Why EtCO2 Levels <35 mmHg Signal High Risk in Trauma Patients
A growing body of literature shows EtCO2 levels closely and inversely correlate with venous lactate, a marker of anaerobic metabolism.
BLUF: Operationally, if your trauma patient has an EtCO2 <35 mmHg, they have a significant risk for occult shock, a 50% chance of needing significant blood transfusion, and a markedly increased need for operating room intervention. Although the data is preliminary, an ETCO2 <35 mmHg should heighten your worry when managing trauma patients.
Cellular CO2 production is a byproduct of aerobic metabolism. In shock states, anaerobic metabolism increases with resultant metabolic acidosis and reduced CO2 production. Some have defined this metabolic acidosis as the “gold standard” to define shock.
There is also trauma literature suggesting an elevated venous lactate can be a marker of occult shock before the appearance of frank hypotension. However, a venous lactate requires a blood test in the trauma bay. If a reduced EtCO2 could be used as a marker for occult shock, we could potentially identify this prehospital.
Unfortunately, the two trauma studies providing evidence that an EtCO2 <35 mmHg correlates with occult shock were each done in a single level 1 trauma center and only on a small number of patients. Generalizing the results from such limited studies is risky.
In a prospective cohort study of 105 penetrating trauma patients at a NYC trauma center, half of whom were stabbed and half shot, if the EtCO2 was <35 mmHg there was a 20-fold increase in operating room intervention. If the venous lactate was >4 the likelihood of going to the OR was only four-fold increased. A systolic blood pressure of 90 mmHg only increased it three-fold.1
The surgeons were not blinded to the EtCO2, so if they believed the working theory that a reduced EtCO2 correlated with occult shock, they may have been more willing to take a trauma patient to the operating room just based on where the injury was anatomically and the reduced EtCO2.
The authors felt EtCO2 worked better than either the elevated venous lactate or hypotensive blood pressures because their young population (average 26.5 years old) could compensate for shock better, so SBP decreases were a late finding in shock.
In the other article to look at this topic, a prospective, observational study of 67 trauma patients at a different level 1 center, where 50% of the patients had blunt trauma, found that an EtCO2 of <35 mmHg was 81% sensitive, but only 50% specific for needing three units of pRBCs in one hour.
It better predicted the need for significant transfusion than an SBP <90 mmHg, venous lactate >4, or Shock Index >0.9. One challenge was that 60% of the patients presented with an EtCO2 <35 mmHg. Only half of them required at least three units pRBCs in one hour.2
One caveat to low EtCO2 readings: a misplaced side stream nasal cannula, displaced endotracheal tube, or hyperventilation can also reduce the measured EtCO2. Both of the above studies verified they didn’t have any significant hyperventilation in their trauma patients. The Stone study2 required the respiratory therapist to verify a stable capnographic waveform for 5 to 10 seconds before accepting the EtCO2.
References:
1Caputo ND, Fraser RM, Paliga A, Matarlo J, Kanter M, Hosford K, Madlinger R. Nasal cannula end-tidal CO2 correlates with serum lactate levels and odds of operative intervention in penetrating trauma patients: a prospective cohort study.PMID: 23117381.
2Stone ME Jr, Kalata S, Liveris A, Adorno Z, Yellin S, Chao E, Reddy SH, Jones M, Vargas C, Teperman S. End-tidal CO2 on admission is associated with hemorrhagic shock and predicts the need for massive transfusion as defined by the critical administration threshold: A pilot study. PMID: 27712903.
