Yesterday, I wrote about one of the weirder measurement systems in medicine, the French (or Charrière) system for catheter sizes. Today, I’ll deal with another arcane system, the gauge system for hypodermic needles.
I’ve always wondered how this worked. Lower numbers somehow indicate bigger needles. And to make it more confusing, there are two conflicting gauge systems, the Birmingham Wire Gauge (aka the Stubs Iron Wire Gauge) and the Stubs Steel Wire Gauge. Confusing, right? And these two are actually a little different.
Hypodermic needle sizes are based on the Birmingham gauge system (G). The Gauge indicates the outer diameter (OD) of the wire used to make the needle, but is inversely related to it. And to top it off, there is no real mathematical formula that relates gauge to the OD. If you are that interested, you have to consult a printed table.
Needle gauge ranges from 7G (4.57mm OD) to 34G (0.18mm OD). There is also little correlation of the inner diameter to gauge, as the wall thickness decreases as the OD of the needle decreases. As an interesting tidbit, there is an International Standards Organization (ISO) standard for single-use needles that determines the color coding of the hub. I always wondered who thought those up!
Okay, so needle gauge is based on the size of the wire used to make them. What about IV catheters? Just to make it more confusing, IV catheter size indicates the size of the needle used to insert them. So an 18G IV catheter has an inner diameter that is almost identical to the OD of its needle. The outside diameter of the catheter will be a tiny bit bigger. Got it?!
So there you have it. Amaze your friends with your arcane knowledge about tubes and needles and catheters!
In the past, I’ve written about the merits of needle vs finger thoracostomy. One of the arguments against needle thoracostomy is that it may not reach into the chest cavity in obese patients. As I mentioned yesterday, use the right needle!
Obviously, the one on top isn’t going to get you very far. The bottom one (10 gauge 3 inch) should get into most pleural spaces.
But what if you don’t have the right needle? Or what if the patient is massively obese and the longer needle won’t even reach? Pushing harder may seem logical, but it doesn’t work. You might be able to get the needle to reach to the pleural space, but the catheter won’t stay in it.
Here’s the trick. First, make the angiocatheter longer by hooking it up to a small (5 or 10cc) syringe. Now prep the chest over your location of choice (2nd intercostal space, mid-clavicular line or 5th intercostal space, anterior axillary line) and make a skin incision slightly larger than the diameter of the syringe. Now place the syringe and attached needle into the chest via your incision. It is guaranteed to reach the pleura, because you can now get the hub of the catheter down to the level of the ribs. Just don’t forget to pull out the catheter once you’ve placed the chest tube!
Tension pneumothorax is an uncommon but potentially lethal manifestation of chest injury. An injury to the lung occurs that creates a one-way valve effect, allowing a small amount of air to escape with every breath. Eventually the volume becomes so large as to cause the lung and mediastinum to push toward the other side, with profound hypotension and cardiovascular collapse.
The classic clinical findings are:
Decreased or absent breath sounds on the affected side
Hyperresonance to percussion
Shift of the trachea away from the affected side
Distended neck veins
You should never diagnose a tension pneumothorax with a chest xray or CT scan, because the diagnosis is a clinical one and the patient may die while these procedures are carried out. Having said that, here’s one:
The arrow points to the completely collapsed lung. Note the trachea bowing to the right.
As soon as the diagnosis is made, the right thing to do is to “needle the chest.” A large bore angiocath should be placed in the second intercostal space, mid-clavicular line, sliding right over the top of the third rib. The needle should then be removed, leaving the catheter.
The traditional large bore needle is 14 gauge, but they tend to be short and flimsy. They may not penetrate the pleura in an obese patient, and will probably kink off rapidly. Order the largest, longest angiocath possible and stock them in your trauma resuscitation rooms.
The top catheter in this photo is a 14 gauge 1.25 inch model. The bottom (preferred at Regions) is a 10 gauge 3 inch unit. Big difference!
The final tip to treating a tension pneumothorax is that a chest tube must be placed immediately after inserting the needle. If the patient is on a ventilator, the positive pressure will slowly expand the lung. But if they are breathing spontaneously, the needle will change the tension pneumothorax into a simple open pneumothorax. Patients with other cardiovascular problems will not tolerate this for long and may need to be intubated if you dawdle.