Monday, February 18, 2019
Corpsman Chronicles XIII: Under Pressure
"Who doesn't like Queen?"
Music is a simple thing, except for all the complicated stuff. I like lots of different music, and I dislike lots of different music. I like the stuff that sounds good to me, and I dislike the stuff that doesn't sound good to me. Often bands and/or musicians I love will produce songs I can't stand, and often bands I can't stand will produce songs I love. Like I said, simple.
Like pressure and pressure gradients.
We turn to the laws of thermodynamics to understand how materials behave in the real world. In these laws we find the explanation and proof of why gas tends to move from a higher concentration to a lower concentration. Down through the long centuries of painfully slow discovery we've come to find that temperature and pressure are similar ways of talking about the same thing, which turns out to be the movement of molecules.
At the beginning of first semester freshman chemistry you learn about ideal gas laws, which presuppose gas molecules existing in a closed and isolated environment, or container. It's really, really cool, because it makes so much sense. Add heat to the system and the molecules move faster, remove heat and they move slower, all the way down to absolute zero where all molecular motion stops. As the molecules move faster or slower they carry more or less kinetic energy, and impart that energy to other molecules as well as the walls of the closed container. The more energy imparted in these collisions, the higher the pressure, and vice versa. If you open a pathway from your closed, gas-containing container to a separate closed but empty container, the gas molecules will move from the one to the other down the pressure gradient from higher pressure in the full container to lower pressure in the empty container. This flow will continue until the pressure in the two containers becomes equal.
Now let's do something different, but along the same lines. We'll take a transparent container filled with ambient air and featuring a small opening on top and a closed stopcock at some location on the side near the bottom of the container. The exact location of the stopcock doesn't matter. The bottom of the container features a plunger which can slide up and down, rather like the plunger in a syringe. This closes the container except for the hole up top. In this configuration the volume of the chamber gets smaller as the plunger moves up and larger as it moves down, but the pressure remains the same because the hole up top allows inside and outside air to equalize. Now we place a balloon inside the container, affixed to the hole at the top with the mouth of the balloon passing through and solidly anchored in place. As the plunger moves up, decreasing the volume of the container, a couple of things happen. The pressure in the container increases as more molecules are squeezed into a smaller space. The pressure inside the container can't equalize because the balloon has blocked the hole. If you look closely you'll see the balloon shrivel up a bit as increasing pressure in the container presses it flat, forcing the few air molecules in the balloon out through its mouth which is affixed to the hole up top.
Next we move the plunger down, increasing the volume of the container. With the top hole blocked by the balloon, the air in the container still can't equalize with the outside air, so the pressure goes down and falls below ambient (room air) pressure. As this happens outside air rushes through the hole and into the balloon, flowing down the pressure gradient from higher ambient pressure to the lower pressure. As we watch, we see the balloon inflate. Air pressure inside the glass container remains below ambient pressure, and the pressure inside the balloon goes up -- expanding the balloon as air flows in -- until it matches room air pressure. When this happens the balloon stops expanding. Our container now has a partially inflated balloon inside.
Now let's connect some tubing to the stopcock and plug the other end into a vacuum pump. When we open the stopcock the pump sucks all the rest of the air out of the container and the balloon expands to completely fill the space inside the container. The reason it expands to fill the container is that the ambient pressure is higher than container pressure, and room air flows down the pressure gradient and into the balloon until the pressure inside the balloon is the same as ambient pressure. When we close the stopcock everything remains the same. Inside the container, in the very small space between the inner container wall and the outer skin of the expanded balloon, there is now a small negative pressure, and it's this negative pressure which allows the balloon to remain expanded, completely filling the container.
In this new configuration, when we move the plunger up and down, air flows in and out of the balloon as the volume gets smaller and larger, following an ever changing pressure gradient. The container continues to maintain a negative pressure so the balloon continues to fill the entire container regardless of the changing volume.
In this way we've created a crude model of our chest and lungs. The container is our chest, and the inside of the container is our pulmonary cavity. The balloon is our lungs, and the plunger is our diaphragm. The neck of the balloon is our trachea, which is affixed to the hole up top which is our nose and mouth.
For the novice student, the universe suddenly leaps into sharp focus and everything makes perfect sense.
Unfortunately, when you come back to class the following Monday you find that there are no closed systems in reality. The universe gets blurry again. You find that getting down into the weeds of real understanding is going to be a hard slog after all.
But the generalizations hold true. The pressure/temperature gradient is a solid part of the foundation, and you can build on that with confidence. More or less.
For the purposes of this post you really don't need to get down into the weeds. Just remember that gasses flow down the concentration gradient (higher to lower concentration/pressure) just as water flows downhill (higher to lower) in a gravitational milieu.
So there I was, seeing sick call on USS Coral Sea. It was February 12, 1988, and we were operating in the Aegean Sea between Greece and Turkey. It was just another ho-hum day at sea. Twelve days earlier the day had begun in much the same way; just another morning sick call. On that day a young sailor had died in the very treatment room where I was seeing sick call today. Ho-hum can fall apart in an instant.
Unbeknownst to most of the crew, our Battle Group had sent a cruiser and destroyer north the day before. All the way north into the Black Sea. USS Caron (DD-970) and USS Yorktown (CG-48) were exercising Freedom of Navigation and yanking Soviet chains in response to National Command Authority tasking. Anybody remember who NCA was back then?
Yorktown was the second Ticonderoga Class Aegis Cruiser launched.
Caron was a Sprucan named after Wayne Caron, a Hospital Corpsman who fell in Vietnam in 1968 and was posthumously awarded the MOH.
In an interesting coincidence, Wayne Cornell died aboard Coral Sea about six months less than twenty years after HM3 Wayne Caron fell in battle at Quang Nam Province, South Vietnam.
Butt I digress.
Anyway, there I was seeing sick call. As I recall, I was auscultating (listening with stethoscope) a patient's chest when one of the Flight Surgeons caught my eye and gave me the crooked finger c'mere sign.
"Might be a medevac in the works," he said.
In the SMO's (Senior Medical Officer) office the story sounded like this: Yorktown had been bumped by a Soviet destroyer, one of Yorktown's crew had broken some ribs and might have a pneumothorax, and we might need a quick-draw medevac. The Soviets were being pretty belligerent and Yorktown wanted to keep her own helos close by, so we'd scoot up there with an HS-17 Sea King and return with the casualty.
"So get suited up, If this thing goes it'll go in a hurry."
Meanwhile in the Black Sea...
I hustled up ladders and down passageways to my squadron paraloft where my flight gear resided. As I entered there were a pair of pilots and a pair of B/N's donning flight gear as well. I greeted them and assumed they were on the flight sked for the next routine go (launch/land cycle), but as I stripped and began squirming into a wet suit (February in the Aegean after all) they disabused me of that notion.
"What's goin' on Doc? They just set the Alert-30 SUCAP."
Which meant these guys would be manning up a pair of Intruders armed and configured for sinking ships. Ouch!
"Yorktown got bumped by a Russian can and they've got an injury, I guess we'll do a medevac if higher pulls the trigger on it."
As I rolled into the helo squadron's ready room I was reviewing chest injury procedures in my mind while my hands were checking supplies in my medevac kit. In particular I was making sure I had a 10 gauge, 8cm angiocath handy and that there were spares. My mind rolled back several years to a different place and a different ship where a similar injury had prompted a medevac.
In the blink of an eye I recalled how the supposedly stable patient had almost croaked on me and caused me to do my very first needle chest decompression.
Back then the call was from a Perry Class frigate.
The injured sailor had "possible fractured ribs" but was otherwise stable and ambulatory. We had to hoist him aboard because there wasn't room for us to land on the frigate. Technically there was room, and Sea Kings had landed on Perrys, but it was an awfully tight fit, and it just wasn't often done, at least in my experience. So we didn't. A complicating factor was that we couldn't (or shouldn't) hoist a fellow with chest trauma using the horse collar, which fits under the arms and around the back.
Therefore we had to hoist him aboard in a stokes litter.
I chafed a bit at the extra complexity involved as well as the increased risk to the injured man should we go in the water. It's quite difficult to get yourself out of a sinking helo and swim if you're strapped into a stretcher. I made up my mind to get him out of the stretcher as soon as he was aboard.
When we slid the stretcher into the helo everything changed. The injured man's eyes were bulging in panic and he was struggling and failing to undo the stretcher straps. He was obviously fighting to breathe and losing the fight.
My mind and body went into hyper drive. I tore the stretcher chest strap free and ripped open the struggling mans shirt. I could clearly see that his trachea was pushed far to the left as high pressure air from a "punctured" right lung remorselessly pressed on his heart and left lung. If the pressure continued to build in his chest he'd be dead very quickly indeed.
Something had changed drastically for my patient from the time we began to hoist the stretcher to when we pulled it into the helo, a period of about 90 seconds. What was it?
In medical terms he'd developed a tension pneumothorax. In plain English, pneumothorax means "air in the chest cavity," and the tension part means that the air is under pressure. A lot of pressure. If we go back to our container and balloon model, there was now air in the space between the inner container wall and the outer balloon wall. This space is supposed to be empty with a slight negative pressure in both our model and in real live humans. Without negative pressure in that space the lung cannot expand fully, which means at the very least a certain level of respiratory impairment.
The air in the in the man's chest was becoming increasingly pressurized, to the point where it was actually starting to push against and deform the mediastinum (heart and great blood vessels, trachea, esophagus, etc.) and the left lung. Everything was being pushed over to the left, and if the trend continued both breathing and heart function would be compromised, almost certainly leading to death.
Air had entered the sailor's chest cavity from his own lung, injured when he broke some ribs. Initially it had probably been a very small leak, unnoticed by both the patient and the frigate's corpsman. Over time, however, the leak had allowed enough air into the chest that the injured right lung began to be squashed. At some point a bit of lung tissue probably folded over the leaky bit, sealing it off. You might think this would be a good thing because it would seal the leak and prevent further air from entering the chest cavity. Unfortunately, what actually happens is that each time the patient breathes in there's enough pressure inside the lung to push the sealing tissue aside and force more air through the leak. On breathing out the pressure in the lung drops and the tissue sealing the leak moves back into place. What you now have is a one-way valve. More air enters the chest with each breath, but none of it can escape. Air pressure in the chest rapidly builds. This is a bad thing.
The fix is simple. You reduce pressure in the chest by letting the excess air out. Implementing the fix is a bit tricky. You have to open a hole in the chest wall to allow the pressure in the chest to equalize with ambient pressure.
In my training I'd been given a couple of unauthorized tools for just such an occasion. These tools -- techniques really -- were in common use in trauma centers and large emergency rooms, but they hadn't generally been authorized for use in the field by either civilian or military paramedics or EMT's. One was the stab thoracostomy, where you literally stab a knife blade through the chest wall. This was a very last resort, as you can imagine, and you'd better be exactly correct in your assessment and diagnosis before you proceed.
I discarded that tool without much thought and turned immediately to the second unauthorized tool, needle decompression. I zipped open my IV bag and selected a 12 gauge, four inch IV catheter. These are neat technology, featuring a flexible catheter fitted tightly over a needle. When starting an IV you'd poke the needle into the vein, then slide the catheter in and withdraw the needle. Hook your IV tubing up to the catheter and tape it down and you're golden.
Although not designed precisely for decompressing a tension pneumothorax, this style of IV catheter is just about perfect for the job. With one end inside the chest and the other end outside, you've got a clean, neat, and small but adequate pathway to let the high pressure air in the chest escape and equalize.
Moment of truth. With my left hand I felt for and found the correct site; second intercostal space on the mid-clavicular line. I lined up the big needle and catheter and quickly but firmly pressed it home. I could immediately feel pressurized air escaping. The injured man was already breathing more easily before I could tape the catheter in place. Whew!
The rest of the medevac was anticlimactic, at least for me. The injured sailor got a chest tube placed and after a couple of days his lung was healed and reinflated, good as new. The ribs took longer to heal, of course, but he returned to his frigate on light duty and carried on.
I got an attaboy and a stern but appropriate warning to be very careful about performing unauthorized procedures and not to let this success go to my head.
As we landed on Yorktown in the Black Sea on February 12, 1988, I rejoiced that we wouldn't have to hoist a stretcher. A Chief met me as I bounced out of the Sea King and escorted me toward the cruiser's tiny sick bay. The Chief was Yorktown's IDC -- Independent Duty Corpsman, and he gave me a rundown on the condition of the injured man.
"I had to decompress him," said the Chief, "he was really starting to get tight." The sailor looked tense and sore, laying there on the exam table with an IV catheter sticking out of his chest. Not a great position to be in, I guess, but he had no idea how lucky he was!
I, however, knew exactly how lucky I was!
As we launched with our very stable patient and headed back to the Med and Coral Sea, Yorktown was turning about and heading south too, escorted by several Soviet warships and a couple of rusty old freighters. The international incident was winding down.
We can do some pretty impressive things in medicine, but at the end of the day we're just providing rudimentary support to the miraculous living body that nature designed and produced.
It's all very simple, except for the complicated stuff.