question on venous o2

[yu-gi-oh]

Po2 taken from internal jugular bulb is 45mmHg during halothane, nitrous,pancuronium anesthesia with nitroprusside infusion to control hypertension. Increase in jugular bulb PO2 to 50mmHg could be caused by except:

a. decrease in body temperature
b. increase in alveolar halothane concentration
c. nitroprusside infusion
d. hyperventilation

answer d.

I thought hypotension would cause decrease in PaO2 and thus cause decrease in venous O2???
I thought hyper ventilation would increase Pao2 and increase venous O2

Any gurus please explain this question???

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[dhb]

Not a guru but here it goes: hyperventilation will not increase PaO2 it will decrease PaCO2 which in turn will decrease CBF which will result in a higher extraction of O2 and a similar or lower (but not higher) jugular bulb PvO2.

dhb

 

[lvspro]

Not a guru but here it goes: hyperventilation will not increase PaO2 it will decrease PaCO2 which in turn will decrease CBF which will result in a higher extraction of O2 and a similar or lower (but not higher) jugular bulb PvO2.

dhb

Exactly. I can elaborate a bit, ust for my own edification.

The cerebral vasculature changes its radius with it's changing needs. Higher CMRO2's produce a concomitant increase in the CBF, and lower CMRO2's do the opposite. Another major player in the cerebral vascular tone is CO2. Studies indicate that the vasculature is sensitive to H+, and that a proportional relationship exists with vasc radius. Hence CO2 is the major player in cerebral flow, not O2. So when a pt is hyperventilated, CO2 is blown off, and the drop in H+ produces a decrease in vascular radius. In fact, many studies are now showing that hyperventilation produces areas of focal/global ischemia. That being said, if you take venous bulb sats and subtract it from arterial sat you will get cerebral O2 consumption. if O2 consumption is held constant, but less blood is passed through the brain, greater extraction will occur to the blood that is actually making its' way thru the brain. Hence, a lower sat will exist in the bulb with hypervent b/c a greater extraction was needed to make up for the lower flow.

 

[periopdoc]

a. decrease in body temperature
b. increase in alveolar halothane concentration
c. nitroprusside infusion
d. hyperventilation

answer d.

I thought hypotension would cause decrease in PaO2 and thus cause decrease in venous O2???
I thought hyper ventilation would increase Pao2 and increase venous O2

Any gurus please explain this question???

I don't know if I qualify as a guru, but since I did all of my neuro with Arthur Lam and/or his protege's (probably 80 or so cranis in the last few years with SjO2 monitoring), I am probably more familiar with SjO2 monitoring than average.

Lets start with the two statements you made at the end of the post. You need to review the basic determinants of PaO2 as ventilation and blood pressure have no effect on PaO2 unless you decrease them to levels incompatible with life.

As far as the options listed

a) Decrease in body temperature will decrease CMRO2 and thus decrease O2 extraction and thus INCREASE SjO2

b) Increase in alveolar halothane concentration will result in cerebral vasodilatation and it will further uncouple oxygen supply from CMRO2. It will INCREASE SjO2

c) Nitroprusside infusion is a tricky selection as a wrong answer as it preferentially decreases systemic vascular resistance over cerebral vascular resistance. However, in clinically appropriate blood pressure ranges, autoregulation is reasonably well maintained during nitroprusside infusion and there should be no change in SjO2

d) Hyperventilation is clearly the correct answer as it is a foundation of neuro-anesthesia that hyperventilation increases cerebral vascular resistance thereby decreasing cerebral blood flow and oxygen delivery despite no change in oxygen extraction. Any hyperventilation will result in a decrease of SjO2 and we use SjO2 monitoring to determine just how low we can go with PaCO2 without starving the brain of oxygen.


- pod

 

[Licoricestick]

Pod - I thought the question the OP raised was about an INCREASE in the jugular bulb PvO2 not a decrease?

I don't know if I qualify as a guru, but since I did all of my neuro with Arthur Lam and/or his protege's (probably 80 or so cranis in the last few years with SjO2 monitoring), I am probably more familiar with SjO2 monitoring than average.

Lets start with the two statements you made at the end of the post. You need to review the basic determinants of PaO2 as ventilation and blood pressure have no effect on PaO2 unless you decrease them to levels incompatible with life.

As far as the options listed

a) Decrease in body temperature will decrease CMRO2 and thus decrease O2 extraction and thus INCREASE SjO2

b) Increase in alveolar halothane concentration will result in cerebral vasodilatation and it will further uncouple oxygen supply from CMRO2. It will INCREASE SjO2

c) Nitroprusside infusion is a tricky selection as a wrong answer as it preferentially decreases systemic vascular resistance over cerebral vascular resistance. However, in clinically appropriate blood pressure ranges, autoregulation is reasonably well maintained during nitroprusside infusion and there should be no change in SjO2

d) Hyperventilation is clearly the correct answer as it is a foundation of neuro-anesthesia that hyperventilation increases cerebral vascular resistance thereby decreasing cerebral blood flow and oxygen delivery despite no change in oxygen extraction. Any hyperventilation will result in a decrease of SjO2 and we use SjO2 monitoring to determine just how low we can go with PaCO2 without starving the brain of oxygen.


- pod

 

[periopdoc]

Pod - I thought the question the OP raised was about an INCREASE in the jugular bulb PvO2 not a decrease?

Not sure what you do not like about my response. It is simply test taking 101. Read the entire question. (Emphasis added)

Po2 taken from internal jugular bulb is 45mmHg during halothane, nitrous,pancuronium anesthesia with nitroprusside infusion to control hypertension. Increase in jugular bulb PO2 to 50mmHg could be caused by except:...

That clearly limits it to two possibilities, Nitroprusside and Hyperventilation because the other two increase SjO2

Test taking 101 says the answer has to be hyperventilation because there is no question that low PaCO2 decreases SjO2, and even if you aren't training in a large neuro institution you should know that hyperventilation is dangerous because it decreases oxygen supply to the brain.

Now either I do not understand what the examiner is asking with C or the examiner doesn't understand the effects of SNP on SjO2. It has been clearly shown in multiple studies to have no effect up or down on SjO2 in anesthetized subjects. In awake subjects SNP has shown a decrease in SjO2, but that study was criticized because the SNP treated subjects also had a decrease in PaCO2 which could have caused the decreased SjO2.

Maybe I wasn't clear in my response as I had fresh salmon on the grill when I was typing and it needed to be flipped so I was in a hurry. MMMMM it was damn good.


- pod

 

[Licoricestick]

My mistake - sorry Pod.
I shouldn't try to post on SDN, cook and study simultaneously!

 

[lvspro]

Not sure what you do not like about my response. It is simply test taking 101. Read the entire question. (Emphasis added)



That clearly limits it to two possibilities, Nitroprusside and Hyperventilation because the other two increase SjO2

Test taking 101 says the answer has to be hyperventilation because there is no question that low PaCO2 decreases SjO2, and even if you aren't training in a large neuro institution you should know that hyperventilation is dangerous because it decreases oxygen supply to the brain.

Now either I do not understand what the examiner is asking with C or the examiner doesn't understand the effects of SNP on SjO2. It has been clearly shown in multiple studies to have no effect up or down on SjO2 in anesthetized subjects. In awake subjects SNP has shown a decrease in SjO2, but that study was criticized because the SNP treated subjects also had a decrease in PaCO2 which could have caused the decreased SjO2.

Maybe I wasn't clear in my response as I had fresh salmon on the grill when I was typing and it needed to be flipped so I was in a hurry. MMMMM it was damn good.


- pod

I am in agreement here... grilled salmon is delicious. Try a lemon dill aioli: Mayo, garlic, lemon juice, salt, dill, capers. food processor for everything w/o capers, and then add capers at the end. Or if yer feeling really saucy throw some roasted red peppers into the processor... oh yeah, the answer looks right here.


It would be kinda nice if we had someone post 2-3 questions a day that we could all come together and discuss like this. It's nice to hear opinions/explanations from people outside of my own institution. We get plenty of q-a discussions at my program, but it's a closed group so people like POD that bring a little extra to the table are very nice resources to have around.

 

[periopdoc]

Recipes on SDN Anesthesia forum??? Whats next home decorating and hairstyling tips? Any of you guys own a curling iron? ok in the interest of increasing SDN's Ying.

Nice recipe. I will give it a shot in Sept when the salmon isn't so fresh and needs a little zip to cover up that "not so fresh" taste. When the salmon is right out of the water like it is now, I pick up some Copper River salmon filets with skin on. Coat both sides with a light dusting of Lowery's salt and garlic powder. Let sit for 15 min while you soak a cedar shake in water. Heat grill to 350 to 400. Place cedar shake on grill for five min. Turn shake over and place filet on it. Cook until the salmon just bronzes over and the juice turns white. Take off the grill and let sit for five minutes. Serve on the shake. This will let the cedar aroma fill your dining area. It is great with grilled squash and zuchini. Just coat with olive oil, fresh ground salt and pepper and grill.


-pod

(disclaimer) being from Alaska, I am a total salmon snob. Copper river is excellent this time of year when it is fresh out of the water, but sucks for storing in the freezer. Try to find a leaner salmon like Kenai River. The farmed ****? I am not sure that stuff is even fish, much less salmon. Get some real Alaska salmon.