So you have a family practitioner doctor who does not have ventilator privileges standing over the bed of a morbidly obese patient who was just intubated and he says, "Gee, what tidal volume should we use. Let's see. How much does she weigh?"
By now you are already rolling your eyes and biting your cheek to prevent yourself from blurting our or, worse, actually slapping the doctor. Yet you're politically patient.
A nurse says, "I bet she's at least 500 pounds."
"Well then, " the good doctor says, "Let's set the tidal volume at 1,000." He looks at you.
While this is all going on you take your trusty ruler from your pocket and measure the length of the patient. You come up with 5 feet 6.5 inches. Based you your hospital's tidal volume protocol of 6-10cc/kg ideal body weight (not actual body weight), you come up with a tidal volume of between 420 and 700.
The doctor leaves the room, and you set up the vent and place the tidal volume at 500. You go low because the patient has a suspected lung problem. You can always adjust it higher later up or down if needed.
This happens very seldom anymore, but this is a scenario that I experienced in the past year. Thankfully we had a ventilator protocol, and the physician giving me the orders did not have ventilator privileges. Usually when this happens it's a family practice doctor or a surgeon. Surgeons are notorious for wanting those high tidal volumes.
And it's understandable, because back when I went through RT School the tidal volumes taught to us were on the high range. We were taught 10-15 cc/kg ideal body weight. And that tidal volume is fine for a completely healthy person. But you have to consider that most people ventilated in the ER and CCU do not have normal lungs, and you are better off ventilating on the low end, and adjusting later.
That said, obese patients do not have larger lungs. If you have a 100 pound lady who is 5 feet 8 inches tall her lungs are basically the same size as a lady who is 200 pounds at 5 feet 8 inches, and the same size as a lady who is 500 pounds at 5 feet 8 inches.
So if you ventilate either of these ladies based on their weight, you may be under ventilating or over ventilating. However, it's safer to under ventilate than over ventilate. If you over ventilate that 500 pound lady you might end up blowing up her lungs. Thus, it is highly possible you just saved this family practice doctor from a major law suit and he didn't even know about it.
You can read a great article about this here at PulmonaryReviews.com. According to this article there are other things we can do to help these patients better ventilate:
Putting the head of bed up 30 degrees so their abdomen is not pressing up against the diaphragm and impeding breathing. Where I work this is part of the ventilator protocol for all patients to diminish the chance of ventilator acquired pneumonia. Likewise, obese patients may become hypoxic in a supine position.
PEEP may also help with hypoxia. According to the above mentioned article, " In a study of nine obese patients who were anesthetized and supine after abdominal surgery, 10 cm H2O of PEEP was shown to markedly improve lung volumes and pulmonary compliance.[2] Those improvements were minimal in a comparison group of normal-weight patients.
For those with poor vasculature, ultrasonography may be very helpful in helping nurses find a vein, and a doppler may also be helpful. A doppler might also be tried if an ABG draw is needed.
Before these patients are intubated the patient should be trialed on BiPAP. I must admit that BiPAP is a machine that is used much more than it was when I started as an RT 10 years ago, and I have seen remarkable results. In many cases the BiPAP may prevent the patient from needing to be intubated.
According to this article you were just in going with the lower tidal volume, as their recommendation is to ventilate at tidal volumes of 5-7 cc/kg ideal body weight.
When it comes to medication, the article notes that "excessive weight-based dosages may be reasonable for medication-related adverse events in morbidly obese patients." Thus, when administering opoids, it is recommended that this be administered in "frequent small doses... until the desired level of pain control is achieved."
When weaning these patients it's best to have them sitting up in a 90 degree angle, or having them in a "reverse trendelenberg" position with their feet on the ground. With some of our newer beds this is possible. Studies, however, show the 45 degree angle worked best for weaning.
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