Errors in First Aid for the USMLE (2007): Pulmonary System

As always, this comes from an email sent to the First Aid Team.

Pulmonary

  1. P.428, Oxygen-hemoglobin dissociation curve
    1. High altitude decreases P02 and decreases Hb saturation. This results in a decrease (or down-shift) in the dissociation curve, not a right shift as described.
  2. P.428, Pulmonary circulation
    1. While exercise does decrease the perfusion limitation, it never reaches the point of diffusion limitation i.e., the SaO2 on an athlete will almost never be lower than it is at rest.
  3. P.431, Obstructive lung disease (COPD)
    1. “[up arrow] FVC” is incorrect. RV increases, FVC is decreased or normal, FEV is decreased, TLC is increased.
    2. Emphysema and Asthma are both listed as having decreased I/E ratios. I assume this means Inspiration/Expiration ratio. This seems impossible as a ratio other than 1 would lead to progressive deflation and collapse of the lungs or the opposite expansion. What this should instead indicate is that in both cases, the breathing is shallow. In any steady state of respiration (including shallow breathing), I/E = 1 and the tidal volume (Vt) is lowered.
  4. P.431, Restrictive lung disease
    1. “([down arrow] VC and TLC)” should say “([down arrow] FVC and TLC)”
  5. P.431, Neonatal respiratory distress syndrome
    1. The opening description describes “lecithin” as the important surfactant. The surfactant is then described as “dipalmitoyl phosphatidylcholine .” I think everyone might benefit if this was changed to:
      1. Surfactant–dipalmitoyl phosphatidylcholine (DPPC, lecithin)
  6. P.432, Obstructive vs restrictive lung disease
    1. The FEV1/FVC ratios are listed for Normal and Obstructive, but not Restrictive even though “>80%” was listed on the previous page. For consistency, I think it should be included here.
    2. The curve for “Normal” is not drawn correctly, listing a ratio of 80% but showing a ratio of 60%.

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11 Responses to Errors in First Aid for the USMLE (2007): Pulmonary System

  1. W. says:

    Regarding #1, I think a right shift is correct. The dissociation curves shift left or right, and we were taught that high altitude leads to a right shift. Perhaps you just call the same thing a downward shift. (If you look at the graph, it’s both right and down.) Either way, I think we’re both referring to the Bohr effect?

  2. Maybe a technicality, but the reason I think there should be a distinction is because a right shift of a normal curve still begins at 100mmHg and ends at 0mmHg. In high altitude, the PO2 is decreased, so the curve would not begin at 100mmHg (as both right and left shifts do).

  3. [...] outlines some of the errors found in First Aid for the USMLE Step I pulmonary section, which should come in handy for those of you preparing for that bitch of an [...]

  4. guitarguy09 says:

    For the restrictive lung disease thing on p. 432, I think you meant “>80%” for FEV1/FVC, NOT less than.

  5. Thanks for the catch. It’s been corrected.

  6. karen says:

    p431 Obstructive…regarding I/E ratios, I think it is referring to time length (rather than volumes) of inspiration vs expiration. The I/E ratio is lower because the patients often spend more time in expiration – they expire slowly (remember – through ‘pursed lips’) in order to avoid airway collapse that may result from forceful expiration.

  7. Joe says:

    Found another one… the diagram of the trachea on page 426 is backwards. The RIGHT main bronchus is, in fact, the one that is shorter, wider, and more upright.

  8. nyu says:

    as to #1 – The tendency will be an initial shift left since there’s no PO2. But the body compensates by increasing 2,3-BPG thereby shifting the curve to the right in order to allow the hemoglobin to release the oxygen to the body cells. Otherwise you would die.

  9. Ian says:

    pg. 428, comment #1

    The right shift is indeed correct. The mechanism is as follows:

    A decrease in partial pressure of oxygen due to decreased barometric pressure causes the medullary respiratory center to increase the respiratory rate. Hyperventilation causes respiratory alkalosis (the goal is to decrease the arterial partial pressure of carbon dioxide so that the partial pressure of oxygen increases). Additionally, respiratory alkalosis increases PFK activity and, thus, glycolysis. An increase in glycolysis increases the synthesis of 2,3-BPG, which facilitates oxygen unloading to tissue. As a side note, remember that respiratory alkalosis causes tetany by increasing the negative charges on albumin; consequently, an increased amount of calcium from the ionized pool becomes bound to albumin.

  10. JC says:

    In response to your point #3.1, regarding the I:E ratio… I think First Aid has got it correct (if poorly explained).

    The I:E ratio is a ratio of times, not volumes. You’re quite right that in steady-state the volume inhaled equals the volume exhaled; but in patients with obstructive lung disease, it takes them a lot *longer* than normals to do the exhaling.

  11. Kevin Klassen says:

    IE ratio refers to inspiratory:expiratory times in the cycle. It is DEFINITELY decreased in obstructive conditions.

    You have a right shift at altitude to do increased production of 2,3-BPG.

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