Welcome to issue #77 of The USMLE Times… A special welcome to the 175 new members who have joined our community this week!
Here’s what’s on tap for issue #77 of the USMLE Times:
Question of the Week (Renal/Pulmonary)
Link to this week’s USMLE training video
Our tweet thread of the week
Question deep-dive & breakdown
Let’s dive in!
A 45-year-old woman with a history of chronic kidney disease presents to the clinic with generalized fatigue and shortness of breath. Her vital signs are as follows: Blood pressure 145/95 mmHg, heart rate 95/minute, respiratory rate 24/minute, and temperature 98.6°F (37°C). Blood tests reveal a pH of 7.25, PaCO2 of 35 mmHg, and HCO3- of 18 mEq/L. Based on these findings, which of the following conditions is most likely the underlying cause of the patient's acid-base disturbance?
A) Respiratory alkalosis
B) Respiratory acidosis
C) Metabolic alkalosis
D) Metabolic acidosis with respiratory compensation
E) Mixed respiratory and metabolic acidosis
The answer & question breakdown is at the bottom of the post.
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ANSWER + QUESTION BREAKDOWN
This patient has chronic kidney disease (CKD) and presents with symptoms suggestive of an acid-base disturbance. The blood gas values indicate a low pH (7.25), which signifies acidosis. The PaCO2 is within the normal range, and the HCO3- is low, indicating a metabolic component. We need to determine the primary cause of the acidosis and whether there is compensation or a mixed disorder.
ANSWER CHOICES:
ANSWER CHOICE A: RESPIRATORY ALKALOSIS
Respiratory alkalosis is characterized by an elevated pH and a decreased PaCO2. This patient has a low pH and a normal PaCO2, which rules out respiratory alkalosis.
ANSWER CHOICE B: RESPIRATORY ACIDOSIS
Respiratory acidosis is characterized by a decreased pH and an increased PaCO2. The patient's PaCO2 is normal, which does not support a diagnosis of respiratory acidosis.
ANSWER CHOICE C: METABOLIC ALKALOSIS
Metabolic alkalosis is characterized by an increased pH and an increased HCO3-. This patient's pH is low, and HCO3- is decreased, which rules out metabolic alkalosis.
ANSWER CHOICE D: METABOLIC ACIDOSIS WITH RESPIRATORY COMPENSATION
Metabolic acidosis is characterized by a decreased pH and a decreased HCO3-. The body compensates by hyperventilating to decrease PaCO2. The patient has a low pH, low HCO3-, and normal PaCO2, indicating metabolic acidosis with partial respiratory compensation (hyperventilation).
ANSWER CHOICE E: MIXED RESPIRATORY AND METABOLIC ACIDOSIS
Mixed acidosis would show a decreased pH, increased PaCO2 (respiratory acidosis), and decreased HCO3- (metabolic acidosis). This patient's PaCO2 is not elevated, which does not support a mixed disorder.
THE VERDICT: The patient’s blood gas values (pH of 7.25, PaCO2 of 35 mmHg, and HCO3- of 18 mEq/L) indicate a primary metabolic acidosis. Chronic kidney disease commonly leads to metabolic acidosis due to the kidneys' reduced ability to excrete acid and regenerate bicarbonate. The normal PaCO2 suggests the patient is attempting respiratory compensation by increasing ventilation to lower CO2 levels, thus helping to buffer the acidosis. This pattern is consistent with metabolic acidosis with respiratory compensation.
FINAL ANSWER: D: METABOLIC ACIDOSIS WITH RESPIRATORY COMPENSATION
That’s it for issue #77 of The USMLE Times!
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