"Tidalling" is a meaningless nontechnical term indicating the rise and fall of fluid (like the tide, I guess) in the tube (also indicated in the water seal chamber) due to the changes in pressure inside the chest with ventilation. It has nothing to do with the presence or absence of a pneumothorax.
This little tutorial started out with a few sample NCLEX questions someone posted, and includes information on intrathoracic mechanics of breathing. I answered this one....
<< On the way to an x-ray examination a client with a chest tube becomes confused and pulls the chest tube out. The nurse's immediate action should be to:
1. Place the client in Trendelenburg position.
2. Hold the insertion site open with a Kelly clamp.
3. Obtain sterile Vaseline gauze to cover the opening.
4. Cover the opening with the cleanest material available.>>
As always in NCLEX-land (and in real life), you're looking for the answer that keeps the patient safest. I know you'd rather cover that hole with something sterile, but what is a greater immediate danger to this unfortunate fellow, an infection (which may not even develop) or a great honking pneumothorax (which certainly will)?
And while we're at it, let's talk about how you know whether to clamp or not to clamp a chest tube that has been disconnected from its drainage device (but is still in the pleural space). To understand this, let's look at the differences between a tension pneumo and a pneumo that isn't a tension pneumo .
Respiratory mechanics first ! When you breathe in, you're not actually pulling air into your lungs with your muscles. You're actually making a suction inside your chest with them (I know this may seem like a distinction without a difference, but stay with me), and the air enters the lungs thru the route provided for it to do so-- your trachea, via your nose or mouth (or trach tube, if you aren’t so lucky).
Your lungs are covered with a slippery membrane called the visceral pleura. The inside of your chest wall has one too, the parietal pleura. They allow the lungs to slip around with chest wall motion, like you can slip two wet glass plates around that are stuck together. Like the two glass plates, they're hard to pry apart due to the surface tension of the wet between them, and that's why the lungs fill the chest cavity and stay there. But just as you can easily pop those glass plates apart if you get a teeny bit of air between them, you can pop the bond between the two pleural layers with air, and if you do, the natural elasticity of the lung will cause it to collapse down to about the size of a goodish grapefruit.
How does the air get in the pleural space where it doesn’t belong? Well, you can do it two ways. One is to play rough with the bad boys (or have surgery, which is, after all, only expensive trauma) and have a sharp object puncture your chest wall and admit air into the pleural space. How does it get in there? Well, you make suction in your chest when you breathe in, and now air has TWO routes to get inside your chest-- down the trachea into the lungs, and thru the hole in the chest wall into the pleural space. This is called a pneumothorax, air in the chest that is outside of the lung. The lung will tend to collapse because the surface tension between the wet layers is now interrupted (remember how the pieces of wet glass can be separated by introducing air between them?) and the lungs are naturally elastic.
The other way to get air into your pleural space is from having blebs/bullae on your lung surfaces, and pop one (or more), or have some other hole in your lung (sharp things again). Then air gets out of your lungs thru the hole(s) and disrupts that pleural side-to-side thing, and there you go again, a pneumothorax. This, however, is called a TENSION pneumothorax, because that air increases with every exhalation (the lung now having two routes to exhale air out of, the trachea and the hole in the lung itself). This allows the lung to collapse on that side, and soon enough pressure (tension) will develop in that half of the chest to push the chest contents over to the other side, compromising blood flow and air exchange in the other lung & heart when it does so. (This is when you see the "tracheal shift.") This is also a bad thing.
So: now both of these fine folks have bought themselves chest tubes. The guy with the chest wall trauma has had his trauma hole sewed up, so when he takes a deep breath air enters his trachea only. He has a water seal on his chest tube so he can't pull air into his chest thru the tube-- the water seal acts like the bend in your sink drain and prevents continuity of the inside and outside places. The suction on the chest tube setup has done its job of removing the air from the pleural space where it didn't belong--it was seen bubbling out thru the water seal and then couldn't get back in. (When all the air is gone from his pleural space, there will be no more airleak in the water seal compartment.) Now, if he disconnects his Pleurevac (or other copyrighted device), he can again take a deep breath and pull air thru the open tube into his pleural space, where it doesn't belong, collapse his lung, and start all over again. THEREFORE, when this guy disconnects his tube, you clamp it IMMEDIATELY, to prevent air from entering the pleural space. He should ALWAYS have those two big old chest tube clamps taped to his Pleurevac (so they go with him to xray and all), just in case he does this.
However, the other guy, with the ruptured blebs and the intact chest wall? Well, his chest tube is pulling air out of the pleural space, but more is still getting in there since he still has a hole in his lung. The idea of the CT is to pull it out faster than he can put it in, and allow the hole to heal up, at which point he will no longer collect air in his pleural space and be all better. Meanwhile, though, you see air bubbling in the waterseal chamber, showing you that there is still air being pulled out of his pleural space. He has “an air leak.” What happens to him if his chest tube gets disconnected?
Well, remember, he still puts air into his pleural space, because there's still a hole in his lung. You put a tube in there to take it out, remember? OK, so what happens if you clamp his tube? Bingo, air reaccumulates in the pleural space all over again, his lung collapses, and things go to hell in a handbasket. This guy should NEVER have clamps at his bedside, because some fool may be tempted to clamp his tube before his airleak seals, and he'll get in trouble all over again. If he pulls his tubing setup apart, have him breathe slowly and shallowly (to minimize the air leaving the hole in his lung and getting trapped in his pleural space) while you quick-like-a-bunny hook him up again to a shiny new sterile setup. But do NOT clamp his tube while your assistant gets it set up for you.