Capnography in the field

Carbon dioxide monitoring is a relatively new addition to the diagnostic tools for emergency medical service care providers. The technology has become reliable enough, and the packaging appropriate for field use (meaning that it is tough enough to survive the brutal treatment that EMS providers dish out), so the timing is right for the implementation of the monitors for improving patient care.

Carbon dioxide measurement technology is used extensively in the hospital for monitoring of all types of critical patients, from the operating rooms and intensive care units, to the Emergency Department and clinics. The most valuable use of carbon dioxide measurement in patients is to monitor the patient’s ability to breathe adequately, and to detect and diagnose abnormalities.

The process of measuring and displaying carbon dioxide is called capnography, and the technology utilized in prehospital care displays respiratory rate based upon the partial pressure of exhaled CO₂ and the patterns of flow of the gas.

Utility of carbon dioxide measurement
Measurement of CO₂ allows the provider to monitor metabolism, circulation, and ventilation in the emergency patient. CO₂ is produced in cells as they burn fuel (mostly hydrocarbons), and is released back into the bloodstream, taken to the lungs, and disposed of into the airways.

Ambient air on earth contains essentially no carbon dioxide, so inhaled air is free of the gas. Exhaled air contains the CO₂, which can be monitored either through an endotracheal tube or alternate airway device or by capturing a small sample at the nose or mouth (think of the automobile exhaust gas sensor process).

The monitor reads the air passing between a light source and a detector, and as the concentration of CO₂ increases, more light is absorbed by the CO₂ and less light is transmitted onto the detector plate. This measures the percentage of carbon dioxide present in the exhaled air. The EMT reading the monitor can view both a waveform and a number. Like reading an EKG, both can have value. In studies that verify the accuracy of the carbon dioxide monitors, the blood levels of carbon dioxide are tightly correlated with blood levels, and change almost instantly.

There are three processes necessary for human life: ventilation, circulation, and metabolism. The measurement and movement of CO₂ will monitor all three of these.

Ventilation is the movement of air in and out of the lungs to inhale oxygen needed for metabolism and rid carbon dioxide and other waste products.

Circulation is the bloodstream movement of material from the cells to the outside world, and vice versa. Circulation requires an effective heartbeat, blood, and an intact movement stream.

Metabolism is the utilization of hydrocarbons by the cells to produce the energy that power tissues, organs, and the body. Metabolism is measured through the quantity of CO₂ being produced, and following this over time. Circulation is measured by the trends in delivery of CO₂ to the lungs. Ventilation is quantified in the transport of CO₂ over time from the pulmonary circulation across the alveoli for exhalation.

Abnormal capnography values can be traced to diseases affecting ventilation, perfusion, or metabolism. It is also used to detect patients that are producing no carbon dioxide (those patients are dead or near death) and to identify when a tube is in the airway.

Capnography provides indisputable documentation of the patient’s ventilatory status, and in a very common use confirms that the endotracheal tube or another advanced airway device is in the column of air that contains CO₂. The stomach and esophagus have typically small amounts of air present, and that air is ambient air, which contains little CO₂. So, the carbon dioxide measurement technology being used to confirm the placement of a tube in the airway will detect carbon dioxide, and let the EMT know the tube is in the correct place.

As such, capnography is a proven and powerful monitor to provide instantaneous assessment of metabolism, circulation, and ventilation. The recently published 2010 ACLS guidelines have elevated the role of continuous waveform CO₂ monitoring during resuscitation, and it is recommended for use by EMS for the verification of endotracheal tube and other airway placement.

Capnography can be used as a noninvasive measurement of cardiac output during cardiac arrest. While performing CPR, carbon dioxide production by cellular metabolism remains constant; therefore, capnography can assist in showing the EMT the effectiveness of chest compressions.

The American Heart Association (AHA) Guidelines call for quality compressions (“push hard, push fast, push deep”). It is important for EMTs to perform quality CPR to keep the CO₂ number up as high as possible. This represents effective cellular perfusion allowing cells to metabolize, and circulation to deliver the carbon dioxide to the lungs.

AHA Guidelines suggest rescuers are directed to switch places every two minutes to maintain effective CPR. By analysis of the CO₂ waveform, the EMT can detect rescuer fatigue before the rescuer is even aware of tiring. In the resuscitation situation, a rapid rise in the ETCO₂ during chest compressions can be the first indicator of return of spontaneous circulation (ROSC), possible even before a pulse is detectable.

In critical care patient situations, capnography allows an early warning of changes in the patient’s cardiopulmonary status, and detects the presence of pulmonary pathology. Capnography can be used to differentiate between the varying causes of respiratory distress often seen by EMS providers in the field, such as asthma, COPD exacerbation, and congestive heart failure.

It can provide EMTs with early warning signs of hypoventilation, apnea, airway obstruction, and hypercarbia before compensatory changes are seen in heart rate and/or blood pressure. By analyzing the CO₂ waveform over time, the EMT can monitor the severity of asthma or COPD and the effectiveness of therapy provided.

Monitoring ETCO₂ can provide an early warning sign of shock. With skilled EMS providers, CO₂ monitoring is useful for monitoring patients with seizure, trauma, head injuries, metabolic problems such as hypothermia, hyperthermia, shock, diabetes, and pulmonary emboli. A patient with a sudden drop in cardiac output will show a diminished CO₂ waveform and a drop in the ETCO₂ number that may occur regardless of any change in breathing rate. In nonintubated patients, capnography is equally relevant. Capnography and pulse oximetry are tools that, when used together, will assist EMS providers in assessing patient status and treatment results in patients with serious cardiac and pulmonary problems.

The available tools
Colorimetric disposable end tidal CO₂ detectors are a single use device. These detectors use an indicator that changes color in response to ETCO₂ concentration. This tool is often integrated into the bag-valve system. If the indicator remains purple after the monitor is attached to the endotracheal tube, carbon dioxide concentration is low (< 0.3 percent). If it turns bright yellow, the concentration is high (>2.0 percent). A beige color represents intermediate concentration (0.5—1.0 percent).

The colorimetric CO₂ detectors have a shelf life of about a year, under normal conditions. When opened, the device will function for up to five hours, but in high humidity, the life span may be as little as 15 minutes. A colorimetric carbon dioxide detector costs less than $20. This very low price and ease of use allow this tool to be routinely employed in every intubation.

Electronic end tidal CO₂ detectors are monitor devices that stand alone, or are built into multi-purpose patient monitors as a module. Electronic CO₂ detectors have capnography technology that uses one of two types of technology: sidestream or mainstream. Sidestream monitors sample the patient’s exhaled breath in a breathing circuit, which is aspirated to a sensor located in the monitor itself. Mainstream monitors utilize a sensor located on an in-line airway adapter and CO₂ is measured directly in the patient’s breathing circuit. The cost of these units is several thousands of dollars, with disposable portions of the apparatus that cost $10 to $20 per use.

Summary
The quality and safety of emergency patient care is improved using capnography. It improves the ability of the provider to assess the patient initially, and monitor changes in patient status with treatment.

References
American Heart Association (AHA) - Highlights of the 2010 AHA Guidelines for CPR and ECC, http://static.heart.org/eccguidelines/pdf/90-1043_ECC_2010_Guidelines_Highlights_noRecycle.pdf

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