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#1
04-07-2015
 USMLE Forums Guru Steps History: Step 1 Only Posts: 305 Threads: 40 Thanked 103 Times in 64 Posts Reputation: 113
doubt in lung ventilation

patient a has a tidal volume of 400ml and her resp rate is 10
patient b TV IS 200ml and RR is 20

who has the higher Po2?

according to the formula....alv ventilation= TV*RR both patients must have same alv ventilation ie 4000ml/min

but patient b has mare rapid breathing so that t s more shallow....so more air is entering into the conducting system rather than into the alveoli right?
can plz anyone explain this concept??

because patient b has more rapid breathing so he must hav lower o2 and more co2. kindly tell me if im wrong

#2
04-07-2015
 USMLE Forums Addict Steps History: Step 1 Only Posts: 141 Threads: 26 Thanked 50 Times in 32 Posts Reputation: 60

You have to keep in mind the dead space which is constant and equal to 150ml the patient breathing 200ml will lose a major portion to dead space leading to low alveolar ventilation which is
(vt-vd)*respiratory rate
This is not the same as total ventilation which is
vt*respiratory rate
So althought the total ventilation maybe greater in the person with greater respiratory rate the alveolar ventilation will be lower leading to a lower po2
 The above post was thanked by: smita (04-07-2015)
#3
04-07-2015
 USMLE Forums Addict Steps History: Step 1 Only Posts: 141 Threads: 26 Thanked 50 Times in 32 Posts Reputation: 60

For memorization purpose you can remember that
RATE = CO2
Depth= O2
 The above post was thanked by: smita (04-07-2015)
#4
04-07-2015
 USMLE Forums Guru Steps History: Step 1 Only Posts: 305 Threads: 40 Thanked 103 Times in 64 Posts Reputation: 113
another doubt

can u also plz tell me what id dynamic compression of airways and how it is tested in the steps?

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