Hypercapnia; calculating dead space requires knowing PCO2

Hypercapnic Failure
* happens due to decreased minute ventilation and increased dead space ventilation

Hypercapnic Failure is hard to detect unless you measure minute ventilation.

Minute ventilation is the volume of gas exchanged by a patient’s lungs per minute and is expressed as: minute ventilation = tidal volume *times* respiratory rate.

Respiratory rate is easy to measure with the help of a stop watch and counting respirations for one minute.

Tidal Volume is the volume of air that is exhaled when breathing out normally. This is impossible to measure, I think. I’m not sure how to measure this without using a ventilator [which you do not want to do]. Maybe this is why doctors do not measure minute ventilation, because they can’t.

Spirometry measures the biggest breathe you can take, not your ordinary breathing. Spirometry does not measure respiratory rate either. [Luckily this is easy to do manually.] So spirometry is not useful for detection of hypercapnia respiratory failure.

No wonder doctors miss the syndrome of respiratory hypercapnic failure.

Remember Hypercapnic Failure * happens due to decreased minute ventilation and increased dead space ventilation

Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution it is possible to measure anatomical and alveolar dead space, employing the concept of mass balance, as expressed by Bohr Equation.

Calculating dead space.

{\displaystyle {\frac {V_{d}}{V_{t}}}={\frac {P_{a\,{\ce {CO2}}}-P_{e\,{\ce {CO2}}}}{P_{a\,{\ce {CO2}}}}}}

where Vd{\displaystyle V_{d}} is the dead space volume and VtV_{t} is the tidal volume;PaCO2{\displaystyle P_{a\,{\ce {CO2}}}} is the partial pressure of carbon dioxide in the arterial blood, andPeCO2{\displaystyle P_{e\,{\ce {CO2}}}} is the partial pressure of carbon dioxide in the expired (exhaled) air.

PaCO2 requires an arterial blood gas test, which is an invasive test.

Altered mental status is a disaster. It is horrible to endure. It makes life impossible. Paula would have gladly had an arterial blood gas test to find out if she had hypercapnic respiratory failure. She was not given that option. Because doctors are not good at detecting altered mental status or hypercapnic respiratory failure.

Because they think its depression and don’t measure vital signs, including respiratory rate…….


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