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End-tidal carbon dioxide monitoring in pediatrics: concepts and technology. MS BhendeDivision of Paediatric Emergency Medicine, Department of Paediatrics, University of Pittsburgh School of Medicine, Children's Hospital of Pittsburgh, Pittsburgh, PA 15213-2583, USA. , USA
Correspondence Address: Source of Support: None, Conflict of Interest: None PMID: 11832614 Keywords: Carbon Monoxide, analysis,Emergencies, Human, Monitoring, Physiologic, methods,Pediatrics, Tidal Volume,
End-tidal CO2 (ETCO2) monitoring allows exhaled CO2 to be measured non-invasively. This methodology was first studied clinically by Smallhout and Kalenda in the 1970s,[1] studied extensively over the last 2 decades and now is being used extensively mainly to verify endotracheal tube (ETT) position and during cardiopulmonary resuscitation (CPR).[2] Critically ill children need to have their airways controlled by endotracheal intubation and their ventilation status optimally managed.[2] PaCO2 (partial pressure of CO2 in the arterial blood) is a direct measurement of ventilation status but is invasive and the data are not continuous. ETCO2 monitoring has been a boon allowing us to measure exhaled CO2 non-invasively by different methods. [2],[3],[4] It has become the standard of care in the operating room, ICU and now being increasingly used in the emergency department and the prehospital setting. [2],[3],[4] This article will review the basic physiology, the technology, both qualitative and quantitative, and discuss capnograms. A companion article will discuss the clinical applications in the paediatric emergency setting.
CO2 is produced by cellular metabolism and is transported to the right heart by the venous system.[1],[2],[3] It is then pumped into the lungs by the heart and then diffuses out into the exhaled air where it can be measured.[1],[2],[3] ETCO2, therefore, is a reflection of metabolism, circulation and ventilation. [1],[2],[3] If any two systems are kept constant, then the changes in ETCO2 will reflect changes in the third system.[1],[2],[5] CO2 production normally remains constant, and when circulation is normal, ETCO2 usually changes with ventilation and therefore approximates PaCO2.[1],[2],[3],[4],[5] During shock or cardiac arrest, ETCO2 levels will be reflective of pulmonary blood flow and cardiac output and not minute ventilation.[5],[6] If ventilation (v) and perfusion (q) are well matched, the ETCO2 will nearly equal PaCO2.[1],[2],[5],[6],[7] It is usually 2-5 mm Hg less than PaCO2. [1],[2],[5],[6],[7] ETCO2 represents PaCO2 from all ventilated alveoli and PaCO2 represents PaCO2 of all perfused alveoli. The v/q ratio is about 0.8, usually because there is some dead space (main airway and alveoli).[1],[2],[5],[7] In non-perfused alveoli, the CO2 concentration is zero and in adequately perfused alveoli the CO2 concentration is normal. So usually the net ETCO2 underestimates the PaCO2 in cases of v/q mismatch, such as shock, CPR, pulmonary embolism, etc. Decreased ventilation will increase PaCO2 and increase ETCO2-PaCO2 difference as in asthma, atelectasis, pneumonia and emphysema.[2],[5],[7] Absent ETCO2 is found in oesophageal intubation.[2],[5],[7]
* Capnometry is the measurement of expired CO2 and provides a numeric display of CO2 tension in mm Hg or %CO2.[8] * Capnography is the graphic representation of expired CO2 over time * Capnograph is the measuring instrument * Capnogram is the waveform displayed by the capnograph * End-tidal CO2 (ETCO2) is the measurement of CO2 at the very end of expiration. It is the maximum concentration of expired CO2 * PaCO2 is partial pressure of CO2 in arterial blood
There are different methods of measuring CO2 in respiratory gases – mass spectroscopy, Raman spectroscopy, infrared (IR) spectroscopy and colorimetric devices using chemically treated filter papers.[5],[9] In the ED and prehospital setting, monitors using infrared spectroscopy and colorimetric detectors are used.[2],[5] The most common method of measuring CO2 is by infrared spectroscopy.[10] Of the common gases in exhaled air, CO2 and water vapour absorb IR light. Water vapour is removed by dehumidifying the sample. In the sensor unit, light from IR measuring source is filtered to include only the bandwidth corresponding to the absorption peak of CO2. The amount of gas absorbed at the CO2 bandwidth is compared to a known CO2 concentration in the reference sample, and the partial pressure of ETCO2 in sample is determined.[2],[5],[9],[10] High concentrations of O2, N2O and anaesthetic gases can change CO2 concentration because of spectral overlap and need to be compensated for or eliminated.[5],[10] Several hand-held models are available that measure CO2 qualitatively, semi-qualitatively or quantitatively using the IR method.[5],[10] Microstream technology that uses sidestream sampling method (described below) was approved by the FDA in 1997.[2],[11] This technology uses a microbeam IR sensor that specifically isolates the CO2 waveform and does not require correction, recalibration or software compensation for N2O, O2 or anaesthetic gases.[2],[11] Position independent adapters, linear flow, vapour-permeable tubing and submicron multisurface filters are used to minimize the frequency of tube occlusion and contamination.[2],[11] The accuracy and resolution of waveform are preserved in low flow systems (e.g. neonates) because the sample flow rate (50 ml/min) and cell volume (15 ml) is small. We validated the microstream capnometer in the paediatric ICU by comparing it with standard mainstream capnometers.[12] The devices measure ETCO2 in torr (normal 38 mmHg) or volume concentration (5%) by dividing the ETCO2 in torr by atmospheric pressure.[2],[7] The CO2 sensors are located directly in the patient’s breathing circuit (mainstream) or remote from the patient as part of CO2 monitoring system (sidestream).[2],[5],[7],[10] Sidestream - Sidestream capnometers have tiny aspiration pumps that aspirate samples from the airway. The sampling tube is connected to a T piece inserted at the ETT in intubated patients or to the nostril or mask in non-intubated patients. Common problems include occlusion with water and secretions, and contamination of the monitor.[2],[10] These may be inaccurate in patients with high respiratory rates and low flow system,[8] as in neonates, which can lead to inadequate flushing of the sample cell and yield falsely low ETCO2 levels and change the shape of the capnogram.[2],[10] Microstream technology (described above) which uses the sidestream sampling method claims to have taken care of this problem and is being currently studied in clinical settings. Mainstream – The IR source and detector are placed in line at the end of the patient’s ETT.[2],[7],[10] There is no need for gas sampling because the cuvette is in line. All mainstream sensors are heated above body temperature to prevent condensation of water vapor which can be uncomfortable and cause burns.[2],[9] This can also cause kinking of the ventilation tubing and traction on the ETT and its use is restricted to intubated patients.[2],[9] Colorimetric – Portable colorimetric detectors are attached between the ETT and ventilation tubing. A pH sensitive, non-toxic chemical indicator is housed in a clear dome that remains purple in room air and changes to yellow in presence of CO2.[2],[13] The colour change is reversible and changes from purple to yellow with each inspiration and expiration in correctly intubated patients. The 38 cc dead space precludes continuous use in infants but can be safely used briefly in patients weighing more than 2 kg to verify ETT position.[13] A paediatric colorimetric detector with dead space of 3 ml is also available and allows brief usage in infants up to 1 kg.[14] Capnoflo resuscitators have the colorimetric strip incorporated in the clear connector of the ventilator bag.[15],[16]
A capnogram is a plot of CO2 concentration over time and has four phases [Figure - 1].[2],[5],[7] Phase I – (AB) is flat and corresponds to late inspiration and early expiration where CO2 free dead space gases are released. Phase II (BC) is the upstroke corresponding to the appearance of alveolar CO2 in expired gas. Phase III (CD) or plateau represents flow of expired air from uniformly ventilated alveoli with nearly constant CO2. Point D is the highest point of the plateau, is called ETCO2, and is the maximal concentration of expired CO2. Phase IV (DE) is the rapid descent of CO2 concentration to baseline during inspiration. A capnogram can be recorded at low speed (0.7 mm/sec) to follow clinical trends or at fast speed (7 mm/sec) to give detailed information.[2],[5] High baseline represents rebreathing of expired CO2.[2],[5] Slow upstroke of Phase II can be due to slow sampling rate, prolonged expiration as in asthma or uneven emptying as in atelectasis. A rising phase III with no plateau is seen in lower airway obstruction as in asthma and the alpha angle between II and III phases increases as the airway obstruction increases.[2],[5] Sudden loss of the capnograph (levels to zero) occur with airway disconnection or total obstruction or oesophageal intubation [Figure - 2].[2],[5] Sudden exponential disappearance of expired CO2 can be seen due to loss of pulmonary blood flow, as in cardiac arrest, or when the patient is placed on cardiopulmonary bypass [Figure - 3].[2],[5] Rising ETCO2 can be seen either due to increased CO2 production, such as in sepsis, malignant hyperthermia or increased metabolism, or due to decreased CO2 elimination as when there is decreased pulmonary compliance.[5] A transient rise of ETCO2 is seen when a tourniquet is released, bicarbonate is infused, or there is transient ROSC during CPR. [5] Sudden exponential reduction of CO2 can be seen in pulmonary embolism and sudden severe hypovolaemia.[5]
ETCO2 monitoring can provide the emergency physician with a valuable tool to assess and monitor patients. It is now mandated by the American Heart Association in the new Paediatric Advanced life Support guidelines that all intubations have to be confirmed by some form of ETCO2 measurement.[17] This will be of utmost importance to avoid the tragedy of undetected oesophageal intubation.[2] Clinical application of the above discussed technology will be reviewed in a companion article.
[Figure - 1], [Figure - 2], [Figure - 3]
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