Capnography : An Overview for EMS personnelBy Robert S. Cole, CCEMT-PAbout the Author: Steve Cole has been involved in EMS and EMS education since 1990. He has workedfor a variety of EMS agencies including volunteer fire, military, hospital based, private, and third service.He is currently employed by Ada County Paramedics (Boise, Idaho),a top-tier and often besieged ThirdService EMS.He has no current conflicts of interest with any portion of this article, and has not received from anycommercial interest for its production. He would like to mention that he is open for some money, should asufficiently insane commercial enterprise wish to invest in his expansive, eclectic, and somewhathaphazard quest for EMS excellence.Comments are welcome. He can be reached by email at: email@example.com“CO2 is the smoke from the flames ofmetabolism.” – Ray Fowler, M.D. Dallas, Street Doc’s SocietyIntroductionCapnography, while seen as a relatively new tool for EMS, was first introduced in 1943.Using infrared technology, EMS has seen the advancement of many tools andtechnologies that have promised revolution in medicine. Ultrasound, cardiac markers,CO-oximetry, and pulse oximetry to name a few. Some have panned out and some havenot. One of the more promising technologies to break into EMS practice has been EndTidal Capnography (ETCO2). Originally conceived as a method of monitoringventilation, it has gained wide acceptance in EMS for airway maintenance andmonitoring in intubated patients. The availability of waveform capnography, as well asrecent breakthroughs in the understanding of capnography as a marker for overallphysiologic function, have broadened the role of capnography in EMS further. Thisarticle will attempt to discuss both the standard and the new roles for ETCO2.Types of CapnographyMost ETCO2 devices usually work on the principle that CO2 absorbs infra-red radiation.A beam of infra-red light is passed across the gas sample to fall onto a sensor. Thepresence of CO2 in the sample leads to a reduction in the amount of light falling on thesensor. The analysis is typically rapid, accurate, and in EMS, fairly accurate. In HAZ-MAT and anesthesia situations, the presence of high concentrations of other gasses (suchas nitrous oxide) will adversely effect the accuracy of the ETCO2.There are three major types of ETCO2 devices, side stream and main stream sampling.
Colorimetric ETCO2This is better described as an ETCO2 detector, rather than capnography. These devicesprovide CO2 detection based on PH changes in airflow. Colorimetric ETCO2 is adisposable, single use ETOC2 strictly for use in tube confirmation. In this role is costeffective, easy, and quick. It does have some limitations. It does not provide any type ofwaveform, quantitative analysis, or even changes specific to CO2. It loses itseffectiveness rapidly. It cannot be used to determine different types of respiratoryabnormalities. Presence of some fluids and moisture in the ETT will give false readingsas well.“End Tidal CO2 reading without a waveform is like a heart rate without anECG recording.” – Bob Page “Riding the Waves”Main-stream ETCO2With mainstream capnography,an infrared sensor is placeddirectly over the passageway ofgas to reflect light across the gassample. The most common useof this method is monitoring theplacement and function of anendotracheal tube (ETT). Whileuncommon, it is possible to hookup mainstream capnography to abreathing patient. Side-stream ETCO2. Side-stream capnography is more versatile than mainstream, as it can be used in a variety of situations besides placement and monitoring of an ETT. In side- stream ETCO2, a sample of gas is extracted from the pathway of gas, and sampled remotely from the airway. It is also quicker to use, requires less calibration by the user, and is more durable than external sensors. Side-stream technology can be used to monitor ETT,as well as respiratory function in patients without an ETT. The most common example ofthis is the nasal cannula ETCO2, which can be used to monitor patients with variousrespiratory conditions, or those receiving treatments that might have respiratory effects.The two main disadvantages of this method are (1) that it is considered slightly lessaccurate, and (2) there is a slight delay in time for the sample to proceed up the tube tothe monitor. Neither of these concerns is considered clinically significant in EMS (yet).
Micro-stream ETCO2Micro-stream technology is a new variation of side-stream technology. The main difference of thistechnology is that only very small amounts of the gassample are required to accurately detect and analyzeCO2 readings.This technology was originally developed for use inneonates with small tidal volumes, but has shownpromise in other patients as well, such as those with adecreased tidal volume or in respiratory extremis.Accuracy of types of ETCO2Generally speaking, mainstream ETCO2 is considered slightly more accurate, but takeslonger to set up (“warm up”) and requires frequent calibration. The differences inaccuracy have only been demonstrated in spontaneously breathing patients, and are notconsidered significant. In addition, both methods are considered accurate for monitoringETT placement. Micro-stream ETCO2 technology shows promise in overcoming thesedifferences in accuracy.Many things can “spoof” ETCO2: The most common is excessive water vapor in theETT, which can cause sensors (especially main stream sensors) to provide “false highs”.Leaky ETCO2 tubing can give false lows. Finally, repeated use of epinephrine (such asseen in cardiac arrest) by ET or IV routes, can cause ETCO2 to drop. Typically adecrease of about 5 torr has been seen. This is not constant through out the study group,and varies by patient. The clinical significance of this beyond its effect on ETCO2monitoring is the subject of several ongoing studies.Basics of CapnographySimply put it is a measurement of the byproduct of cellular metabolism in the humanbody. CO2 is produced by cellular metabolization through the process known as the KrebCycle. It is then exhaled through the respiratory process where we can measure it. Thepoint where it is measured is at the end of the airway, therefore it is called end tidalmeasurement. ETCO2 can be measured in either torr, or mmHg.Basic capnography is not only the measurement of ETCO2, but analysis of the phasesof exhalation (measured indirectly by monitoring ETCO2) in the respiration cycle.It is also affected by a variety of respiratory states and perfusion conditions; which can beindirectly assessed by detailed analysis of ETCO2. Key PointBasic capnography is not only the measurement of ETCO2, but analysis of the phases ofexhalation (measured indirectly by monitoring ETCO2) in the respiration cycle.What is normal?ETCO2 30-45 mm Hg is the normal value for capnography. Many iatrogenic factors canalter this reading. For example, imperfect positioning of nasal cannula capnofilters maycause distorted readings. Unique nasal anatomy, obstructed nares and mouth breathersmay skew results and/or require repositioning of cannula. In addition, oxygen by mask
may lower the reading by 10% or more. Finally, as discussed elsewhere, use of nitrousoxide or excessive water vapor may cause false high readings.What is the waveform?Similar to the ECG, the waveform in ETOC2 describes function over time and volume.The waveform can be divided into inspiratory (phase 0) and expiratory segments. Theexpiratory segment, is divided into phases I, II and III, and occasionally, phase IV, whichrepresents the terminal rise in CO2 concentration. The angle between phase II and phaseIII is the alpha angle. The nearly 90 degree angle between phase III and the descendinglimb is the beta angle.A waveform illustrating phase 4 of the capnogram:Another view of the representative parts of the waveform (using different terminology):
When looking at ETCO2, 5 things are measured: • Frequency: ETCO2 provides an objective analysis of the rate of breathing. • Rhythm: By assessing the rhythm in ETCO2 periodic apnea, cheyne stokes, boyts, and other respiratory patters become easily identifiable. • Baseline: Evaluation of the baseline can give clues regarding inter-thoracic pressure and other resistance to ventilation issues. • Shape: The shape of the waveform gives clues regarding the exhalation cycle itself. An asthmatic with an acute asthma attack, for example, will present with a “shark fin” shape to their waveform. • Height: The height of a waveform is representative of the level of CO@ being measured. Various factors result in either increased or decreased/absent ETCO2.ETCO2 increased : Technical errorsCO2 output Pulmonary perfusion Alveolar Ventilation Machine faultsFeverMalignant Hypoventilation Inadequate O2 flowshyperpyrexia Increased cardiac Bronchial intubation Leaks in breathingSodium bicarbonate output Partial airway systemuse Increased blood obstruction Faulty ventilatorTourniquet release in pressure Rebreathing of airway Faulty valvescompression injuries gasETCO2 decreased: Technical errorsCO2 output Pulmonary perfusion Alveolar Ventilation Machine faultsHypothermia Reduced cardiac output Hyperventilation Circuit disconnectionEpinephrine Use Hypotension Apnea Sampling tube leak Hypovolemia Total airway obstruction Pulmonary embolism Partial airway obstruction
Accidental tracheal Cardiac arrest extubationVentilation/Perfusion mismatch and ETCO2If ventilation or perfusion are unstable, a Ventilation/Perfusion (V/Q) mismatch canoccur. This will alter the correlation between Arterial CO2 (PaC02) and ETCO2.This V/Q mismatch can be caused by blood shunting such as occurs during atelectasis(perfusing unventilated lung area) or by dead space in the lungs (Ventilating unperfusedlung area) such as occurs with a pulmonary embolism or hypovolemia.Uses of capnographyCapnography and intubation“When exhaled CO2 is detected (positive reading for CO2) in cardiac arrest,it is usually a reliable indicator of tube position in the trachea.” - TheAmerican Heart Association 2005 CPR and ECG GuidelinesA relatively new concept, ETCO2 monitoring to confirm and monitor ETT placement,was first reported in 1991. One early use was to prevent inadvertent placement of agastric tube into the airway instead of the esophagus. Since then, it has evolved tobecome the standard of care for the confirmation of tube placement.While most research has focused on maintenance of ETT, the ETCO2 has been shownuseful in studies involving the LMA. In theory it should also be acceptable in doublelumen airway devices such as the PTLA and Combitube, as well as new airway deviceslike the King LTD.ETCO2 is considered mandatory in many services for ETT. Therefore, EMS providersshould document their use of continuous ETCO2 monitoring and when possible attachwave form strips to their PCRs. This provides an objective and concrete documentationof tube placement. This is especially important if tube placement is later called intoquestion by other agencies or the ER. This will timestamp and document your tube asgood.A good tube will produce a clearly identifiable waveform, but the details of the waveformwill depend on the underlying medical conditions of the patient. Vukmir et al reported asensitivity and specificity of 100% for endotracheal tube localization by capnography.
A Properly Placed ETTAn esophageal placement (AKA “Gut tube”) should not produce any waveform. Example(A) below shows sudden dislodgement. Example (b) shows a gut tube from the start.A:B: A misplaced ETTIn low cardiac output states (like shock, cardiac arrest or inadequate chest compressions)ETCO2 may not be detected. If a patient has had carbonated beverages, or if mouth tomouth ventilation has been attempted, CO2 may be detected after esophageal intubation(false positive). The EtCO2 should rapidly decrease to zero (within 3-6 breaths) in thissituation, and the wave form will not be ‘Normal’ looking.Capnography as a diagnostic toolBroncheospasm/Asthma: ETCO2 has proven diagnostic value in the determination ofbroncheospasm in reactive airway states (as opposed to CHF, hyperventilation, and otherstates). This has increasing importance given that recent research calls into doubt theability of providers (and even physicians) to adequately differentiate between pneumonia,asthma/COPD, and CHF based on assessment alone.ETCO2 values change with the severity of the broncheospasm. With a mild asthma, theCO2 will drop (below 35) as the patient hyperventilates to compensate. As the asthmaworsens, the C02 levels will rise to normal. When the asthma becomes severe, and thepatient is tiring and has little air movement, the C02 numbers will rise to dangerous levels(above 60).The characteristic waveform of broncheospasm is the “Shark Fin”. The Shark Fin is termfor the classic shape of the waveform seen in acute broncheospasm. It is manifested by amore acute upward of phase II and III than is normally seen, making the ALPHA angleless acute and the BETA angle more acute.
Successful treatment will lessen or eliminate the shark fin shape and return the ETCO2 tonormal range. Therefore, documenting ETCO2 before and after a breathing treatment isuseful in documenting treatment.It is important to note that a recent study published in the Annals of Emergency Medicinefailed to show the efficacy of ETCO2 in detecting asthma in pediatrics. Future studies arepending.
KEY POINT:ETCO2 is not an effective diagnostic tool for asthma in pediatrics.Hypoventilation: This may be due to over sedation, head injury, or other issues.A real example of hypoventilation. Note the normal shape of the waveform, but theelevated levels of CO2.Hyperventilation: This may be due to anxiety, a panic attack, or well compensatedrespiratory distress.Return of spontaneous ventilation during paralysis: ETCO2 is useful for monitoring apatients level of paralysis, and may detect the need for further paralysis or sedationbefore other methods commonly used in EMS. This is usually evident by “notches” inthe waveform.
In this capnogram, the notches are notable between the normal waveforms.Air trapping/overventilation: Mechanical over ventilation in some patients causes anumber of issues effecting coronary perfusion, ROSC, and barotraumas. This willmanifest itself by an elevation of the baseline over time. When this is seen, thenprolonged exhalation times are needed.Equipment Malfunction: The ETCO2 can detect a failure in the ventilation circuit,particularly a leaky cuff, detached BVM, or failing one-way valve.
Detection of sudden cardiac arrest: While uncommon, the sudden loss of pulses, whilean electrical rhythm remains (such as seen in a tension pneumo-thorax) does happen. Thisis even harder to detect in an intubated and paralyzed patient.Return of Spontaneous Circulation: A sudden increase in the height in an ETCO2waveform may indicate a sudden increase in blood flow in cardiac arrest, usually fromROSC.PEA vs. Severe Shock: White R.D. et al used EtCO2 measurement in out-of hospitalcardiac arrest. They concluded that capnography can detect the presence of pulmonaryblood flow even in the absence of major pulses (pseudo-pulseless electrical activity-PEA) and also can rapidly indicate changes in pulmonary blood flow (cardiac output)caused by alterations in cardiac rhythm. This can be helpful in determining the differencebetween a true low-flow state and actual PEA.Capnography as a monitoring toolETCO2, like any “tool” is not, and should not, be considered the ultimate monitoringdevice. It is however very useful in select situations. Its effectiveness is increased whenused with other monitoring devices, such as SPO2. One anesthesia study showed that useof both ETCO2 and SPO2 together would have prevented 93% of anesthesia “mishaps”.This is not to imply that SPO2 is accurate or reliable by itself either, and as a general ruleETCO2 is more “timely” in its detection of problems than SPO2.Another study used ETCO2 as a detector for compressor fatigue in cardiac arrest. This isbased on the thought that quality compressions produce better ETCO2 readings. Whenthe quality of compressions fall, then ETCO2 will fall as well. Therefore as a matter oftroubleshooting, when ETCO2 decreases, switching compressors should be considered. What does the literature say?In a study of 291 intubated head injured patients, 144 had ETCO2 monitoring. Patientswith ETCO2 monitoring had lower incidence of inadvertant severe hyperventilation(5.6%) than those without ETCO2 monitoring (13.4%). Patients in both groups with
severe hyperventilation had significantly higher mortality (56%) than those without(30%). –Davis, The Use of Quantitative End-Tidal Capnometry to Avoid InadvertantSevere Hyperventilation in Patients with Head Injury After Paramedic Rapid SequenceIntubation, -Journal of Trauma, April 2004Trauma - A 2004 study of blunt trauma patients requiring RSI showed that only 5 percentof patients with ETCO2 below 26.25 mm Hg after 20 minutes survived to discharge. Themedian ETCO2 for survivors was 30.75. - Deakin CD, Sado DM, Coats TJ, Davies G. “Prehospital end-tidal carbon dioxide concentration and outcome in major trauma.” Journal of Trauma. 2004;57:65-68.Use of ETCO2 in intubated patients is not just useful in tube confirmation. It can be usedas a guide to ventilation rates. A target ETCO2 of 30-35 is acceptable and desirable.Decreasing the ventilatory rate in an intubated patient is acceptable to achieve this goal. Key Point An ETCO2 of 30-35 is a desirable goal of ventilationCapnography as a predictor of survival in SCASeveral studies have looked at ETOC2 as a predictor of survival of cardiac arrest. One oflarger studies was conducted overseas with a study population of nearly 400 patients. Inthis study, the researchers looked at both initial and final ETOC2. They found that theaverage initial ETCO2 in the ROSC was 18 torr vs 6.75 torr in the non-ROSC group. Inaddition, the final ETCO2 for ROSC group was on average 26 torr vs 7.5 for the non-ROSC group. The most important finding was that no patient with an initial ETCO2 of 9or less survived to admission.Another study noted that "An end-tidal carbon dioxide level of 10 mmHg or lessmeasured 20 minutes after the initiation of advanced cardiac life support accuratelypredicts death in patients with cardiac arrest associated with electrical activity but nopulse. Cardiopulmonary resuscitation may reasonably be terminated in such patients.”(Levine, 1997) It should be noted than most normal arrests that fail to be resuscitatedafter 20 minutes survive, regardless of ETCO2.Several other studies have found dramatic differences in ETCO2 in survivors and nonsurvivors of SCA, but no consensus has yet been reached on what constitutes a survivableETCO2. In addition, it remains unclear if ETCO2 is truly an independent predictor, orsimply a result of other factors (such a quality of CPR) that affect ROSC.Therefore, while future studies will be needed before ETCO2 alone can be used asjustification to terminate resuscitation (especially in special resuscitation groups likehypothermia), use of this technology does hold promise, and should be consideredimportant in the termination thought process and included in discussion with medicalcontrol. KEY POINT:
LOW ETCO2 alone is not indication to terminate resuscitation, but it is animportant consideration. High ETCO2 may be an indicator to continueprolonged resuscitation.Pitfalls of capnographyThe greatest potential for failure is focusing on the ETCo2 and not the patients overallclinical picture. The ages old mantra “Treat the patient, not the monitor” is more true thanever.Other pitfalls include not using ETCO2 on ETT patients. This is now considered by manyexperts as the “gold standard” and failing to do so opens up the medic to accusations oftube displacement.Summary Basic capnography is not only the measurement of ETCO2, but analysis of the phases ofexhalation (measured indirectly by monitoring ETCO2) in the respiration cycle. In this, itis as useful, and as complex, as electrocardiography. It has been proven accurate and insome cases definitive in many clinical concerns facing the EMS community today. Whilestill an emerging technique, it is likely that future paramedics will be stressing overcapnography classes as much as they do over cardiology or pharmacology today.
GlossaryCapnography – A graphic representation of the measurement of carbon dioxide (CO2) inexhaled breath.Capnometer – The numeric measurement of CO2.Capnogram – CO2 waveforms which can be of two types: FCO2 can be plotted againstexpired volume (SBTCO2 curve /volume capnogram/ CO2 expirogram) or against time(time capnogram) during a respiratory cycle. Most EMS monitors are TIME capnograms.End Tidal CO2 (ETCO2 or PetCO2) - the level of (partial pressure of) carbon dioxidereleased at end of expiration.
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