Recognizing Trouble Before Blood Pressure Falls

What capnography may be indicating

The blood pressure reads 118/74. Heart rate is 104. SpO2 is 98%. Respiratory rate is 22.

The patient is awake, answers questions appropriately, and mainly complains of weakness, nausea, and feeling unwell. On those numbers alone, nothing immediately suggests shock.

Then the capnography catches your eye. ETCO2 is 24 mmHg. A few minutes later, it drops to 21. Then to 19.

Which vital sign matters most now?

The call

A crew is dispatched to a private residence for a 67-year-old woman with weakness and dizziness. Her daughter says she has been ill for several days with what the family believed was a urinary tract infection. By the time help arrives, she is too weak to walk to the bathroom.

Initial assessment shows a blood pressure of 118/74, heart rate of 104, respiratory rate of 22, SpO2 of 98% on room air, and a temperature of 101.7°F. She is pale but alert. Her lungs are clear, and she denies chest pain.

At first glance, nothing appears catastrophic. A nasal capnography cannula is placed, and the waveform is consistent.

ETCO2 measures 24 mmHg. Five minutes later, it is 21. Her blood pressure is now 110/68. She still seems reasonably stable. It would be easy to assume there is time, but that assumption may be wrong.

Blood pressure does not equal perfusion

EMS providers are trained to see hypotension as a sign of shock. The problem comes when hypotension becomes the definition of shock.

Shock is really a failure of tissue perfusion and oxygen delivery, not simply a low blood pressure. The body can compensate with a faster heart rate, vasoconstriction, and redistribution of blood flow. For a while, arterial pressure may stay fairly normal even as the patient worsens.

Research involving injured patients supports looking beyond blood pressure. Prehospital ETCO2 has been linked to mortality and massive transfusion needs, including in patients who were normotensive before reaching the hospital.

ETCO2 does not replace blood pressure. It adds another piece to the physiologic picture.

ETCO₂ is more than a ventilation number

Many EMS clinicians first learn capnography as a way to confirm endotracheal tube placement. That remains vital, but ETCO2 can reveal much more.

For CO2 to appear on the capnogram, several steps must occur: metabolism, circulation, pulmonary perfusion, ventilation, and exhaled CO2.

Cells create CO2. Circulation carries it to the lungs. Blood delivers it to the alveoli, and ventilation removes it. Because of that, ETCO2 is affected by metabolism, circulation, and ventilation. If cardiac output and pulmonary blood flow fall, less CO2 reaches the alveoli, and ETCO2 may drop as perfusion worsens. But one important warning remains: a low ETCO2 does not automatically mean shock.

Hyperventilation can lower ETCO2. So can metabolic acidosis with respiratory compensation, pulmonary embolism, ventilation-perfusion problems, and technical issues. That is why the number alone is not treated. The patient is. One ETCO2 reading provides information; a trend tells a story.

Back to the patient

Her ETCO2 was 24, then 21, and now 19 mmHg. Her respiratory rate has risen to 26 while her blood pressure remains 108/66. Put the whole picture together: 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 very useful.

It made the clinician ask why.

Why is the ETCO2 low? Why is it continuing to fall? Why is the respiratory rate increasing? Why does she look sicker than the blood pressure suggests?

Research has explored this relationship in suspected sepsis. Hunter and colleagues evaluated a prehospital screening approach that combined ETCO2 of 25 mmHg or less with systemic inflammatory response criteria. Lower ETCO2 was associated with greater illness severity and mortality. Earlier research also found relationships between prehospital ETCO2, lactate, bicarbonate, anion gap, and in-hospital mortality.

Again, ETCO2 is not making the diagnosis. It is supplying another clue.

Stop treating vital signs as isolated facts

One of the easiest mistakes in EMS is viewing vital signs as separate items of data.

• BP: acceptable
• SpO2: good
• Heart rate: slightly elevated
• Respirations: a little fast
• ETCO2: low

Each reading may be explainable on its own. Taken together, they may describe a patient heading in the wrong direction.

Good clinical assessment asks:

• Does the blood pressure fit the patient’s appearance?
• Does the ETCO2 fit the ventilation pattern?
• Are the numbers improving or worsening?
• What physiologic process could tie these findings together?

The monitor does not think. The clinician does.

Run the call again

Return to the bedroom. Same patient. Same initial blood pressure of 118/74. Same SpO2 of 98%. This time, though, the ETCO2 of 24 is recognized as something worth investigating.

The patient is reassessed. The ETCO2 falls to 21. The respiratory rate rises. The blood pressure remains above the usual hypotensive threshold.

Instead of waiting for blood pressure to justify concern, the crew recognizes a patient who may be deteriorating. Reassessment becomes more frequent, appropriate vascular access is obtained, infection and other causes continue to be evaluated, treatment is started according to local protocol, transport decisions are made accordingly, and the receiving hospital is informed of the concern. If the blood pressure later drops, it is not a surprise.

The physiology was already changing, and that is the difference.

Before the blood pressure drops

Capnography will not diagnose shock on its own. Neither will a blood pressure cuff or a pulse oximeter. These tools provide information; the clinician must connect it.

It is reassuring to see 118/74 and 98% on the monitor. Sometimes those numbers truly reflect stability. At other times, 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 the patient does not, 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 a patient is losing ground will not be a blood pressure of 78/40. It may be a small number on the monitor quietly drifting from 28 to 24 to 20.

The question is whether you recognized what it was saying before the blood pressure fell.

REFERENCES

1. 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.
2. 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.