#76 - THE WEEKLY USMLE TIMES
Highlights for the week of May 20, 2024
Welcome to issue #76 of The USMLE Times… A special welcome to the 219 new members who have joined our community this week, and a special thanks to the 12 students who booked their 1-on-1 sessions with me over the past week.
Here’s what’s on tap for issue #76 of the USMLE Times:
Question of the Week (Pulmonary)
Link to USMLE Step 1 prep “Starter Playlist”
Our tweet thread of the week
Question deep-dive & breakdown
Let’s dive in!
A 62-year-old man with a history of COPD presents to the emergency department with acute shortness of breath and cough. His vital signs are as follows: Heart rate 110/minute, respiratory rate 26/minute, blood pressure 135/85 mmHg, temperature 98.2°F (36.8°C), and oxygen saturation 88% on room air. Arterial blood gas analysis shows a PaCO2 of 60 mmHg and PaO2 of 55 mmHg. Which of the following is the most likely cause of this patient's increased physiologic dead space?
A) Increased alveolar ventilation
B) Decreased alveolar perfusion
C) Decreased respiratory rate
D) Increased pulmonary vascular resistance
E) Alveolar collapse
The answer & question breakdown is at the bottom of the post.
📺 WATCH
💬 TWITTER THREAD OF THE WEEK
READ THE FULL THREAD HERE.
Click HERE To Follow Us On Twitter/X
🔗 LINKS TO PREVIOUS ISSUES
USMLE Times Issue #75 - CLICK HERE
USMLE Times Issue #74 - CLICK HERE
USMLE Times Issue #73 - CLICK HERE
USMLE Times Issue #72 - CLICK HERE
ANSWER + QUESTION BREAKDOWN
The question presents a patient with a history of COPD (Chronic Obstructive Pulmonary Disease) who is experiencing an acute exacerbation of his condition. The vital signs and arterial blood gas analysis indicate respiratory distress and hypoxemia. The question asks for the most likely cause of increased physiologic dead space.
Physiologic dead space refers to the volume of air that does not participate in gas exchange. It includes the anatomic dead space (conducting airways) and the alveolar dead space (non-perfused or poorly perfused alveoli). In COPD patients, the alveolar dead space is often increased due to a ventilation-perfusion mismatch.
ANSWER CHOICES:
ANSWER CHOICE A: Increased alveolar ventilation
Increased alveolar ventilation would generally improve gas exchange and reduce physiologic dead space. Thus, it would not contribute to an increase in physiologic dead space.
ANSWER CHOICE B: Decreased alveolar perfusion
Decreased alveolar perfusion leads to increased dead space, as alveoli are ventilated but not perfused. This is common in COPD due to the destruction of the pulmonary capillary bed and increased pulmonary vascular resistance.
ANSWER CHOICE C: Decreased respiratory rate
A decreased respiratory rate would not directly increase physiologic dead space. It might lead to hypoventilation, causing hypercapnia and hypoxemia, but it does not increase the volume of non-perfused alveoli.
ANSWER CHOICE D: Increased pulmonary vascular resistance
Increased pulmonary vascular resistance can reduce perfusion to certain lung areas, increasing alveolar dead space. This mechanism is seen in COPD due to pulmonary hypertension and vascular remodeling.
ANSWER CHOICE E: Alveolar collapse
Alveolar collapse (atelectasis) leads to reduced alveolar ventilation, which decreases ventilation rather than increasing dead space.
THE VERDICT: The main cause of increased physiologic dead space in COPD patients is decreased alveolar perfusion due to ventilation-perfusion mismatch. The destruction of alveolar walls and chronic inflammation reduce the surface area available for gas exchange and decrease alveolar perfusion. This increases alveolar dead space, as the air in these poorly perfused alveoli does not participate in gas exchange. The patient's hypoxemia and hypercapnia are a result of this ventilation-perfusion mismatch and the increased physiologic dead space.
The most likely cause of this patient's increased physiologic dead space is decreased alveolar perfusion.
FINAL ANSWER: B. Decreased alveolar perfusion
That’s it for issue #76 of The USMLE Times!
If you enjoyed it and found it helpful, please consider hitting that little heart button, telling me if you got the answer right in the comments, and sharing it with someone you know who would also benefit from it (use the little pink ‘share’ button below).
Thanks for tuning in!
Struggling or need some help with your USMLE Step 1 prep? Click HERE
