The Student Room Group

Respiratory physiology

1. At which lung capacity is there no pressure difference between the alveoli and body surface?

2. What effect will breathing at an increased FRC have upon lung compliance?
decreases it, as the gradient decreases?

3. Patient short of breath on mild exercise, has pulmonary evaluation:

From spirometer trace and determination of FRC, how would you calculate residual volume (RV)? [earlier, we’re told FRC, FVC and FEV1 have been determined]

TLC=6.4L, RV=2.0L (both greater than 100% expected).
FVC =4.0L which is 93% expected; FEV1=2.4L
He breathes slowly with large tidal volumes.
Is this restrictive or obstructive pulmonary disease, explain why?

I thought it was obstructive, as obstructive can be when FEV1 is < 80% of FVC. (and 2.4 is < 80% of 4L). Also, in restrictive disease tidal volume tends to be low.

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When you talk about oxygen saturation of say 90% - does this just mean that 90% of Hb have at least 1 molecule of O2 bound to it? It's the "at least" bit I'm unsure about.

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At altitude do you get hypocapnia? (if hypoxia -> hyperventilate -> CO2 blown off?)
Reply 1
Q3 - all of the picture points to an obstructive picture (FEV1/FVC, slow breathing, large tidal volumes) - specifically it is most likely to be asthma (due to the only minor reduction in FVC).

O2 Sats: an SaO2 of 90% indicates that 90% of Hb binding sites are occupied by oxygen - each Hb molecule can, at most, bind 4 molecules of o2

/edit: Yes, prolonged exposure to hypobaric hypoxic conditions would eventually lead to tachypnoea to the extent where co2 would be 'blown off' causing hypocapnia
Reply 2
idiopathic

From spirometer trace and determination of FRC, how would you calculate residual volume (RV)? [earlier, we’re told FRC, FVC and FEV1 have been determined]


Functional residual capacity = Residual Volume + Expiratory reserve volume