Capnography : An Overview for EMS personnel

By Robert S. Cole, CCEMT-P

About the Author: Steve Cole has been involved in EMS and EMS education since 1990. He has worked for 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 Third Service EMS.

He has no current conflicts of interest with any portion of this article, and has not received from any commercial interest for its production. He would like to mention that he is open for some money, should a sufficiently insane commercial enterprise wish to invest in his expansive, eclectic, and somewhat haphazard quest for EMS excellence.

Comments are welcome. He can be reached by email at: colemedic@hotmail.com

“CO2 is the smoke from the flames of metabolism.”

– Ray Fowler, M.D. Dallas, Street Doc’s Society

Introduction

Capnography, 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 and technologies that have promised revolution in medicine. Ultrasound, cardiac markers, CO-oximetry, and pulse oximetry to name a few. Some have panned out and some have not. One of the more promising technologies to break into EMS practice has been End Tidal Capnography (ETCO2). Originally conceived as a method of monitoring ventilation, it has gained wide acceptance in EMS for airway maintenance and monitoring in intubated patients. The availability of waveform capnography, as well as recent breakthroughs in the understanding of capnography as a marker for overall physiologic function, have broadened the role of capnography in EMS further. This article will attempt to discuss both the standard and the new roles for ETCO2.

Types of Capnography

Most 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. The presence of CO2 in the sample leads to a reduction in the amount of light falling on the sensor. 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 (such as 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 devices provide CO2 detection based on PH changes in airflow. Colorimetric ETCO2 is a disposable, single use ETOC2 strictly for use in tube confirmation. In this role is cost effective, easy, and quick. It does have some limitations. It does not provide any type of waveform, quantitative analysis, or even changes specific to CO2. It loses its effectiveness rapidly. It cannot be used to determine different types of respiratory abnormalities. Presence of some fluids and moisture in the ETT will give false readings as well.

“End Tidal CO2 reading without a waveform is like a heart rate without an ECG recording.” – Bob Page “Riding the Waves”

Main-stream ETCO2With mainstream capnography, an infrared sensor is placed directly over the passageway of gas to reflect light across the gas sample. The most common use of this method is monitoring the placement and function of an endotracheal tube (ETT). While uncommon, it is possible to hook up mainstream capnography to a breathing 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 of this is the nasal cannula ETCO2, which can be used to monitor patients with various respiratory conditions, or those receiving treatments that might have respiratory effects. The two main disadvantages of this method are (1) that it is considered slightly less accurate, and (2) there is a slight delay in time for the sample to proceed up the tube to the 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 this technology is that only very small amounts of the gas sample are required to accurately detect and analyze CO2 readings. This technology was originally developed for use in neonates with small tidal volumes, but has shown promise in other patients as well, such as those with a decreased tidal volume or in respiratory extremis.

Accuracy of types of ETCO2Generally speaking, mainstream ETCO2 is considered slightly more accurate, but takes longer to set up (“warm up”) and requires frequent calibration. The differences in accuracy have only been demonstrated in spontaneously breathing patients, and are not considered significant. In addition, both methods are considered accurate for monitoring ETT placement. Micro-stream ETCO2 technology shows promise in overcoming these differences in accuracy. Many things can “spoof” ETCO2: The most common is excessive water vapor in the ETT, 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 as seen in cardiac arrest) by ET or IV routes, can cause ETCO2 to drop. Typically a decrease 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 ETCO2 monitoring is the subject of several ongoing studies.

Basics of CapnographySimply put it is a measurement of the byproduct of cellular metabolism in the human body. CO2 is produced by cellular metabolization through the process known as the Kreb Cycle. It is then exhaled through the respiratory process where we can measure it. The point where it is measured is at the end of the airway, therefore it is called end tidal measurement. ETCO2 can be measured in either torr, or mmHg.Basic capnography is not only the measurement of ETCO2, but analysis of the phases of 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 be indirectly assessed by detailed analysis of ETCO2.

Key PointBasic capnography is not only the measurement of ETCO2, but analysis of the phases of exhalation (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 can alter this reading. For example, imperfect positioning of nasal cannula capnofilters may

cause distorted readings. Unique nasal anatomy, obstructed nares and mouth breathers may 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 nitrous oxide 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. The expiratory segment, is divided into phases I, II and III, and occasionally, phase IV, which represents the terminal rise in CO2 concentration.  The angle between phase II and phase III is the alpha angle.  The nearly 90 degree angle between phase III and the descending limb 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 :

CO2 output Pulmonary perfusion Alveolar VentilationTechnical errors Machine faults

Fever Malignant hyperpyrexiaSodium bicarbonate useTourniquet release in compression injuries

Increased cardiac output Increased blood pressure

Hypoventilation Bronchial intubationPartial airway obstructionRebreathing of airway gas

Inadequate O2 flowsLeaks in breathing systemFaulty ventilatorFaulty valves 

ETCO2 decreased:

CO2 output Pulmonary perfusion Alveolar VentilationTechnical errors Machine faults

HypothermiaEpinephrine Use

Reduced cardiac output HypotensionHypovolemiaPulmonary embolismCardiac arrest

Hyperventilation ApneaTotal airway obstructionPartial airway obstructionAccidental tracheal

Circuit disconnection Sampling tube leak

extubation 

Ventilation/Perfusion mismatch and ETCO2If ventilation or perfusion are unstable, a Ventilation/Perfusion (V/Q) mismatch can occur. 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 unperfused lung area) such as occurs with a pulmonary embolism or hypovolemia.

Uses of capnography

Capnography 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.” - The American Heart Association 2005 CPR and ECG Guidelines

A 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 a gastric tube into the airway instead of the esophagus. Since then, it has evolved to become the standard of care for the confirmation of tube placement. While most research has focused on maintenance of ETT, the ETCO2 has been shown useful in studies involving the LMA. In theory it should also be acceptable in double lumen airway devices such as the PTLA and Combitube, as well as new airway devices like the King LTD. ETCO2 is considered mandatory in many services for ETT. Therefore, EMS providers should document their use of continuous ETCO2 monitoring and when possible attach wave form strips to their PCRs. This provides an objective and concrete documentation of tube placement. This is especially important if tube placement is later called into question by other agencies or the ER. This will timestamp and document your tube as good.A good tube will produce a clearly identifiable waveform, but the details of the waveform will depend on the underlying medical conditions of the patient. Vukmir et al reported a sensitivity and specificity of 100% for endotracheal tube localization by capnography.

A Properly Placed ETT

An 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 ETT

In 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 to mouth 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 this situation, and the wave form will not be ‘Normal’ looking. 

Capnography as a diagnostic toolBroncheospasm/Asthma: ETCO2 has proven diagnostic value in the determination of broncheospasm in reactive airway states (as opposed to CHF, hyperventilation, and other states). This has increasing importance given that recent research calls into doubt the ability 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, the CO2 will drop (below 35) as the patient hyperventilates to compensate. As the asthma

worsens, the C02 levels will rise to normal. When the asthma becomes severe, and the patient 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 term for the classic shape of the waveform seen in acute broncheospasm. It is manifested by a more acute upward of phase II and III than is normally seen, making the ALPHA angle less acute and the BETA angle more acute.

Successful treatment will lessen or eliminate the shark fin shape and return the ETCO2 to normal range. Therefore, documenting ETCO2 before and after a breathing treatment is useful in documenting treatment.

It is important to note that a recent study published in the Annals of Emergency Medicine failed to show the efficacy of ETCO2 in detecting asthma in pediatrics. Future studies are pending.

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 the elevated levels of CO2.

Hyperventilation: This may be due to anxiety, a panic attack, or well compensated respiratory distress.

Return of spontaneous ventilation during paralysis: ETCO2 is useful for monitoring a patients level of paralysis, and may detect the need for further paralysis or sedation before other methods commonly used in EMS. This is usually evident by “notches” in the waveform.

In this capnogram, the notches are notable between the normal waveforms.

Air trapping/overventilation: Mechanical over ventilation in some patients causes a number of issues effecting coronary perfusion, ROSC, and barotraumas. This will manifest itself by an elevation of the baseline over time. When this is seen, then prolonged 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, while an electrical rhythm remains (such as seen in a tension pneumo-thorax) does happen. This is even harder to detect in an intubated and paralyzed patient.

Return of Spontaneous Circulation: A sudden increase in the height in an ETCO2 waveform may indicate a sudden increase in blood flow in cardiac arrest, usually from ROSC.

PEA vs. Severe Shock: White R.D. et al used EtCO2 measurement in out-of hospital cardiac arrest.  They concluded that capnography can detect the presence of pulmonary blood 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 difference between 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 monitoring device. It is however very useful in select situations. Its effectiveness is increased when used with other monitoring devices, such as SPO2. One anesthesia study showed that use of 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 rule ETCO2 is more “timely” in its detection of problems than SPO2. Another study used ETCO2 as a detector for compressor fatigue in cardiac arrest. This is based on the thought that quality compressions produce better ETCO2 readings. When the quality of compressions fall, then ETCO2 will fall as well. Therefore as a matter of troubleshooting, 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. Patients with 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 Inadvertant Severe Hyperventilation in Patients with Head Injury After Paramedic Rapid Sequence Intubation,

-Journal of Trauma, April 2004

Trauma - A 2004 study of blunt trauma patients requiring RSI showed that only 5 percent of patients with ETCO2 below 26.25 mm Hg after 20 minutes survived to discharge. The median 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 used as 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 PointAn ETCO2 of 30-35 is a desirable goal of ventilation

Capnography as a predictor of survival in SCASeveral studies have looked at ETOC2 as a predictor of survival of cardiac arrest. One of larger studies was conducted overseas with a study population of nearly 400 patients. In this study, the researchers looked at both initial and final ETOC2. They found that the average initial ETCO2 in the ROSC was 18 torr vs 6.75 torr in the non-ROSC group. In addition, 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 9 or less survived to admission. Another study noted that "An end-tidal carbon dioxide level of 10 mmHg or less measured 20 minutes after the initiation of advanced cardiac life support accurately predicts death in patients with cardiac arrest associated with electrical activity but no pulse. Cardiopulmonary resuscitation may reasonably be terminated in such patients.” (Levine, 1997) It should be noted than most normal arrests that fail to be resuscitated after 20 minutes survive, regardless of ETCO2. Several other studies have found dramatic differences in ETCO2 in survivors and non survivors of SCA, but no consensus has yet been reached on what constitutes a survivable ETCO2. In addition, it remains unclear if ETCO2 is truly an independent predictor, or simply 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 as justification to terminate resuscitation (especially in special resuscitation groups like hypothermia), use of this technology does hold promise, and should be considered

important in the termination thought process and included in discussion with medical control.

KEY POINT:LOW ETCO2 alone is not indication to terminate resuscitation, but it is an important consideration. High ETCO2 may be an indicator to continue prolonged resuscitation.

Pitfalls of capnographyThe greatest potential for failure is focusing on the ETCo2 and not the patients overall clinical picture. The ages old mantra “Treat the patient, not the monitor” is more true than ever. Other pitfalls include not using ETCO2 on ETT patients. This is now considered by many experts as the “gold standard” and failing to do so opens up the medic to accusations of tube displacement.

Summary

Basic capnography is not only the measurement of ETCO2, but analysis of the phases of exhalation (measured indirectly by monitoring ETCO2) in the respiration cycle. In this, it is as useful, and as complex, as electrocardiography. It has been proven accurate and in some cases definitive in many clinical concerns facing the EMS community today. While still an emerging technique, it is likely that future paramedics will be stressing over capnography classes as much as they do over cardiology or pharmacology today.

Glossary

Capnography – A graphic representation of the measurement of carbon dioxide (CO2) in exhaled breath.

Capnometer – The numeric measurement of CO2.

Capnogram – CO2 waveforms which can be of two types: FCO2 can be plotted against expired 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 dioxide released at end of expiration.

References

Lisa Evered, MD;Francine Ducharme, MD;G. Michael Davis, MD; Martin Pusic, MD,

Can we assess asthma severity using expiratory capnography in a pediatric emergency department? Can J Emerg Med 2003;5(3):169-170

Levine R, End-tidal Carbon Dioxide and Outcome of Out-of-Hospital Cardiac Arrest, New England Journal of Medicine, July 1997

Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Review article. Canadian J Anaesth 1992;39:6:617-32.

Ornato J P et al: Multicenter study of a portable, hand-size, colorimetric end-tidal carbondioxide detection device.  Ann Emerg Med, May 1992; 21: 518-523. 

Garnett A R, Germin C A, Germin A S: Capnographic wavefoms in esophageal intubations: effect of carbonated beverages.  Ann Emerg Med 1989; 18:387-90. 

Vukmir R B, Heller M B, Stein K L: Confirmation of endotracheal tube placement: A miniaturized infrared qualitative carbondioxide detector.  Ann Emerg Med July 1991; 20:726-729. 

Tobias JD, Flanagan JF, Wheeler TJ, Garrett JS, Burney C.  Noninvasive monitoring of end-tidal CO2 via nasal cannulas in spontaneously breathing children during the perioperative period.  Crit Care Med 1994;22:11:1805-8.

Ornato J P, Garnett A R, Glauser F L, Virginia R: Relationship between cardiac output and the end-tidal carbondioxide tension.  Ann Emerg med 1990, 19: 1104-1106. 

White R D, Asplin B R: Out of hospital quantitative monitoring of end-tidal carbondioxide pressure during CPR.  Ann Emerg Med January 1994; 23;25-30. 

Lewis L M, Sthothert J, Standeven J et al : Correlation of end-tidal carbondioxide to cerebral perfusion during CPR.  Ann Emerg Med 1992; 21:1131-4. 

Callaham M, Barton C: Prediction of outcome of CPR from end-tidal carbondioxide concentration.  Crit Care Med 1990; 18:358-362. 

Chase P B, Kern k B, Sanders A B et al: Effects of graded doses of epinephrine on both non invasive and invasive measures of myocardial

perfusion and blood flow during cardio-pulmonary resuscitation. Crit care Med 1993; 21:413-9. 

Callaham M, Barton C, Mathay M: Effect of epinephrine on the ability of end-tidal carbondioxide readings to predict initial resuscitation from cardiac arrest.  Crit Care Med 1992; 20:337-343. 

Ward k R, Yealy D M:  End-tidal carbondioxide monitoring in emergency medicine, part2: Clinical applications.  Academic Emergency Med 1998; 5: 637-646. 

Kalenda Z: Capnogram as a guide to the efficacy of cardiac massage.  Resuscitation 1978; 6:259-63.