient may not adequately exhale and remove carbon dioxide effectively. Now, the removal or exhalation of carbon dioxide out of the body is called ventilation. A patient who is not adequately ventilating exhibits increased levels of carbon dioxide since they're not ventilating it out of the body. This patient is at risk of respiratory or ventilatory failure and requires mechanical ventilation.

Lungs with accumulating CO2 and increased a needle with increased PCO2.

Figure 3. Abnormal respiration, such as inadequate ventilation, causes less carbon dioxide (CO2) to be removed from the lungs resulting in increased arterial carbon dioxide (PaCO2) levels.

Decreased PaO2

Or perhaps the patient might be ventilating adequately, but the oxygen that's breathed into the lungs may not effectively distribute to the body. You see, the adding of oxygen into the blood is called oxygenation. A patient who is not fully oxygenating exhibits decreased levels of PaO2 on the ABG. In this case, the lungs may need the support of a mechanical ventilator, not because of poor ventilation, but because of poor oxygenation.

Lungs with a lack of oxygen and increased a needle with decreased PO2.

Figure 4. Abnormal respiration, such as inadequate oxygenation, causes less oxygen (O2) to be inhaled into the lungs (and diffused into the blood), resulting in decreased arterial oxygen (PaO2) levels.

So a mechanical ventilator can help the patient ventilate (i.e., get rid of excess carbon dioxide) or oxygenate (i.e., get adequate oxygen) or both.

 

What else do I need to know about mechanical ventilation?

Unfortunately, supporting a patient's breath is more complicated than it seems. For example, in terms of duration, how long should an inhalation last—half a second, one second, two seconds? Or, how much volume should the patient receive—100 milliliters,1000 milliliters? And how often should the patient receive the volume—10 times per minute, 20 times per minute? And by the way, what oxygen concentration should you give the patient—50%, 100%? And what about the mode of ventilation, or how much pressure can the lung tolerate?

Stopwatch, pressure gauge, volumetric flask, ventilator screen, waveform on graph. Icons.

Figure 5. Many variables need to be determined when placing a patient on a ventilator: duration, oxygen concentration, volume, pressure, respiratory rate, and mode.

You see, answering all of these essential questions and more are vital to successfully providing adequate mechanical ventilation. The answers to these questions and others are covered elsewhere in this guide.

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