Abstract

This article gives a short review of the basic definitions of capnography and its use. The introduction gives an overview of the historical development of this procedure. Technical features of the method are presented, followed by several definitions for understanding the basic terms needed to realize the applications of capnography. The last section is a descriptive part that explains the most important clinical applications of capnography, the strengths and limitations of this method. This article distinguishes capnography applications as a single procedure and its benefits as a complimentary procedure.

Key words: capnography, monitoring, ventilation, end-tidal CO2

Introduction

Capnography is a simple method of monitoring the concentration or partial pressure of carbon dioxide (CO2) in the respiratory gases. The fundamentals of capnography use were established in 1943 by Luft who discovered that CO2 could absorb infrared (IR) radiation. (1) Capnography was first used in Holland in 1978, and subsequently its usefulness was approved for monitoring during anesthesia. Nowadays, capnography is a standard of care for monitoring patient safety in anesthesia, but it has not yet been accepted for routine use in emergency department procedural sedation and analgesia. (1)
Several procedures are available for monitoring airway CO2. The first procedure is by using a side stream sample measured through a rapidly responding infrared CO2 analyzer or measured through a mass spectrometer. The second procedure is direct measurement of CO2 values through an infrared analyzer at the end of the endotracheal tube. These procedures correspond to the term capnography or airway CO2 monitoring. (2)

Basic terms

Capnography is a graphic display of CO2 concentration during the respiratory cycle, while capnometry is a numerical display of CO2 concentration during the respiratory cycle.
This method of monitoring directly shows the elimination of CO2 by the lungs and indirectly reflects the production of CO2 by tissues and CO2 circulatory transport to the lungs. Capnography is a non-invasive and accurate method. The need for arterial blood sampling can be significantly reduced. (3)

Clinical applications

An extensive Pubmed search lists 46 clinical applications of capnography overall, and can be divided into six major categories: Airway, Breathing, Circulation, Anesthetic Delivery Apparatus, Homeostasis and Non-perioperative. (4)
Information obtained through capnography includes: CO2 production; lung perfusion; alveolar ventilation; respiratory patterns and elimination of CO2 from the anesthesia breathing circuit and ventilator. The illustration that follows clearly demonstrates how capnometry, as a method, can be applied to reflect the body’s overall physiology and the crucial parameters of the vital functions (figure 1).

Capnography directly measures the ventilatory performance of the lungs and indirectly presents measurements on the performance of metabolism and circulation. For example, an increased metabolism will increase the production of carbon dioxide increasing the ETCO2. A decrease in cardiac output will lower the delivery of carbon dioxide to the lungs, decreasing the ETCO2. (5) Thus, it gives us a rapid and reliable method to detect life-threatening conditions such as malposition of tracheal tubes, ventilatory failure, circulatory failure and defective breathing circuits.
An important use of capnography is as a non-invasive assessor of proper endotracheal tube placement. (6) An advantage of capnography is that it provides an immediate picture of patient apnea, while pulse oximetry is delayed for several minutes. Pulse oximetry is insufficient for postoperative respiratory monitoring. (7) It is better to use capnometry for postoperative patients because it is easy to use and useful for monitoring patients’ breathing. However, capnometry must be improved in its wearability and detection capability. Therefore, it is not used often for postoperative patients as a respiratory monitor. Capnography and pulse oximetry, used in conjunction, could have helped in the prevention of 93% of avoidable anesthesia mishaps according to the American Society of Anesthesiologists (ASA). So, capnography and pulse oximetry should be used as complimentary procedures for preventive purposes. Also, as a complementary procedure to arterial blood gas analyses, capnography provides estimates of the inefficiency of ventilation. The American Heart Association (AHA) affirmed the importance of using capnography to verify tube placement in their 2005 CPR and ECG Guidelines. (8) The AHA also notes in their new guidelines that capnography, which indirectly measures cardiac output, can also be used to monitor the effectiveness of CPR and as an early indication of return of spontaneous circulation. (9) The figure below presents the response of cardiac output under the influence of epinephrine administration as one of the CPR procedures measured through the ETCO2 (figure 2).

During resuscitation, exhaled CO2 is a better guide to the presence of circulation than an electrocardiogram, pulse or blood pressure. Exhaled concentrations are helpful in determining which patients are likely to be successfully resuscitated. The patient is more likely to be resuscitated if the concentration of exhaled CO2 is greater than 10-15 mmHg.
Finally, it has been suggested that capnography can help in weaning patients off mechanical ventilation, but the prospective value of airway CO2 monitoring in this clinical situation is unclear. (10)

References

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03. Moon RE. Respiratory Monitoring. In: Miller RD, editor. Anaesthesia. 5th ed. New York: Cherchill Livingstone; 2000. p. 1255-95.
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05. Badal JJ, Loeb RG, Trujillo DK. A simple method to determine mixed exhaled CO2 using a standard cycle breathing circuit. Anesth Analg 2007;105:1048-52.
06. Stein N, Matz H, Schneeweiss A. An evaluation of a transcutaneous and an end-tidal capnometry for noninvasive monitoring of spontaneously breathing patients. Respir Care 2006;51:1162-6.
07. Adams AP, Atkinson RS. Capnography and pulse oximetry. Anaesth Analg 1989;2:155-75.
08. Fusch J, Schumer C, Giesser J. Detection of tracheal malpositioning of nasogastric tubes using endotracheal cuff pressure measurement. Acta Anaesthesiol Scand 2007;51:1245-9.
09. Krauss B, Hess DR. Capnography for procedural sedation and analgesia in the emergency department. Ann Emerg Med 2007;50:172-81.
10. Bhavani Shankar K, Kumar AY. Terminology and the current limitations of time capnography. J Clin Monit 1995;11:175-82.

Figure 1. Capnometry is a method applicable for overall assessment of the body’s physiology and the crucial parameters of the vital functions.

Figure 2. Response of cardiac output under the influence of epinephrine administration as one of the CPR procedures measured through the ETCO2.

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