USMLE Step 1 Respiratory Physiology Review 61 12 Shunt

USMLE Step 1 Respiratory Physiology Review 61 12 Shunt

On these “Gold Standard Step 1 Facts” pages you will find Free:

  • USMLE Audio Review files from our “Gold Standard USMLE Reviews”
  • Transcriptions of those files
  • And videos (as they become available)

The idea is that you can review for the USMLE online by:

  • Listening to the Audio
  • Following along with the transcription
  • Or by watching the video (if available)

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Let’s start our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.

Play USMLE Audio MP3  61 12 Shunt  Below

Begin 61 12 Shunt Transcription

In healthy lungs, what percent of cardiac output bypasses gas exchange in the alveoli?

  • From 2 to 5 percent

What is this called? It starts with the letter S

  • Shunt

One kind of shunt that can occur in the lungs happens when ventilation to an area of the lungs is 0. What is this kind of shunt called? It starts with an A

  • Anatomic shunt or anatomical shunt

What is the composition of the blood leaving an unventilated region of the lung?

  • A composition of blood leaving an unventilated lung region is the same as mixed venous blood

What is the ventilation perfusion ratio for lung region with no ventilation?

  • The ventilation perfusion ratio equals zero when ventilation is zero.

Anatomic shunt can also have extra pulmonary causes. Another kind of shunt that can occur in the lungs is caused by a ventilation perfusion imbalance. This one starts with a letter P. What is it?

  • Physiologic shunt or physiological shunt is caused by ventilation perfusion imbalance.

What is the ventilation perfusion ratio when a physiological shunt occurs?

  • The ventilation perfusion ratio is greater than zero but less than one

What causes such a VQ ratio?

  • Excessive perfusion or inadequate ventilation will cause a VQ ratio greater than zero but less than one.

What effect does shunted blood have on the oxygen tension of systemic arterial blood?

  • Shunted blood causes the oxygen tension of systemic arterial blood to fall below alveolar oxygen tension

Student doctor, please pause the tape and summarize what we’ve discussed about shunts.

  • Even in healthy lungs from 2 to 5 percent of cardiac output bypasses gas exchange in the alveoli. An anatomic shunt happens when ventilation to an area of the lungs is zero. The composition of blood leaving an unventilated lung region is the same as mixed venous blood. The VQ ratio equals zero when ventilation is zero. Anatomic shunt can also be extra pulmonary. Another kind of shunt that can occur in the lungs is caused by a ventilation perfusion imbalance. These are called physiological shunts. In the presence of a physiological shunt, the VQ ratio is greater than zero but less than one. Excessive perfusion or inadequate ventilation will cause a VQ ratio greater than zero but less than one. Shunted blood causes the oxygen tension of systemic arterial blood to fall below alveolar oxygen tension.

 

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