1. Metabolism is the utilization of hydrocarbons to produce energy and power tissues, organs and the entire body. This process must be continuous for life to be sustained. It is important to note, however, that several hours after cardiac arrest, some residual metabolism (liver, skeletal muscles, skin, etc.) will continue to produce CO2.
2. Circulation. Blood must be moving in order to deliver CO2 from the tissues to the alveoli. Circulation requires blood, an effective heartbeat and blood pressure. Preload plus afterload equals circulation.
3. Ventilation. Air must move in and out of the alveoli effectively to get rid of carbon dioxide and other waste products, and to inhale fresh oxygen.
Capnography as a Tool for Measuring Metabolism, Circulation and Ventilation
Capnography is a powerful tool for providing continuous real-time objective data to qualify and quantify the status of metabolism, circulation and ventilation. Metabolism is assessed by determining the quantity of CO2 being exhaled and following this over time. Circulation is measured by the trends in delivery of CO2 to the lungs. Capnography is the only technology able to display respiratory rate based upon the partial pressure of exhaled CO2 while providing the unique pattern recognition of CO2. Capnography quantifies patient ventilation in terms of transport of CO2 from the pulmonary circulation across the alveoli for exhalation. It also provides a graphic picture of the patient’s ventilatory status, presents an early warning of changes in the patient’s cardiopulmonary status, supplies indisputable documentation of the patient’s airway patency, and alerts clinicians to the presence of pulmonary pathology. Abnormal capnography values can be traced to diseases affecting ventilation, perfusion or metabolism.
Continuous waveform capnography is recommended to EMS for verification of endotracheal tube (ETT) placement.1 The 2010 ACLS guidelines have elevated the role of capnography to recommending continuous waveform CO2 monitoring during resuscitation.
Capnography is can also provide valuable cardiopulmonary information to assist paramedics in caring for non-intubated patients.
Capnography & Intubated Patients
Capnography provides a reliable and objective method to confirm endotracheal intubation. The presence of a waveform demonstrates proper placement, even during CPR. This has been well documented over the past few years.1,2
Capnography can also be useful in detecting a change in position of the endotracheal tube. In EMS, this is important, as the endotracheal tube may be accidentally displaced while moving the patient. Given some of the untoward and noisy environments where cardiac arrest occurs, confirming ETT placement by lung sounds alone can be problematic and sometimes even deceiving, especially in smaller patients. A 2005 study compared prehospital intubations using continuous capnography to confirm tube placement with those not using capnography. The group using capnography achieved correct tube placement in 100% of their patients. The group practicing without the benefit of capnography had a misplaced tube (unrecognized esophageal or supraglottic) in 23% of their patients.1
Recent studies have dulled the significance of endotracheal intubation, and many EMS providers are relying more on alternative methods of securing the airway.3 Capnography is just as easily applied to alternative airways like the Combitube, King airway or laryngeal mask airway (LMA). As with the endotracheal tube, proper and improper ventilation can be monitored with alternative airways and corrected where necessary. It is imperative to ensure the device is providing sufficient protection of the airway to allow effective rescuer ventilation.4
The capnography waveform assists in determining proper ventilation with any device that securely attaches to a bag-valve mask. No matter which device is in use, capnography can provide immediate indication of the loss of proper position or function. As with endotracheal intubation, when using alternative airways it is critical to continuously monitor the airway and assess ventilatory status. Waveform capnography is able to support those clinical assessment needs and can indicate the need to adjust ventilatory support.2