Capnography Outside the Operating Rooms

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 emergency room procedures, 6% had apnea that required positive pressure ventilation and 75% had hypoxia (oxygen saturation <90%) at some point during the procedure.13Analysis of Procedural Sedation in the Community Emergency Department (ProSCED) data results of procedures performed in emergency rooms in community hospitals (1,000 patients in 14 hospitals) revealed an overall complication rate of 4.1%, with 1.1% of patients requiring assisted ventilation.14In a study of endoscopic retrograde pancreaticocholangiography procedures where 96% of patients (3,058/3,179) received sedation supervised by gastroenterologists, the overall mortality was 0.06%. Midazolam, promethazine, and meperidine were used for sedation in these cases. One hundred twenty-four patients required reversal agents for respiratory depression and somnolence.15This subgroup was associated with morbidity and mortality (6% and 1.6% respectively). Replacing some of the drugs used in the above study with propofol has not eliminated the risk of hypoxia.16It is ironic that more stringent standards of monitoring are not enforced when sedation is administered by nonanesthesiologists, some of whom may not be as adept as anesthesiologists in securing and maintaining ventilation. Furthermore, many procedure sites are located far from main operating room personnel who often provide rescue help. Therefore, it is necessary that monitoring standards for procedural cases performed outside of the operating rooms be reevaluated. The ASA and AAGBI have issued revised standards in 2011 to monitor ventilation by capnography to enhance safety of patients undergoing sedation, irrespective of the location of procedural sedation. The recommendation of continual monitoring of ventilation by capnography during moderate-to-deep sedation is based on the fact that it is difficult to predict how an individual patient will respond to an administered sedative. In a randomized controlled trial in patients undergoing propofol sedation in the emergency room, one group of treating physicians had access to capnography and the other group did not. Hypoxia was defined as an oxygen saturation less than 93% for 15 s or greater, and respiratory depression was defined as a PETCO2of 50 mmHg or greater, an absolute increase or decrease from baseline of 10% or greater in PETCO2values, or a loss of carbon dioxide waveform for 15 s or greater. Hypoxia was observed in 25% of subjects with capnography and 42% with blinded capnography (P = 0.035). Another interesting observation of significance in this study was that changes in capnography forewarned of respiratory depression in all cases of hypoxia (sensitivity 100%, specificity 64%). The median time from capnographic evidence of respiratory depression to hypoxia was about 60 s (range 5–240 s).17A recent meta-analysis reviewed studies involving adverse respiratory events during procedural sedation, and whether or not capnography was used in these cases. In the cases where capnography was not used, pulse oximetry and visual inspection of chest rise was the standard for monitoring patients receiving sedation. The results showed that respiratory depression was approximately 17 times more likely to be detected in procedural sedation cases when capnography was used in combination with pulse oximetry and visual inspection of chest rise compared to the group without capnography.18Furthermore, a recently published study demonstrated that hypoxia occurs even in the most routine gastrointestinal endoscopy procedures. Capnography triggers early intervention and decreases the incidence of oxygen desaturation during colonoscopy performed under propofol sedation (capnography-open arm) compared to a group where capnography was blinded (capnography-blinded arm).19Furthermore, the positive effect of capnography was even more pronounced when comparing the incidence rates of severe hypoxemia (oxygen saturation <85%). The incidence of these events was decreased by more than half in the capnography group when compared with the group using standard monitoring only. These studies demonstrate the importance and utility of capnography during procedural sedation. It must be that there may be of expiratory gases with oxygen or air resulting in lower than normal end-tidal carbon dioxide is important under these is the of changes from the baseline changes in waveform or changes in respiratory should the sedation to monitor the patient for airway or respiratory depression (fig. If respiratory are on visual 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 often the of during capnography for sedation procedures outside the operating room. The is on anesthesiologists to the group 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 cases of sedation. In a statement issued this state that mortality in gastrointestinal endoscopy is approximately which to be the mortality is surgical that a patient is times more likely to during a procedure performed with sedation than as a result of general Capnography use with pulse oximetry was by ASA not based on a randomized trial that these of decreased but on a that capnography with pulse oximetry have 93% of anesthesia that to anesthetic morbidity and the of randomized controlled studies the of pulse oximetry and capnography on we have seen the of in anesthetic mortality from in to in procedures are not as as ASA that of were endoscopy and capnography have some of American Society of with the utility of capnography in endoscopic retrograde pancreaticocholangiography and endoscopic procedures requiring sedation, but in the use of capnography for routine endoscopy procedures. However, a recent study demonstrated and that capnography decreased the incidence of hypoxia by more than during a routine of these will the of and other physicians in recognizing the value of capnography in the near the how do we the about capnography for sedation outside of operating The the most common events in the overall are respiratory are more likely to without capnography than with and capnography the of events that are likely to to hypoxia without of hypoxia is not limited to endoscopic retrograde pancreaticocholangiography and endoscopic procedures but also occurs during routine procedures such as the physician performing the procedure 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 cases 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 issued by the of and and to have in sedation practice all across the our has to monitor ventilation with capnography in all patients requiring sedation irrespective of the and of personnel are in revised of the use of waveform capnography not for of tracheal tube but also to monitor the of chest For given ventilation in an monitor of cardiac output generated by chest The of is difficult to with other but it is demonstrated on the capnography by an abrupt increase in the waveform capnography also of tube than pulse The role of capnography in effective has been known for many the and of this into have about from data and and be a that may have to this Nonetheless, capnography has been into and will a more to into routine given the of and the of on the of the National in United AAGBI also the use of capnography in all patients undergoing was by AAGBI that capnography is not a standard on resuscitation but that should be made to one for the capnography should not be but should have positive waveform (fig. A capnography during should the group of a tracheal Although a recent study demonstrated that patients who of had higher PETCO2values compared to who did not have it is difficult to the value of PETCO2values during is because the values of are not on the of but also on the cause of cardiac such as or pulmonary these there is evidence that can be used as a for et al. performed a study in of cardiac outside of the were and PETCO2values were obtained during that a of less than mmHg or less the of in patients with cardiac are also being to determine the value of the of PETCO2values in patients with and without at and but not at or 5 without had and under the than with The end-tidal carbon dioxide greater than mmHg at all time between 5 and was the variable most likely to predict (sensitivity = specificity = The of this study that using the of resuscitation can be in less than in of the cases the of is a considerable interest to use PETCO2values during to determine the of The for the of in patients with cardiac PETCO2values to or greater than mmHg during as one of the the current recommendation of using capnography for substantial data will be generated in the future to determine the value of during to predict such studies are capnography should be used to of A during may suggest or chest by the It should also the to other to cardiac output generated during such as 2011 case report the value of capnography during in a who cardiac and in a that effective was being performed and the resuscitation continued with resuscitation for when and were the end-tidal carbon dioxide was in the mmHg during These of carbon dioxide were with effective chest pulmonary blood flow of When the to an there was no but a PETCO2of mmHg that had and was The patient and was on the with no or FOX News and National Public Radio this in to the public about capnography and its value during on the current of and AAGBI, we have a capnography unit on our The unit is on at the of the that it by the time the at the In addition, the has a for an difficult many recognizing the value of there has not been a to capnography to monitor ventilation in use of capnography in ICUs from to is more often used in some countries because of the was by the of and the Society of to the safety of patients being for by anesthesiologists in the medical of intensive care emergency and The results of the National by the of Anaesthetists and the Society of United should physicians to use capnography more often in this is not a randomized it is a study in which the data of airway during in the and in the emergency were and of of airway emergency surgical ICU prolonged ICU were obtained from all National over a of The data from the study that in there were airway from the patients undergoing general anesthesia monitored with continuous resulting in a rate of in there were 18 from ICU patients receiving ventilation, resulting in a rate of in These data that it is times more likely to have an airway in an ICU where capnography is not used compared with an operating room where continual capnography is the standard of is more is the of the study group that of ICU airway or have likely been if continuous capnography had been observation from ICU and emergency data was the This to where intubation was not Capnography was not used in of the In the a capnography was as being due to cardiac The of the National three recommendations to capnography in capnography should be used for of all irrespective of continuous capnography should be used in all ICU patients with tracheal who are intubated and and may be to the rapid of routine these not its capnography is not used, the clinical for not using it should be and reviewed 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 routine monitor in ICU there is often variable ICU patients have substantial cardiac or respiratory compared to many routine anesthesia cases in the operating room, and the of and hypoxia are more likely to be in ICU patients compared to healthy ASA or 2 the data 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 and these cardiac have the to is to use capnography as an to monitor the of cardiac and use of capnogram shape in airway or tracheal use of to end-tidal as a surrogate of alveolar dead diagnosis of of the tube in the in use of capnography during the apnea to confirm use of capnography as a to of during the high associated with blood and substantial interest is being in using capnography to changes in dead space during the course of the ICU using either the or the monitoring 2011), the AAGBI their about the National Society of United also published a for Capnography in and made a recommendation to use capnography in all patients during the procedures of or endotracheal intubation when performed in the ICU and in all patients who ventilation during or For continual use of capnography during ventilation in the society was to a recommendation lack of evidence that continuous capnography reduced the of due to an airway during routine ventilation, and further into this the lack of evidence of the value of capnography during routine ventilation in the it must be that it is used endotracheal may not be detected in a to rescue A in the ICU when the tube in a patient with a difficult airway the patient was being is with any monitoring technology, the overall of capnography should be into rather than one individual capnography more often will also to use this device in a If the use of capnography increases with the above we are to have more data in the near future about its in an ICU a case is made that all patients requiring ventilation are to be monitored with it is then to monitor these patients being transported to and from ICUs and between carbon dioxide waveforms during transport the of airway and ventilation. In one of during or were detected by pulse oximetry and ventilation is an abrupt decrease in PETCO2values must be it may be due to decreased cardiac In the some have issued to emergency medical their personnel to necessary in the interpretation of capnography and to use capnography as was about respiratory depression in patients receiving continuous or to morbidity and on the the incidence of respiratory depression from to to capnography monitoring during sedation procedures, there is evidence that capnography can provide an early of respiratory depression in these In a study of patients receiving and monitored with pulse oximetry and the incidence of desaturation (oxygen saturation <90%) and rate more than from to patient required rescue positive pressure ventilation. The recommendations that monitoring the ventilation of patients receiving with capnography because it is the most of However, the of this to patients requires further in technology to decrease and the system more for the A is to pulse oximetry and capnography to greater with less Nonetheless, patients should from this is a of when capnography will be used more often outside of the operating rooms. from other specialties are more of the value of capnography in patient safety. are other and of capnography monitoring such as the role of PETCO2values at and during in patients with heart to predict cardiac are to respiratory and personnel in the interpretation of capnography The of capnography should their to produce cost-effective capnography that are of display and of carbon dioxide Capnography should be a in future intensive care because the current of their use in in the ICUs should of of carbon with and interpretation of capnography Anesthesiologists are with capnography and should be the forerunners in to capnography outside of the operating rooms in their of and and of and for the and in the of this