The blood pressure is 118/74. Heart rate is 104. SpO2 is 98%. Respiratory rate is 22.
Your patient is awake; answering questions appropriately; and complaining mostly of weakness, nausea and “just not feeling right.” Nothing about those numbers immediately screams shock.
Then you look at the capnography. ETCO2: 24 mmHg. A few minutes later, it is 21. Then 19.
Now which vital sign has your attention?
| RIDE THE WAVE WEEK: Using capnography to guide care
The call
You are dispatched to a private residence for a 67-year-old woman complaining of weakness and dizziness. Her daughter tells you she has been sick for several days with what the family assumed was a urinary tract infection. Today she became too weak to walk to the bathroom.
Your initial assessment reveals a blood pressure of 118/74, heart rate of 104, respiratory rate of 22, SpO2 of 98% on room air, and temperature of 101.7°F. She is pale but alert. Her lungs are clear, and she denies chest pain.
Nothing looks catastrophic. You place a nasal capnography cannula. The waveform is consistent.
ETCO2 is 24 mmHg. Five minutes later, it is 21. Her blood pressure is now 110/68. She still looks reasonably stable. It would be easy to think, we have time. But do we?
Blood pressure does not equal perfusion
EMS providers are trained to recognize hypotension as a sign of shock. The danger comes when we begin using hypotension as the definition of shock.
Shock is fundamentally a problem of inadequate tissue perfusion and oxygen delivery, not simply low blood pressure. The body can compensate through increased heart rate, vasoconstriction and redistribution of blood flow. For a period, arterial pressure may remain relatively preserved while the patient’s condition deteriorates.
Research involving injured patients supports the value of looking beyond blood pressure. Prehospital ETCO2 has been associated with mortality and massive transfusion requirements, including among patients who were normotensive in the prehospital environment
ETCO2 does not replace blood pressure. It gives us another piece of the physiologic puzzle.
ETCO₂ is more than a ventilation number
Many EMS clinicians first learned capnography as a tool for confirming endotracheal tube placement. That remains critically important, but ETCO2 can tell us much more.
For CO2 to appear on the capnogram, several things must happen: Metabolism → circulation → pulmonary perfusion → ventilation → exhaled CO2
Cells produce CO2. Circulation transports it to the lungs. Blood delivers it to the alveoli, and ventilation removes it. That means ETCO2 is influenced by metabolism, circulation and ventilation If cardiac output and pulmonary blood flow decrease, less CO2 may reach the alveoli.ETCO2 can therefore fall as perfusion worsens. But here is the important clinical warning: A low ETCO2 does not automatically mean shock.
Hyperventilation can lower ETCO2. So can metabolic acidosis with respiratory compensation, pulmonary embolism, ventilation-perfusion abnormalities and technical problems That is why we don’t treat the capnography number. We treat the patient. One ETCO2 value gives you information. A trend gives you a story.
Back to our patient
Her ETCO2 was 24. Then 21. Now it is 19 mmHg. Her respiratory rate has increased to 26, while her blood pressure remains 108/66. Look at the whole picture: fever, suspected infection, tachycardia, increasing respiratory rate, weakness, low ETCO2 and a downward trend.
The capnography did not diagnose sepsis. It did not diagnose shock. It did something extremely valuable.
It made you ask why.
Why is the ETCO2 low? Why is it continuing to fall? Why is her respiratory rate increasing? Why does she appear sicker than her blood pressure suggests?
Research has examined this relationship in suspected sepsis. Hunter et al. evaluated a prehospital sepsis screening approach incorporating ETCO2 of 25 mmHg or less with systemic inflammatory response criteria. Lower ETCO2 was associated with greater illness severity and mortality. Earlier research also demonstrated relationships between prehospital ETCO2, lactate, bicarbonate, anion gap and in-hospital mortality [1].
Again, ETCO2 is not diagnosing the disease. It is providing another clue.
Stop looking at vital signs individually
One of the easiest mistakes in EMS is treating vital signs as separate pieces of information.
- BP: acceptable
- SpO2: good
- Heart rate: slightly elevated
- Respirations: a little fast
- ETCO2: low
Individually, each finding may be explainable. Together, they may describe a patient moving in the wrong direction.
Good clinical assessment asks:
- Does the blood pressure match the patient’s appearance?
- Does the ETCO2 match the ventilation pattern?
- Are the numbers improving or deteriorating?
- What physiologic process could connect these findings?
The monitor does not think. But you do.
Now run the call again
Let’s go back to the bedroom. Same patient. Same initial blood pressure of 118/74. Same SpO2 of 98%. But this time, you recognize the ETCO2 of 24 as something worth investigating.
You reassess. It falls to 21. Respiratory rate increases. The blood pressure remains above the traditional hypotensive range.
Instead of waiting for the blood pressure to give you permission to become concerned, you recognize a potentially deteriorating patient. You increase reassessment frequency, obtain appropriate vascular access, continue evaluating for infection and other causes, initiate treatment according to local protocols, make an appropriate transport decision and communicate your concerns to the receiving hospital. If her blood pressure eventually falls, you are not surprised.
You were already watching her physiology change. That is the difference.
Before the blood pressure drops
Capnography will not diagnose shock for you. Neither will a blood pressure cuff or pulse oximeter. They provide information. The clinician must connect it.
There is comfort in seeing 118/74 and 98% on the monitor. Sometimes those numbers accurately reflect a stable patient. Sometimes the body is working very hard to keep them looking normal.
Good clinicians recognize abnormal numbers. Great clinicians recognize abnormal patterns. So, when the blood pressure looks fine but something about your patient doesn’t, look again. Look at the patient. Look at the respiratory rate. Look at the waveform. Look at the ETCO2. Then ask the most important question in clinical medicine:
Why?
Because sometimes, the first warning that your patient is losing the battle will not be a blood pressure of 78/40. It may be a small number in the corner of your monitor quietly moving from 28 ... to 24 ... to 20.
The question is whether you recognized what it was telling you before the blood pressure dropped.
REFERENCES
- Hunter CL, Silvestri S, Ralls G, Bright S, et al. (2016). A prehospital screening tool utilizing end-tidal carbon dioxide predicts sepsis and severe sepsis. American Journal of Emergency Medicine, 34(4), 813–819.
- Hunter CL, Silvestri S, Dean M, Falk JL, Papa L. (2013). The sixth vital sign: Prehospital end-tidal carbon dioxide predicts in-hospital mortality and metabolic disturbances. American Journal of Emergency Medicine.
