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.
Feb 27, '04
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.
Last edit by nilepoc on Feb 27, '04
Feb 28, '04
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