Physiology of normal breathing vs mechanical ventilation

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Our lungs have 23 generations of airway branches. Surface area increases as we travel deeper within the lung. The functional unit of the lungs are the alveoli, responsible for oxygenation and ventilation, which together have a surface area of around 70 m².

For healthy individuals, total lung capacity (TLC, after maximal inspiration) is 6–8 liters. Residual volume (RV, after maximal expiration) is 2–2.5L. The difference between the two is called the vital capacity (VC), 4–6L. Functional residual capacity (FRC, after normal expiration) is 3–4L, which decreases when lying down, during anesthesia, etc. In short, everything that causes atelectasis. Anatomical dead space (Vd) is around 100–150ml, does not contribute to ventilation and is about 30% of tidal volume (Vd/Vt = 0.3). This obviously increases when we add stuff that does not contribute to ventilation, like an endotracheal tube, ventilator tubing, etc. Alveolar minute ventilation is thus around 5L/min.

When we breathe, our respiratory muscles (mainly the diaphragm and intercostal muscles) pull our thoracic cavity outward. This causes a negative intrathoracic (or pleural) pressure, which results in the flow of air from outside (via the mouth) into the lungs.

Our diaphragm is the strongest and thus most important muscle responsible for this. So:

In normal breathing, the dorsal and basal parts of the lung are best ventilated, because that's where the diaphragm is. So it's a good thing nature made sure that they have the highest density of blood vessels compared to other parts of the lungs. Optimal respiratory physiology :)

Side note: the apical parts of the lung are usually called the non-dependent parts, whereas the dorsobasal parts are called the dependent parts. By which they mean: dependent on gravity, as gravity and the weight of the lung act on ventilation by increasing pleural pressure at the base (making it less negative) and thus reducing the alveolar volume. In other words, minute ventilation in the dorsobasal parts is dependent on gravity, but it isn’t in the apical parts. Yes, it confused the hell out of me too and whoever came up with this should be deprived of coffee for the entire year.

Now with mechanical ventilation, air is pushed in by the ventilator. And as air likes to travel via the path of least resistance, this usually means that the apical parts of the lung are better ventilated relative to the dorsobasal parts (which contain all the blood vessels!) when compared to normal breathing.

Now imagine your patient has sick ARDS lungs, which usually affects the dorsobasal lung areas disproportionately. This phenomenon is even more exaggerated! The apical parts of the lung receive the majority of the tidal volume, which should have you triggered to consider the following:

  1. The regions with least perfusion are best ventilated (V/Q mismatch)
  2. The regions with the most perfusion are the worst ventilated (V/Q mismatch)
  3. The regions with the least resistance (apical) are at risk for ventilator induced lung injury (VILI), as they receive most of the volume and pressure generated by the ventilator.

Did I mention V/Q mismatch? ;)

Now, does that mean we shouldn't ventilated our patients? Of course not! But we should be aware that our life saving treatment can have detrimental effects on the lungs as well, so we can try and minimalize these negative effects.

So the next time you're ventilating a patient, ask yourself if their lungs are either homogeneously or heterogeneously affected by their disease?

  • If homogeneously (eg. pulmonary edema due to congestive heart failure), the air traveling in the lungs is probably spread evenly.
  • If heterogeneously (eg. pneumonia, some ARDS), beware of overdistension and VILI in certain areas of the lung

Stay tuned for my other writings to learn how to achieve safe mechanical ventilation.

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I'm an intensivist and clinical pharmacologist, spreading the love for and knowledge of acute and critical care medicine on YouTube

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