Capnography: Not Just for Cardiac Arrest and Sedation Anymore

Five conditions where this respiratory assessment could come in handy.

You just walked in to your evening shift in the small, ten bed community ED. Two patients await you:

• The first is a 22-year-old male who presents with tachycardia, nausea, and vomiting. He has a history of type 1 diabetes, with a home glucose reading of 530 mg/dL. You place him on capnography for an end-tidal carbon dioxide (EtCO2), which demonstrates a reading of 15 mm Hg. You quickly diagnose diabetic ketoacidosis while ordering a liter of lactated ringer’s and other laboratory tests.
• Your second patient is a 56-year-old female with right flank pain, dysuria, and frequency. She is tachycardic, hypotensive, and febrile, and urinalysis shows signs consistent with urinary tract infection. You start a bolus of intravenous fluids with IV ciprofloxacin. You place the patient on capnography, finding an EtCO2 of 18 mm Hg.

Capnography 101
Capnography is the noninvasive measurement of carbon dioxide partial pressure, displayed in several different formats: colorimetric/qualitative, quantitative (number), and as a function of time with waveform. EtCO2 has a variety of uses in emergency medicine, with further uses coming to light in recent literature. The normal range for EtCO2 approximates 35-45 mm Hg, and the normal capnography waveform consists of several phases based on inspiration or expiration [^1-7].

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Inspiration starts with phase 0, with clearing of CO2. Exhalation begins with phase I and consists of the anatomical dead space, followed by phase II which is a rapid rise in CO2 as the breath reaches the upper airway. Phase III of the waveform during exhalation is the alveolar plateau, with the highest value of EtCO2 during the respiratory cycle [^1-7].

The waveform and absolute value of EtCO2 is useful in a variety of conditions. Capnography shines in verifying and monitoring placement of endotracheal tube placement. Other than direct visualization, this is one of the best ways to confirm placement, rather than fogging of the tube, chest wall movement, or bilateral breath sounds [^1-3,8,9]. In cardiac arrest, EtCO2 can be used to assess for return of spontaneous circulation (ROSC) with an abrupt increase in EtCO2 readings to 30-40 mm Hg [^1-4,10-16]. Values greater than 20 mm Hg during resuscitation also predict ROSC, while values less than 10 mm Hg indicate poor prognosis and poor chest compression quality [^10-16]. The final common use in the ED includes procedural sedation, in which waveform capnography allows assessment of ventilation, and it may detect hypoventilation and apnea earlier than clinical assessment or pulse oximetry [^17-20].

But what about our two patients: one with DKA and the other sepsis? How else can capnography be used?

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Other Potential Uses for Capnography
Capnography can be used in many other conditions to help you with diagnosis and clinical assessment, especially in those who are critically ill.

The 22-year-old male’s VBG returns with pH 7.1, bicarbonate 12, and potassium 3.7, and your bedside glucose POC reveals a serum glucose of 498.

Diabetic Ketoacidosis (DKA)
The patient in Case 1 has a presentation consistent with DKA. Capnography can be used for diagnosis of metabolic acidosis and DKA. As acidosis worsens, bicarbonate decreases, resulting in respiratory compensation to blow off CO2 [^21-27]. This decreases EtCO2 in correlation with severity of metabolic acidosis and DKA. However, keep in mind that a specific number should not be used to rule in or rule out DKA [^21-27]. Studies suggest levels greater than 36 mm Hg may be able to rule out metabolic acidosis or DKA, while levels less than 25 mm Hg demonstrate a specificity over 80% for diagnosis [^21], and levels less than 20 mg Hg may be able to rule in DKA [^27].

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The patient from Case 2 has a presentation concerning for sepsis from pyelonephritis. Her lactate returns at 4.2 mmol/L, and antibiotics are running…

Sepsis
Similar to metabolic acidosis from DKA, acidosis and severity of sepsis correlate with elevation in lactate and worse mortality [^28-32]. As lactate and acidosis worsen, EtCO2 decreases. Levels less than 35 mm Hg correlate with lactate > 4 mmol/L [^29], while levels less than 31 mm Hg correlate with higher mortality [^31,32]. However, EtCO2 is not ready for primetime in sepsis. Let’s face it, your clinical assessment and gestalt still remain at the top for these patients.

Trauma
If EtCO2 correlates with lactate and acidosis, could EtCO2 be used in trauma? EtCO2 values correlate with lactate and need for surgery in penetrating trauma [^33], with levels less than 30 mm Hg predicting risk of severe injury and need for blood product transfusion [^34,35]. Levels less than 25 mm Hg are associated with decreased cardiac output and worse mortality [^36,37].

Let’s say we’re trying to figure out whether your patients will respond to further IV fluid resuscitation. Sure, you can complete a passive leg raise, but you don’t have a way to evaluate cardiac output in your small community ED. Can capnography help?

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Volume Responsiveness
Several tools have been looked at for predicting whether patients will respond to IV fluids. Recently, passive leg raise while cardiac output is monitored looks reliable, but most EDs don’t have cardiac output monitors sitting around. However, most EDs do have EtCO2. An increase by 2 mm Hg or 5% of the baseline value with passive leg raise can rule in fluid responsiveness, but failure to increase EtCO2 does not suggest the patient won’t respond to fluid [^38-41]. Other studies suggest EtCO2 outperforms pulse pressure variation, heart rate, and blood pressure as markers for volume responsiveness in mechanically ventilated patients [^42-44].

Both patients show an elevation greater than 5 mm Hg in EtCO2 with passive leg raise, and you order more IV fluid resuscitation. Your management and IV fluid provided has improved both patients’ hemodynamic status and capnography readings. You call the hospitalist for admission.

What about other respiratory conditions, such as pulmonary embolism (PE), asthma, or chronic obstructive pulmonary disease (COPD)?

Pulmonary Embolism
PE isn’t always an easy diagnosis. Evaluation in the ED usually takes the form of risk stratification and clinical gestalt. PE can demonstrate several key findings on waveform. In the affected segment of the lung, ventilation usually remains the same, though perfusion decreases. This increases blood partial pressure of CO2 (PaCO2) and the gradient between PaCO2 and EtCO2 (EtCO2 decreases with PE) [^1-6]. Further study is needed for PE before everyday use in the ED. Studies suggest it may be useful if combined with clinical gestalt or Wells score to rule out PE [^45-48]. For example, combined Wells score < 4 and EtCO2 greater than 36 mm Hg has a negative predictive value for PE of 98% [^47,48]. Another study suggests low risk based on clinical gestalt combined with EtCO2 greater than 32 mm Hg has a sensitivity of 100% for PE rule out [^49].

Asthma
A 16-year-old female is quickly brought to Bed 2 with RR 30. She has decreased breath sounds bilaterally, but some small wheezes. In gasping, two-word sentences, she says she has required intubation for her poorly-controlled asthma in the past. You start her on continuous nebs, steroids, and oxygen. Her EtCO2 is 52 mm Hg, concerning for hypercarbia.

Obstructive respiratory conditions such as asthma and COPD demonstrate a specific appearance on waveform capnography, similar to a shark fin, due to bronchospasm and airway obstruction [^50-55]. Early in obstructive airway exacerbation, tachypnea may result in decreased EtCO2 values, while severe exacerbations or patients with hypercarbia and respiratory acidosis demonstrate elevated EtCO2 levels [^55-57]. Similar to other diseases discussed, capnography may be useful in combination with other clinical assessments.

After your treatments, her EtCO2 has improved to 41 mm Hg, and her respiratory rate has actually decreased to 24. Her lungs now have diffuse wheezes, but at least now you can hear the wheezing! You prepare to admit her to the hospital, as her history and current status are concerning for poor outcome.

Take Home Points
Where does that leave us for capnography? This monitoring device has many potential uses, but several are still under study and require further refinement. EtCO2 is reliable in ETT placement, ETT confirmation, evaluation for ROSC, and monitoring for hypoventilation in procedural sedation. Literature suggests it correlates with lactate and acidosis: as lactate increases and acidosis worsens, EtCO2 decreases. Capnography is promising for use in conditions with metabolic acidosis (DKA, sepsis), PE, obstructive lung disease, fluid responsiveness, and trauma, but it should only assist your clinical judgment at the bedside in its current form.

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