July 31, 2012
by mariannahadji

Hi all.

I have been approached to give an opinion on the answer to question 3C of the Mar 2010 physics paper.

The question is as shown

The key (as I see it) is this. If you are given information, you need to use it!

So, this is how I approached it

**Given Information**

**500ml tidal volume**

**40% venturi at 8lpm – blocked venturis (so assume 100% coming out of venturi?)**

**rr 30/min**

**IE 1:2** ** **

**What is average inspired oxygen?**

** Minute volume = 6000ml = 100ml/sec BUT only inspiring 33% of the time therefore inspiratory flow = 300ml/sec**

**mask flow 8000ml/min = 133ml/sec**

**insp time 20secs**

**exp time 40secs**

** ?how much is rebreathed?**

**assuming zero volume in mask? **

**Initial calculation from last night not accurate. The final calculation as I see it is as follows.**

**We require 300ml/sec but only 133ml/sec is being supplied. So the inspired volume/second is made up as follows – 267ml entrained air at Fi of 0.21 and 133ml supplied at Fi****O2 of 1. **

**Thus, 56ml of the entrained air is oxygen added to the 133ml of supplied oxygen giving you 189ml/sec of oxygen inspired out of the 300ml/sec total which calculates out to a percentage of 63%.**

That is how I see it.

Comments welcome. I guess the take home point is that, if your respiratory rate increases significantly, your true inspired oxygen concentration may well drop. I have made a number of assumptions here. Firstly, the mask doesn’t fit so tightly that expired gas can’t escape. Secondly, the volume of the mask is negligible for purposes of calculation.

Have fun!

Mike