When is Capnography Useful in the ED? Part II

Authors: Brit Long, MD (@long_brit, EM Attending Physician, San Antonio, TX) and Michael Vivirito, RN, CEN (Joint Base Elmendorf Richardson Medical Center, Emergency Medicine) // Edited by: Alex Koyfman, MD (@EMHighAK)

A 33-year-old female with a history of type 1 diabetes presents with polyuria, nausea/vomiting, and generalized weakness. Her blood glucose is 420. She is tachycardic and tachypneic, but the rest of her VS are normal. You place her on waveform capnography, which demonstrates EtCO₂ of 16 mm Hg.

 You order a liter of LR and ask the nurse to obtain a VBG, ECG, CBC, urinalysis, HCG, electrolyte panel, and ketones to start.  You diagnose DKA, and the nurse looks at you in surprise and asks about the capnography.

We know capnography may be utilized for cardiac arrest, verification of endotracheal tube placement, and procedural sedation, but what other uses are possible? For Part I, see http://www.emdocs.net/capnography-useful-ed-part/

EtCO₂ has been studied in critically ill patients and in trauma.^1-7 EtCO₂ correlates with blood pressure, lactate, pH, bicarbonate, and base excess. Studies have demonstrated that mortality increases as EtCO₂ levels decrease.^8-10  This post will provide some background on capnography use in trauma, metabolic acidosis, sepsis, PE, obstructive lung disease, and fluid responsiveness. You may be surprised at what the literature reveals…

Trauma

In trauma, EtCO₂ levels less than 25 mm Hg are associated with decreased blood pressure and cardiac output, and more importantly, increased mortality.^11,12  For patients with head trauma, high serum carbon dioxide results in cerebral vasodilation, while lower values result in vasoconstriction.¹² Capnography can be used to prognosticate outcomes and risk of mortality in severe trauma.  In a study evaluating mortality in blunt trauma patients undergoing intubation, survivors on average demonstrated EtCO₂ 4 mm Hg higher than nonsurvivors (30.8 mm Hg in survivors and 26.3 mm Hg in nonsurvivors).¹³ In penetrating trauma, EtCO₂ demonstrates a strong inverse relationship with lactate, with an odds ratio of 20.4 for requiring emergent operation.¹⁴ If levels are less than 30 mm Hg, increased risk of severe injury is likely, and decreased levels also predict need for blood transfusion.^15,16 Capnography should not be used in isolation as a marker for injury, but only with other markers for severe trauma.

Metabolic Acidosis

Yes, that’s right…Capnography can be used in the diagnosis of metabolic acidosis, predominantly associated with diabetic ketoacidosis (DKA) and gastroenteritis.^17-22 As acidosis worsens, bicarbonate decreases and results in increased minute ventilation for respiratory compensation.^17-22 This decreases EtCO₂. The lower the EtCO₂ the more severe the acidosis, and EtCO₂ correlates with venous pH and bicarbonate.^17,18,22 There are a variety of cutoff levels for EtCO₂. Solmeinpur et al. found a sensitivity of 90% and specificity of 90% for DKA with a capnography value of 24.5 mmHg for adults.¹⁸ Dierde et al. found 29 mm Hg in pediatric patients with DKA demonstrates a sensitivity of 83% with specificity of 100% for diagnosis.¹⁹ A level of 36 mm Hg demonstrates a sensitivity close to 100%, while less than 20 mm Hg may possess a specificity of 100% .²³ For acidosis in general, levels less than 25 mm Hg and greater than 36 mm Hg display a specificity of 84% for metabolic acidosis and sensitivity of 98% for excluding metabolic acidosis.¹⁷ As you can see, further study is needed in determining optimal values, but low EtCO₂ supports a diagnosis of DKA or metabolic acidosis. 

Sepsis

Sepsis is a common diagnosis, and emergency physicians use a variety of assessments to assist with diagnosis and resuscitation, such as lactate.^24-26 Similar to metabolic acidosis, capnography and EtCO₂ decrease as lactate increases, and literature suggests capnography levels can be used as a marker for resuscitation in sepsis.^27-31 One study finds EtCO₂ levels less than 25 mm Hg to occur in 78% of patients, and this level was associated with higher mortality.²⁷ Other literature finds a level less than 35 mm Hg to correlate with lactate > 4 mmol/L and SOFA score > 2.²⁸ Capnography is associated with serum bicarbonate, lactate, and anion gap, and EtCO₂  levels are significantly lower in those patients who suffer mortality. A level less than 31 mmol/L demonstrated a 93% sensitivity for mortality in sepsis.^30,31 Hunter et al. evaluated EtCO₂  as a prehospital and triage marker, finding levels less than 25 mm Hg to be a reliable marker for sepsis, with area under the curve (AUC) 99% for sepsis, 80% for severe sepsis, and 70% for mortality.²⁷ However, other studies suggest capnography may not correlate with lactate and outcomes.²⁹ More research is needed, and a normal capnography reading should not be used to rule out sepsis.²⁹ Low EtCO₂ levels can be used to assess resuscitation and prognosticate in conjunction with other measures.

Pulmonary Embolism (PE)

PE can be a challenging diagnosis, often relying on clinical gestalt and risk stratification.³² Capnography can demonstrate a specific type of waveform in patients with PE. While ventilation remains approximately the same, perfusion is decreased in the segment supplied by the affected blood vessel.^33-38 Alveolar dead space increases, and expired CO₂ decreases. This increases blood PaCO₂ and the EtCO₂ gradient.^33-38 The waveform may display flattened alveolar plateau and decreased EtCO₂.

A meta-analysis finds sensitivity 80%, specificity 49%, and area under the summary receiver operating characteristic curve (AUROC) 84% for diagnosis of PE using capnography.³⁷ The authors of this study state capnography may be used to rule out PE if used in patients stratified as low risk for PE.^37-39 When combined with Wells score < 4, EtCO₂ > 36 mm Hg possesses a negative predictive value of 98%.^39,40  Another study sought to combine clinical gestalt, D dimer, and capnography, with a level of 32 mm Hg and low risk by stratification possessing a sensitivity of 100% for ruling out PE.⁴¹  Other areas of research include alveolar dead space measurement and alveolar dead space fraction via capnography, but these are not ready for primetime.^38,39,42-44 Capnography for diagnosis of PE requires more study, especially the potential to combine this with clinical gestalt and/or D dimer.

Asthma, COPD, and Heart Failure

Pulmonary conditions such as asthma, COPD, and heart failure can also be challenging in the ED. Along with clinical assessment, EtCO₂ can provide important information with regards to disease severity and assessing treatment response.^45-49 Bronchospasm and airway obstruction demonstrate a shark-fin appearance (with steep phase III).^49,50 Early in disease, EtCO₂ will slightly decrease as the patient begins to compensate. However, if the exacerbation becomes severe and the patient worsens, EtCO₂ will increase.^2-6,51-54 In prehospital patients with asthma or COPD, EtCO₂ high levels (> 50 mm Hg) and low levels (< than 28 mm Hg) are associated with greater rates of intubation, ICU admission, and mortality.^51-55  Similar to the prior conditions, EtCO₂ should only be used as an adjunctive measure to follow resuscitation and treatment. It may help diagnose or suggest a specific condition when used with history, exam, and other assessments.

Seizures

EtCO₂ can assess ventilation in patients who have seized and are post-ictal. Similar to procedural sedation, capnography can detect hypoventilation faster than pulse oximetry (which missed 5 patients with pulse oximetry > 97% and EtCO₂ greater than 70 torr).⁵⁶

Fluid Responsiveness

Investigators have sought a reliable tool to assess fluid responsiveness, and recently passive leg raise while monitoring cardiac output has demonstrated promise.^57-59  EtCO₂ has been evaluated in mechanically ventilated patients, and patients with fluid responsiveness demonstrate an increase by at least 5% in EtCO_2.^57-60 Increase by 2 mm Hg demonstrates high specificity (93-96%), but sensitivity is 60%.⁶⁰   No increase or increase by less than 2 mm Hg demonstrates a NPV of 86%.^61,62 A recent study finds AUC 83% for predicting fluid responsiveness using capnography, and capnography outperformed pulse pressure variation, systolic blood pressure, heart rate, and mean blood pressure in patients undergoing mechanical ventilation.⁶³

Capnography Interpretation

We have discussed a significant number of conditions, and capnography provides important information on patient status. However, it should only be used as an adjunct to the rest of your clinical assessment. The table below shows what conditions can change EtCO₂. Remember, there are several components of the waveform that should be considered (http://www.emdocs.net/capnography-useful-ed-part/).^1-7

Capnography Limitations

Capnography possesses several important limits. This assessment works best for patients with a specific ventilation, perfusion, or metabolism issue. Patients with mixed pathophysiology (yes, our patients in the ED) create challenges for interpretation.^1-7,64,65 Low tidal volumes, poor perfusion, the specific type and location of the device (mainstream versus sidestream devices), and false readings (stomach content, carbonated beverages) can hamper the use of this test.^1-7,64-67 As discussed, capnography should only supplement other markers and clinical assessment. It is not ready for use in isolation.

Key Takeaways

– Capnography possesses many uses in the ED, but many of these are under study.

– EtCO₂ is reliable in monitoring ETT placement, ETT confirmation, monitoring compression adequacy in CPR, evaluating for ROSC, and detecting hypoventilation in procedural sedation (see Part I).

– EtCO₂ is associated with lactate levels: as lactate elevates, EtCO₂ typically decreases.

– In states with acidosis and poor perfusion, such as DKA, hemorrhage, trauma, and sepsis, capnography can be used as an adjunct to clinical assessment.

– EtCO₂ is promising for use in PE, specifically as a tool for further risk stratification, but it is not ready for primetime.

– Capnography can be used to assess treatment response in conditions with airway obstruction, as well as for hypoventilation in the post-ictal state.

– Elevation > 2 mm Hg with IV fluid load may suggest fluid responsiveness.

– Further study is needed for this promising clinical marker.

References/Further Reading

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