{"id":3577,"date":"2023-02-23T02:54:17","date_gmt":"2023-02-23T02:54:17","guid":{"rendered":"https:\/\/blog.georgiaemsacademy.com\/?p=3577"},"modified":"2023-05-10T00:44:15","modified_gmt":"2023-05-10T00:44:15","slug":"petco2-capnometry","status":"publish","type":"post","link":"https:\/\/blog.georgiaemsacademy.com\/?p=3577","title":{"rendered":"PETCo2 = Capnometry"},"content":{"rendered":"\n
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Capnometry is a non\u2010invasive monitoring technique which allows fast and reliable insight into ventilation, circulation, and metabolism. In the prehospital setting it is mainly used to confirm correct tracheal tube placement. In addition it is a useful indicator of efficient ongoing cardiopulmonary resuscitation due to its correlation with cardiac output, and successful resuscitation. It helps to confirm the diagnosis of pulmonary thromboembolism and to sustain adequate ventilation in mechanically ventilated patients. In patients with hemorrhage, capnometry provides improved continuous hemodynamic monitoring, insight into adequacy of tissue perfusion, optimization within current hypotensive fluid resuscitation strategy, and prevention of shock progression through controlled fluid administration.<\/p>\n\n\n\n

Managing the critically ill patient is one of the most challenging aspects of prehospital emergency care. The nature of work in such circumstances demands methods which rapidly recognize patients at risk. Capnometry, as a non\u2010invasive method, offers fast and reliable insight into certain pathophysiological processes and aids in the clinical evaluation of critically ill patients, especially those in cardiac arrest and requiring cardiopulmonary resuscitation, and assists in the monitoring patients after endotracheal intubation and mechanical ventilation.<\/p>\n\n\n\n

For many years aggressive fluid resuscitation has been a mainstay in the early treatment of the patient in hemorrhagic shock. But in recent years hypotensive fluid resuscitation before surgical repair of uncontrolled thoraco\u2010abdominal hemorrhage has proven to be superior over aggressive fluid resuscitation. Methods like capnometry could allow better additional discrimination between therapeutic demands, leading to more precise fluid resuscitation in the prehospital setting. In fact, changes in end tidal carbon dioxide partial pressure (Petco2<\/sub>) promptly reflect circulatory compromise and as such help in timely recognition of patients at risk.<\/p>\n\n\n\n

Basic physiology of carbon dioxide and end tidal carbon dioxide measurements<\/h2>\n\n\n\n
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Carbon dioxide (CO2<\/sub>) is the product of cellular aerobic metabolism. It diffuses easily from cells into blood and erythrocytes and is transported to the lungs by venous blood through the function of cardiac output. Under normal conditions of circulation and ventilation the partial pressure of CO2<\/sub> approaches 50\u2005mm\u2005Hg at the level of tissues, and 45\u2005mm\u2005Hg in the venous blood. The difference between the latter and alveolar CO2<\/sub> partial pressure (Paco2<\/sub>) which is around 40\u2005mm\u2005Hg, is responsible for diffusion of CO2<\/sub> into the alveoli. There, CO2<\/sub> is eliminated from the body with minute ventilation. Arterial CO2<\/sub> partial pressure (Paco2<\/sub>) normally varies from 35\u201345\u2005mm\u2005Hg.<\/p>\n\n\n\n

Approximately 7% of CO2<\/sub>\u00a0is dissolved in blood. The rest of it diffuses into erythrocytes where 23% of CO2<\/sub>\u00a0reversely bonds to hemoglobin, and the rest forms carbonic acid with water after carbonic anhydrase mediated enzymatic reaction. Carbonic acid dissolves into hydrogen and bicarbonate ion. The former is buffered by deoxygenated hemoglobin in tissues, and the latter can be exchanged with plasma chloride ions. In lung capillaries these reactions are reversed: bicarbonate enters erythrocytes, binds to hydrogen ions recently released from oxygenated hemoglobin, and dissolves into CO2<\/sub>\u00a0and water.<\/p>\n\n\n\n

Capnometry in monitoring and diagnostics<\/h2>\n\n\n\n

Confirmation of correct endotracheal tube placement is of paramount importance because esophageal intubation increases morbidity and mortality if unrecognized. Capnometry, and capnography with typical waveform in particular, are the most reliable methods to confirm tracheal intubation.<\/p>\n\n\n\n

In cardiac arrest Petco2<\/sub> values decrease abruptly almost to zero. They increase with the start of chest compressions during CPR, and as spontaneous circulation resumes they return to normal or above normal levels. As the cardiac arrest occurs, cardiac output no longer serves as a conduit between the large peripheral tissue compartment where production of CO2<\/sub> is generated, and the smaller central pulmonary compartment where elimination of CO2<\/sub> takes place. The result is accumulation of CO2<\/sub> in the tissues and a drastic fall in alveolar and consequently end tidal CO2<\/sub>. Therefore initial values of Petco2<\/sub> after endotracheal intubation and resumed ventilation are low. Initiation of chest compressions restores a fraction of normal cardiac output and begins to wash CO2<\/sub> out of the peripheral tissue compartment, where CO2<\/sub> is now produced in part by aerobic metabolism and in part by anaerobic metabolism. The latter is caused by buffering of metabolic acids by bicarbonate, which leads to carbonic acid production and its dissociation to CO2<\/sub> and water.<\/p>\n\n\n\n

What can you figure out with Petco2<\/sub>?<\/h3>\n\n\n\n
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Return of Spontaneous Circulation<\/h4>\n\n\n\n

Establishing cardiac output with CPR re\u2010establishes CO2<\/sub>\u00a0transport and causes alveolar CO2<\/sub>\u00a0tension to increase which ultimately results in increased Petco2<\/sub>\u00a0values. But cardiac output produced by closed chest compressions is only a fraction of normal cardiac output, so the Petco2<\/sub>\u00a0values are usually lower than normal. Return of spontaneous circulation is usually indicated by a rapid increase in Petco2<\/sub>\u00a0since normal or near\u2010normal cardiac output increases alveolar and therefore end tidal CO2<\/sub>\u00a0partial pressure. \u00a0However, initial and final values of Petco2<\/sub>\u00a0of \u2a7d10\u2005mm\u2005Hg during CPR proved to predict mortality, <\/sup>which makes capnometry also an important tool in the prognostication of cardiac resuscitation. <\/sup>\u00a0Capnometry in CPR thus provides information about efficacy of chest compressions, return of spontaneous circulation, and prognosis of outcome after cardiac arrest.<\/p>\n\n\n\n

Capnometry can also help to differentiate an asphyxic from primary cardiac arrest, \u00a0detect waning neuromuscular blockade, and serves as an additional diagnostic tool in pulmonary thromboembolism. \u00a0In patients with asphyxia as the cause of cardiac arrest, initial Petco2<\/sub>\u00a0values after endotracheal intubation are much higher than in patients with primary cardiac arrest due to ventricular fibrillation and\/or ventricular tachycardia. Asphyxial cardiac arrest leads to continued cellular production of CO2<\/sub>\u00a0and continued cardiac output before cardiac arrest that allows delivery of CO2<\/sub>\u00a0to the lungs which ultimately increases alveolar CO2<\/sub>.\u00a0This in turn is reflected as high Petco2<\/sub>\u00a0once ventilation is resumed during CPR. However, the difference in Petco2<\/sub>\u00a0between primary and secondary cardiac arrest is no longer present after 1\u2005min of CPR. \u00a0The initial Petco2<\/sub>\u00a0difference thus serves as a good diagnostic tool for reassuring emergency physicians to take measures appropriate for asphyxial cardiac arrest, since the hypoxia is a potentially reversible cause of cardiac arrest.<\/p>\n\n\n\n

In pulmonary thromboembolism Petco2<\/sub>\u00a0values are characteristically lower than normal because of diminished pulmonary perfusion and increased alveolar dead space, and consequently decreased CO2<\/sub>\u00a0elimination capability. Retention of CO2<\/sub>\u00a0leads to increased venous carbon dioxide partial pressure (Pvco2<\/sub>) and Paco2<\/sub>, which ultimately increases arterial to end tidal CO2<\/sub>\u00a0gradient.\u00a0This helps further in reaching a correct diagnosis, especially in silent pulmonary embolism.<\/p>\n\n\n\n

In hemorrhage, changes in Petco2<\/sub>\u00a0reflect hemodynamic changes.\u00a0This allows not only prompt recognition of circulatory compromise but also more precise fluid resuscitation. Capnometry is the only continuous non\u2010invasive monitoring method in the prehospital setting which gives indirect information about tissue perfusion through its correlation with oxygen supply dependency during hemorrhagic shock. \u00a0It is also of value in predicting outcome from major trauma and trauma surgery where P(a\u2013et)co2<\/sub>\u00a0difference >10\u2005mm\u2005Hg can predict mortality even if blood pressure is normalised.<\/p>\n\n\n\n

Petco2<\/sub>\u00a0values can be measured in intubated and non\u2010intubated patients,\u00a0even though hyperventilation can alter measurements. The latter is successfully achieved with a nasal cannula,\u00a0especially in children.\u00a0Measurements taken through a facial mask are less accurate compared to a nasal cannula because of increased dead space, which decreases the amount of CO2<\/sub>\u00a0in expired air. \u00a0Oxygen supplementation can affect measurements of Petco2<\/sub>, but not significantly.<\/p>\n\n\n\n

However, there are some limitations in the interpretation of Paco2<\/sub>\u00a0on the basis of Petco2<\/sub>\u00a0measurements in critically ill patients. In any cardiorespiratory dysfunction, including breathing patterns which cause incomplete alveolar emptying, and some pharmacological interventions which are used in treating low flow states, P(a\u2013a)co2<\/sub>\u00a0and consequently P(a\u2013et)co2<\/sub>\u00a0difference increases due to ventilation\u2013perfusion mismatch. \u00a0These limitations do not support Petco2<\/sub>\u00a0measurements as a substitute for Paco2<\/sub>\u00a0measurements.<\/p>\n\n\n\n

Nevertheless, in the prehospital setting it is sudden changes of Petco2<\/sub>\u00a0that make capnometry an important tool for early detection of mainly circulatory and\/or ventilatory compromise in the critically ill, and not the relationship between Petco2<\/sub>\u00a0and Paco2<\/sub>. A sudden decrease in Petco2<\/sub>\u00a0is caused by sudden hypotension, sudden hyperventilation, massive pulmonary embolism, occlusion of the endotracheal tube, and disconnection or leakage in the artificial ventilatory system, while decreased CO2<\/sub>\u00a0production and hyperventilation cause a more gradual decline in Petco2<\/sub>.\u00a0A sudden increase in Petco2<\/sub>\u00a0is caused by a sudden increase in cardiac output, such as when spontaneous circulation returns after CPR, and following an injection of sodium bicarbonate, while hypoventilation and increased CO2<\/sub>\u00a0production cause more gradual increases in CO2<\/sub>. \u00a0Therefore, a final interpretation of Petco2<\/sub>\u00a0must be made after all parameters which affect measurements have been accounted for\u2014that is, ventilation and circulatory parameters, metabolism, and technical errors.<\/p>\n\n\n\n

Sublingual capnometry shows great promise for the early recognition of patients at risk of tissue hypoperfusion, hypoxia, and shock progression,\u00a0because it avoids disturbances of measurements caused by hyperventilation.<\/p>\n\n\n\n

Petco2<\/sub>\u00a0as an indicator of hemorrhage and tissue perfusion adequacy<\/h3>\n\n\n\n

In uncontrolled hemorrhage, cardiac output and blood pressure decrease. In early stages of hemorrhage, cardiac output decreases before changes in blood pressure can be detected. The relationship between cardiac output and Petco2<\/sub>, even in low flow states, makes Petco2<\/sub>\u00a0an indicator of changes in cardiac output. Hemodynamic changes lead to immediate changes in Petco2<\/sub>; this makes Petco2<\/sub>\u00a0a good indicator of an early stage hemorrhage due to early detection of changes in cardiac output.<\/p>\n\n\n\n

According to Fick’s principle, cardiac output equals the relationship between oxygen consumption and arteriovenous difference in oxygen concentration. Decreased cardiac output leads to decreased tissue oxygen consumption. Under physiological conditions oxygen consumption is constant because oxygen supply is greater than its consumption. A critical decrease in oxygen supply makes oxygen consumption supply\u2010dependent. This marks the initiation of anaerobic metabolism. Increased lactate, hypercapnia in gastric mucosa blood vessels, and increased arteriovenous difference in CO2<\/sub>\u00a0partial pressure all indicate tissue hypoperfusion and hypoxia at this point. Constant minute ventilation, changes in Petco2<\/sub>\u00a0correlate well with changes in oxygen consumption in hemorrhagic shock, and these changes also indicate the onset of oxygen supply dependency during hemorrhagic shock.\u00a0Fluid resuscitation increases oxygen consumption due to its increased delivery, which ultimately increases Petco2<\/sub>.<\/p>\n\n\n\n

Capnometry guided fluid resuscitation in uncontrolled internal hemorrhage<\/h2>\n\n\n\n
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The strategy of fluid resuscitation in hypovolemic shock caused by hemorrhage has been thoroughly debated in recent years. Aggressive fluid resuscitation dilutes clot factors, dislodges formed blood clots, and leads to further bleeding, especially in parenchymal organ trauma.\u00a0These factors are associated with increased blood loss and mortality, \u00a0so that aggressive fluid therapy may be harmful to patients with uncontrolled thoraco\u2010abdominal hemorrhage. However, aggressive fluid resuscitation is still considered appropriate in controllable hemorrhage (head and isolated extremity trauma) and in trauma patients without a palpable pulse.\u00a0In patients with uncontrolled thoraco\u2010abdominal hemorrhage, limited or hypotensive fluid resuscitation avoids detrimental effects of an early aggressive resuscitation, decreases mortality, and is currently considered more appropriate for short periods before surgical repair, even at the expense of tissue perfusion.<\/p>\n\n\n\n

Nevertheless, at an early stage of hemorrhage blood pressure can still be normal, even with substantial blood loss, and significant delay in fluid resuscitation, based on clinical signs and normal blood pressure measurement alone, can be detrimental because of decreased tissue perfusion and shock progression. Changes in Petco2<\/sub>\u00a0are an immediate reaction to hemorrhage, and any decrease in cardiac output would cause Petco2<\/sub>\u00a0to decrease, indicating hemorrhage in progress. It is an intriguing thought that capnometry guided fluid resuscitation could avoid delay in initiating fluid resuscitation due to prompt detection of these hemodynamic changes. It could also help to enhance tissue perfusion by adjusting further fluid administration, while keeping the blood pressure in appropriate ranges at the same time. This could significantly improve not only the clinical evaluation but also the fluid administration strategy in patients with uncontrolled hemorrhage.<\/p>\n\n\n\n

On the other hand, there is also a possibility of severe under\u2010resuscitation as a potential complication of hypotensive fluid resuscitation strategy in hemorrhagic shock. Capnometry could prevent this complication through prompt detection of a decrease in Petco2<\/sub>. In order to maintain normal or near normal Petco2<\/sub>\u00a0values, and tissue perfusion as well, increased amounts of infused fluids help to overcome this problem. More research is needed to confirm the usefulness of capnometry in decision making in patients with uncontrolled internal hemorrhage.<\/p>\n\n\n\n

Conclusion<\/h2>\n\n\n\n

In the prehospital setting capnometry is an important tool in the diagnosis, monitoring, and prediction of outcome. Even though there are some limitations of Petco2<\/sub>\u00a0monitoring in critically ill patients, sudden changes in Petco2<\/sub>\u00a0promptly reflect circulatory and\/or ventilatory compromise. Prompt reaction to acute hemodynamic changes in patients with constant minute ventilation, together with other measures of clinical evaluation, helps in timely detection of patients at risk in the prehospital setting. For that reason capnometry, especially sublingual, can also be an important non\u2010invasive monitoring method in patients with uncontrolled internal hemorrhage in the prehospital setting, together with the control of fluid resuscitation through indirect control over tissue perfusion, especially in limited fluid resuscitation. This could ultimately help to impede shock progression and further bleeding at the same time.<\/p>\n","protected":false},"excerpt":{"rendered":"

Capnometry is a non\u2010invasive monitoring technique which allows fast and reliable insight into ventilation, circulation, and metabolism. In the prehospital setting it is mainly used to confirm correct tracheal tube placement. In addition it is a useful indicator of efficient ongoing cardiopulmonary resuscitation due to its correlation with cardiac output, and successful resuscitation. It helps […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[27],"tags":[115],"_links":{"self":[{"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=\/wp\/v2\/posts\/3577"}],"collection":[{"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3577"}],"version-history":[{"count":5,"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=\/wp\/v2\/posts\/3577\/revisions"}],"predecessor-version":[{"id":4153,"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=\/wp\/v2\/posts\/3577\/revisions\/4153"}],"wp:attachment":[{"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3577"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3577"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.georgiaemsacademy.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3577"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}