Okay back to our problem.
There really are two solutions. The first, as we often must do in medicine, we have to make some assumptions. The assumption we are going to make is that the patients' compliance is linear. This is seldom true but often true enough for the range of ventilating volumes and pressures we are using with most patients. Like all assumptions you need to be cognoscente of making it and realize that it may not apply or alter your observations at the bedside.
Okay with that said, we now assume a constant lung compliance for the patient. The present "C" (compliance) of the patient is 25ml/cmH2O. Now we know the equation for calculating is; C= volume / pressure. Using this equation we can solve for the change in pressures with the increased volume. I like to set this up as a proportion;
present C 25 = Change in volume 800ml / unknown new pressure in lungs X
Cross multiply to solve for the unknown pressures X; so now
unknown pressure X = 800ml / 25 ml/cmH2O
X = 32 cmH2O
So this pressure, 32cmH2O is the expected increase in pressure that would result from increasing the lung volume from FRC (functional residual capacity) with an additional 800ml. You can think of it as inflating a stiff balloon with 800ml of volume.
Now let us consider the second answer, what if the lung compliance is not linear? How would this affect our estimation? It is possibly a valid answer to say that the exact pressures could not be calculated without measuring the dynamic compliance of the lungs. We will discuss and demonstrate this in class. So if the actual lung compliance of the patient at the higher volume is lower, then our calculated pressure would be low. The actual pressures in the patients' lungs would be higher than calculated.
If the actual patients' lung compliance is higher at the larger volume, and this can be true, then our calculated pressures would be high. The actual lung pressures would be lower than our calculated value.
Comments or questions let me know below.
Added Later: Graph of idealized lung compliance, lower zone = low compliance with alveolar collapse, middle zone =best compliance with open lung, upper zone = low compliance due to alveoli over inflation.
Al
So is it okay to say that compliance is inversely proportional to the change in pressure if you assume that compliance is not linear. If that is the case would the compliance be directly proportional to the change in pressure assuming a linear compliance?
ReplyDeleteOne last thing, what affect does increasing the volume have on the patient? is there a certain point were you could damage a persons lungs with too much volume? And when you increase the volume are you doing that in terms of adding gas like oxygen?
Hello Anonymous:
ReplyDeleteYes compliance is inversely proportional to the pressures in the lungs but the concept of the none linear element is that compliance changes as the volume and pressure changes in the lungs. For many patients low lung volumes, where many alveoli are collapsed the compliance is low. Then as the volume is increased and more alveoli are opened up (above the lower inflection point) the compliance increases. Now when the lungs are almost completely filled and many alveoli are filled or overfilling and completely stretched the compliance decreases. Our goal with mechanical ventilation is to use an appropriate level of PEEP to keep the lungs inflated to prevent the majority of the alveoli from collapsing and to use small enough volumes so we are not over distending the lungs. Ventilate in this optimal compliance zone.
This leads into the second part of your comment about too much volume. Yes too high a volumes would over distend the alveoli and potentially cause rupture. It seems from the literature and my understanding this rarely happens and it is more important to try and maintain the alveoli open with adequate PEEP to prevent lung injury. No matter what we do, lung injury will result from mechanical ventilation and oxygen use.
Thanks for your questions.
Al
Thanks for answering the questions it is making more sense. However, can you clarify what you mean when you say ventilate in this optimal compliance zone, I don’t quite understand.
ReplyDeleteSo the purpose of PEEP is to keep the airway from collapsing on exhalation, then would that mean that the more sever the alveolar collapse the more pressure you need to open them back up. Also, does that mean you can undo the damage of atelectasis?
Hi Anonymous:
ReplyDeleteI have a coffee in my hand my texts are at the office but I would like you to find the section on compliance of the lung in your ANPH 221 text. It would help our discussion if you could see the “S” shaped curve that represents lung compliance in your text. I like the one in the Egans text, page 221, but many find this one confusing. The basic idea is that low lung compliance is represented by a near linear line, higher lung compliance is represented by a more vertical line on the graph. It is this zone in the middle with the higher lung compliance that I am referring to as the optimal zone for ventilation. The lower zones and upper zones have lower compliance as previously discussed.
Your comments about alveolar collapse and atelectasis are very insightful! We can reverse these effects with PEEP and help to prevent that lung damaging condition, that also limits gas exchange effectiveness.
WOW thanks for the opportunity to discuss this!
Al
Hi Anonymous:
ReplyDeleteNote I have posted a very crude compliance curve on the original "Solutions Solutions" above and let me know if it helps.
Al
Thanks so much... It's coming together and making sense. Yeah!!
ReplyDeleteYes the compliance curve helps a lot... it's good becasue it gives you another way of looking at the material
ReplyDeleteonce again thank you