If you’re going to paint by numbers, at least paint by the right ones.
Dr. Zaffer Qasim put out a paper recently that got my wheels turning (thanks for the shout out, Doc!). It is about placing an art line in the common femoral artery and catching the patient in shock before it blows up in our faces. It made me think of how we in the prehospital setting can see shock coming without the use of an art line. I will argue that it would be really nice to have one for these patients and eliminate the guesswork and the math, but that’s a talk for another day.
Check out their paper here and another one by Perera that puts the spotlight on NIBP measures and compares the accuracy against an art line. It certainly raises my eyebrows to read these.
Murali S, Cannon J, Joergensen S, et al. Video review analysis of early common femoral arterial access in trauma: Can we identify occult shock? Injury. 2026 Mar 25:113169. doi: 10.1016/j.injury.2026.113169. Epub ahead of print. PMID: 41896154.
Perera, Y., Raitt, J., Poole, K. et al. Non-invasive versus arterial pressure monitoring in the pre-hospital critical care environment: a paired comparison of concurrently recorded measurements. Scand J Trauma Resusc Emerg Med 32, 77 (2024). https://doi.org/10.1186/s13049-024-01240-y
The Burr Under My Saddle
It is a well known fact that a drop in blood pressure is not a sign that the patient has suddenly gone into shock, but that we have suddenly noticed. Another way to say that is that we have missed the more subtle signs that the patient was in shock. Their ability to compensate gave us a false sense of security that the patient was stable…
Or worse… we knew that there was a problem and we waited for the hypotension to give us permission to treat. This is the reality for the majority of prehospital and critical care transport programs I have seen. They think:
"I can't do x until the number changes to y."
…and it is a terribly flawed way to approach sick patients. It is flawed because so many other things happen that warn of impending badness that we can use to support our actions, even if they fall out of the black and white language of a guideline or protocol.
As it pertains to patients in shock, how do we think better? How can we use those spaces between the boxes in the protocol book to come to better conclusions and deliver better care for our patients? I say, you maximize the clinical information you are getting. We do not, generally, have A-lines available to us (with the exception of the CCT world). How can we make better decisions with less invasive monitoring?
"Use your head and your hands," as a colleague once told me.
If you want to catch shock in the occult phase (aka Class I) instead of it catching you in the decompensated phase (Class III-IV). You need to pay attention to everything on your screen. When you consider that oscillometric blood pressure monitoring via NIBP is not an exact SBP or DBP that is either under or over estimating the true blood pressure… and the worse the patient’s shock state, the less reliable those numbers are, one should be compelled to evaluate what the numbers are telling you instead of just looking for numbers.
A more eloquent way to put it: Stop looking at vital signs and start seeing physiology.
1.) Pulse Pressure3-5
Perhaps the best way to make sense of the patient's hemodynamics that your instructor never taught you. The pulse pressure is the relationship between the systolic BP and the diastolic BP, calculated like this: PP = SBP-DBP. What this is a picture of is the volume of blood ejected by the LV (SBP), and the patient's vascular tone (DBP).
At first, in shock, the pulse pressure is narrow. Meaning that the SBP and DBP are closer to each other (generally <30-40). Which makes some physiologic sense as the initial response to hypoperfusion (or hypovolemia in hemorrhage) is to vasoconstrict in an attempt to increase (or maintain as close to normal) the output of the LV.
This is the first sign of badness you should look for. A BP of 110/90 may look sort of reassuring. The MAP is well above concerning thresholds… but that pulse pressure is super narrow. Perhaps an indication of maximal compensation. DBP of 90 should scream that the SVR or vascular system is maxed out. Perhaps they will respond to volume resuscitation, but that is not an absolute truth.
The question you should have is: They are compensating for how much longer?
In later stages of shock, the pulse pressure widens. These numbers are getting farther apart, meaning that we may have corrected the volume problem (evident by a good SBP), but the patient is losing their ability to vasoconstrict. Dr. Sara Crager suggests that this is a sign of catecholamine exhaustion and you are about to "suddenly notice" that the patient is on their way to a hemodynamic catastrophe (everything she does is awesome btw @ icuedu.org).
In practice, if I see a SBP of 98 but the DBP is 55, 60, etc. I am already moving toward something more aggressive. Why so aggressive? Vascular tone is 2/3's of the MAP.
(Note: To calculate MAP: MAP = DBPx2 + SBP / 3).
The DBP has double the value of the SBP and therefore is the biggest contributor to MAP and stabilization. In my opinion, DBP is the trigger for aggressive vasopressor support, not the SBP. Maybe a better way to say that is, look at the MAP, then look at the DBP, and then the SBP.
Just as important is how FAST are they changing.
2.) TREND the Shock Index6-7
If you want to use the standard Shock Index (HR/SBP) or the Modified Shock Index (mSI, HR/MAP). I am a fan of the mSI since it takes the DBP into account and gives a more complete picture than the SBP alone does. Having said that, in a tight situation I will 100% look to see if the HR is > than the SBP. I know the SI or mSI is at least 1 and the patient is in trouble. What I love about this measurement is that it is OBJECTIVE. You do math and the number is what it is. There is no room for "well, they don't feel cold to me," or "that isn't pale" meanwhile the patient looks like ET when they found him in the creek.
Maybe the shock index is the number we should be using to trigger escalations in care.
A single number should never get you to pull the trigger on a big move. A five or ten minute trend in the wrong direction should absolutely have you making a big move to stabilize the patient. Also concerning is how fast it changes. Did the MAP trend down over a 20 or 30 minute transport, or did it drop precipitously over 5 or 6 minutes? You should absolutely be concerned with the velocity of the changes, not just that the numbers are not looking good.
Speaking of numbers, a SI of 0.7 - 0.9 is SHOCK. They just have intact compensatory mechanisms. When it is >1.0, you are late to make the big move. That range of 0.7-0.9 is the window of opportunity to make a significant impact and make an aggressive move. At 1.0, you are forced to chase the patient's physiology. The sooner you start trying to move the needle, the more likely it is to move. The longer you wait to make a move the less likely it is to work as well as the patient needs it to.
3.) ETCo28-11
What other tool gives you a glimpse of ventilation status, cardiac output, and metabolism all at once? We don't have one. ETCo2 for the critically ill patient is not an option, in my opinion. There is just too much valuable information there.
ETCo2 has a direct correlation to the patient’s cardiac output.
In one study that looked at pre-hospital ETCo2 as a predictor of in-hospital mortality. The results were rather eye opening for me when I read this paper. It overwhelmingly outperformed other standard vitals in predicting in-hospital mortality and survival to discharge. The survivors had an average ETCo2 of 24, while the non-survivors consistently had an ETCo2 of 25 or less (average was 25). What I think is an important take-away here is that these patients were not differentiated by illness like the other studies (which agree with this one) that looked specifically at shock, trauma, sepsis, and metabolic disorders.
You are specifically looking for low end tidal numbers in the absence of a dramatic tachypnea. Are these patients tachypneic? Sure. It is expected. But… hypotension (SBP 80) + ETCO2 < 25-30 should not have you thinking about respiratory issues, it should scream “imminent cardiovascular collapse” at you. In fact, there is a roughly 3:00-4:00 window for you to act and avoid a cardiac arrest with that combination of vitals (specifically the SBP <80).
When the hemodynamics, metabolism, and ventilation are all normal, then you can trust ETCo2 readings to be purely a reflection ventilatory status.
| Metric | Compensated | Uncompensated |
| Pulse Pressure | Narrowing and <30 | Very low/Unreadable |
| Shock Index | 0.7-0.9 | >1.0 |
| ETCo2 | >30 | <25-28 |
This rounds out part 1 of a series on shock. There are so many signs that a patient is in shock and in trouble before their blood pressure reaches some arbitrary number. Most importantly, a single number matters not. Trends are the most important thing, and the VELOCITY of those trends is the key detail to pay attention to. Pro tip: set your NIBP to cycle at the lowest time interval available on your monitor.
Fun fact to cue up your brain for part 2: a patient in shock can demonstrate a lactate >2 (an indicator of tissue hypoperfusion) with a SBP >100.
References:
(Google Gemini was used to assist with the formatting and referencing of this article. At Pulse Check, we believe in high-performance education. We use AI to help synthesize massive amounts of clinical data and structure our layouts, but every clinical take, every 'dirt-dive' analogy, and every final word is vetted and written by a human paramedic. We use the tech so we can spend more time on the medicine. The conclusions here are wholly our own.)
1. Perera, Y., Raitt, J., Poole, K. et al. Non-invasive versus arterial pressure monitoring in the pre-hospital critical care environment: a paired comparison of concurrently recorded measurements. Scand J Trauma Resusc Emerg Med 32, 77 (2024). https://doi.org/10.1186/s13049-024-01240-y
2. Murali S, Cannon J, Joergensen S,et al. Video review analysis of early common femoral arterial access in trauma: Can we identify occult shock? Injury. 2026 Mar 25:113169. doi: 10.1016/j.injury.2026.113169. Epub ahead of print. PMID: 41896154.
3. Jouffroy R, Gilbert B, Tourtier JP, et al. Prehospital pulse pressure and mortality of septic shock patients cared for by a mobile intensive care unit. BMC Emerg Med. 2023 Aug 25;23(1):97. doi: 10.1186/s12873-023-00864-0. PMID: 37626302; PMCID: PMC10464421.
4. Putowski Z, Teboul JL, Castro R, et al. Can we reliably use pulse pressure as a surrogate for stroke volume? Physiological background and potential clinical implications for shock resuscitation. Crit Care. 2025 Jun 19;29(1):249. doi: 10.1186/s13054-025-05490-9. PMID: 40537864; PMCID: PMC12178023.
5. Crager S. The physiology of hemodynamic collapse. Ann Emerg Med. 2021;78(4):S15-S16.
6. Koch E, Lovett S, Nghiem T, et al. Shock index in the emergency department: utility and limitations. Open Access Emerg Med. 2019 Aug 14;11:179-199. doi: 10.2147/OAEM.S178358. PMID: 31616192; PMCID: PMC6698590.
7. Singh A, Ali S, Agarwal A, et al. Correlation of shock index and modified shock index with the outcome of adult trauma patients: a prospective study of 9860 patients. N Am J Med Sci. 2014 Sep;6(9):450-2. doi: 10.4103/1947-2714.141632. PMID: 25317389; PMCID: PMC4193151.
8. Öztürk Örmeci G, Yiğit Ö, Eray O. Utility of ETCO2 to predict hemorrhagic shock in multiple trauma patients. Turk J Med Sci. 2022 Feb;52(1):206-215. doi: 10.3906/sag-2103-206. Epub 2022 Feb 22. PMID: 36161601; PMCID: PMC10734833.
9. Kheng CP, Rahman NH. The use of end-tidal carbon dioxide monitoring in patients with hypotension in the emergency department. Int J Emerg Med. 2012 Jul 24;5(1):31. doi: 10.1186/1865-1380-5-31. PMID: 22828152; PMCID: PMC3585511.
10. Hunter CL, Silvestri S, Ralls G, et al. The sixth vital sign: prehospital end-tidal carbon dioxide predicts in-hospital mortality and metabolic disturbances. Am J Emerg Med. 2014 Feb;32(2):160-5. doi: 10.1016/j.ajem.2013.10.049. Epub 2013 Nov 4. PMID: 24332900.
11. Richard A, Johns J, Wolfe A, et al. Systolic Blood Pressure Threshold for HEMS-Witnessed Arrests. Air Med J. 2018 Mar-Apr;37(2):104-107. doi: 10.1016/j.amj.2017.11.014. Epub 2018 Jan 12. PMID: 29478573.