Beyond the Basics: Capnography

CEU Review Form Capnography (PDF)Valid until March 3, 2008

All levels of EMS personnel are taught that a patent airway is imperative to effective patient management. Endotracheal intubation is often used to secure and maintain a patent airway in the prehospital setting. Following endotracheal intubation, the EMS provider must assess and continually reassess tube placement, especially after any patient movement.

Initially, placement is confirmed by visualization of the tube passing through the glottic opening and between the vocal cords. Once the tube is placed, auscultation is performed over the epigastrium to listen for gastric sounds. If none are present, the lungs are auscultated for presence and equality of breath sounds.

A device that can provide invaluable information regarding initial and continuous monitoring of endotracheal tube placement is the end-tidal CO₂ monitor, which provides a numeric carbon dioxide value in addition to a continuous waveform.

RESPIRATORY PHYSIOLOGY AND GAS TRANSPORT
To understand capnography, it is important to review some basic respiratory physiology. Whether a patient is ventilating spontaneously under his or her own control or you are delivering ventilations via a bag-valve device, air travels through the respiratory tract until it reaches the alveoli--very thin-walled structures surrounded by capillaries. This allows for gas exchange between the oxygen-enriched air in the alveoli and the oxygen-depleted blood in the capillary that is also high in carbon dioxide. As a general rule, gas moves from a high to low concentration; therefore, oxygen diffuses from the alveoli into the capillary as carbon dioxide exits the capillary and enters the alveoli. The blood exiting the pulmonary capillaries has a high partial pressure of oxygen and a low partial pressure of carbon dioxide. This blood is then transported back to the left side of the heart, where it is ejected into the aorta, arteries and, eventually, arterioles. The arterioles are the terminal ends of the artery and are the entry point into the capillary bed. The cells have a lower partial pressure of oxygen and higher partial pressure of carbon dioxide. Based on the general rule of gas diffusion, oxygen leaves the capillary and moves into the cell as carbon dioxide diffuses from the cell into the capillary bed. The blood exits the capillary via a venule and enters a vein. The venous blood has high levels of carbon dioxide as a result of normal cellular metabolism.

When a cell metabolizes glucose in the mitochondria in the presence of oxygen, known as aerobic metabolism, it produces water and carbon dioxide as its byproducts. The carbon dioxide must be transported to the alveoli so it can be exhaled. Carbon dioxide is transported in the blood in three ways: 1) dissolved in plasma; 2) attached to the hemoglobin molecule forming carbaminohemoglobin (HbCO₂); and 3) in the converted form of bicarbonate (HCO₃-). Approximately 7% is transported dissolved in plasma, 23% as carbaminohemoglobin and 70% as bicarbonate.

Carbon dioxide enters the red blood cells and attaches with water. Carbonic anhydrase converts the water and carbon dioxide to carbonic acid, which is split into a hydrogen molecule and bicarbonate:

H₂O + CO₂  ¨ H₂CO₃-  ¨ H⁺ + HCO₃-

A chloride shift then occurs. As chloride enters the red blood cells, the bicarbonate leaves and is transported in the plasma.

When blood reaches the pulmonary circulation, the bicarbonate reenters the red blood cells as chloride exits. The bicarbonate combines with hydrogen and forms carbonic acid, which splits into water and carbon dioxide:

H₂O + CO₂  © H₂CO₃-  © H⁺ + HCO₃-

The carbon dioxide diffuses from the cells, leaves the capillaries and is eliminated through exhalation.