Capnography Outside the Operating Rooms

Abstract

HISTORICALLY, anesthesiologists seem to be the forerunners in implementing tools and standards for safety in the medical fraternity. In the United States, since 1985, there has been a dramatic decrease in the malpractice insurance premiums of anesthesiologists. Such a decrease has not been seen in other medical or surgical specialties over this time frame. Thanks to the foresight of the American Society of Anesthesiologists (ASA), Anesthesia Patient Safety Foundation (APSF), Association of Anaesthetists of Great Britain and Ireland (AAGBI), and the Association of Anesthesiologists in Holland, capnography was embraced and incorporated into the standards of monitoring during anesthesia to enhance patient safety. Currently, anesthesiologists in many developing countries follow these recommendations (India, Government of Andhra Pradesh Order, AST/775/F25/dated September 2011. Capnography is mandatory for laparoscopic surgeries for reimbursement). Although capnography has become an integral part of anesthesia care in operating rooms for more than 25 yr, its value has been limited to these situations and is not well appreciated beyond these confinements. It is not uncommon in our practice to observe an intubated and ventilated patient, originally monitored with capnography in the operating room, but then transported to the intensive care unit (ICU) without capnography. It is even more surprising that many ICUs do not have capnography either to confirm endotracheal intubation or to continually monitor ventilation. As anesthesiologists, we use capnography to monitor sedation in the operating room because we appreciate that the line between consciousness and unconsciousness is very thin, and the patient can drift from one state to another. However, in many institutions, capnography is not used to monitor ventilation during sedation for procedures performed particularly by nonanesthesiologists outside of the operating rooms. One of the obvious reasons seems to be a lack of a single society overseeing the safety of outside-the-operating-room procedures the way ASA and AAGBI do in the operating room. Nonetheless, in the last 2 yr there has been a surge in understanding and recognizing the value of capnography outside of the operating rooms.1,2This “Clinical Concepts and Commentary” will summarize physiology and clinical interpretation of capnography and update the current status of capnography outside of the operating rooms, including public and media awareness, and suggest probable future directions.Infrared technology is by far the most common and cost-effective method of carbon dioxide measurement and monitoring. Efforts have been made to decrease the response time and increase the accuracy of infrared technology to produce superior capnography waveforms even in premature babies with small tidal volumes and rapid respiratory rates.3The carbon dioxide values are usually displayed as partial pressure (PCO2). Depending on the location of the carbon dioxide measuring device, there are two sensor types: mainstream and sidestream. In mainstream sensors, the adaptor housing the sensor is interposed between the tracheal tube and breathing circuit, and the measurement of carbon dioxide is made across the airway. In sidestream technology, the respiratory gases are aspirated via an adaptor and a 6-foot sampling tube to the monitor housing the infrared sensor. The transport of gases to the infrared measuring device results in a delay of 1–4 s in carbon dioxide measurement and display of capnograms in sidestream capnography.A conventional capnogram in adults results in a more or less identical shape in all healthy individuals. Any variation from this requires an analysis to determine a physiologic or pathologic cause of the variation. Carbon dioxide waveforms can be plotted against time (time capnogram, fig. 1A) or expired volume (volumetric capnogram, fig. 1B). Time capnography is used more commonly in clinical practice. A time capnogram has two important segments: inspiratory (phase 0) and expiratory.4–6The expiratory segment is further divided into three phases (I, II, III), and an occasional phase IV (fig. 1C), based on the physiology of carbon dioxide evolution from the lungs and airways. Phase I does not contain any expired carbon dioxide (dead space gases, zero PCO2). In phase II, the PCO2rises rapidly as carbon dioxide–laden alveolar gas replaces zero-carbon dioxide dead space gas. Phase III is the alveolar plateau that represents the evolution of carbon dioxide from alveoli. If all the alveoli had the same PCO2, the alveolar plateau would be perfectly flat. In reality, there is a considerable spatial and temporal mismatch in the lungs resulting in variable V/Q ratios and thus, variable PCO2. Usually the alveoli with lower V/Q ratios and with longer time constants (containing relatively more carbon dioxide) contribute to the later part of phase III. This results in a slight upward slope of the alveolar partial pressure “plateau.” Therefore, the slope indirectly represents the V/Q status of the lungs. Hence, the height and slope of the alveolar plateau provide valuable information about ventilation, perfusion, and, more importantly, the V/Q relationship. When there is substantial variation in the V/Q ratio as a result of airway caliber changes, the slope of phase III is exaggerated and can also be associated with a prolonged phase II (fig. 2A). Under these circumstances, the angle between phase II and phase III (the α angle), which is generally 100°, is increased. The therapeutic effect of bronchodilators can be judged by the changes in phase II, phase III, and the α angle. The height of the alveolar plateau is related to the ratio of cardiac output to alveolar ventilation. For a given ventilation, the alveolar plateau height increases or decreases with abrupt changes in cardiac output.7,8The maximum PCO2at the end expiration is displayed as a numerical value, called the end-tidal PCO2(PETCO2). The values vary between 35 and 40 mmHg. At the end of phase III, the PCO2decreases rapidly to zero when carbon dioxide–free gas is inhaled during inspiration. The angle between phase III and the inspiratory downstroke is generally 90° (the β angle) (fig. 1A). However, this angle increases in the presence of rebreathing (fig. 2, Band C). Occasionally, at the end of phase III, there may be a terminal upward blip (fig. 1C), which is generally seen in the capnograms of children, pregnant, or obese patients (phase IV).5,9,10The rapid initial emptying of alveolar gas compartments containing rather constant carbon dioxide concentrations is responsible for the near-horizontal initial part of phase III in the carbon dioxide trace. However, as the expiratory flow decreases toward the end of expiration, the carbon dioxide content of the expired air increases markedly and thus produces a terminal steep rise or upward blip in the carbon dioxide tracing. This is because, in the latter part of expiration, the delayed alveolar emptying results in higher carbon dioxide concentrations due to the continuous release of carbon dioxide into the alveoli. Normally, the alveolar gases with high carbon dioxide remain within the airways and are not analyzed by the carbon dioxide sensor near the mouth. However, the use of a large tidal volume and low-frequency ventilation enables these gases to reach the carbon dioxide sensor that registers a high carbon dioxide concentration. Pregnant subjects who normally have a reduced functional residual capacity, a low total thoracic compliance, and an increased carbon dioxide production are likely to exhibit phase IV in the capnograms during general anesthesia and intermittent positive pressure ventilation using large tidal volumes.5Because the PCO2is plotted against expired volume in a volume capnogram, the waveform can be related to various components of tidal volume (fig. 1B). However, there is no inspiratory component in this curve. In both the time capnogram and the volume capnogram, PaCO2to PETCO2difference can be used as a surrogate of physiologic dead space (fig. 1B). The normal PaCO2-PETCO2gradient is about 5 mmHg. This is due to the mixing of alveolar gases containing carbon dioxide with dead space gases containing no carbon dioxide. The understanding of the above physiology is vital to the interpretation of capnography.Clinical information can be obtained from three sources in capnography: numerical value of PETCO2, shape of the capnograms, and the difference between PETCO2and PaCO2. Numerical values should be used as a tool in the differential diagnosis (table 1). On the other hand, the shapes of the capnograms offer more specific diagnostic clues (fig. 2A–O). It is difficult to use capnography as a diagnostic tool by itself. However, if the changes in PETCO2values or variations in the carbon dioxide waveforms are used in conjunction with accompanying data, such as heart rate, blood pressure, respiratory flow, pulmonary inflation pressures, and minute volumes, the diagnostic accuracy of capnography can be enhanced. Tautz et al. 11described a case report where there was a gradual increase in PETCO2values during anesthesia in a 55-yr-old man. This was later associated with increasing heart rate and body temperature. A systematic and methodical check of the patient’s hemodynamic and respiratory variables and the anesthesia machine did not reveal any defect in the anesthetic system or airway obstruction. The PETCO2values continued to increase to 65 mmHg, despite minute ventilation of 18 l/min. A diagnosis of malignant hyperpyrexia was made. Treatment for malignant hyperpyrexia was initiated, and rapid resolution of hypercarbia and hyperthermia was achieved.The PaCO2-PETCO2gradient, a surrogate of physiologic dead space, is valuable in assessing the V/Q relationship. A changing gradient denotes unstable circulatory hemodynamics or variable alveolar ventilation as a result of dynamic changes in compliance or resistance in the lungs. If the gradient stabilizes over the course of clinical management, it can be surmised that stability of alveolar ventilation and perfusion has been achieved. This valuable utility of capnography is underused in ICU settings.In the last 2 yr, the ASA (New standards of Basic Anesthesia Monitoring, effective July 2011), AAGBI (Updated statement from AAGBI, May 2011), and American Heart Association (2010) have revised and updated their recommendations on the use of capnography outside of the operating room locations. Recent studies have also highlighted the morbidity and mortality related to the underuse of capnography in ICUs.1,12In addition, public interest has been generated with FOX News and National Public Radio publicizing the role of capnography in cardiopulmonary resuscitation (CPR).With advances in interventional radiology, electrophysiology, and cardiac catheterization, there has been a substantial growth in the number of sedation-requiring procedures being performed outside of the operating rooms. For many of these procedures, sedation is provided by nurses under the supervision of the physician performing the surgical, radiologic, or endoscopic procedure. It is well known that hypoxia occurs in these procedural sedation cases. When midazolam and ketamine were used for 77 room procedures, had that positive pressure ventilation and had hypoxia at during the of in the results of procedures performed in rooms in patients in an rate of with of patients a of endoscopic procedures where of patients sedation by the mortality was and were used for sedation in these cases. One patients for respiratory and was associated with morbidity and mortality and of the used in the above with has not the of is that more standards of monitoring are not when sedation is by of may not be as as anesthesiologists in and ventilation. many are far from operating room who provide Therefore, it is that monitoring standards for procedural performed outside of the operating rooms be The ASA and AAGBI have revised standards in to monitor ventilation by capnography to enhance safety of patients of the location of procedural The of monitoring of ventilation by capnography during sedation is based on the that it is difficult to an patient will to an In a in patients sedation in the room, one of had to capnography and the other did was as an less than for s or and respiratory was as a or an increase or decrease from of or in or a of carbon dioxide waveform for s or was in of subjects with capnography and with capnography of in this was that changes in capnography of respiratory in all of hypoxia The time from of respiratory to hypoxia was about s studies respiratory during procedural and or not capnography was used in these cases. In the where capnography was not and of rise was the for monitoring patients The results that respiratory was more likely to be in procedural sedation when capnography was used in with and of rise to the without a that hypoxia occurs even in the most Capnography and decreases the of during performed under sedation to a where capnography was the positive effect of capnography was even more when the of The of these was by more than in the capnography when with the using monitoring studies the and utility of capnography during procedural It be that there may be of expiratory gases with or air resulting in lower than normal end-tidal carbon dioxide is important under these is the of changes from the changes in waveform or changes in respiratory should the sedation to monitor the patient for airway or respiratory (fig. If respiratory are on such as can the partial airway as a result of sedation and increase the recommendations of ASA and AAGBI to ASA and AAGBI and are not Society of This is the of during capnography for sedation procedures outside the operating room. The is on anesthesiologists to the of the that capnography is a vital tool for patient safety. American Society of are not in with ASA or AAGBI standards that capnography should be used in all of In a statement this state that mortality in is which to be the mortality is surgical that a patient is more likely to during a performed with sedation than as a result of general Capnography use with was by ASA not based on a that these of but on a that capnography with have of anesthesia that to anesthetic morbidity and the of studies the of and capnography on we have seen the of in anesthetic mortality from in to in procedures are not as as ASA that of were and capnography have of American Society of with the utility of capnography in endoscopic and endoscopic procedures but in the use of capnography for However, a and that capnography the of hypoxia by more than during a of these will the of and other in recognizing the value of capnography in the near the do we the about capnography for sedation outside of operating The the most common in the are respiratory are more likely to without capnography than with and capnography the of that are likely to to hypoxia without of hypoxia is not limited to endoscopic and endoscopic procedures but also occurs during procedures such as the physician performing the should be not to have or patient in the performing the using capnography and with the use and interpretation of capnograms will sedation to capnograms more during difficult case of as a result of procedural sedation have been in and also during and because the current by the for and the to procedural sedation in an it is to follow ASA standards and monitor ventilation with in of the current recommendations of the by the of and and to have in sedation practice all across the our has to monitor ventilation with capnography in all patients sedation of the and of are in revised of the use of waveform capnography not for of tracheal tube but also to monitor the of For given ventilation in an monitor of cardiac output generated by The of is difficult to with other but it is on the capnography by an abrupt increase in the waveform capnography also of tube than The role of capnography in effective has been known for many the and of this into have about from and and be a that may have to this Nonetheless, capnography has been into and will a more to into given the of and the of on the of the National in United AAGBI also the use of capnography in all patients was by AAGBI that capnography is not a on resuscitation but that should be made to one for the capnography should not be but should have positive waveform (fig. 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three recommendations to capnography in capnography should be used for of all of continuous capnography should be used in all ICU patients with tracheal who are intubated and and may be to the rapid of these not its capnography is not the clinical for not using it should be and all clinical who in ICU should interpretation of capnography. should on of airway or can that anesthesiologists have all in the above but we all do under the of capnography at in the operating rooms. there are other reasons capnography should be a monitor in ICU there is variable ICU patients have substantial cardiac or respiratory to many anesthesia in the operating room, and the of and hypoxia are more likely to be in ICU patients to healthy ASA or 2 the from ICUs also that endotracheal tube in ICU patients for a substantial number of or the on and American Heart Association recommendations the use capnography not for the tracheal of the endotracheal tube but also for the of to the National of of cardiac of in 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