Capnography as a Clinical Tool

The capnography waveform is a key vital sign when determining treatment for patients in the field

Carbon dioxide (CO2) is a waste gas, so why do we care about it as long as we and our patients can get it out of the body? Just like an automobile’s performance can be monitored by a probe put in the exhaust pipe, an evolving technology allows emergency providers to precisely monitor the performance of a number of critical human processes in an ill or injured patient. This article describes the use of technology to monitor metabolism, circulation and ventilation in the emergency patient.

The Relevance of CO2

Long ago, Greek philosophers believed we had tiny internal combustion engines inside our bodies that produced “smoke,” or capnos. It turns out, the Greeks were correct. Our internal combustion engines are really mitochondria that are fueled by hydrocarbons (sugars, fats and proteins) essential in the human diet. After eating and movement through the digestive processes, sugars enter the mitochondria, where they are “combusted” and carbon dioxide is “exhausted.” Once CO2 is transported to the lungs via the circulatory system, it is exhaled with alveolar air.

The gases we breathe in are measured in partial pressure. So, a person standing at sea level under normal weather conditions would have 1 atmosphere of pressure being exerted by the gases in the air he breathes. An atmosphere of pressure is equivalent to 760 millimeters of mercury (mmHg). If the atmosphere contains approximately 79% nitrogen and 20% oxygen, the partial pressure of each is 600 mmHg nitrogen and 150 mmHg oxygen. The approximately 10 mmHg of air pressure remaining is composed of all of the other gases and vapors—mostly water vapor. Inhaled air contains essentially no carbon dioxide, while exhaled air is rich with CO2. The portion of pressure being exerted by carbon dioxide is called partial pressure and is typically 35–45 mmHg in the healthy person at rest. Partial pressure can be monitored on either the intubated or non-intubated patient. The monitoring process can be either discrete (one time or quick look) or continuous.

Capnography works by capturing exhaled air and redirecting it into the capnography device. The air then passes between a light and a detector that measures how much light is shining on it. As the concentration of CO2 increases, more light is absorbed by the CO2 and less light is transmitted onto the detector plate. This increased light absorption directly correlates with the percentage of carbon dioxide. The monitor presents the CO2 concentration to the capnographer as both a number and a waveform. The respiratory rate can be very accurately estimated and reported by measuring the tides between CO2 peaks.

Blood levels of carbon dioxide are as critically important as blood oxygen levels. In fact, oxygen loading onto hemoglobin and transport to the tissues is highly dependent on the tight regulation of CO2. Furthermore, CO2 can act as a molecular signal affecting both nervous and smooth muscle tissues.

Ventilation and Oxygenation

Ventilation and oxygenation are interrelated, but represent distinct processes. Different diseases can affect the processes in different ways. Oxygenation involves loading hemoglobin with oxygen for delivery to the tissues, while ventilation addresses clearance of CO2 from the blood. Technologies for monitoring oxygenation have been extraordinarily useful to EMS providers for the past 20 years. More recently, technologies are being developed to enhance EMS’s abilities to measure the effectiveness of ventilation. Measuring exhaled CO2 can provide valuable insight into metabolism and circulation. Take, for example, the moderately sick asthma patient demonstrating a “shark fin” pattern with an elevated EtCO2 but 100% oxygen saturation. This patient is oxygenating well, but not ventilating effectively.

There are three physiological processes for human life: metabolism, circulation and ventilation.

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