Editor’s note: Capnography is one of the most powerful tools in the EMS toolbox. It tells us what’s happening with a patient’s ventilation, perfusion and metabolism — instantly and continuously. But to use it effectively, you have to understand what the waveforms are telling you. Learn how to decode capnography with our Infographic: Understanding normal and abnormal capnography waveforms
Medical professionals agree that capnography is an important non-invasive, low-risk tool for treating and monitoring patients, but it should always be interpreted alongside the patient’s overall clinical picture, vital signs and other assessment data to make informed decisions.
| MORE: Infographic: Understanding normal and abnormal capnography waveforms
“Capnography gives different information that can paint a more detailed clinical picture, especially in conjunction with other vital sign markers such as SpO2,” noted Dr. Maria Mulligan-Buckmiller, MD, a Johns Hopkins University EMS fellow. “It can also be a more sensitive and timely indication of worsening status, for example, in hypoxia resulting from inadequate respirations or even apnea, the end tidal will likely become abnormal more promptly than the SpO2.”
“When you look at end-tidal, you’re seeing the exhaust from cellular respiration,” noted Dr. Rishi Kundi, chief of vascular and endovascular trauma surgery at the R Adams Cowley Shock Trauma Center on the campus of the University of Maryland Medical Center in Baltimore and medical director for Maryland’s Go-Team. “End-tidal depends on oxygen delivery to tissues through arterial perfusion, aerobic cellular respiration throughout the body, delivery of CO2 to the venous system, delivery of venous blood to the heart and ventilation.”
“A low end-tidal means, among other things, you’re not perfusing well. You’re not producing normal levels of CO2 because you’re not delivering normal levels of oxygen to the tissues,” Kundi added. “Poor perfusion manifests in the prehospital context primarily as cold and pale, potentially mottled extremities and unresponsiveness. Ultimately, poor perfusion is accompanied by an absence of the usual signs of life.”
According to Mulligan-Buckmiller, who works at the Johns Hopkins Howard County Medical Center in Columbia, Maryland and is involved in the medical direction of Howard County(Maryland) Fire and Rescue Services, the uses for capnography have greatly expanded in recent years, including assessing:
- Adequate respirations
- Endotracheal tube placement
- Response and effectiveness of mechanical ventilation
- Achievement of return of spontaneous circulation (ROSC)
- Cerebral vasoconstriction/vasodilation in head trauma
- Perfusion status in both trauma and shock situations
Identifying shock with capnography
In shock, capnography is a critical, non-invasive tool that provides real-time data on tissue perfusion and ventilation by continually monitoring the patient’s end-tidal CO2 (EtCO2), the CO2 level in exhaled air. A decrease in EtCO2 can signal worsening shock due to reduced blood flow and tissue perfusion. A normal range should be 35-45 mmHg for adults; levels below that in a shock patient indicate significant distress and the need for aggressive management.
“In shock, an end tidal <28.5 mmHg can indicate need for massive transfusion protocols. It has been thought that end tidal can be a better indicator of poor prognosis even over systolic blood pressures,” Mulligan-Buckmiller said. “A decreasing end tidal can clue you in to inadequate perfusion, worsening volume status, failing cardiac output even before skin color changes, hypotension on blood pressure, altered mental status.”
In the prehospital setting, a patient’s CO2 will improve/increase in shock situations when the volume and perfusion status of a patient improve, according to Mulligan-Buckmiller.
“This can be accomplished in several ways:transfusion of blood, transfusion of fluids, vasopressors, tamponade of bleeding,” Mulligan- Buckmiller said. “As the shock is treated, the ability of the body to deliver the CO2 to the lungs should increase.”
Kundi emphasized the steps to improve perfusion are the steps of advanced prehospital care. “Everything we do- fluid, blood, pressors and so on — aims to preserve perfusion to the body in the order of brain, heart, viscera and extremities,” Kundi said.
Capnography caveats
“Therapies must be tailored to the individual patient. Examples of perfusion-based strategies could include intravenous crystalloids, blood (products) and vasopressors. If sepsis is suspected as the source of shock, then antibiotics can be considered if those medications are in the EMS/prehospital scope of practice,” Dr. Benjamin Lawner, DO, MS, EMT-P, University of Maryland School of Medicine, Department of Emergency Medicine, associate professor said. “It is critically important to take patients’ specific factors into account, since medical history affects the response to any therapy. A patient with decompensated heart failure, for example, may not benefit greatly from excessive intravenous fluids.”
Lawner is also the medical director for Baltimore City Fire Department and Maryland ExpressCare Critical Care Transport Team, University of Maryland Medical Center ground and aviation critical care service.
Mulligan-Buckmiller notes that if there is a V/Q mismatch, dead space ventilation or pulmonary perfusion concerns, end tidal might not appropriately correlate with a, arterial blood gas (ABG).
“Before you rely on end tidal especially in terms of directing titration of ventilators, it is important to confirm that the ABG and the end tidal are similar. The advantages of end tidal over an ABG is the availability of continuous data, but if the treating team can perform an ABG, these should be performed at regular intervals to ensure the reliability of end tidal regarding the true PCO2 of the patient,” Mulligan-Buckmiller added.
“There have also been several studies showing that end tidal can be drastically different (>10mmHg) than the PCO2 level, causing increased hypercarbia and resulting acidosis in patients in shock. This underlines the point that while end tidal can be a helpful marker of changing status, improving perfusion it should not be taken as a reliable indicator of the true acid-base status of the patient alone,” Mulligan-Buckmiller said. “Other studies have shown that this discordance will only increase as shock progresses, underlining the point that the end tidal should be seen as additional data that should not take priority over the holistic clinical picture of the patient.”
Lawner added, “it is more about augmenting perfusion than improving EtC02. Patients who are significantly acidotic may exhibit rapid breathing to compensate. Respiratory compensation lowers the level of EtC02 and may not need to be immediately corrected depending upon the treatment scenario.”
| MORE: Capnography: A vital sign for every EMS patient
Pre-hospital and interfacility transfer capnography considerations
“Capnography is mandatory for intubated patients and is used to monitor patient perfusion while enroute to definitive care. Existing prehospital and interfacility transport protocols suggest that values can inform treatment strategies for patients experiencing shock and respiratory related emergencies,” Lawner said.
“Capnography is a unique modality in that it is a surrogate marker for patient perfusion,” Lawner said. “It is a continuous variable that can be ‘trended’ and therefore provides information about responses to treatment.”
Additional uses for capnography in EMS
In the prehospital setting, capnography can be used for:
- Confirming endotracheal tube placement. Capnography provides an objective measurement of exhaled carbon dioxide (CO2), which helps verify that an ET tube is correctly placed in the trachea and not the esophagus.
- This is crucial during intubation to ensure proper oxygenation and ventilation.
- Monitoring CPR quality. Capnography can be used to assess the effectiveness of chest compressions during CPR by measuring the level of CO2 in exhaled air.
- A sudden increase in CO2 levels can indicate (ROSC).
- It can also help identify inadequate chest compressions, prompting adjustments to improve CPR quality.
- Assessing respiratory and circulatory status. Capnography can help detect changes in patient’s ventilation, perfusion and metabolism by monitoring CO2 levels.
- Changes in the CO2 waveform can indicate issues like airway obstruction, pulmonary embolism or inadequate tissue perfusion.
- This information allows clinicians to make informed decisions about treatment and patient management.
- Additional capnography medical monitoring uses. Capnography is a valuable tool for monitoring patients with various conditions, including respiratory distress, shock and opioid overdose.
- Uses include both intubated and spontaneously breathing patients.
Lawner suggested when applying EtCO2 values to an individual clinical scenario, consider asking focused, closed ended questions:
- Is the endotracheal tube still in place?
- Is the patient in shock or is there evidence of low perfusion?
- Is there evidence of severe obstructive disease (e.g., asthma)?
Mulligan-Buckmiller emphasized, “capnography should be used in all patients with concern for respiratory impairment, in all patients with supraglottic or endotracheal airways, all patients on mechanical ventilation, all patients who received sedation/medication that could impair respiratory status, patients with head trauma (especially in head bleeds), patients in cardiac arrest and shock.”
“Within the Maryland EMS system, EtC02 is an ALS skill. Typically, ALS clinicians have access to end tidal equipment. Speaking for systems that I am directly involved with, all ALS transport and critical care ambulances/helicopters can monitor capnography,” Lawner said. “I think it is important to look at how BLS clinicians can utilize this important technology. For example, end tidal waveforms can confirm an adequately placed supraglottic airway.”
