do you fight a V/Q mismatch

  1. somthing clinical for a change:
    I have recently read about the phenomena that exsits where patients in a (lateral recumbent position) for example will have a v/q mismatch. I qualify this with the explination that due to gravity , perfussion will prefer the dependant lung while ventilation would prefer the independant lung do to the increased tone of muscles of expiration which causes greater recoil of the lung thus increasing the compliance of the independant alveoli compared to that of the dependant lung alveoli.
    now. what if I had to drop a lung? then what. what if i have an open chest which would only amplify the mismatch do the the increased distensbility of the independant lung. I am in awe as to how anesthesia providers blance these physiologic problems. I am sure we'll learn more down the road but due to my impatients I want an answer. how do you deal with a mismatch like this. what if one lung is sick.. yeesh... it just gets more and more complicated.
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  2. 27 Comments

  3. by   nilepoc
    Had this discussion with an attending just last week.

    You can do passive ventilation (like CPAP) of the independant lung, Just pass oxygen to it, but do not inflate it. To understand this, look up apnic ventilation, it turns out that you can maintain someones sat in the presence of complete apnea by just exchanging the gases through the application of oxygen through an ET. There is a time limit of about twenty minutes based on body habitus and size.

    Don't change your vent settings for one lung ventilation, (I still don't understand this).

    Have the surgeon temporarily clamp the pulmonary arteries of the independant lung. This will reduce shunting by a great deal, and increase your zone two and decrease zones one and three.

    Use some peep.

    I am sure there are some other techniques, but this is what I remember off the cuff. It usually takes me about three times of studying something to remember it well.

    craig
    Last edit by nilepoc on Feb 27, '04
  4. by   alansmith52
    ooo. i like the clamping of the pulm arteries to decrease the mismatch I havn't even thought of that. I am front loaded so i cant visualize much but i dont understand gas exchange durring apena.. ill look it up. it seems ther would still need to take place some sort of alveolar ventilation to keep the gas tensions normal.
  5. by   nilepoc
    Think diffusion and entropy, along with FRC
  6. by   Oldsalt
    One lung ventilation can create havoc on your anesthetic techniques d/t once collapse occurs a fair respectable r to l shunt develops in combination with and increase PA-a gradient - both of which predisposes the patient to hypoxemia......

    But the limiting factors of HPV offsets that but careful management of factors that we can impact (i.e. Hypocapnia, dilators, inhaled anesthetics, High Paw, low FIO2 levels, etc...) are also very important.

    But all of these factors and anesthetic considerations are widely available - so prep yourself and enjoy doing these cases.

    Take away points -

    Correct confirmation of tube placement is vital both after auscultation but more importantly after positioning (by Fiber Optic is the only definitive method)

    Know and act immediately to signs of poor oxygenation - be proactive and communicate your concerns- suction generously and reconfirming positioning

    Re-inflate intermittently, CPAP, PEEP, change TV/Rate - request early ligation of the ipsilateral pulm. Artery.

    Extubate conservatively (review of PFTs, history, labs, ect...will provide potential clues of needs of postop ventilation)...

    Most of all...... Have fun
  7. by   Tenesma
    actually apneic oxygenation can keep sats normal for up to 3.5 hours....

    if you want to be bold and correct the mismatch you can use Nitric Oxide!!!

    In my practice, i don't worry about the collapsed lung and will often tolerate sats in the high 80s for long periods of time (no biggie - unless they have cardiac disease) --- otherwise i just put the collapse lung on a bit of peep, and that usually fixes things.... or I just tell the surgeon (preferably at a non-critical point) that i will be re-expanding the collapsed lung a few times, before collapsing it...

    if you do thoracic anesthesia long enough, you will come to find that sats in the 70s and the 80s aren't always all that bad
  8. by   Brenna's Dad
    Anemia, significant blood loss, or decreased CO would be the other areas where 70-80 Sats would not be tolerable of course.

    In my vast thoracic experience (hehe) I consider four things:

    1. Maintain normal volumes to the down lung (decreases the V/Q mismatch)

    2. CPAP to the deflated lung (as already mentioned)

    3. Instillation of low flow O2 (3l for example) through a suction catheter placed in the deflated lung brochial tube. However, it is necessary to ensure that you dont create a complete seal at the suction port and end of inflating the lung. (ie. Passive oxygenation as already mentioned)

    4. Intermittent ventilation of the deflated lung after alerting the surgeon.

    I find passive oxygenation to be extremely effective and apparently offering excellent surgical conditions... in my vast experience (hehe).
  9. by   Tenesma
    a quick correction: long sats of 70s and 80s aren't bad when compared to 50s...
  10. by   alansmith52
    nice to see that when there is a post your interseted in you guys reply. I like hearing all the stuff i will be getting into later.
    thanks yall
  11. by   fence
    Whew! You guys are scarring the hell out of me.....lol. I can't wait to start gaining this knowledge. Thanks for the inspiring posts.
  12. by   gaspassah
    a little clarification for me please.
    i did my first thoracotomy last week. while i understand the interventions mentioned here, it is only if there is a need in the presence of V/Q mismatch. is this correct? the case i had went smoothly with no significant drops in O2 sats. so we didnt need to do any of these interventions, i just want to make sure i didnt miss something of importance.
    d
  13. by   Brenna's Dad
    Exactly gaspasser. If it aint broke, dont fix it.
  14. by   WntrMute2
    It is only if there is a need in the presence of V/Q mismatch. is this correct? the case i had went smoothly with no significant drops in O2 sats. so we didnt need to do any of these interventions, i just want to make sure i didnt miss something of importance.


    Actually, there WILL be a V/Q mismatch whether you see it or not. In the lateral position the normal blood flow to each lung is 60% to the dependent lung and 40% to the nondependent lung. The shunt flow is 10% divided equally between the 2 lungs (5% in each lung). So the non dependent lung has 35% of total cardiac output participating in gas exchange and the dependent lung has 55%. Once the nondependent lung is deflated, all blood flowing thru that lung is shunt flow and without HPV, 35% would need to be added to the 10% of usual shunt flow equalling 45% of cardiac output having a V/Q ratio that is closer to 0. HOWEVER, since HPV is assumed to shunt 50% of the nondependent lung's flow to the dependent ventilated flow, 17.5% (35%/2) plus the usual 5% shunt is therefore directed to the dependent lung. Now add in the 5% shunt from that lung and the total is assumed to be 27.5% shunt flow and 72.5% participates in gas exchange. A maximum PaO2 of 150 on an FiO2 of 1.0 is the usual maximum attainable. So, while not always clinically apparent, a V/Q mismatch does occur. Therefore one needs to avoid inhibiting HPV. Avoid SNP, NTG, Hydralazine, high MACs of volatile agents. Nitric Oxide reverses HPV so I'n not sure why that would help when you want HPV (Tenesma) Any other drugs I haven't mentioned??
    BTW, I had board questions revolving around these concepts, so study hard if you are near the big quiz.
    Last edit by WntrMute2 on Feb 28, '04

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