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Capnography
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Utilization of Technology: Unrealized Potential
Capnography A Key Underutilized Technology Tom Ah rens, DNS, RN , CCRN , CS, Helen Wijeweera, RN , MSN , and Shawn Ray, BSN , RN , CCRN
C apnography, the measurement of exhaled carbon dioxide, is one of the most underutilized technologies in critical care. While capnography has been available for decades, only in the recent past has the use of capnography grown to the point that at least eight applications of this technology are now possible (see Box 1, page 52). Capnography allows the clinician several key advantages in its use: it is noninvasive, easy-to-use, easy to maintain, and relatively inexpensive. In addition, capnography has impressive potential to reduce costs, both from a clinical and legal perspective. From these advantages, capnography use should be widespread in acute and critical care. However, the use of capnography is still limited. The reason for this limitation may stem from a lack of education and understanding on the potential applications of this technology. In this article, a review of the uses of capnography and its impact on patient outcome and costs, illustrates why this technology has such great promise in the assessment of acute and critically ill patients.
From the Department of Critical Care (TA) and Surgical Intensive Care Unit (SR), Barnes-Jewish Hospital; and Veteran's Administration Medical Center (HW) , St. Louis, Missouri
Physiology of Capnography During exhalation, carbon dioxide (co 2) is eliminated in a characteristic manner that creates a waveform with distinct features (Fig. 1). During initial exhalation, little co 2 is present since the initial exhalation is primarily from deadspace areas (upper airways that do not participate in gas exchange). After this initial expiration, co 2 pressures start to increase, reflecting the contribution of gas exchange regions of the lung. The pattern culminates with the emptying of distal alveoli. This end portion of the co 2 elimination is visible on the capnogram waveform with a value called the end tidal co 2 (Petcoz) level. Under normal lung physiology conditions, where ventilation and perfusion are relatively equally matched, the Petco 2 level closely approximates the arterial (alveolar) co 2 level (Pacoz) . The slope of the waveform is important to monitor in that it reflects alveolar emptying patterns. If the slope is smooth and progressive with a plateau at the end, a normal capnogram is present (Fig. 2). If the slope is rapid and terminates abruptly, it may indicate inadequate alveolar emptying (Fig. 3). If inadequate alveolar emptying is present, the Petco 2 w ill not approximate the Paco 2 • However, circumstances exist where even if a normal capnogram is present, the ability of Petco 2 to reflect Paco 2 may be
CR ITICAL CARE NURSING CLINICS OF NORTH AMERICA I Volume 11 I Number 1 I March 1999
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Figure 1.
Normal capnogram appearance.
limited. The key to Petco2's ability to reflect Paco 2 is in a normal ventilation (V) to perfusion (Q) relationship. This relationship is not discernible by noting the waveform alone. However, it is discernible by understanding the clinical situation of any given patient.
Figure 2.
Accuracy and Safety in the Measurement of co 2 Capnography is measured by several methods, including infrared technology or mass spectrometry. The primary method of mea-
First capnogram illustrates normal carbon dioxide elimination pattern.
CAPNOGRAPHY
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!] /] /\ i Figure 3.
Inadequate alveolar emptying as reflected by the capnogram.
surement in acute and critical care is with the infrared technique. With this technique, either a sidestream (aspirating gas to an off patient analyzer) or mainstream (measuring the gas at the patient) analyzing technique is employed. Infrared sensing of co 2 is well-documented in the literature in terms of accuracy. 11 38. 10· •2. 'i6 Both technologies allow for accurate measurement of co 2 as well as respiratory rate. Both side-stream and mainstream techniques sample exhaled air as close to the patient as possible (Fig. 4). Sidestream measurement offers an advantage in that the sampling device is vety lightweight and can be used in nonintubated patients. The primary
Figure 4.
disadvantage is that it is prone to obstruction by moisture in the sampling circuit. 1'i Mainstream sampling has advantages in real time measurement and fewer problems with moisture obstructing the measurement of co2• In terms of safety, there are virtually no clinical dangers with this technology. The sampling of exhaled gas from the patient is taken with technology that has no harmful effects to the patient. With sidestream technology on the nonintubated patient, the aspiration tubing at times can feel uncomfortable. With mainstream technology, the weight of the sensor may pull on the ventilator tubing and increase the chance for a disconnec-
Mainstream co2 analyzer.
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Box 1 EIGHT CLINICAL APPLICATIONS OF CAPNOGRAPHY
• Avoiding esophageal intubations during endotracheal tube (ET) placement • Avoiding tracheal intubations during nasogastric (NG) placement • Prognosis and adequacy of resuscitations • Reducing arterial blood gas (ABG) use • Detecting physiologic deadspace changes • Identifying end expiration with hemodynamic waveforms • Identifying alveolar emptying patterns • Assessing sedation/paralytic therapy Each of these applications has its value in patient management. Each application has variable degrees of potential impact on patient outcome. The impact of each application will be clear as each application is discussed later in text.
location of the endotracheal tube simple. If the endotracheal tube is in the trachea or epiglottis, a normal capnogram can be seen (Fig. 5). If the ET is outside the trachea (e.g., esophagus) , a flat line will appear. Capnography can sense the placement of the endotracheal tube within seconds. 43 The use of capnography in assessment of ET placement has become so accepted that it is the standard in anesthesia when tracheal intubations are attempted. 58 Several papers have been published on the accuracy and reliability of capnography to safely detect correct placement of ETs. 22• 25 • 32 • 52 For this reason alone , capnography should be available in every clinical setting where tracheal intubations occur.
Avoiding Tracheal lntubations During NG Tube Placement tion. This disconnection will be immediately sensed if the alarms are appropriately set.
Avoiding Esophageal lntubations During ET Placement Perhaps the simplest and most powerful impact of capnography is in the area of identifying the location of the ET during intubation. During intubation of a patient's trachea , a critical step is to ensure correct placement in the trachea. Placement in the esophagus will lead to inadequate breathing with the potential for brain damage or death. Capnography makes
Figure 5.
While uncommon, it is possible that during placement of an NG tube inadvertent placement in the trachea could occur. If such an event were to occur, the complications could range from minor to catastrophic. The worse case scenario would be the placement of the NG tube in the trachea when the purpose of the NG placement was for enteral nutrition. Placement of enteral flu id in the lungs would cause aspiration, resulting in anything from a pneumonitis to adult respiratory distress syndrome and death. Avoiding NG placement in the esophagus can occur through the use of capnography. Attaching the capnography sensor to the end
Use of capnography with nasogastric tube placement.
CAPNOGRAPHY of the NG tube will immediately confirm if the NG tube is in the trachea or the esophagus (Fig. 6). 17 The incidence of NG placement in the lungs is unknown. However, since capnography allows accurate, immediate detection of a potentially dangerous situation, its use in this application may be warranted. Further research in this area would better clarify the value of capnography on avoiding tracheal intubations during NG placement.
Predicting Survival During Cardiopulmonary Arrests Assessment of survival during cardiopulmonary resuscitation is an inexact science. Understanding the potential a patient has for survival is important for several reasons but primarily for determining when to continue or stop a resuscitation effort. The most common method for determining when a code can be stopped now is based on individual clinician judgment of factors surrounding the code (e.g., diagnosis) , potential for survival, patient/ family wishes, response to code efforts, and length of time the code has been in progress. It would be helpful to clinicians to have
a more objective measure for the potential success during the resuscitation effort. Capnography, specifically end tidal co 2 (Petco 2) levels, has the potential for being an objective measure of survival during a cardiopulmonary arrest. This potential is the result of Petco 2 levels having a strong correlation with cardiac output. Weil et al were one of the first to note that exhaled co 2 levels, specifically the peak or end tidal co 2 (Petco 2) correlated with cardiac output. 55 The theory for this correlation is that as pulmonary blood flow (and therefore cardiac output) decreased, an increase in deadspace occurs. The drop in exhaled co 2 would reflect this drop in pulmonary blood flow. Capnography measures this decreased co 2 . Exhaled co 2 reflects the amount of co 2 returning to the lungs, the result of substrate metabolism and pulmonary blood flow. Under normal circumstances (normal blood flow to the lungs), alveolar co 2 values are about the same as a1terial co 2 levels. The normal difference between arterial and Petco 2 values is about 1 to 5 mm Hg, with the exhaled co 2 almost always being lower than arterial. 47 Any condition that reduces pulmonary blood flow relative to ventilation will cause this gradient to widen. The greater the difference between
Capnography Wave indicating Lung Placement
No Capnography Wave Indicating Non-Lung (esophageal) Placement
Figure 6.
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Flat line on capnogram indicates nonlung placement of tube.
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the arterial and exhaled co2 level, the more severe the disruption between pulmonary blood flow and ventilation. Weil et al's observations of Petco 2 levels reflecting cardiac output were extended by other investigators. Isserles and Breen as well as Shibutani et al and Idris et al also found Petco 2 levels correlated well with cardiac output.26· 28· 50 In the same year as Weil et al's observation, Sanders et al first published the ability of capnography to predict outcome in a cardiopulmona1y arrest. 46 This paper was followed by many others, including Garnett et al, Dohi et al, Ornato et al, Falk et al, Sanders et al, Kern et al, Callaham and Barton, Lambert et al, Asplin et al, Angelos et al, Cantineau et al, Ward, and White et al. 2·3, 8• 10· 14• J9, 20. 30, 33, 39, 47. s4. 57 All of these authors demonstrated that capnography has a potential correlation with survival in cardiopulmonary arrests. Levine and colleagues demonstrated with out of hospital cardiac arrests that if the exhaled co2 level did not return to 20 mm Hg after 20 minutes, mortality was 100%. 34 These studies suggest that capnography might be an important aid to the clinician when evaluating the success of the resuscitation efforts. A few case histories illustrate how easily capnography might be employed in a cardiopulmonary arrest situation. CASE HISTORY 1
Does the capnography waveform (Fig. 7) indicate adequate CPR? Answer: Yes, the Petco 2 value of 28 indicates a probable adequate cardiac output with CPR (Fig. 7). CASE HISTORY 2
A 66-year-old woman is brought into the emergency room (ER); cardiopulmonary resuscitation (CPR) is in progress. She was found "down" in her house by her husband. Paramedics have been doing CPR for more than 20 minutes. Her capnography wave shows a value of 12 mm Hg. How would you assess the adequacy of the resuscitation effort?
Answer: Resuscitation is not effective for this patient. In addition, the likelihood of survival at this point is very poor. Since the Petco 2 is only 12, a very low cardiac output is likely (Fig. 8). Given the length of time of resuscitation, irreversible brain injury may be present now.
Using Capnography to Predict Arterial co 2 and Avoid Blood Gas Measurement In normal lungs, exhaled co 2 levels closely approximate arterial co 2 levels. Specifically, at end expiration the maximum partial pressure of co2 (Petco2) occurs and it is this point that correlates with arterial co 2 levels.31 When lungs are normal, ventilation (V) at the alveolar level is matched by alveolar perfusion (Q), a term referred to as V/ Q ratio. As long as ventilation matches perfusion, the Petco2 level will be within 1 to 5 mm Hg of the arterial co 2. The difference between the arterial and Petco 2 level is often referred to as the Paco 2 to Petco2gradient. If the Paco2to Petco 2gradient is normal, then the Petco 2 value can be used as a reflection of Paco2. Several papers have been published on the use of the Paco 2 to Petco 2 gradient in clinical practice. The gradient between Paco 2 and Petco2 has clinical value but must be used with caution, based on these papers. A brief review of each of the papers illustrates the reason for caution when using the Paco2 to Petco 2 gradient. Baraka and colleagues recorded continuous Petco2 of 13 patients undergoing laparoscopic cholecystectomy and found good correlation between Petco2 and Paco2 during carbon dioxide insufflation. Their conclusion was that Petco 2can be used to monitor arterial oxygenation and co2 elimination.4 Bongard et al found in 41 postoperative surgical patients that the Petco2 was lower than the Paco 2 by an average of 2.8 mm Hg. 6 The study concluded that "on-line" Petco 2 measurement was a useful substitute for routine ABG determination of Paco 2in selected patients. Several other studies also concluded that Petco 2 is a good indicator for Paco2 in several populations. Liu studied nonintubated weaned patients and found that Petco 2was on an average 3.6 mm Hg less than Paco2 .35 Wright et al found Petco2 was a useful tool during conscious sedation.6
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Figure 8.
Practice example. A 66-year-old woman is brought into the ER. CPR is in progress.
tubated postoperative coronary artery bypass graft (CABG) patients were studied. Thrush studied 10 postoperative CABG patients, finding the sensitivity of Petco2 for hype rcarbia was 95% and concluding that Petco2 can be safely used for ventilator weaning. 52 Withington found in postoperative CABG patients that the correlation between Paco2 and Petco 2 was 0.64 and 0.79 for a mass spectrometer and 0.67 and 0.81 for the infrared analyzer. 59 The authors concluded that once Paco 2 to Petco 2 gradients are established , capnometry is sufficie ntly reliable to e nable weaning and allow a reduction in the use of arterial blood gas analysis. Sharma and colleagues studied 21 patients unde rgoing elective carniotomies, finding a constant relationship between Paco 2 and Petco 2. They concluded that once the Paco 2 to Petco 2 difference has been established , the Petco 2 can be used as a reliable guide to estimate Paco 2 during neurosurgical procedures. 4 Other studies found that Petco 2 and Paco 2 correlated well if the patient was stable or without lung disease (conditions that would alter the V/ Q ratio). 24 • 37 However, not all research has been conclusive in indicating Petco 2 be used as the sole predictor of Paco 2 • Russell and Graybeal, studied nine intubated trauma patients and found that a positive correlation of statistical significance between Paco 2 and Petco 2 existed in 78% of patients, with 15% with false negatives and 12% false positives, although these changes were small.44 In another study by Russell and
Graybeal, the Paco 2 and Petco 2 of 35 craniotomy patients were studied. The authors found a significant positive correlation exists between Paco2 and Petco2• However, with subsequent measurements, 18% of the time the changes were opposite . These changes were small and not always clinically significant. 45 The authors concluded that ABG analysis is necessary during these procedures. However, in the latter study it is recognized that the duration of mechanical ventilation and the presence of spontaneous breaths may have been factors in this analysis. In 24 patients being weaned from mechanical ventilation, Hess and colleagues found that Petco 2 signiflcantly correlated with Paco 2. However, when changes of >5 mm Hg in Paco2 occurred, Petco2 incorrectly indicated the direction of change in 30% of cases. Hess suggests that Petco 2 should not be used in isolation to wean patients following cardiac surgery.23 However, the authors acknowledge that this inability of Petco 2 to predict Paco2 may stem from the patient population type, changes in ventilation, or changes in pulmonary blood flow. 27 This potential change in pulmonary blood flow and the changes of ventilation were also recognized by other authors as the difflculty of Petco 2 as a predictor of Paco 2. Many studies recognized that Petco2 could predict Paco2 in stable patients, but because of the inequality of ventilation- pe rfusion ratios and inconsistent Paco 2 to Petco2 gradients in critically ill patients, Petco 2 may not always be the sole predictor of Paco 2• As indicated
CAPNOGRAPHY in the above studies, Petco 2 may reflect Paco2 in healthy lungs. However, the recognition of the need to first establish the Paco 2 to Petco 2 gradient in healthy patients and that Petco 2 does not reflect Paco2 due to a widening gradient in all patients, indicates another potential use for Petco2 monitoring, that is, the identification of changes in physiologic deadspace.
Identifying Changes in Physiologic Deadspace and Detecting Pulmonary Emboli The Paco2 to Petco2 gradient increases as the deadspace increases, or as ventilationperfusion mismatching increases. Deadspace occurs if there is a ventilation-perfusion mismatch, as occurs with pulmonary emboli.41 A normal ventilation/ perfusion match, designed to allow optimal 0 2 and co 2 exchange to occur, must be matched with the lungs. 9· 61 Ventilation is the volume of airflow per minute CL/min) through the lungs. This is also known as minute ventilation. Minute ventilation (Ve) is composed of the sum of alveolar ventilation (VA), air that participates in gas exchange, and deadspace ventilation (Vd), air that does not participate in gas exchange (VA+ Vd =Ve). All healthy individuals have anatomic deadspace, parts of the airway that do not participate in gas exchange (e.g., nasooro pharynx, trachea, bronchus). Perfusion is blood flow through the pulmonary vasculature. Blood flows into the pulmonary capillaries at the alveolar level where co2 in the blood is exchanged for oxygen in the alveoli. In some diseased lungs there also exists alveolar deadspace, where ventilation occurs at the alveolar level, but reduced perfusion takes place. 5· 36· 51 When an increased deadspace is present, the reduced pulmonary blood flow can be detected with capnography. As pulmonary blood flow decreases in one area, it increases in another to compensate for the obstructed blood flow region. As an increased blood flow occurs in this new area, increased ventilation must occur to match the increased perfusion, or gas exchange is affected. The increase in ventilation also means more air is going to the deadspace area. The net result is that during exhalation, the pressure of co2 is reduced. This reduced co2 pressure is measured by cap-
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nography. As reduced co 2 pressure occurs, the gradient between the arterial and end tidal co 2 level falls. Capnography, through the identification of the Paco2 to Petco2 gradient, can thereby reflect Vd. Owing to the ability for capnography to reflect changes in deadspace, it has been proposed in the literature that capnography be utilized as a diagnostic test for pulmonary embolisrn. 48 Many studies support this application of capnography. Yamanaka and Sue studied 17 patients undergoing mechanical ventilation, finding that while PETco2 may be a poor estimate of Paco 2, it can be utilized as a measurement of VdNT (17). 61 Later, in a prospective study, Chopin and colleagues studied 44 adult chronic obstructive pulmonary disease (COPD) patients suspected of having acute respiratory failure secondary to pulmonary emboli (PE). The results showed that 17 patients had PE, and 17 patients did not have PE as diagnosed by pulmonary angiography. The Paco 2 to Petco2 was significantly different in the +PE and - PE patients. The positive predictive value was 74%, but the negative predictive value was 100%, with specificity of 65% and sensitivity of 100%. 12 Carroll concluded that the capnographic trend curve can detect a 1-mL pulmona1y emboli. 11 In this experimental study, the capnograms of 24 mechanically ventilated patients were recorded during a simulation of a 1-mL PE by inflation of the balloons of their pulmonary artery catheters (PACs). The results showed a positive predictive value of 98% and negative predictive value of 89%. In the canine model, several studies have also shown that capnography can be a useful tool in detecting PE. Byrick and colleagues fou nd that capnography is a useful tool intraoperatively to detect PEs; however, pulmonary artery pressure (PAP) is more sensitive in detecting fat and marrow microemboli. 7 English and colleagues fo und that Petco2 in correlation with PACs provides "sensitive and reliable detection of air embolus and quantitative evaluation of embolus size." 18 Drummond and colleagues found that any acute change in Petco2 of equal or greater than .2% during "steady-state" conditions could be attributed to air emboli.16 Petco 2 can identify a widening Pacoi-Petco2 gradient. Therefore, capnography can be utilized to identify deadspace, thus pulmonary emboli. Potentially
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Petco 2 may be utilized, in coordination with other assessment tools, to replace V/ Q scans and the invasive pulmonary angiography.
Identifying End Expiration During Hemodynamic Waveform Analysis Capnography can ease the interpretation of hemodynamic waveforms by clearly identifying end expiration (Fig. 9). 1 While no studies have been performed in this area to date, Ahrens reports how the use of capnography can improve finding end expiration for waveform analysis. Capnography use in finding expiration is relatively simple. At the peak of the capnogram (what should be the end tidal level), expiration has stopped and inspiration is about to begin. At this location, end expiration is present. The clinician should read the waveform immediately below the end tidal level to find the end expiration location. Capnography use in finding end expiration is almost foolproof, but not quite. At times, the patient will generate an inspiratory effort that changes pleural pressure well before the capnogram reflects the inspiration. Be careful when using capnography to find end expiration. Make sure the point that should align with end expiration is consistent with what is displayed on the hemodynamic waveform.
Figure 9.
The Cost of Capnography-Does the Cost Justify its Use? Like any technology, capnography acquisition is associated with a purchase price. Does this purchase price allow for an institution to recoup the cost through improved clinical practice? In terms of capnography, the purchase price is actually very reasonable. A review of capnography and its associated costs as well as its benefits will help illustrate why capnography is a reasonably priced technology. Direct costs to acquiring capnography costs vary depending on the type of monitor purchased, for example, hand held, stand alone, or incorporated into a bedside monitor. Capnography units can range from less than $2000 per module to up to $5000 in terms of purchase price. In addition, the cleaning or replacing of disposable equipment is necessary. However, the capnography modules can last for years with little maintenance. Table 1 gives a review of capnography costs. Capnography can directly provide benefits in four areas: 1. Avoiding legal encounters during intubation 2. Avoiding costs associated with futile resuscitation efforts 3. Reducing frequency of ABGs 4. Reducing V/ Q scans and pulmonary angiography
Capnogram overlapped on hemodynamic waveform to ease interpretation of end expiration.
CAPNOGRAPHY
Table 1
COST/BENEFIT COMPARISON WITH CAPNOGRAPHY
Capnography Costs Per unit/per year Purchase price* Disposable Cleaning Education costs t
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Yearly Capnography Benefits
$2000-$5000 $100-$1000 $100-$500 $5000
Legal Resuscitation ABG reduction V/Q scan and pulmonary angiography reduction
$0-$1,000,000 $10, 000- $300,000 $1000-$10' 000 $1000-$10' 000
• Each module lasts for years.
t Most costs incurred in start up education.
A review of these four areas gives an idea of the potential impact of capnography on costs.
should the patient have an unnecessarily induced pneumonia, ARDS, or death.
Avoiding Legal Encounters During Intubation
Avoiding Costs Associated with Futile Resuscitation Efforts
The occurrence of difficult intubations is not common. A well-trained clinician can perform intubations of the trachea relatively easily. However, there will always be a few cases each year that present a substantial challenge to even a well-prepared clinician. In these circumstances, the use of capnography not only helps avoid clinical complications but also legal complications. Legal complications can result from difficult intubations owing to consequences of death or brain damage. It is not uncommon for a hospital risk management department to encounter a case involving difficult intubations. If the hospital has to defend such a case, the minimum costs are likely to be about $25,000 with the potential, including damages, to be well in excess of $1,000,000. If a hospital is self-insured, much of this cost will be incurred by the hospital. Even if the occurrence of such an incident is rare, if legal action occurs just once in the lifespan of a capnography module, the module will easily pay for itself many times over. The clinical costs associated with improper intubation are also considerable. Assuming the patient does not die from a misplaced endotracheal tube, there is a strong likelihood the outcome will at minimum increase the stay in the hospital. Considering ICU costs are about $1,000 per day, the clinical costs can quickly mount. Similar, although not as striking, are the costs avoided with placing an ET in the lungs. Both legal and clinical costs can be substantial
The costs avoided by using capnography to predict outcome can be substantial. The major impact of the use of capnography is in the prehospital phase. Levine and colleagues illustrated this savings in a ve1y convincing argument. They identify that only a small percentage of patients who arrest outside the hospital actually survive to go home. A study in support of Levine is one by Kellermann et al. 29 Kellerman and colleagues noted that only 3 patients out of a total of 758 who were transferred to the hospital emergency room in refractory cardiac arrest were successfully resuscitated. Unfortunately, all three had substantial neurologic deficits. Gray et al provided fu1ther evidence of the potential for futility when dealing with out-of-hospital arrests.21 They note that in a study of 185 patients who could not have spontaneous circulation restored from outside the hospital, 16 eventually did achieve spontaneous circulation restoration in the e mergency department (ED). Again, unfortunately, the outcome for this effort was not rewarded since all 16 patients died in the hospital. However, the costs associated with the resuscitation efforts were substantial. Hospital costs for attempting resuscitation in patients who could not be resuscitated in the ED is likely between $100,000 and $150,000 for the 169 patients who could not be resuscitated in the ED. Pe rhaps more importantly, the costs are even higher for the patients who survived the ED. The cost for the 16 SLllvivors in the ED was $180,908. It is
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important to keep in mind that all 16 patients eventually died despite the hospital's best efforts. Gray and colleagues claim that if this data are extrapolated to the entire country, over $1,000,000,000 is lost in futile care efforts. Hospitals employing capnography in prehospital (and possibly in-hospital resuscitation as well) have the potential to save up to $300,000 while not adversely affecting patient outcome. Reducing Frequency of ABGs , V/Q Scans, and Pulmonary Angiography
Avoiding unnecessary blood gases and ventilation perfusion scans can lead to a savings that is equal to the cost of capnography modules. However, these savings are small in comparison to the savings achieved with intubation and resuscitation efforts. Blood gases, which cost about $5, have to be reduced in a large number before substantial clinical savings will be realized. Capnography will reduce blood gases if used properly. Perhaps the best example is in the avoidance of "routine" ABGs. The following case history helps to illustrate this point. CASE HISTORY 3 A 59-year-old man with congestive heart
failure (CHF) is on mechanical ventilation with a rate of 12, total rate of 14. His Flo 2 is .50, pulse oximeter is .92, and the Petco 2
is 26. The minute ventilation (Ve) is 7 LPM (normal 5 to 10 LPM). This information is 12 hours old. Current readings indicate no ventilator changes and the Spo2 now is .91, Petco 2 is 26. Is a routine ABG necessary at this time? Answer: No. Since the Petco 2 and minute ventilation are unchanged, the Paco 2 is unlikely to have increased. In this instance, the use of capnography avoided the need for a routine blood gas. V/ Q scans and pulmonary angiography are more costly and can be avoided by performing a Paco 2 to Petco 2 gradient measurement prior to performance. These tests involve the use of additional personnel, frequently requiring a transport to the radiology department and the interpretation of results by a radiologist. All these factors make tests such as V/ Q scans and angiography labor intensive and more costly. While the exact costs of these tests are difficult to identify, the costs will be at least in the area of $100, with the potential to be higher than lower. Once again, several of these tests can be avoided with the net benefit being a savings in excess of the capnography costs. However, these savings will not be in the region of intubation and resuscitation. From a prioritization perspective, it is more efficient to focus on the applications of capnography that will generate more of a financial and clinical impact. The applications with less powerful impact can be developed at a later date when the initial clinical applications have already achieved success.
SUMMARY Based on the multiple applications and the potential cost savings, every ICU should have enough capnography for all intubations and probably for all mechanically ventilated patients. Of the multiple clinical applications of capnography, most attention should be focused on its use with intubation and resuscitation. Other applications, such as blood gas and ventilation-perfusion scan reduction, should be instituted after the primary areas have been implemented. While capnography modules may appear to be expensive at first glance, an analysis of their clinical application reveals they can save the hospital hundreds of thousands of dollars beyond the purchase price.
CAPNOGRAPHY
REFERENCES 1. Ahrens TS: Effects of mechanical ventilation on hemodynamic waveform analysis. Critical Care ursing Clinics of North America 3:629-639, 1991 2. Angelos MG , DeBehnke DJ, LeasurejE: Arterial blood gases during cardiac arrest: Markers of blood flow in a canine model. Resuscitation 23:101-111, 1992 3. Asplin BR, White RD: Prognostic value of end tidal carbon dioxide pressures during out-of-hospital cardiac arrest. Ann Emerg Med 25:756-761, 1995 4. Baraka A, Jabbour S, Hammoud R, et al: Can pulse oximet1y and end-tidal capnography reflect arterial oxygenation and carbon dioxide elimination during laparoscopic cholecystectomy' Surg Laparosc Endosc 4:353-356, 1994 5. Bern RM, Levy MN: Physiology, ed 3. SL Louis, MO, CV Mosby, 1993 6. Bongard F, Wu Y, Lee TS, et al: Capnographic monitoring of extubated postoperative patients. ] Invest Surg 7(3):259-264, 1994 7. Byrick RJ, Colin Kay J, Brendan Mullen]: Capnography is not as sensitive as pulmonary artery pressure monitoring in detecting marrow microembolism. Anesthesia Analogue 68:94-100, 1989 8. Callaham M, Barton C: Prediction of outcome of cardiopulmonary resuscitation from end-tidal carbon dioxide concentration. Crit Care Med 18:358-362, 1990 9. Campbell RS, Branson RD, Burke WC, et al: AARC clinical practice guideline: Capnography/capnometry during mechanical ventilation. Respiratory Care 40(12): 1321- 1324, 1995 10. Cantineau JP, Lambert Y, Merckx P, et al: End-tidal carbon dioxide during cardiopulmonary resuscitation in humans presenting mostly with asystole: A predictor of outcome. Crit Care Med 24(5):791796, 1996 11. Carroll GC: Capnographic trend curve monitoring can detect 1-mL pulmonary emboli in humans. J Clin Monit 8:101-106, 1992 12. Chopin C, Fesard P, Mangalaboyi J, et al: Use of capnography in diagnosis of pulmonary embolism during acute respiratory fa ilure of chronic obstructive pulmonary disease. Crit Care Med 18(4):353-357, 1990 13. DePauw C, Poelaertj, Rolly G, e t al: Clinical evaluation of pulse oximetry monitors on critically ill patients. 4:33-39, 1990 14. Dohi S, Takeshima R, Matsumiya N: Carbon dioxide elimination during circulatory arrest. Crit Care Med 15:944-946, 1987 15. Doyle DJ: Time constant related distortion in sidestream capnography. Can] Anaesth 37:S70, 1990 16. Drummond JC, Prutow RJ , Scheller MS: A comparison of the sensitivity of pulmonary artery pressure, e ndtidal carbon dioxide, and end-tidal nitrogen in the detection of venous air embolism in the dog. Anesthesia Analogue 64:688-692, 1985 17. D'Souza CR, Kilam SA, D'Souza U, et al: Pulmo nary complications of feeding tubes: A new technique of insertion and monitoring malposition. Can] Surg 37:404-408, 1994 18. English JB, Westenskow D, Hodges MR, et al: Comparison of venous air embolism monitoring methods in supine dogs. Anesthesiology 48:425-429, 1978
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19. FalkjL, Rackow EC, Weil MH: End-tidal carbon dioxide concentration during cardiopulmonary resuscitation. Engl] Med 318:607-611, 1988 20. Garnett AR, Ornato JP, Gonzalez ER, et al: End tidal carbon dioxide monitoring during cardiopulmonary resuscitation. JAMA 257:512-515, 1987 21. Gray WA, Capone Rj , Most AS: Unsuccessful emergency medical resuscitation: Are continued efforts in the emergency department justified' N Engl j Med 325: 1393-1398, 1991 22. Guggenberger H, Lenz G, Federle R: Early detection of inadvertent oesophageal intubation: Pulse oximetry vs. capnography. Acta Anaesthesiol Scand 33:112-115, 1989 23. Hess D, Schlottag A, Levin B, et al: An evaluation of the usefulness of end-tidal Pco 2 to aid weaning from mechanical ventilation following cardiac surgery. Respiratory Care 36:837-843, 1991 24. Hoffman RA, Krieger BP, Kramer MR, et al: End-tidal carbon dioxide in critically ill patients during changes in mechanical ventilation. American Review of Respiratory Disease 140:1265-1268, 1989 25. Holland R, Webb RK, Ruchman WB: The Australian incident monitoring study. Oesophageal intubation: An ana lysis of 2000 incident reports. Anaesth Intensive Care 21:608-610, 1993 26. Idris AH, Staples ED, O'Brien DJ, et al: End-tidal carbon dioxide during extremely low cardiac output. Ann Emerg Med 23:568-572, 1994 27. Isert P: Control of carbon dioxide levels during neuroanaesthesia: Current practice and an appraisal of our reliance upon capnography. Anaesth Intensive Care 22:435-441, 1994 28. Jsserles SA, Breen PH: Can changes in end-tidal Pco2 measure changes in cardiac output? Anesth Analg 73:808-814, 1991 29. Kellermann AL, Hackman BB, Somes G: Predicting the outcome of unsuccessful prehospital advanced cardiac life support. JAMA 270:1433-1436, 1993 30. Kern KB, Sanders AB, Voorhees WO, et al: Changes in expired end-tidal carbon dioxide during cardiopulmonary resuscitation in dogs: A prognostic guide for resuscitation efforts. J Am Coll Cardiol 13:11841189, 1989 31. Knill RL: Practical co2 monitoring in anesthesia. Can j Anaesth 40:R40-R44 , 1993 32. Ko FY, Hsieh KS, Yu CK: Detection of airway co2 partial pressure to avoid esophageal intubation. Acta Paediatr Sinica 34:91- 97, 1993 33. Lambert Y, Cantineau JP, Merckx P, et al: Influence of end-tidal co, monitoring on cardiopulmonary resuscitation. Anesthesiology 77:(Suppl)Al081 , 1992 34. Levine RL, Wayne MA, Miller CC: End tidal carbon dioxide and outcome of out-of-hospital cardiac arrest. N Engl J Med 337:301-306, 1997 35. Liu SY, Lee TS, Bongard F: Accuracy of capnography in nonintubated surgical patients. Chest 102:15121515, 1992 36. Misasi RS, KeyesjL: Matching and mismatching ventilation and perfusion in the lung. Crit Care Nurse 16 23-40, 1996 37. Morley TF, Giaimo J, Maroszan E, et al: Use of capnography for assessment of the adequacy of alveolar ventilation during weaning from mechanical ventilation. American Review of Respiratory Disease 148: 339- 344, 1993
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38. Myles PS, Story DA, Higgs MA, et al: Continuous measurement of arterial and end-tidal carbon dioxide during cardiac surgery: Pa-ETco2 gradient. Anaesth Intensive Care 25(5):459-463, 1997 39. Ornato JP, Garnett AR, Glauser FL: Relationship between cardiac output and the end-tidal carbon dioxide tension. Ann Emerg Med 19:1104-1106, 1990 40. Paulus DA: Capnography. International Anesthesiology 27:167-175, 1989 41. Robbins PA, Conway J, Cunningham DA, et al: A comparison of indirect methods for continuous estimation of arterial Pco 2 in men. J Appl Physiol 68:1727-1731, 1990 42. Raemer DB, Calalang I: Accuracy of end-tidal carbon dioxide tension analyzers. J Clini Monit 7:195-208, 1991 43. Roberts WA, Maniscalco WM , Cohen AR, et al: The use of capnography for recognition of esophageal intubation in the neonatal intensive care unit. Pediatr Pulmonol 19:262-268, 1995 44. Russell GB , Graybeal JM: Reliability of the arterial to end-tidal carbon dioxide gradient in mechanically ventilated patients with multisystem trauma. J Trauma 36:317-322, 1994 45. Russell GB, Graybeal JM: The arterial to end-tidal carbon dioxide difference in neurosurgical patients during craniotomy. Anesth Analg 81:806-810, 1995 46. Sanders AB , Ewy GA, Bragg S, et al: Expired Pco2 as a prognostic indicator of successful resuscitation from cardiac arrest. Ann Emerg Med 14:948-955, 1985 47. Sanders AB, Kern KB , Otto CW, et al: End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation: A prognostic indi cator for survival. JAMA 262:1347-1351, 1989 48. Severinghaus JW, Stupfel M: Alveolar dead space as an index of distribution of blood flow in pulmonary capillaries. J Appl Physiol 10:335-348, 1957 49. Sharma SK, McGuire GP , Cruise CJE: Stability of the arterial to end-tidal carbon dioxide difference during anesthesia for prolonged neuro-surgical procedures. Can] Anaesth 42:498-503, 1995
50. Shibutani K, Muraoka M, Shirasaki S, et al: Do changes in end-tidal Pco2 quantitatively reflect changes in cardiac output' Anesth Analg 79:829833, 1994 51. Slonim NB, Hamilton LH: Respiratory Physiology, ed 5. St. Louis, MO, CV Mosby, 1991 52. Szmuk P, Ezri T: Capnography and cuff inflation to aid blind tracheal intubation [letter]. Anaesthesia 50:662, 1995 53. Thrush DN, Mentis SW, Downs JB: Weaning with end-tidal co, and pulse oximetry. J Clin Anesth 3:456460, 1991 54. Ward KR, Menegazzi]J, Zelenak RR, et al: A comparison of chest compressions between mechanical and manual CPR by monitoring end-tidal Pco 2 during human cardiac arrest. Ann Emerg Med 22:669-674, 1993 55. Weil MH, Bisera J, Trevino RP , et al: Cardiac output and end-tidal carbon dioxide. Crit Care Med 3:9079, 1985 56. Weingarten M: Respiratory monitoring of carbon dioxide and oxygen: A ten year perspective. J Clin Monit 6:217-225, 1990 57. White RD , Asplin BR: Out-of-hospital quantitative monitoring of end-tidal carbon dioxide pressure during CPR. Ann Emerg Med 23:25-30, 1994 58. WilliamsonJA, Webb RK, Cockingsj, et al: The Australian incident monitoring study: The capnography: Applications and limitations: An analysis of2000 incident reports. Anaesth Intensive Care 21:551-557, 1993 59. Withington DE, Ramsay JG , Saoud AT, et al: Weaning from ventilation after cardiopulmonary bypass: Evaluation of a non-invasive technique. Can J Anaesth 38:15-19, 1991 60. Wright SW: Conscious sedation in the emergency departments: The value of capnography and pulse oximetry. Ann Emerg Med 21:551-555 , 1992 61. Yamanaka MK, Sue DY: Comparison of arterial-endtidal Pco 2 difference and cleadspace/ ticlal volume ratio in respirato1y failure. Chest 92:832-835, 1987
Address reprint requests to Tom Al1fens, DNS, RN, CCRN, CS Clinical Specialist Department of Critical Care Barnes-Jewish Hospital One Barnes-Jewish Hospital Plaza St. Louis, MO 63110
Utilization of Technology: Unrealized Potential
Capnography A Key Underutilized Technology Tom Ah rens, DNS, RN , CCRN , CS, Helen Wijeweera, RN , MSN , and Shawn Ray, BSN , RN , CCRN
C apnography, the measurement of exhaled carbon dioxide, is one of the most underutilized technologies in critical care. While capnography has been available for decades, only in the recent past has the use of capnography grown to the point that at least eight applications of this technology are now possible (see Box 1, page 52). Capnography allows the clinician several key advantages in its use: it is noninvasive, easy-to-use, easy to maintain, and relatively inexpensive. In addition, capnography has impressive potential to reduce costs, both from a clinical and legal perspective. From these advantages, capnography use should be widespread in acute and critical care. However, the use of capnography is still limited. The reason for this limitation may stem from a lack of education and understanding on the potential applications of this technology. In this article, a review of the uses of capnography and its impact on patient outcome and costs, illustrates why this technology has such great promise in the assessment of acute and critically ill patients.
From the Department of Critical Care (TA) and Surgical Intensive Care Unit (SR), Barnes-Jewish Hospital; and Veteran's Administration Medical Center (HW) , St. Louis, Missouri
Physiology of Capnography During exhalation, carbon dioxide (co 2) is eliminated in a characteristic manner that creates a waveform with distinct features (Fig. 1). During initial exhalation, little co 2 is present since the initial exhalation is primarily from deadspace areas (upper airways that do not participate in gas exchange). After this initial expiration, co 2 pressures start to increase, reflecting the contribution of gas exchange regions of the lung. The pattern culminates with the emptying of distal alveoli. This end portion of the co 2 elimination is visible on the capnogram waveform with a value called the end tidal co 2 (Petcoz) level. Under normal lung physiology conditions, where ventilation and perfusion are relatively equally matched, the Petco 2 level closely approximates the arterial (alveolar) co 2 level (Pacoz) . The slope of the waveform is important to monitor in that it reflects alveolar emptying patterns. If the slope is smooth and progressive with a plateau at the end, a normal capnogram is present (Fig. 2). If the slope is rapid and terminates abruptly, it may indicate inadequate alveolar emptying (Fig. 3). If inadequate alveolar emptying is present, the Petco 2 w ill not approximate the Paco 2 • However, circumstances exist where even if a normal capnogram is present, the ability of Petco 2 to reflect Paco 2 may be
CR ITICAL CARE NURSING CLINICS OF NORTH AMERICA I Volume 11 I Number 1 I March 1999
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Figure 1.
Normal capnogram appearance.
limited. The key to Petco2's ability to reflect Paco 2 is in a normal ventilation (V) to perfusion (Q) relationship. This relationship is not discernible by noting the waveform alone. However, it is discernible by understanding the clinical situation of any given patient.
Figure 2.
Accuracy and Safety in the Measurement of co 2 Capnography is measured by several methods, including infrared technology or mass spectrometry. The primary method of mea-
First capnogram illustrates normal carbon dioxide elimination pattern.
CAPNOGRAPHY
51
!] /] /\ i Figure 3.
Inadequate alveolar emptying as reflected by the capnogram.
surement in acute and critical care is with the infrared technique. With this technique, either a sidestream (aspirating gas to an off patient analyzer) or mainstream (measuring the gas at the patient) analyzing technique is employed. Infrared sensing of co 2 is well-documented in the literature in terms of accuracy. 11 38. 10· •2. 'i6 Both technologies allow for accurate measurement of co 2 as well as respiratory rate. Both side-stream and mainstream techniques sample exhaled air as close to the patient as possible (Fig. 4). Sidestream measurement offers an advantage in that the sampling device is vety lightweight and can be used in nonintubated patients. The primary
Figure 4.
disadvantage is that it is prone to obstruction by moisture in the sampling circuit. 1'i Mainstream sampling has advantages in real time measurement and fewer problems with moisture obstructing the measurement of co2• In terms of safety, there are virtually no clinical dangers with this technology. The sampling of exhaled gas from the patient is taken with technology that has no harmful effects to the patient. With sidestream technology on the nonintubated patient, the aspiration tubing at times can feel uncomfortable. With mainstream technology, the weight of the sensor may pull on the ventilator tubing and increase the chance for a disconnec-
Mainstream co2 analyzer.
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Box 1 EIGHT CLINICAL APPLICATIONS OF CAPNOGRAPHY
• Avoiding esophageal intubations during endotracheal tube (ET) placement • Avoiding tracheal intubations during nasogastric (NG) placement • Prognosis and adequacy of resuscitations • Reducing arterial blood gas (ABG) use • Detecting physiologic deadspace changes • Identifying end expiration with hemodynamic waveforms • Identifying alveolar emptying patterns • Assessing sedation/paralytic therapy Each of these applications has its value in patient management. Each application has variable degrees of potential impact on patient outcome. The impact of each application will be clear as each application is discussed later in text.
location of the endotracheal tube simple. If the endotracheal tube is in the trachea or epiglottis, a normal capnogram can be seen (Fig. 5). If the ET is outside the trachea (e.g., esophagus) , a flat line will appear. Capnography can sense the placement of the endotracheal tube within seconds. 43 The use of capnography in assessment of ET placement has become so accepted that it is the standard in anesthesia when tracheal intubations are attempted. 58 Several papers have been published on the accuracy and reliability of capnography to safely detect correct placement of ETs. 22• 25 • 32 • 52 For this reason alone , capnography should be available in every clinical setting where tracheal intubations occur.
Avoiding Tracheal lntubations During NG Tube Placement tion. This disconnection will be immediately sensed if the alarms are appropriately set.
Avoiding Esophageal lntubations During ET Placement Perhaps the simplest and most powerful impact of capnography is in the area of identifying the location of the ET during intubation. During intubation of a patient's trachea , a critical step is to ensure correct placement in the trachea. Placement in the esophagus will lead to inadequate breathing with the potential for brain damage or death. Capnography makes
Figure 5.
While uncommon, it is possible that during placement of an NG tube inadvertent placement in the trachea could occur. If such an event were to occur, the complications could range from minor to catastrophic. The worse case scenario would be the placement of the NG tube in the trachea when the purpose of the NG placement was for enteral nutrition. Placement of enteral flu id in the lungs would cause aspiration, resulting in anything from a pneumonitis to adult respiratory distress syndrome and death. Avoiding NG placement in the esophagus can occur through the use of capnography. Attaching the capnography sensor to the end
Use of capnography with nasogastric tube placement.
CAPNOGRAPHY of the NG tube will immediately confirm if the NG tube is in the trachea or the esophagus (Fig. 6). 17 The incidence of NG placement in the lungs is unknown. However, since capnography allows accurate, immediate detection of a potentially dangerous situation, its use in this application may be warranted. Further research in this area would better clarify the value of capnography on avoiding tracheal intubations during NG placement.
Predicting Survival During Cardiopulmonary Arrests Assessment of survival during cardiopulmonary resuscitation is an inexact science. Understanding the potential a patient has for survival is important for several reasons but primarily for determining when to continue or stop a resuscitation effort. The most common method for determining when a code can be stopped now is based on individual clinician judgment of factors surrounding the code (e.g., diagnosis) , potential for survival, patient/ family wishes, response to code efforts, and length of time the code has been in progress. It would be helpful to clinicians to have
a more objective measure for the potential success during the resuscitation effort. Capnography, specifically end tidal co 2 (Petco 2) levels, has the potential for being an objective measure of survival during a cardiopulmonary arrest. This potential is the result of Petco 2 levels having a strong correlation with cardiac output. Weil et al were one of the first to note that exhaled co 2 levels, specifically the peak or end tidal co 2 (Petco 2) correlated with cardiac output. 55 The theory for this correlation is that as pulmonary blood flow (and therefore cardiac output) decreased, an increase in deadspace occurs. The drop in exhaled co 2 would reflect this drop in pulmonary blood flow. Capnography measures this decreased co 2 . Exhaled co 2 reflects the amount of co 2 returning to the lungs, the result of substrate metabolism and pulmonary blood flow. Under normal circumstances (normal blood flow to the lungs), alveolar co 2 values are about the same as a1terial co 2 levels. The normal difference between arterial and Petco 2 values is about 1 to 5 mm Hg, with the exhaled co 2 almost always being lower than arterial. 47 Any condition that reduces pulmonary blood flow relative to ventilation will cause this gradient to widen. The greater the difference between
Capnography Wave indicating Lung Placement
No Capnography Wave Indicating Non-Lung (esophageal) Placement
Figure 6.
53
Flat line on capnogram indicates nonlung placement of tube.
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the arterial and exhaled co2 level, the more severe the disruption between pulmonary blood flow and ventilation. Weil et al's observations of Petco 2 levels reflecting cardiac output were extended by other investigators. Isserles and Breen as well as Shibutani et al and Idris et al also found Petco 2 levels correlated well with cardiac output.26· 28· 50 In the same year as Weil et al's observation, Sanders et al first published the ability of capnography to predict outcome in a cardiopulmona1y arrest. 46 This paper was followed by many others, including Garnett et al, Dohi et al, Ornato et al, Falk et al, Sanders et al, Kern et al, Callaham and Barton, Lambert et al, Asplin et al, Angelos et al, Cantineau et al, Ward, and White et al. 2·3, 8• 10· 14• J9, 20. 30, 33, 39, 47. s4. 57 All of these authors demonstrated that capnography has a potential correlation with survival in cardiopulmonary arrests. Levine and colleagues demonstrated with out of hospital cardiac arrests that if the exhaled co2 level did not return to 20 mm Hg after 20 minutes, mortality was 100%. 34 These studies suggest that capnography might be an important aid to the clinician when evaluating the success of the resuscitation efforts. A few case histories illustrate how easily capnography might be employed in a cardiopulmonary arrest situation. CASE HISTORY 1
Does the capnography waveform (Fig. 7) indicate adequate CPR? Answer: Yes, the Petco 2 value of 28 indicates a probable adequate cardiac output with CPR (Fig. 7). CASE HISTORY 2
A 66-year-old woman is brought into the emergency room (ER); cardiopulmonary resuscitation (CPR) is in progress. She was found "down" in her house by her husband. Paramedics have been doing CPR for more than 20 minutes. Her capnography wave shows a value of 12 mm Hg. How would you assess the adequacy of the resuscitation effort?
Answer: Resuscitation is not effective for this patient. In addition, the likelihood of survival at this point is very poor. Since the Petco 2 is only 12, a very low cardiac output is likely (Fig. 8). Given the length of time of resuscitation, irreversible brain injury may be present now.
Using Capnography to Predict Arterial co 2 and Avoid Blood Gas Measurement In normal lungs, exhaled co 2 levels closely approximate arterial co 2 levels. Specifically, at end expiration the maximum partial pressure of co2 (Petco2) occurs and it is this point that correlates with arterial co 2 levels.31 When lungs are normal, ventilation (V) at the alveolar level is matched by alveolar perfusion (Q), a term referred to as V/ Q ratio. As long as ventilation matches perfusion, the Petco2 level will be within 1 to 5 mm Hg of the arterial co 2. The difference between the arterial and Petco 2 level is often referred to as the Paco 2 to Petco2gradient. If the Paco2to Petco 2gradient is normal, then the Petco 2 value can be used as a reflection of Paco2. Several papers have been published on the use of the Paco 2 to Petco 2 gradient in clinical practice. The gradient between Paco 2 and Petco2 has clinical value but must be used with caution, based on these papers. A brief review of each of the papers illustrates the reason for caution when using the Paco2 to Petco 2 gradient. Baraka and colleagues recorded continuous Petco2 of 13 patients undergoing laparoscopic cholecystectomy and found good correlation between Petco2 and Paco2 during carbon dioxide insufflation. Their conclusion was that Petco 2can be used to monitor arterial oxygenation and co2 elimination.4 Bongard et al found in 41 postoperative surgical patients that the Petco2 was lower than the Paco 2 by an average of 2.8 mm Hg. 6 The study concluded that "on-line" Petco 2 measurement was a useful substitute for routine ABG determination of Paco 2in selected patients. Several other studies also concluded that Petco 2 is a good indicator for Paco2 in several populations. Liu studied nonintubated weaned patients and found that Petco 2was on an average 3.6 mm Hg less than Paco2 .35 Wright et al found Petco2 was a useful tool during conscious sedation.6
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Practice example. A 66-year-old woman is brought into the ER. CPR is in progress.
tubated postoperative coronary artery bypass graft (CABG) patients were studied. Thrush studied 10 postoperative CABG patients, finding the sensitivity of Petco2 for hype rcarbia was 95% and concluding that Petco2 can be safely used for ventilator weaning. 52 Withington found in postoperative CABG patients that the correlation between Paco2 and Petco 2 was 0.64 and 0.79 for a mass spectrometer and 0.67 and 0.81 for the infrared analyzer. 59 The authors concluded that once Paco 2 to Petco 2 gradients are established , capnometry is sufficie ntly reliable to e nable weaning and allow a reduction in the use of arterial blood gas analysis. Sharma and colleagues studied 21 patients unde rgoing elective carniotomies, finding a constant relationship between Paco 2 and Petco 2. They concluded that once the Paco 2 to Petco 2 difference has been established , the Petco 2 can be used as a reliable guide to estimate Paco 2 during neurosurgical procedures. 4 Other studies found that Petco 2 and Paco 2 correlated well if the patient was stable or without lung disease (conditions that would alter the V/ Q ratio). 24 • 37 However, not all research has been conclusive in indicating Petco 2 be used as the sole predictor of Paco 2 • Russell and Graybeal, studied nine intubated trauma patients and found that a positive correlation of statistical significance between Paco 2 and Petco 2 existed in 78% of patients, with 15% with false negatives and 12% false positives, although these changes were small.44 In another study by Russell and
Graybeal, the Paco 2 and Petco 2 of 35 craniotomy patients were studied. The authors found a significant positive correlation exists between Paco2 and Petco2• However, with subsequent measurements, 18% of the time the changes were opposite . These changes were small and not always clinically significant. 45 The authors concluded that ABG analysis is necessary during these procedures. However, in the latter study it is recognized that the duration of mechanical ventilation and the presence of spontaneous breaths may have been factors in this analysis. In 24 patients being weaned from mechanical ventilation, Hess and colleagues found that Petco 2 signiflcantly correlated with Paco 2. However, when changes of >5 mm Hg in Paco2 occurred, Petco2 incorrectly indicated the direction of change in 30% of cases. Hess suggests that Petco 2 should not be used in isolation to wean patients following cardiac surgery.23 However, the authors acknowledge that this inability of Petco 2 to predict Paco2 may stem from the patient population type, changes in ventilation, or changes in pulmonary blood flow. 27 This potential change in pulmonary blood flow and the changes of ventilation were also recognized by other authors as the difflculty of Petco 2 as a predictor of Paco 2. Many studies recognized that Petco2 could predict Paco2 in stable patients, but because of the inequality of ventilation- pe rfusion ratios and inconsistent Paco 2 to Petco2 gradients in critically ill patients, Petco 2 may not always be the sole predictor of Paco 2• As indicated
CAPNOGRAPHY in the above studies, Petco 2 may reflect Paco2 in healthy lungs. However, the recognition of the need to first establish the Paco 2 to Petco 2 gradient in healthy patients and that Petco 2 does not reflect Paco2 due to a widening gradient in all patients, indicates another potential use for Petco2 monitoring, that is, the identification of changes in physiologic deadspace.
Identifying Changes in Physiologic Deadspace and Detecting Pulmonary Emboli The Paco2 to Petco2 gradient increases as the deadspace increases, or as ventilationperfusion mismatching increases. Deadspace occurs if there is a ventilation-perfusion mismatch, as occurs with pulmonary emboli.41 A normal ventilation/ perfusion match, designed to allow optimal 0 2 and co 2 exchange to occur, must be matched with the lungs. 9· 61 Ventilation is the volume of airflow per minute CL/min) through the lungs. This is also known as minute ventilation. Minute ventilation (Ve) is composed of the sum of alveolar ventilation (VA), air that participates in gas exchange, and deadspace ventilation (Vd), air that does not participate in gas exchange (VA+ Vd =Ve). All healthy individuals have anatomic deadspace, parts of the airway that do not participate in gas exchange (e.g., nasooro pharynx, trachea, bronchus). Perfusion is blood flow through the pulmonary vasculature. Blood flows into the pulmonary capillaries at the alveolar level where co2 in the blood is exchanged for oxygen in the alveoli. In some diseased lungs there also exists alveolar deadspace, where ventilation occurs at the alveolar level, but reduced perfusion takes place. 5· 36· 51 When an increased deadspace is present, the reduced pulmonary blood flow can be detected with capnography. As pulmonary blood flow decreases in one area, it increases in another to compensate for the obstructed blood flow region. As an increased blood flow occurs in this new area, increased ventilation must occur to match the increased perfusion, or gas exchange is affected. The increase in ventilation also means more air is going to the deadspace area. The net result is that during exhalation, the pressure of co2 is reduced. This reduced co2 pressure is measured by cap-
57
nography. As reduced co 2 pressure occurs, the gradient between the arterial and end tidal co 2 level falls. Capnography, through the identification of the Paco2 to Petco2 gradient, can thereby reflect Vd. Owing to the ability for capnography to reflect changes in deadspace, it has been proposed in the literature that capnography be utilized as a diagnostic test for pulmonary embolisrn. 48 Many studies support this application of capnography. Yamanaka and Sue studied 17 patients undergoing mechanical ventilation, finding that while PETco2 may be a poor estimate of Paco 2, it can be utilized as a measurement of VdNT (17). 61 Later, in a prospective study, Chopin and colleagues studied 44 adult chronic obstructive pulmonary disease (COPD) patients suspected of having acute respiratory failure secondary to pulmonary emboli (PE). The results showed that 17 patients had PE, and 17 patients did not have PE as diagnosed by pulmonary angiography. The Paco 2 to Petco2 was significantly different in the +PE and - PE patients. The positive predictive value was 74%, but the negative predictive value was 100%, with specificity of 65% and sensitivity of 100%. 12 Carroll concluded that the capnographic trend curve can detect a 1-mL pulmona1y emboli. 11 In this experimental study, the capnograms of 24 mechanically ventilated patients were recorded during a simulation of a 1-mL PE by inflation of the balloons of their pulmonary artery catheters (PACs). The results showed a positive predictive value of 98% and negative predictive value of 89%. In the canine model, several studies have also shown that capnography can be a useful tool in detecting PE. Byrick and colleagues fou nd that capnography is a useful tool intraoperatively to detect PEs; however, pulmonary artery pressure (PAP) is more sensitive in detecting fat and marrow microemboli. 7 English and colleagues fo und that Petco2 in correlation with PACs provides "sensitive and reliable detection of air embolus and quantitative evaluation of embolus size." 18 Drummond and colleagues found that any acute change in Petco2 of equal or greater than .2% during "steady-state" conditions could be attributed to air emboli.16 Petco 2 can identify a widening Pacoi-Petco2 gradient. Therefore, capnography can be utilized to identify deadspace, thus pulmonary emboli. Potentially
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Petco 2 may be utilized, in coordination with other assessment tools, to replace V/ Q scans and the invasive pulmonary angiography.
Identifying End Expiration During Hemodynamic Waveform Analysis Capnography can ease the interpretation of hemodynamic waveforms by clearly identifying end expiration (Fig. 9). 1 While no studies have been performed in this area to date, Ahrens reports how the use of capnography can improve finding end expiration for waveform analysis. Capnography use in finding expiration is relatively simple. At the peak of the capnogram (what should be the end tidal level), expiration has stopped and inspiration is about to begin. At this location, end expiration is present. The clinician should read the waveform immediately below the end tidal level to find the end expiration location. Capnography use in finding end expiration is almost foolproof, but not quite. At times, the patient will generate an inspiratory effort that changes pleural pressure well before the capnogram reflects the inspiration. Be careful when using capnography to find end expiration. Make sure the point that should align with end expiration is consistent with what is displayed on the hemodynamic waveform.
Figure 9.
The Cost of Capnography-Does the Cost Justify its Use? Like any technology, capnography acquisition is associated with a purchase price. Does this purchase price allow for an institution to recoup the cost through improved clinical practice? In terms of capnography, the purchase price is actually very reasonable. A review of capnography and its associated costs as well as its benefits will help illustrate why capnography is a reasonably priced technology. Direct costs to acquiring capnography costs vary depending on the type of monitor purchased, for example, hand held, stand alone, or incorporated into a bedside monitor. Capnography units can range from less than $2000 per module to up to $5000 in terms of purchase price. In addition, the cleaning or replacing of disposable equipment is necessary. However, the capnography modules can last for years with little maintenance. Table 1 gives a review of capnography costs. Capnography can directly provide benefits in four areas: 1. Avoiding legal encounters during intubation 2. Avoiding costs associated with futile resuscitation efforts 3. Reducing frequency of ABGs 4. Reducing V/ Q scans and pulmonary angiography
Capnogram overlapped on hemodynamic waveform to ease interpretation of end expiration.
CAPNOGRAPHY
Table 1
COST/BENEFIT COMPARISON WITH CAPNOGRAPHY
Capnography Costs Per unit/per year Purchase price* Disposable Cleaning Education costs t
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Yearly Capnography Benefits
$2000-$5000 $100-$1000 $100-$500 $5000
Legal Resuscitation ABG reduction V/Q scan and pulmonary angiography reduction
$0-$1,000,000 $10, 000- $300,000 $1000-$10' 000 $1000-$10' 000
• Each module lasts for years.
t Most costs incurred in start up education.
A review of these four areas gives an idea of the potential impact of capnography on costs.
should the patient have an unnecessarily induced pneumonia, ARDS, or death.
Avoiding Legal Encounters During Intubation
Avoiding Costs Associated with Futile Resuscitation Efforts
The occurrence of difficult intubations is not common. A well-trained clinician can perform intubations of the trachea relatively easily. However, there will always be a few cases each year that present a substantial challenge to even a well-prepared clinician. In these circumstances, the use of capnography not only helps avoid clinical complications but also legal complications. Legal complications can result from difficult intubations owing to consequences of death or brain damage. It is not uncommon for a hospital risk management department to encounter a case involving difficult intubations. If the hospital has to defend such a case, the minimum costs are likely to be about $25,000 with the potential, including damages, to be well in excess of $1,000,000. If a hospital is self-insured, much of this cost will be incurred by the hospital. Even if the occurrence of such an incident is rare, if legal action occurs just once in the lifespan of a capnography module, the module will easily pay for itself many times over. The clinical costs associated with improper intubation are also considerable. Assuming the patient does not die from a misplaced endotracheal tube, there is a strong likelihood the outcome will at minimum increase the stay in the hospital. Considering ICU costs are about $1,000 per day, the clinical costs can quickly mount. Similar, although not as striking, are the costs avoided with placing an ET in the lungs. Both legal and clinical costs can be substantial
The costs avoided by using capnography to predict outcome can be substantial. The major impact of the use of capnography is in the prehospital phase. Levine and colleagues illustrated this savings in a ve1y convincing argument. They identify that only a small percentage of patients who arrest outside the hospital actually survive to go home. A study in support of Levine is one by Kellermann et al. 29 Kellerman and colleagues noted that only 3 patients out of a total of 758 who were transferred to the hospital emergency room in refractory cardiac arrest were successfully resuscitated. Unfortunately, all three had substantial neurologic deficits. Gray et al provided fu1ther evidence of the potential for futility when dealing with out-of-hospital arrests.21 They note that in a study of 185 patients who could not have spontaneous circulation restored from outside the hospital, 16 eventually did achieve spontaneous circulation restoration in the e mergency department (ED). Again, unfortunately, the outcome for this effort was not rewarded since all 16 patients died in the hospital. However, the costs associated with the resuscitation efforts were substantial. Hospital costs for attempting resuscitation in patients who could not be resuscitated in the ED is likely between $100,000 and $150,000 for the 169 patients who could not be resuscitated in the ED. Pe rhaps more importantly, the costs are even higher for the patients who survived the ED. The cost for the 16 SLllvivors in the ED was $180,908. It is
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important to keep in mind that all 16 patients eventually died despite the hospital's best efforts. Gray and colleagues claim that if this data are extrapolated to the entire country, over $1,000,000,000 is lost in futile care efforts. Hospitals employing capnography in prehospital (and possibly in-hospital resuscitation as well) have the potential to save up to $300,000 while not adversely affecting patient outcome. Reducing Frequency of ABGs , V/Q Scans, and Pulmonary Angiography
Avoiding unnecessary blood gases and ventilation perfusion scans can lead to a savings that is equal to the cost of capnography modules. However, these savings are small in comparison to the savings achieved with intubation and resuscitation efforts. Blood gases, which cost about $5, have to be reduced in a large number before substantial clinical savings will be realized. Capnography will reduce blood gases if used properly. Perhaps the best example is in the avoidance of "routine" ABGs. The following case history helps to illustrate this point. CASE HISTORY 3 A 59-year-old man with congestive heart
failure (CHF) is on mechanical ventilation with a rate of 12, total rate of 14. His Flo 2 is .50, pulse oximeter is .92, and the Petco 2
is 26. The minute ventilation (Ve) is 7 LPM (normal 5 to 10 LPM). This information is 12 hours old. Current readings indicate no ventilator changes and the Spo2 now is .91, Petco 2 is 26. Is a routine ABG necessary at this time? Answer: No. Since the Petco 2 and minute ventilation are unchanged, the Paco 2 is unlikely to have increased. In this instance, the use of capnography avoided the need for a routine blood gas. V/ Q scans and pulmonary angiography are more costly and can be avoided by performing a Paco 2 to Petco 2 gradient measurement prior to performance. These tests involve the use of additional personnel, frequently requiring a transport to the radiology department and the interpretation of results by a radiologist. All these factors make tests such as V/ Q scans and angiography labor intensive and more costly. While the exact costs of these tests are difficult to identify, the costs will be at least in the area of $100, with the potential to be higher than lower. Once again, several of these tests can be avoided with the net benefit being a savings in excess of the capnography costs. However, these savings will not be in the region of intubation and resuscitation. From a prioritization perspective, it is more efficient to focus on the applications of capnography that will generate more of a financial and clinical impact. The applications with less powerful impact can be developed at a later date when the initial clinical applications have already achieved success.
SUMMARY Based on the multiple applications and the potential cost savings, every ICU should have enough capnography for all intubations and probably for all mechanically ventilated patients. Of the multiple clinical applications of capnography, most attention should be focused on its use with intubation and resuscitation. Other applications, such as blood gas and ventilation-perfusion scan reduction, should be instituted after the primary areas have been implemented. While capnography modules may appear to be expensive at first glance, an analysis of their clinical application reveals they can save the hospital hundreds of thousands of dollars beyond the purchase price.
CAPNOGRAPHY
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Address reprint requests to Tom Al1fens, DNS, RN, CCRN, CS Clinical Specialist Department of Critical Care Barnes-Jewish Hospital One Barnes-Jewish Hospital Plaza St. Louis, MO 63110