All images created by the author.
You’re three hours into the quietest Friday you’ve had in months. Your partner is reading something on their phone, you’re halfway through a crossword puzzle in pen, because you live dangerously.
You are posted up at Engine Company 2 for the time being and the station smells like someone made actual soup on the stove. Not the canned kind. Real soup, with actual fresh ingredients in it. You make a mental note to find out who’s responsible and extend them a formal thank you. And then the tones drop and your radio goes off.
You’re being dispatched to a multi-vehicle collision at the exit 42 off ramp to I-70. Before the dispatcher finishes the address, your partner is already on their feet. The highway is six minutes out in normal conditions. Traffic splits open for you for what feels like the first time ever. Some people pull over immediately, some freeze completely, and tonight there’s a lot of both.
You arrive to find three vehicles involved: a pickup that appears to have clipped the median, a sedan that hit the pickup, and a compact that rear-ended the sedan. Tower 1 is already working the driver’s door on the sedan with significant intrusion on the driver’s side. Law enforcement is managing a growing crowd of stopped cars.
The tower officer waves you over and is pointing to the driver side of the car they are working on. You grab your bag of tricks and box of life and head over. You take a quick 360 of the car and note the intrusion and what appears to be in and out of consciousness and breathing fast and shallow. The Tower gets the door right after you hop in through the rear passenger side.
The driver is a male who appears to be in his early sixties. Partially conscious, eyes slowly opening and closing, but not alert. He isn’t tracking to your voice or penlight when you check his pupils. Which are equal. You ask him his name and get a faint “Larry”. His front airbag deployed. But that is all he had with this being a later model sedan.
The steering wheel has visible deformity and you cut his seatbelt off with your sweet multi-tool you’ve been dying to use. You do a quick head to toe as best you can while maintaining C-Spine. Larry clearly doesn’t like you feeling his left chest wall.
You grab his wrist: radial pulse present, fast, and quite thready and you feel the patented shock stickiness (A.K.A. diaphoretic and clammy). You look at his chest. It’s moving, but ever so slightly. It looks almost as if the left side rises less than the right with each breath. His respiratory rate is somewhere above 30, fast and shallow.
You look at his neck while your partner gets the monitor on. JVD, subtle, but it’s there when he breaths in. You prepare to move. You opt to just get him in the unit to allow for better assessment and intervention. He has an airway, breathing and has a pulse with no major external hemorrhage. You are worried about his chest and pelvis from the assessment, mechanism and vitals.
The Tower helps you slide the long board under him with minimum manipulation. Your partner is already on the radio calling the trauma alert. You get in the back of the rig in what feels like 30 seconds.
You let Larry know you are going to cut his clothes and check the rest of him out. Your partner tilts the head of the cot up to 30 degrees in case of head injury while they are also putting on the leads (who says we can’t multitask).
Your partner turns the monitor.
You reach for your stethoscope. You listen to the right side and hear shallow but present breath sounds. You move to the left and don’t think you hear anything. No…..wait……very diminished on the left. Not absent but very diminished. Sounds like a NREMT test question right? Your partner hands you the ultrasound so you can confirm what you here. What do you think based on the presentation and vitals? I don’t have any slide!
You let your partner know “Left side tension. I’m going to needle.” They look over to you handing your the harpoon as if they were two steps ahead. I mean, that’s what good partners are all about.
You locate the second intercostal space, midclavicular line on the left. You clean it quickly, confirm your landmarks rib above, rib below, go above the lower rib to avoid the neurovascular bundle and advance the needle. You just got those new fandangled decompression needles with the float valve when you hit the air dense space.
Since “the rush of air” is only for the movies and not the easiest thing to appreciate with sweaty gloves, in the back of a idling ambulance on the side of an interstate. You see the float bounce and you know you’re in.
Your partner finishes the pelvis and legs for you and says they are unremarkable. You thank them, let them know they can start towards university and you peak over at the monitor while calling in your trauma alert.
Your partner starts to hop out the back and mentions the “End Tidal is coming up!”
You reply with, “Nice! I see it. HR is coming down too. Let’s roll, bro.”
His eyes open slightly. He tries to say something.
Larry asks, “…what happened.”
You let him know “You were in an accident. We’re taking care of you. You’re going to be okay. Don’t try to talk right now.”
He blinks slowly. His radial pulse is stronger now. You note that. You also note the time: from scene arrival to needle, four minutes and forty seconds. The number speaks for itself.
The Circle Back
Let’s reconstruct the picture before we talk intervention. Unequal chest rise. Diminished left breath sounds with slight (not major) jugular venous distention (noted on inspiration). BP of 82/64, MAP well below 65. Tachycardia of 138. SpO2 of 88%. Mechanism: high-speed collision with left-sided chest wall involvement and steering wheel deformity.
But most importantly an EtCO2 of 18 mmHg, with a flattened, low-amplitude waveform.
That capnography number isn’t just low, it’s telling you the most important thing happening in this patient right now: circulation is failing. Here’s the physiology. CO2 is produced by cellular metabolism and transported by the venous blood back to the right side of the heart, this return is heavily reliant on the negative pressure made when you breath in.
It “sucks” blood back into and up the thorax. It then pumps to the pulmonary system, where it crosses into the alveoli and is exhaled. For CO2 to show up in the exhaled breath, blood has to get to the lungs. If cardiac output drops, less CO2 reaches the alveoli, and the exhaled value drops.
This patient is breathing thirty-four times per minute, and his EtCO2 is eighteen. The air is moving. The blood is not.
In tension pneumothorax, air enters the pleural space—through a lung laceration, rib fracture, or chest wall injury—and cannot escape. Each breath adds more. The pressure builds. The affected lung collapses.
The great vessels, especially the vena cava begin to kink and compress (get pinched). The right heart can no longer fill. Cardiac output drops. The body compensates with tachycardia and vasoconstriction, which buys minutes. When those minutes run out, cardiac arrest follows.
The waveform you’re looking at has low amplitude because there is simply less CO2 being delivered. It’s not an artifact. It’s not a probe issue. It’s cardiovascular collapse displayed in real time, one exhaled breath at a time.
The Pathophysiology Snapshot
Tension pneumothorax is one of the few immediately reversible causes of traumatic cardiac arrest. As the tension develops and venous return to the right heart drops, CO2 delivery to the alveoli falls in direct proportion to the fall in cardiac output.
The patient may be breathing rapidly compensating through increased rate and more shallow depth but the EtCO2 stays critically low because there is little to no perfusion to bring CO2 to the exchange surface.
Post-decompression, the rise in EtCO2 is one of the earliest and most reliable signs of hemodynamic improvement often preceding a visible change in blood pressure or heart rate. You are watching the heart fill, and the cardiac output recover, all through the lens of a single exhaled number climbing back toward normal.
What was Larry at? Eighteen. Say it out loud. His end-tidal CO2 was 18mmHg, in a patient who is breathing thirty-four times per minute. He was working hard. He was not getting anything out of it.
For the POCUS you want to see lung sliding. It looks like a ripple or sparkle going across the chest between the parietal pleura and visceral pleura. It let’s you know there isn’t pressure between the space and the serous fluid can allow the two layers to move. Some call it ants marching.
*** Street Trick: In trauma with a low EtCO2 and a flat, small-amplitude waveform: think two things before anything else. Is the blood getting there? And is the chest the reason it isn’t? Low EtCO2 in trauma is a perfusion problem until you find another answer. The capnography will also be the first thing to tell you that your needle or therapy worked.
Last thing to leave you with for another shift. If Larry just broke ribs and it hurt to inhale what would his breathing do? What would his depth of breath do? Would you get good lung sounds through your horrible agency bought $22 stethoscope? But would the EtCO2 fall?
Lastly the heart sits tilted forward and to the left. In a driver who had left lateral impingement be concerned for cardiac contusion and potential dysrhythmia or tamponade. More on that later.
Chris Kroboth has been a career paramedic/firefighter for over 17 years and in EMS for over 23. He has been in prehospital and in-hospital education for the past 18 years. His last assignment before returning to operations was as the EMS training captain in charge of continuing education programs and certification. He is also affiliate faculty with the Virginia Commonwealth University Paramedic Program. He is the U.S. clinical education manager for iSimulate and also facilitates national conference clinical challenges to include EMS World, ENA and NTI.
Another great article written by Chris. The breakdown of the physiology behind each of his scenarios he writes about it excellent and easy to understand.