“Macro” level dead space – imagine that a patient suddenly develops a segmental pulmonary embolism.

For any given tidal volume, there is less effective CO2 clearance.

“Micro” level dead space – scarred alveoli due to ARDS become inefficient at CO2 clearance.

Excess dead space can arise at a “micro” level or a “macro” level: Dead space is essentially wasted breath – the body moves gas in and out of the chest, but it achieves no CO2 clearance. Thus, any patient who is desaturating despite high concentrations of inhaled oxygen likely has a shunt.ĬO2 clearance depends on the amount of gas entering and leaving the lungs every minute, which removes CO2 from the body.Ī key variable here is dead space – which is gas that is inhaled and exhaled but which does not participate in CO2 clearance. A hallmark of shunt physiology is that it is poorly responsive to increased levels of oxygen (because blood isn’t coming anywhere close to ventilated alveoli). Examples include an anatomic abnormality (e.g., ventricular septal defect) or complete dysfunction of parts of the lung (e.g., mucus plugging of one lobe of the lung).

Shunt physiology: Blood flows from the right ventricle to the left ventricle without ever coming into contact with oxygenated alveoli at all.

Increasing the concentration of inhaled oxygen generally fixes this problem – because even poorly ventilated areas of the lung will now get enough oxygen. Taken together, the net result is hypoxemia. Alternatively, blood flowing to areas of the lung with excess ventilation will not be saturated over 100%. Blood going to portions of the lung which are inadequately ventilated will be starved of oxygen. Ventilation-perfusion mismatch (V-Q mismatch): Some parts of the lung have excess ventilation, whereas other parts have inadequate ventilation.

This generally occurs in one of two ways: Hypoxemia occurs when blood somehow passes from the right ventricle into the left ventricle without being fully oxygenated. These problems typically run together, but under certain situations they can become divorced. The key to understanding this is thinking about oxygenation and CO2 clearance separately. understanding the paradox of happy hypoxemia However, COVID is causing us to re-think how to manage this physiology. This isn’t anything especially new – we have occasionally seen this since time immemorial. Happy hypoxemia is severe hypoxemia (poorly responsive to supplemental oxygen) without dyspnea.

Physiological shunt vs dead space