How does mechanical ventilation work during an operation?

An anesthetic is necessary to make sure that a surgical procedure doesn't cause any pain. It is sometimes enough to just anesthetize (numb) one part of the body, such as the edges of a wound, only one arm, or the lower abdomen. However, a general anesthetic is usually needed for more extensive surgery. It deactivates our consciousness and the pain receptors in our body. Depending on how strong the anesthetic is, it can also stop our breathing. A ventilator then has to take over our breathing.

A mask is placed on your face which firmly covers the nose and mouth. A ventilator can be attached to the mask to constantly provide fresh, oxygen-rich air to the airways.

A manual resuscitator or bag-valve-mask (BVM) can be attached to the mask for a short period or in emergencies. The anesthetist pumps it rhythmically to push the air from the bag into the airways. Valves make sure that every time they let go, the exhaled air escapes and the bag fills back up with fresh air.

Doctors also refer to mask ventilation as “non-invasive” ventilation.

Laryngeal mask airways (LMAs) are now used for a lot of procedures. The mask resembles an ear with two tubes attached. It is not placed over your mouth and nose, but rather inserted into the throat above the larynx. To do so, the doctor has to feed the mask in through your mouth. Once the right position is found, a little air is fed into the inflatable edge of the mask through the thinner tube. It then covers the larynx better. The ventilation device or the manual resuscitator is then connected to the thicker tube.

Because the laryngeal mask airway is deployed deep inside the throat, doctors also refer to it as “invasive ventilation.” It is only introduced once the anesthetic has started to work, so the person being operated on doesn't notice anything.

Intubation usually refers to when the anesthetist introduces a flexible plastic tube (called a tracheal or endotracheal tube) into the windpipe. A ventilator is then connected to the other end of the tube. A manual resuscitator can be used instead as a temporary solution.

Intubation is also referred to as invasive ventilation. As when the laryngeal mask airway is inserted, the person being operated on doesn't feel the tube being put in because the anesthetic is already taking effect.

Ventilators regularly pump air into the lungs via tubes. The anesthetist can control exactly what should happen during ventilation, for instance:

• how long each breath lasts,
• how much air flows into the lungs,
• the pressure in the airways, or
• the oxygen concentration of the air.

Anesthetic gases and moisture can also be mixed in with the ventilating air.

The physician can keep an eye on all key measurements using a monitor and then adjust the device when needed.

One complication is that the contents of the stomach can flow back up the food pipe into the mouth and throat and then enter the windpipe. That is called aspiration. Patients need to have an empty stomach before an operation to minimize the risk of that happening.

This risk is even lower when intubation is used compared to a ventilation mask or a laryngeal mask airway, because the end of the tube that is introduced into the windpipe is surrounded by an inflatable plastic cuff. As soon as it has been inserted far enough, the cuff is filled with a little air. That fixes the tube in place and seals off the windpipe. This makes it impossible for the contents of the stomach to pass by the tube and enter the windpipe. After surgery, the air has to be let out of the cuff before the tube is removed.

More medication is needed during intubation than for mask ventilation though, which can affect circulation. Injuries during intubation and hoarseness and irritations of the throat are also possible following ventilation. Complications can also occur if the tube is not inserted correctly into the windpipe. For example, it might be inserted into the adjacent opening of the food pipe or be pushed too far down into the lungs. That is why doctors need good visibility when inserting the tube. A hook-shaped instrument with a light on it called a laryngoscope can be helpful. It keeps the airways open while also illuminating the throat. The doctors can then check that the tube is actually going into the windpipe. They also listen to the lungs and analyze the exhaled air after intubation as an additional check that the tube has been positioned correctly.

Side effects of ventilation during an operation such as damage to the lungs or circulatory system or infections with hospital germs are rare, and are more of an issue with longer term mechanical ventilation or if the lungs are already damaged.

A number of factors dictate which type of ventilation is used when:

• the type of operation,
• the type of anesthetic needed, and
• the condition of the person being operated on.

Individual risk of complications is the most important factor. If a fast operation is needed, there is not enough time to wait for the person's stomach to be empty. Intubation is then usually used to minimize the risk of aspiration. Ventilation and laryngeal masks are also an option for scheduled procedures. Intubation is the general rule for longer, more complicated operations, even if the patient has an empty stomach.

It is also possible that the insertion of a tube is difficult or unsuccessful. The doctors then have to consider: if mask ventilation can be used instead, whether a different type of anesthetic is possible, or if the procedure can be postponed.

Even without general anesthetic, you can also fall into a deep unconscious state where you can no longer breathe unaided, for instance due to severe head injuries or significant blood loss. Experienced first aiders can use mouth-to-mouth resuscitation in those types of emergency situations to make sure the unconscious person is still getting enough oxygen. As soon as experts take over, ventilation can be continued using a bag-valve-mask or ventilator.

Lung problems are often the reason people have to be mechanically ventilated. If they are significantly impaired, for instance due to severe pneumonia, normal breathing will no longer be enough to provide the body with sufficient oxygen. Ventilation therapy is helpful then.