MECHANICS OF BREATHING AND PEEP

Why Low Lung Volumes are Bad

• The Functional Residual Capacity (FRC) is the lung’s physiologic reserve, it is a reservoir.
• Loss of chest wall or lung compliance causes reduced FRC.

I am convinced that most people know far more about respiratory physiology than they think. For example, imagine that you are lying in bed – it’s a Sunday morning and a member of your family comes in and sits on your chest. Most people will respond to this provocative assault with a comment such as: “get off my chest I cannot breathe”.
The extra weight of someone sitting on your chest is, indeed, hard work. Consider a slightly different situation: the person announces that they are going to sit on your chest; you have time to prepare. What do you do? Do you 1. Blow all of the air out of your lungs, or 2. Take a deep breath in and hold it? You do the latter. This is because – with the extra weight on your chest wall – it is easier to breath when lung volumes are high than when they are low.

Take another situation: a man is in a restaurant eats a large piece of steak and it promptly lodges in his glottis, and he begins to choke.
What happens if no intervention takes place: after 3 to 5 o minutes he will become hypoxic, bradycardic and have a cardiac arrest.
Why doesn’t this take place immediately? Because of physiologic reserve: we maintain a reservoir of air in our lungs to give us such a buffer against injury. If this man were to have been breathing 100% oxygen for 2 or 3 minutes prior to this event, then it may have been 10 or 12 minutes before his hypoxic arrest. Indeed this is what anesthesiologists do when they anticipate a difficult airway: a paralyzed patient who is difficult to intubate may become rapidly hypoxic.
By asking the patient to breath pure oxygen, the other gases that are normally in the airway (principally nitrogen) are effectively washed out, and the patient’s ventilatory reservoir is filled with oxygen.

What is this reservoir: we call it the Functional Residual Capacity (FRC) – it’s what remains in the lungs at the end of a normal tidal breath (2.5 liters). It is a space in which gas exchange continues to occur throughout the respiratory cycle, which makes sense as blood is continuously passing through the lungs. If there were no FRC, we would be continuously reabsorbing carbon dioxide.
The FRC is a scientific volume: it exists at the end of a passive expiration. At this point, the tendency for the lungs to collapse inwards is balanced by the tendency for the chest to spring outwards.
What holds the two of these together is a small vacuum in the pleural space. If there is a disruption in this mechanism – such as a stabbing injury to the chest – then the lung collapses and the chest enlarges (pneumothorax).

Clinical Scenario

►An anesthesiologist is called for an emergency Caesarian section, under general anesthesia for a woman with a cord prolapse. The woman is given pure oxygen to breath for 3 minutes. Anesthesia is induced, but almost immediately the woman starts to desaturate. The endotracheal tube is placed and manual ventilation is performed. The woman’s SpO2 comes back up.

What just happened?

● Due to the presence of a term fetus in the woman’s abdomen, the diaphragm is considerably elevated, compressing the lungs and grossly reducing the FRC. This woman has minimal physiologic reserve. This is not an uncommon circumstance in intensive care where patients frequently present with tense ascites and pleural effusions, thus compressing lung tissue, and reducing the physiologic oxygen reservoir, the FRC.

The functional residual capacity is determined by the compliance of the lung and chest wall. Compliance is the rate of change of volume in response to pressure. Anything that reduces the outward mobility of the chest wall reduces its compliance: examples of this are severe obesity or constrictive bandages. Likewise, anything that reduces the volume of the lungs reduce their compliance, examples of this are pulmonary edema, fibrosis, consolidation or increased intra-abdominal pressure.

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