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Learn more about Capnographs

What is a Capnograph?

Introduction

A Capnograph is a device that measures expired carbon dioxide using infrared spectroscopy.

It displays the peak concentration of carbon dioxide occurring at the end of expiration, known as the end-tidal CO2(ETCO2) measurement, either as a percentage or as a partial pressure.

This is currently the optimal method of continuously monitoring the adequacy of ventilation and circulation in infant, paediatric and adult patients.

ETCO2 can be of value in the assessment of ventilation, metabolism, and of a patient’s circulation status.

Glossary

Capnography – Measurement of carbon dioxide with a graphic as well as numeric display

Capnogram – A graphical waveform display of carbon dioxide concentration over time

CO2 – Carbon Dioxide, a by-product of cellular metabolism that is exhaled during the respiratory cycle

Carbon Dioxide Physiology

Carbon dioxide (CO2) is a waste product of normal cellular metabolism. CO2 leaves the cells and is carried by the venous blood to the heart (circulation) and lungs (respiration). Once CO2 reaches the lungs, it is eliminated in the process of exhalation.

In order for CO2 to be effectively eliminated from the body, there must be adequate blood flow to the lungs, adequate gas exchange across the alveolar-capillary membrane, and adequate ventilation of the lungs to “blow off” the CO2. Therefore, changes in respired CO2 may reflect alterations in metabolism, circulation, respiration, the airway or breathing system.

Metabolism, changes in ETCO2 can be a reliable indicator in metabolic changes.

Metabolic conditions that may increase ETCO2: fever, sepsis, shivering and convulsions

Metabolic conditions that may decrease ETCO2: hypothermia, paralytics and sedation

Malignant hyperthermia is a hypermetabolic state with a massive increase in CO2 production. The increase occurs early, before the rise in temperature. Early detection of this syndrome is one of the most important reasons for routinely monitoring CO2

Circulation, a decrease in ETCO2 is seen with a decrease in cardiac output if ventilation remains constant. ETCO2 transport to the lungs is dependent on adequate cardiovascular function; any factor that alters cardiovascular function can affect CO2 transport to the lungs

ETCO2 can alert clinicians to changes in the cardiovascular function of the patient with a “normal” respiratory status,

Cardiac conditions that may decrease ETCO2: hypovolemia, hypotension

Respiratory, ETCO2 can be a guide for determining the ventilation requirement of a patient

Changes in respiratory function will affect the removal of CO2 from the lung thus affecting the ETCO2

Measurement Techniques

Infrared Absorption (IR), is the most common technique in measuring ETCO2.

The principle is based on the fact that CO2 molecules absorb infrared light energy of specific wavelengths, with the amount of energy absorbed being directly related to the CO2 concentration. When an IR light beam is passed through a gas sample containing CO2, the electronic signal from a photodetector can be obtained. This signal is then compared to the energy of the IR source and calibrated to accurately reflect CO2 concentration in the sample.

Measurement Methods

Mainstream vs. Sidestream Sampling – Mainstream and sidestream sampling are the two basic configurations of CO2 monitoring. Each term refers to the position of the actual measurement device (often referred to as “the IR bench”) relative to the source of gas being sampled.

Mainstream method utilises a sensor or infrared measuring device placed directly in-line between the ventilator breathing circuit and the ET tube. Mainstream generally provides a fast response time and the elimination for the need of water traps.

Sidestream method requires a gas sample to be aspirated from the patient’s airway and transported to the senor inside a monitor by means of a pump. This type of system can be used on non-intubated patients while utilising a variety of sampling cannulas.

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