Question 30

[McatCrusher]

Hi,
I think the correct answer here should be C. Solution says B but it does not make sense as thoracic actually gets smaller as diaphragm goes up!!!!I dont know...thats how i got the answer...

[admin]

The correct answer is B.

Clearly, using CPR is not natural so we have to work this out a bit. If you apply pressure to the abdomen upward, I think you will agree that this would lead to the diaphragm rising.

If there was no obstruction (blockage) of the airway, then air would flow out like water through a hose. But, because there is an obstruction, it is like someone standing on a soft hose, the hose will increase in volume because of the added pressure. The pressure rises until the obstruction is removed. Then we get a normal expiration with the volume of the chest cavity (thoracic) being reduced.

[admin]

Granted that the water/hose analogy is flawed because there is a source of more water. OK, let's move it to a balloon. Imagine a soft balloon with air inside (we are graduating from the water stuff!). Draw somewhere in the middle of the balloon a circumferential line (the balloon's "waist") such that the chest is above and the abdomen is below the line. Apply pressure from below (abdomen) thus the upper part gets more voluminous. Why? because the balloon is tied = the obstruction. Release the obtruction, the "chest" will decrease in size as the balloon "expires." (of course, for humans, when the diaphram rises, it is the liver, spleen and bowel which rise to occupy space in the thoracic cavity but the effect is similar to the balloon analogy)

The thoracic cavity MUST increase in size because the diaphram is moving up, occupying space but the air has nowhere to go yet. Once critical pressure has been obtained then the foreign body is dislodged and the "balloon" deflates. Of course, the chest wall is not anywhere as flexible as a balloon so of course the terms we are using are relative.

Analyze the reverse for a moment. If the diaphram went up, what mechanism explains the thorax getting smaller before an expiration? The only reason that physiologically your thorax decreases in size is precisely because you are expiring thus volume of air decreases as it leaves your body. That can't happen while the obstruction is still there.

[mcat_premed3832]

The previous response has the following incorrect assumptions: (a) the thoracic cavity is an inflexible container; (b) upward abdominal thrust does not push bowel into the chest cavity thus reducing abdominal volume -> increasing thoracic volume until a maximum is achieved at which point pressure increases.

Consider:
(a) upward thrust -> (b) increased thoracic pressure -> (c) object dislodged

Let's consider what happens between (a) and (b): as the abdominal contents enter the thoracic cavity, lungs are compressed to a degree, the volume of the thoracic cavity increases to its maximum, now all the thrust is transmitted as increased pressure.

The correct answer is B.

[mcat_premed3832]

You correctly applied the relationship with PV and you correctly described what happens in passive respiration but the problem is your conclusion is not what the answer is saying.

"Let's just say for some magical reason the upward thrust increases the lung volume (as in choice B)."

This is not what answer choice B is suggesting (thoracic volume is very different from lung volume). In fact, I agreed with you above, that the lung volume must decrease then pressure increases until the object is dislodged. But how do you you decrease lung volume with someone who is not breathing? Keep in mind, the lung is changing volume, but it's not in a hardened container. It's in a cavity that can increase to a degree or decrease to a degree. The question is, with abdominal contents suddenly being pushed into the chest, what will the chest cavity naturally do? Increase to a degree.

Oh, about the passive breathing model: as you mentioned passive breathing (expiration for example) means that lung volume decreases as the diaphragm relaxes and the thoracic cavity reduces in size ("recoil"); but, if you can't breathe then those events cannot occur in the natural way. So this manoeuvre forces the diaphragm up, pushes abdominal contents into the chest with the objective to reduce lung volume, but as a consequence of this violent act, the thoracic cavity gives way to its maximum size until the increased thrust is only transmitted to increase pressure in the "collapsing" lung.

[mcat_premed3832]

The correct answer is still B!

Note the quote from the Explanation: "The events in expiration normally include decreasing the size of the thoracic cavity and relaxation or raising of the diaphragm (BIO 12.4)."

That is "normal" expiration. Having someone apply pressure to your abdomen resulting in your bowels/stomach/liver being pushed up into your chest cavity does not constitute "normal" expiration. The details are described above as to how this increased "stuff" being pushed in the chest leads to increased chest volume (increased thoracic cavity) followed by/in concert with compression of the lungs leading to forced expiration.

The real MCAT loves to take something you know well, and then twist one thing about it to see if you can make the adjustment on your feet. If that's all you get out of this question, then that's more than good enough.

[mcat_premed3832]

You put your finger on the issue. During the discussion above the words used in the question became something else. The issue is much more simple: let's try an experiment! Take your fist and hand and perform an auto-Heimlich but not too fast! If you try it on yourself, what would you say happens to your thoracic size? Bigger? Definitely.

The question never asked about thoracic volume (that came up in the discussion in this thread and was never clarified).

[mcat_premed3832]

I know what you mean but I think that the most common definition of chest size is based on appearance (how big?) which refers to circumference.