Respiratory acidosis

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Respiratory acidosis


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Respiratory acidosis

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USMLE® Step 1 style questions USMLE

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A 62-year-old man comes to his primary care physician for a routine visit. The patient has no acute complaints, but he feels out of breath after walking 3-4 blocks. The patient has a 50-pack-year smoking history and consumes one pack of cigarettes per day. His temperature is 37.0°C (98.6°F), pulse is 77/min, blood pressure is 159/83 mmHg, and respirations are 12/min. Physical exam reveals a barrel shaped chest, bilateral wheezing, and a prolonged expiratory phase. Routine labs are ordered. Which of the following sets of findings would be expected given this patient’s clinical presentation?

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Respiratory acidosis p. 616


With respiratory acidosis, “acidosis” refers to a process that lowers blood pH below 7.35, and “respiratory” refers to the fact that it’s a failure of the respiratory system carrying out its normal pH- balancing job.

Normally, during an inhalation, the diaphragm and chest wall muscles contract to pull open the chest and that sucks in air like a vacuum cleaner. Then, during an exhalation, the muscles relax, allowing the elastin in the lungs to recoil, pulling the lungs back to their normal size and pushing that air out.

Ultimately, the lungs need to pull oxygen into the body and get rid of carbon dioxide CO2. CO2 binds to water H2O in the blood and forms H2CO3 carbonic acid, which then dissociates into hydrogen H+ and bicarbonate ions HCO3-.

So, in order to prevent pH fluctuations, the CO2 concentration, or the partial pressure of CO2, called PCO2, needs to be kept within a fairly narrow range.

For this reason, lungs maintain the ventilation rate they need to get rid of CO2 at the same rate that it’s created by the tissues.

If PCO2 levels starts to rise and pH starts to fall, chemoreceptors that are located in the walls of the carotid arteries and in the wall of the aortic arch start to fire more, and that notifies the respiratory centers in the brainstem that they need to increase the respiratory rate and the depth of breathing.

As the respiratory rate and depth of each breath increase, the minute ventilation increases - that’s the volume of air that moves in and out of the lungs in a minute.

The increased ventilation helps move more carbon dioxide CO2 out of the body, reducing the PCO2 in the body, which raises the pH.

In respiratory acidosis, the normal mechanism of ventilation is disturbed, and minute ventilation becomes inadequate to balance the pH.


Respiratory acidosis is a type of acid-base imbalance that occurs when the lungs fail to eliminate excess CO2, which builds up in the blood, causing blood pH to fall below 7.35. It can be caused by a variety of causes, including lung diseases, such as chronic obstructive pulmonary disease (COPD) or asthma, which can limit the amount of oxygen that reaches the lungs and reduce the ability to exhale CO2.

Other causes of respiratory acidosis include brain injury or disease, which can disrupt the normal control of breathing, and certain medications, such as opioids, which can slow down breathing. Symptoms of respiratory acidosis can include shortness of breath, fatigue, confusion, and drowsiness. In severe cases, it can lead to coma and even death. The diagnosis of respiratory acidosis is made by measuring the levels of CO2 and pH in the blood, and it is treated by addressing the underlying cause and providing supportive care, such as oxygen therapy or mechanical ventilation.


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