Airway Wall Pressure

In normal respiration, the patient’s diaphragm and intercostal pull on the lungs to make them larger to create a vacuum that pulls air into the lungs. Since the lungs are creating a vacuum (negative pressure) to pull in the air, there is little or no chance of over-inflation in increased pressure on the structure of the lungs or rib cage. As an added safety measure, the Herring Breuer Reflex (a reflexive expiration to prevent over-inflation related lung injury). With this system of checks and balances, the lungs tend to work with little or no risk of barotrauma (physical damage to body tissues caused by a difference in pressure between a gas space inside, or contact with, the body and the surrounding gas or liquid) to the lung or thoracic cavity.

In artificial or mechanical respiration, the air is pushed into the lungs by clinicians or rescuers at higher pressure. In order to facilitate chest rise, a much larger volume of air is required to fill the dead space in the pharynx, trachea, and to a lesser degree the alveoli. The patient has the same maximum volume whether they are breathing or not. This increased volume leads to damage to the lungs, alveoli, and This higher pressure can lead to damage to the lungs, alveoli, and alveolar blood vessels. To make matters worse, the increased volume leads to greater expansion of the chest than normal.

Esophageal Opening Pressure

In normal respiration, the patient’s diaphragm and intercostal pull on the lungs to make them larger to create a vacuum that pulls air into the lungs. Since the lungs are creating a vacuum (negative pressure) to pull in the air, the negative pressure pulls the air into the lungs and not the esophagus. Since the air flow is generated by the vacuum created by the lungs, the air will flow only towards the vacuum. This relationship makes it nearly impossible for normal respirations to result in air being pulled into the esophagus and stomach, despite the fact that the esophagus is completely open to this air flow.

In artificial or mechanical respiration, the air is pushed into the lungs by clinicians or rescuers at higher pressure. In order to facilitate chest rise, a much larger volume of air is required to fill the dead space in the pharynx, trachea, and to a lesser degree the alveoli. Since some of the highly pressurized air is used to fill the dead space, some of the air is likely to travel down the esophagus and into the stomach. The air will collect in the stomach, until the stomach is full of air. When the amount of air collected in the stomach is sufficient to significantly increase the pressure in the stomach, the air will evacuate through the esophagus, it will escape through the esophagus carrying gastric contents with it (vomit).

Over Ventilation (Either By Rate of Volume)

When clinicians or rescuers ventilate too fast or deliver too much tidal volume into the lungs, complications are likely to occur. The first complication is the worst, and often unseen. The increased pressure in the chest will apply pressure to the vena cava. The vena cava is the main venous return for the body’s blood supply to return to the heart to be reoxygenated in the lungs. The expansion and contraction of the lungs applies alternating pressure to the vena cava, resulting in a vacuum that pulls blood to the heart. When the pressure remains higher than usual, more pressure is placed on the vena cava resulting in a restriction of blood flow to the heart. Less blood arriving at the heart means less blood being pumped out of the heart. This condition is known as obstructive shock. This phenomenon is also seen in tension pneumothorax.

Another complication that occurs is gastric distension that will result in explosive vomiting and further compromise to the airway.