USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 45-year-old man comes to the emergency department with left lower quadrant pain, fever. This is the 4th episode this year. A diagnosis of recurrent diverticulitis is made and a left colectomy is scheduled for tomorrow. During anesthesia, an arterial blood gas is drawn and shows:
PaCO2: 70 mm Hg
HCO3-: 30 mEq/L.
Which of the following best describes the patient's acid-base disorder?
Content Reviewers:Rishi Desai, MD, MPH
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.
This could be due to a number of problems. Sometimes, the problem is not in the lungs themselves, but in the respiratory centers of the brainstem.
It may also be due a neuromuscular disorder like myasthenia gravis, where the nerves don’t effectively stimulate the muscles to contract.
Sometimes the diaphragm or chest wall muscles didn’t work properly, which can happen after severe trauma, or due to obesity when the chest wall is too heavy for the muscles to lift.
Another reason is airway obstruction, which might happen if a child swallows an object like a peanut and it lodges in the right mainstem bronchus, preventing that lung from fully ventilating.
Finally, there might be impaired gas exchange between the alveoli and the capillary.
That might happen if alveoli are damaged from chronic obstructive pulmonary disease, or if fluid accumulates within the alveoli like in pneumonia, or if fluid collects between the alveoli and the capillary walls like in pulmonary edema.
- "Physiology" Elsevier (2017)
- "Human anatomy & physiology" Pearson Education/Benjamin Cummings (2013)
- "Human physiology" McGraw-Hill Education (2016)
- "Kaplan USMLE Step 1 Lecture Notes" Kaplan (2017)
- "First Aid for the USMLE Step 1 2017 (27th edition)" McGraw-Hill Education / Medical (2017)
- "Step-up to medicine" Wolters Kluwer (2016)
- "Medical physiology" Elsevier (2017)
- "Textbook of medical physiology" Saunders (2011)