Resp & Ventilation

| Jump To Section:

• Must-Haves
• Ventilation and Perfusion
• Mechanical Ventilation

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| Must-Haves

Several things need to be true for us to spontaneously breathe oxygen:

• The surrounding air must contain sufficient oxygen
• The airway must be open
• The neuromuscular hardware required to expand the rib cage must be functional
• The lungs must also with expand the chest wall, creating a pressure gradient
• Once air is in the lungs, oxygen must be able to diffuse across alveoli
• There must be sufficient blood vessels to pick up this oxygen

If any of these assumptions break, the whole system falls apart.

Thankfully, most of the time, we aren’t underwater, or being strangled, or having neuromuscular problems, or suffering from a pneumothorax.

So let’s focus on the two remaining parameters:

| Ventilation and Perfusion

Ventilation (V) is air flow to alveoli, and perfusion (Q) is blood flow to alveolar capillaries.

Without enough perfusion, there's no blood to pick up the oxygen. Without enough ventilation, there's no oxygen to pick up.

We call an imbalance between these two processes a "VQ mismatch". There are two types:

• Dead space: when ventilation exceeds perfusion
• Shunt: when perfusion exceeds ventilation

The conducting airways of the lungs aren't supposed to facilitate gas exchange ("anatomical" dead space), but "alveolar" dead space can be caused by many factors, such as:

• Arterial obstruction (e.g. pulmonary embolism)
• Low pulmonary perfusion pressure
• Alveolar pressure exceeding capillary hydrostatic pressure
• Vascular destruction/remodelling (e.g. pulmonary fibrosis)

Ventilation, on the other hand, can be outsourced.

| Mechanical Ventilation

Gases flow down pressure gradients, from high to low.

Usually, when we inspire, our bodies create a negative pressure gradient in the lungs, helping air to flow in. But when this system breaks, mechanical ventilation can be used.

Ventilators are machines that create positive pressure in the proximal airway, such as in an endotracheal tube. Because of this, the relative pressure in the distal airways decreases, allowing air to flow toward the alveoli without needing respiratory effort.

The amount of pressure required depends on:

• Compliance: change in lung volume per unit change in pressure. The higher the compliance, the less pressure required for any given flow.
• Resistance: required pressure gradient per unit airflow. The higher the resistance, the more pressure required for any given flow.

To prevent alveoli from collapsing during expiration, ventilators produce Positive End-Expiratory Pressure (or PEEP). And to ensure enough oxygen makes it into the circulation, the Fraction of Inspired Oxygen (FiO₂) can be adjusted as needed.

To modulate all this, ventilators manage one of two primary targets:

• Volume-Control Ventilation (VCV). A set “tidal” volume of air is inspired with each breath, ensuring a consistent amount of air enters the lungs per unit time.
• Pressure-Control Ventilation (PCV). During inspiration, airway pressure is set to a single value. This means the alveoli are open earlier and for longer with each breath, favouring better gas exchange.

But none of this matters if the oxygenated blood can’t reach the required tissues.

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