A recent study in Resuscitation explores end tidal CO2 measurements in cardiac arrest patients. The authors try to identify what impact various factors have on the numbers you see on your screens. They discuss what implications this could have for managing cardiac arrests.
End tidal CO2 levels (ETCO2) mirrors our patients´ carbon dioxide production and the transport to and elimination of CO2 from the lungs. By measuring ETCO2-levels in cardiac arrest (CA) we have marker mirroring the efficiency of the artificial circulation we provide in advanced life support.
The presence of capnograph CO2 trace in cardiac arrest tells us the endotracheal tube is in the right place. It tells us the cells´metabolism to some extent is working. It tells us the circulation to some extent transports CO2 to the lungs where it to some extent is eliminated. It indirectly give us an idea of the quality of the life support we provide.
It has been taken further than that. The actual ETCO2-level has been used to quantify the quality of CPR. Researchers have suggested using a drop in ETCO2 as a cue to swap the person providing compressions. Return of Spontaneous Circulation (ROSC) results in sudden characteristic rise in ETCO2. This rise could possibly replace pulse controls for identifying ROSC in our algorithms.
Importantly it has been suggested using ETCO2-levels for making decisions on the futility of CPR. ETCO2 lower than 1,5kPa in CA resus is strongly associated with failure of achieving ROSC and death..
Such interpretation might be stretching it. Quality of CPR i.e. our artificial circulation is far from the only factor.
The cause of CA matters. The presenting rhythm matters. CPR quality matters. Bystander CPR matters. Time factors matter.
The city of Bergen and the Haukeland universityhospital is a major HEMS hub and think-tank in Norway. The authors Heradstveit et al obtained data from 575 HEMS patients who suffered non-traumatic cardiac arrest. All patients had cardiac arrest on HEMS arrival and achieved ROSC or were pronounced dead on scene. All were intubated and had ETCO2 recordings.
Factors that were subsequently related to ETCO2 measurements were initial heart rhythm, cause of arrest, presence of bystander CPR, ROSC or no ROSC.
Average ETCO2-levels related to cause and ROSC are summarised in table 2 on your right.
It is striking how average ETCO2-levels are significantly higher in the patients where ROSC was finally achieved.
This is hardly news. It is well described in the literature and is the basis for researchers suggesting making predictions on futility of CPR. But if you take into account the other factors that influence ETCO2, the notion of using ETCO” values for deciding on futility becomes less realistic.
ETCO2-levels depend on the cause. Average ETCO2-levels in CA due to a respiratory cause is significantly higher than CA due to a cardiac cause. CA due to pulmonary embolism has the lowest ETCO2. This is likely due to tissue CO2 accumulation and poor ventilation rather than better perfusion in respiratory CA.
ETCO2 depends on the presenting rhythm.
Patients presenting with asystolia is likely to have a higher ETCO2 than patients with VT/VF or PEA. This should partly be a reflection of the higher incidence of asphyxia in systolic patients.
Finally ETCO2 was correlated to the estimated time after cardiac arrest. As one would expect ETCO2 reliably decreases with time without ROSC as tissues and cells malfunction and die.
Take home message
The study demonstrates the impact of the many factors deciding ETCO2 and how making assumptions based on the quantified ETCO2-measurements is complicated.
There is a host of factors affecting the numbers on my screens. With all these confounding factors it is impossible to rely on quantified ETCO2 to make assumptions on the quality of CPR or futility of CPR. We must ensure and optimise constant high quality CPR no matter the ETCO2.
However, careful assessment of the value in the individual setting could hint at a likely cause. This could be useful in some settings.
For example, diagnosing pulmonary embolism in CA is notoriously difficult and also a potentially reversible condition. A suspiciously low ETCO2 could strengthen the indication for thrombolysis in a patient with a suspected pulmonary embolism.
Factors complicating interpretation of capnography during advanced life support in cardiac arrest-A clinical retrospective study in 575 patients.
Heradstveit BE, Sunde K, Sunde GA, Wentzel-Larsen T, Heltne JK.Resuscitation. 2012 Jul;83(7):813-8. Epub 2012 Feb 25.