Dead Space Ventilation

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DreamMachine

DreamMachine

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During general anesthesia with spontaneous ventilation, a patient has a Vd/Vt of 0.5. Which is most likely to decrease this ratio?

a. acute bronchospasm
b. continuous positive airway pressure
c. decreased tidal volume
d. increased cardiac output
e. pulmonary embolism

Please discuss.

Source: ITE 1996 Book A Q 46

****...I meant to put this in the keyword forum...
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DreamMachine said:
During general anesthesia with spontaneous ventilation, a patient has a Vd/Vt of 0.5. Which is most likely to decrease this ratio?

a. acute bronchospasm
b. continuous positive airway pressure
c. decreased tidal volume
d. increased cardiac output
e. pulmonary embolism

Please discuss.

Source: ITE 1996 Book A Q 46

****...I meant to put this in the keyword forum...
Click to expand...

Vt=Tidal volume
Vd=Dead space
Dead space= ventilation without perfusion.
Vd/Vt=0.5, and as Vd=>0, or Vt approaches infinity, the ratio=>0

Hence:
a. Bronchospasm= Decrease Vt= increased ratio
b. CPAP= decreased Vt= increased ratio
c. Vt decrease= increased ratio
d. Inc CO= Decreased Vd= decrease ratio
e. PE= increased Vd, and increase ratio

The reason inc CO decreases dead space is that the larger volumes/flows through the pulmonary vasculature results in engorgement of the west zone 1 of the lung. This means that areas which typically don't have perfusion, but plenty of ventilation are now being perfused and ventilated and therefore decreasing the amount of functional dead space. The subsequent decrease in Vd results in a lower dead space-tidal volume ratio and the answer to this question.


If my reasoning is correct, then the answer is good, but I could be wrong.

One other thing, the reason we even have this ratio is...

Bohr equation is used to quantify the ratio of physiological dead space to the total tidal volume, and gives an indication of the extent of wasted ventilation.
Vd / Vt = (PaCO2 − PeCO2) / PaCO2

As per wikipedia :)
 
Last edited:
Yeah dude! You are ready for writtens.

Does CPAP decrease tidal volume? Not sure. Regardless, it makes sense that Vd would increase with CPAP, since there is more of West Zone 1.
 
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lvspro said:
Vt=Tidal volume
Vd=Dead space
Dead space= ventilation without perfusion.
Vd/Vt=0.5, and as Vd=>0, or Vt approaches infinity, the ratio=>0

Hence:
a. Bronchospasm= Decrease Vt= increased ratio
b. CPAP= decreased Vt= increased ratio
c. Vt decrease= increased ratio
d. Inc CO= Decreased Vd= decrease ratio
e. PE= increased Vd, and increase ratio

The reason inc CO decreases dead space is that the larger volumes/flows through the pulmonary vasculature results in engorgement of the west zone 1 of the lung. This means that areas which typically don't have perfusion, but plenty of ventilation are now being perfused and ventilated and therefore decreasing the amount of functional dead space. The subsequent decrease in Vd results in a lower dead space-tidal volume ratio and the answer to this question.


If my reasoning is correct, then the answer is good, but I could be wrong.

One other thing, the reason we even have this ratio is...

Bohr equation is used to quantify the ratio of physiological dead space to the total tidal volume, and gives an indication of the extent of wasted ventilation.
Vd / Vt = (PaCO2 − PeCO2) / PaCO2

As per wikipedia :)
Click to expand...


how is cpap different from peep? I know that peep increases tidal volume.. so wouldn't cpap then increase it as well? or is it because cpap is continuous then you don't exhale as much and thus your tidal volume decreases.
 
My first inclination was to jump to bronchospasm, since that will reduce the surface area and thus the volume of deadspace (bronchioless spasm in bronchospasm), which would make the numerator (and thus the ratio) smaller.

Even if you include the lost volume in both the deadspace and tidal volume, the math still makes the ratio less. Lets say the ratio is 1/2, and the bronchoconstriction is going to substract .25 from each of them, then (1-.25)/(2-.25) = 0.75/1.75 = 0.42. 0.42 is still smaller than 0.50.

Is the bronchospasm complete and thus reducing all air that would otherwise have passed to the alveoli? Then it would increase the numerator.
 
Impromptu said:
My first inclination was to jump to bronchospasm, since that will reduce the surface area and thus the volume of deadspace (bronchioless spasm in bronchospasm), which would make the numerator (and thus the ratio) smaller.

Even if you include the lost volume in both the deadspace and tidal volume, the math still makes the ratio less. Lets say the ratio is 1/2, and the bronchoconstriction is going to substract .25 from each of them, then (1-.25)/(2-.25) = 0.75/1.75 = 0.42. 0.42 is still smaller than 0.50.

Is the bronchospasm complete and thus reducing all air that would otherwise have passed to the alveoli? Then it would increase the numerator.
Click to expand...

Your explanation implies bronchioles are the only factor in Vd. You've also got alveoli, trachea and ETT. Bronchospasm does nothing to change the ETT/tracheal contribution to Vd.
 
I was gonna leave out the explanation to the others, but I'll hit 'em just fot the heck of it.

a. Bronchospasm doesn't really reduce the surface area. It is an obstructive process that is worse w/exhalation, and with time the pt becomes so inflated that diaphragmatic excursion is impaired. With diaphragmatic impairment the logical outcoe is a loss of the ability to ventilate, and hence, smaller tidal volumes.

b.CPAP: the idea behind CPAP is a constant pressure holding the airways open during SPONTANEOUS VENTILATION. Now, if you think about the expiratory cycle, it's a passive event. If the result of the passive event is movement of air out of the trachea, then it logically follows that anything that increases the pressure gradient (cpap produces a pressure opposing the tracheal expiratory pressure) in the opposite direction will decrease the flow. Decreased flow is decreased volume, which is a decreased Vt. One other thing, CPAP during GA (like peep) doesn't recruit new alveoli, it just prevents the closure of airways, and alveoli. That being said, Vd isn't really effected either way, but a decrease in Vd is prevented b/c CPAP is maintaining the status quo of the ventilation: perfusion ratio.

c.PEEP: Is a burst of pressure at the END of and expiratory cycle in an INTUBATED AND MECHANICALLY VENTILATED pt, that serves to prevent alveolar/small airway collapse collapse (think prevention of reaching closing capacity volume). The Vt is not really increased in this, and it's common to make that assumption. Some think that b/c you get a burst of volume at the end of the exp cycle, then you are taking in larger volumes and hence a greater Vt, and also b/c you have a larger number of alveoli, you ventilate more. The flaw here is that the P burst during end expiration prevents closure of airways and prevents the exhalation of the final 20-30 cc which translates to a slightly smaller Vt. Also you dont actually recruit alveoli with peep, all you do is hold open the ones that are already open. If you want to increase the # of alveoli, then you have to use a recruitment manuever such as holding a 40-50 mmH2O for 30 seconds or sitting pt up, using a sigh cycle on the vent, or something else.

d. Explanation above

e. With a PE you effectively eliminate the pulmonary blood flow, and ventilation w/o flow is the definition of Vd n hence increased Vd.

Again, I don't know the answer for sure, but logic and reason guided these answers. If something is off, please post it so I/we can learn from it.

BTW tnx 4 saying I'm ready for boards, but I don't think u can ever be too ready for those durned things.

:luck:
 
Last edited:
proman said:
Why do you think PEEP increases tidal volume?
Click to expand...


well peep keeps more alveoli open thusthere's more tidal volume.. plus I have seen in it happen on multiple patients in the OR.. we'd set the vent on pressure control for a a few min. and monitor the tidal volume then we'd add peep and lower the pressure so that the combined pressure would be the same, the tidal volume would significantly increase.


edit:
I guess it makes sense that peep doesn't recruit new alveoli and just prevents the open ones from closing like lvspro said above.. however, i have seen the increase in tidal volume on multiple patients so I am a believer... and all those patients have healthy lungs and were none smokers... maybe i'll do a study on that once i am a resident.
 
Last edited:
RussianJoo said:
well peep keeps more alveoli open thusthere's more tidal volume.. plus I have seen in it happen on multiple patients in the OR.. we'd set the vent on pressure control for a a few min. and monitor the tidal volume then we'd add peep and lower the pressure so that the combined pressure would be the same, the tidal volume would significantly increase.


edit:
I guess it makes sense that peep doesn't recruit new alveoli and just prevents the open ones from closing like lvspro said above.. however, i have seen the increase in tidal volume on multiple patients so I am a believer... and all those patients have healthy lungs and were none smokers... maybe i'll do a study on that once i am a resident.
Click to expand...

I'd be cautious generalizing those observations. Anesthesia ventilators are much different than ICU ventilators (as in worse). PEEP should have no effect on tidal volume. It will affect oxygenation and venous return/cardiac output.
 
proman said:
I'd be cautious generalizing those observations. Anesthesia ventilators are much different than ICU ventilators (as in worse). PEEP should have no effect on tidal volume. It will affect oxygenation and venous return/cardiac output.
Click to expand...

these were anesthesia ventilators. not ICU vents...we did this on only two patients and it worked for both of them.. try it out on one of your patients tomorrow and see if the same thing happens.
 
RussianJoo said:
these were anesthesia ventilators. not ICU vents...we did this on only two patients and it worked for both of them.. try it out on one of your patients tomorrow and see if the same thing happens.
Click to expand...

Which ventilator are you using. Some of the older vents do not automatically adjust related parameters when one is adjusted.

The other explanation is that machine adds the peep volume to the final tidal volume. So, if you set it to pressure of 20mm H2O, and deliver say 500cc, then reduce it to 15mm H2O, and 375cc with peep 6, and if the peep burst delivers 150 cc, the final Vt that the computer will calculate will be 525. This gives the illusion that a larger volume was being delivered when in reality the added volume is just that used during the expiratory-inspiratory lull to hold open the airways with a pressure setting of 6. The volume wasn't actually used to ventilate, it was used to prevent airway closure, kind of like a machine method of pursing your lips.

I think the Datex-Ohmeda site has a discussion about this. Check it out to confirm my answer.
 
Last edited:
lvspro said:
Which ventilator are you using. Some of the older vents do not automatically adjust related parameters when one is adjusted.

The other explanation is that machine adds the peep volume to the final tidal volume. So, if you set it to pressure of 20mm H2O, and deliver say 500cc, then reduce it to 15mm H2O, and 375cc with peep 6, and if the peep burst delivers 150 cc, the final Vt that the computer will calculate will be 525. This gives the illusion that a larger volume was being delivered when in reality the added volume is just that used during the expiratory-inspiratory lull to hold open the airways with a pressure setting of 6. The volume wasn't actually used to ventilate, it was used to prevent airway closure, kind of like a machine method of pursing your lips.

I think the Datex-Ohmeda site has a discussion about this. Check it out to confirm my answer.
Click to expand...

unfortunately i am no longer in my anesthesia rotation so i can't comment on the brand and model of the vents that we use, but they didn't look old, which really doesn't mean anything. However, I was trying this out with an attending and he would ask me what I would expect to happen, and he didn't correct me on this, so that's why I thought this was correct. I'll definitely look into this some more when I do my next anesthesia rotation in 1 month.
 
dhb said:
depends on the parameters of your mechanical ventilation.
by keeping more alveoli open it decreases dead space
Click to expand...

regarding PEEP - won't it actually increase dead space by over pressurizing already open airways (as in west zone 1) to further decrease perfusion?

i know this is an old thread but ITE is around the corner!
 
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