(b) The concentration of CO2 in exhaled gas depends only on the body's CO2 production and the volume of gas exhaled per minute (the minute ventilation).(a) The amount of CO2 clearance in the lungs will always equal the CO2 production by the body.In an equilibrium state, the amount of CO2 produced by the body tissues must be cleared by the lungs.Two general principles can guide us here: #1) at steady state, cardiac output does not affect etCO2 For patients with severe lung disease, the gap can be much greater. In patients with normal lungs, the gap is typically ~3-10 mm. Any increase in dead space (e.g., due to severely injured lungs), will widen the gap. (2) The gap between the etCO2 and the PaCO2 is a reflection of the amount of dead space.□ For the purpose of everyday critical care practice, it's reasonable to assume that the PaCO2 is above the etCO2. Extraordinarily rarely, etCO2 can be slightly higher than arterial CO2 in pregnant patients with otherwise normal lungs. (1) Arterial CO2 should be higher than etCO2.This leads to two foundational principles of etCO2:.#3) By the time gas reaches the endotracheal tube, the end-tidal CO2 concentration will be lower than the arterial CO2 tension.#2) As gas flows from this alveolus out of the lung, it will be diluted by dead space gas that will have a lower CO2 concentration (since this dead space gas doesn't absorb CO2 from the blood).#1) Within the high-functioning alveolus, the CO2 pressure will be equal to the arterial CO2 pressure.The best way to conceptualize this is to imagine gas flowing from a high-functioning alveolus into the ventilator.The relationship between etCO2 and arterial CO2 (PaCO2) The amount of alveolar dead space may change over time, as lung disease improves or deteriorates. For example, increased dead space is seen in pulmonary embolism, in pneumonia or other parenchymal lung diseases such as aspiration, or in obstructive lung diseases such as asthma.
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