End Tidal CO2 (EtCO2) and Capnography Prepared by Shane Barclay MD

Definitions EtCO2: is the maximum partial pressure of CO2 at the end of the breath. In healthy adults this is about 36-40 mm Hg. Capnogram: a plot of EtCO2 versus time. Usually displayed as a wave form. EtCO2 gives a number. Capnography gives a number and a waveform which is more reliable and gives additional information.

End tidal Co2 (EtCO2) EtCO2 consists of two contributing factors: The level of CO2 in the blood. (PaCO2) The dead space in the lungs ie areas not being perfused. The major reason for dead space in an ER patient is poor perfusion: ie poor cardiac output.

End tidal Co2 (EtCO2) So when you consider a patient’s EtCO2 you have to consider both the level of CO2 in the blood and the cardiac status of the patient. Because of this, EtCO2 is NOT directly correlated with arterial CO2

End tidal Co2 (EtCO2) So the thing to remember is the EtCO2 does NOT equal the PaCO2 but the PaCO2 will be at least as high as the EtCO2

End tidal Co2 (EtCO2) In otherwise normal patients, using blood gases, the PaCO2 will be around 36-40 mmHg. In these patients, using EtCO2 monitors will usually give you a number very close to that 40 mmHg. However in the ER we use EtCO2 on patients with respiratory and cardiac disease.

End tidal Co2 (EtCO2) So we have said the value of EtCO2 will be equal to or greater than the PaCO2, but NOT less. So clinically, if the EtCO2 is 20 we know that the arterial CO2 will be at least 20 (but could be 30, 50, 70 etc), but we will not know exactly what without an ABG or venous blood gas. Conversely, if the EtCO2 is 70, the arterial CO2 will be at least 70 or higher. These patients need faster ventilations (not more tidal volume!). If they are on a ventilator, get the EtCO2 down to 40 or so, then do Blood Gases to find out the exact CO2 level.

A quick reminder on blood gases If you only want to know the blood CO2 level, you don’t need to do an arterial blood gas. You can use a 3 or 5 cc syringe and take 1 cc of blood from any vein. The lab can then do a ‘venous blood gas’, which will give you an accurate CO2 level but not an accurate Oxygen level.

Types of Devices to detect etco2 Qualitative EtCO2 colorimetric types use litmus paper to detect EtCO2. They go on the ET tube. Quantitative EtCO2 ‘in line’ detectors can go on the ET tube or any supraglottic airway device. They display either a waveform and/or a numerical value. Nasal cannula ‘in line’ detectors are also available.

Colorimetric CO2 Detectors Yellow = positive for CO2

Colorimetric EtCO2 detectors – Things to Know. Detects ‘acid’, of which CO2 is acidic. They turn from purple to YELLOW if they detect ‘acid’. Need at least 6 breaths or ventilations to confirm if they are positive for CO2. There can be CO2 from prior BVM etc in the stomach that will initially turn the test a ‘false positive’. Also, if the patient has vomited into the ET tube, the stomach acid will often register as a ‘false positive’. Lastly, if you have given Epinephrine down the ET tube, it is acidic and can give a false positive.

Nasal Cannula CO2 detectors This kind can have supplemental oxygen Attached.

Nasal Cannula CO2 detectors Caution: Patients will often breath through their mouth, nose or both. The nasal cannula will only register CO2 being expelled through the nose, so you may have a falsely low reading if they are mouth breathing.

In Line ETCO2 monitor Color will vary depending on make and supplier.

EtCO2 monitor – in line These monitors are ideal for attachment to Endotracheal tubes. They can also be attached to BVM, NIV masks, King tubes and LMAs. They have not been well studied in the literature but it is generally felt the values will be fairly accurate. If the EtCO2 value is high, the patient needs increase ventilation rate. If low, you need a blood gas to truly determine the CO2 value.

EtCO2 monitor – in line If using with a NIV mask and the values seem wrong, you can either do a blood gas or apply a nasal prong CO2 detector under the mask. Usually this will not affect the seal. However if using the nasal prong probe, just remember, to be of any value the patient must exhale through their nose.

So what does capnography look like?

Physiology When you inhale and exhale, and measure partial pressure of CO2 at the mouth, you get the following tracing.

normal Capnogram

Caprnography You can look at the EtCO2 value for information on each breath. You can look at the slope of the tracing for information, as it will be a gradual up-sloping in COPD/Asthma Often most importantly is the ‘trend’. If the EtCO2 values are going up over time (and the SpO2 is going down), this is BAD. The patient may be fatiguing out or going into full respiratory failure.

So how does EtCO2 apply clinically? USES: ET tube confirmation. Adequacy of ventilation with any Supraglottic airway device. Monitoring for hypoxia/apnea in procedural sedation. Assess ventilation during CPR Detecting return of spontaneous circulation during CPR. Determining prognosis during CPR/resuscitation.

ET Tube confirmation Qualitative colorimetric monitors can be inaccurate in cardiac arrest intubated patients because the cardiac output is usually too low to register CO2 on these devices. Quantitative in line EtCO2 monitors however are accurate. Even during CPR you should see a waveform on the Capnogram when the ET tube is in the trachea.

Use with Supraglottic airway devices The Quantitative in line EtCO2 monitors can be used with any supraglottic device. As mentioned prior though, the numbers may not be as accurate as with and ET tube.

Procedural sedation If the patient is breathing room air (ie 21%) then the pulse oximeter is a good measurement of oxygenation and can detect desaturations. However if you have pre-oxygenated the patient say for 5 minutes with a non rebreather mask and their PaO2 is way over 100 mmHg, then the Oxygen Saturation monitor can lag far behind the patients ventilations. So with the use of EtCO2 you can see the patient is breathing with each breath. If they continue to breath with the same EtCO2 value as when they started the procedure, then they are probably fine. Watch for a trend in the EtCO2.

Assessing ventilation during CPR If you are doing CPR and have a patient intubated, use the in line quantitative EtCO2 to first confirm correct ET placement. Then during CPR, if you are doing effective CPR, the EtCO2 should be between 10-20 mmHg.

Detecting return of spontaneous circulation (ROSC) An abrupt increase in EtCO2 may indicate return of spontaneous circulation (ROSC). Increase in pulmonary circulation brings more CO2 into lungs for elimination

Determining prognosis for CPR A number of studies have shown that if after 20 minutes of cardiac arrest, if a patient’s EtCO2 is 10 mmHg or less and you are doing correct chest compressions, survival is near zero. So if after 20 minutes of effective CPR, the EtCO2 is less than 10 and the EDE cardiac shows no cardiac wall motion, you can virtually ‘call’ the code.

Summary EtCO2 is an easy and reliable method of monitoring a patient’s ventilatory status. EtCO2 is an accurate method of confirming ET tube placement. It should be used for ALL intubations. EtCO2 does not equal PaCO2. The PaCO2 will be at least the value of the EtCO2 but can be much higher. You need venous or arterial blood gases to confirm. Be aware of the limitations of the colorimetric CO2 detection devices – takes 6 breaths to confirm, other ‘acids’ can give false positives.

What the Doctors need to Know. Know where the detectors are and how and where to hook them to the various monitors. You should be using EtCO2 for all Intubations, Cardiac arrests, Procedural Sedation and Supraglottic airway devices. If you aren’t the nurses may say something. Your lawyers will thank them. Often the absolute value isn’t as important as the EtCO2 trend.

What the nurses need to Know. Know where the detectors are and how and where to hook them to the various monitors. The doctors should be using EtCO2 for cardiac arrest, procedural sedation, any intubated patient or a patient with a Supraglottic airway device. If they aren’t using one, say something! Their lawyers will thank you. Watch for the trend of EtCO2, especially in procedural sedation. If the EtCO2 is going up, the patient needs attention. A rising EtCO2 can signal respiratory fatigue and possible failure.

The END