END TIDAL CO2
While a relatively new practice in the emergency department (now fast becoming standard of care) the measure of end tidal CO2 (ETCO2) or capnography has been part of OR anesthesia since the 1970's in Europe and the 1980's in the US. The ED is the perfect place to benefit from capnography in the performance of procedures, and we should be using it on a regular basis. We will briefly review the physiology and interpretation along with applications, benefits and limitations of end tidal CO2 monitoring and analysis. First the formal definition of capnography is "the noninvasive measurement of the partial pressure of CO2 in exhaled breath expressed as the CO2 concentration over time." This allows ED physicians to better monitor the respiratory status, the metabolic status and resuscitation efforts in order to aid in the treatment and prevention of hypoxemia of our patients in real time.
The physiology of respiration or ventilation is simply the exchange of oxygen and carbon dioxide at a molecular and cellular level. We can measure the CO2 level in the exhaled breath and determine how efficiently or effectively the patient's pulmonary system or circulatory system is performing. The amount of exhaled CO2 varies depending on respiratory rate, heart rate, and disease pathology. A normal value is between 35-45 mm Hg. End tidal CO2 monitors measure partial pressure or concentration utilizing either mainstream or side stream configuration. In mainstream configuration, the measurement is taken directly from the airway via a hub sensor at the end of the endotracheal tube so it can be used for intubated patients. In the side stream configuration, the measurement is from respiratory gas via nasal or nasal/oral cannula by taking a small sample from the exhaled breath to a sensor in the monitor. This configuration can be used for intubated and non-intubated patients. We see these in prehospital patients all the time when brought in by various EMS units.
End tidal CO2 monitors are either qualitative or quantitative. Quantitative monitoring gives exact numeric value (e.g. 30 mm Hg) or an exact number and a waveform, where as qualitative monitoring reports the range in which ETC falls by a color change on the device. The equipment we are most familiar with that uses the qualitative reporting is the colorimetric ETCO2 detector that is applied to the end of the ETT during intubation that uses litmus paper to turn colors (<3 mm Hg the paper is purple, >15 mmHg the paper is yellow) if the ETCO2 values are in the proper range, indicating proper tube placement.
Besides color change as a visual indicator there is the ETCO2 waveform, which has 4 phases.
Phase 1 Dead space ventilation. This represents the beginning of exhalation.
Phase 2 Ascending phase. This represents the rapid rise in CO2 concentration in the breath stream.
Phase 3 Alveolar plateau. This represents the CO2 concentration reaching uniform level in entire breath.
Phase 4 Inspiratory phase.
· ETT Placement
· Monitoring of tube placement during transport
· Indicator of return of spontaneous circulation (ROSC)
· Determining adequacy of ventilation
· Titrating ETCO2 in head injury
· Gauging effectiveness in of resuscitation and progression during cardiac arrest
· Monitoring patients for inadequate respirations during procedural sedation
Verification of endotracheal tube placement is of utmost importance, as incorrect placement will have disastrous consequences. ETCO2 monitoring can help the clinician recognize improper ETT placement. A normal 4-phase waveform indicates the tube is through the vocal cords. A flat line waveform usually indicates an esophageal placement. A flat waveform can also be a result of prolonged cardiac arrest, poor pulmonary blood flow, tube obstruction or malfunction of equipment. It is important to realize that in a person with spontaneous circulation the sensitivity or accuracy approaches 100% for tracheal placement, the sensitivity in a cardiac arrest varies from 62 to 100%. Overall, capnography seems to be very accurate in determining tracheal intubation tube location. Capnography's specificity for esophageal intubation in cardiac patients is not certain. ETCO2 also helps monitor patients during transportation. If ETCO2 goes down or you lose your good 4-phase waveform, it could indicate a dislodged tube.
In 1998, there was a landmark study that showed human ETCO2 correlated with cardiac output during CPR. During cardiac arrest, the ETCO2 reflects pulmonary blood flow and can be used as a gauge of the effectiveness of cardiac compressions. A peak or sudden rise in the ETCO2 is the earliest sign of ROSC and may precede the return of a palpable pulse or blood pressure. This rise is the CO2 that accumulated during CPR in the cells being transported to the lungs and exhaled. The AHA guidelines for cardiac resuscitation emphasize chest compression without interruption until a perfusing rhythm has been reestablished. Monitoring capnography waveforms and ETCO2 values can allow for uninterrupted CPR by virtually eliminating the need to stop compressions to check for a pulse. Prognosis during an arrest: if after 20 minutes of CPR your ETCO2 is less than or equal to 10 mm Hg, you can accurately predict death in the adult patient.
Use of capnography during trauma or increased cranial pressure (ICP) allows for monitoring the patient to avoid accidental hyperventilation, decreasing the chance of further damage to the brain. Remember high CO2 (PaCO2 >50) results in cerebral vasodilatation and low CO2 (PaCO2 <30) results in cerebral vasoconstriction. Both of these states are harmful, so by using ETCO2 monitoring, you can keep the patient eucapnic decreasing chance of further injury by giving proper ventilations.
In the spontaneously breathing patient, we can use ETCO2 monitoring to keep apprised of real time ventilations and any adverse events, so we can make changes or adjustments before catastrophe occurs. Some applications in the spontaneously breathing patient can be:
· Rapid assessment in the critically ill patient
· Determining the adequacy of ventilation in the seizing, obtunded or unconscious patient
· Using it to provide prognostic indicators for patients with septic shock or sepsis
· Determining the adequacy of ventilation during procedural sedation
In the critically ill or seizing patient, a "normal waveform denotes a patent airway and spontaneous breathing", "normal ETCO2 levels (35-45 mm Hg) signify adequate perfusion. Capnography is the only monitoring modality that is reliable and accurate in the actively seizing patient because the capnography waveform is determined entirely by respiratory activity and not confused by muscle activity or movement artifact.
Procedural sedation is not a benign event and the medications used can cause serious respiratory depression that can be very subtle to notice until too late. Watching a pulse ox level can be misleading and cause a false sense of security as the patient can keep a normal saturation level until it falls precipitously. Capnography can give us a better modality in which we see respiratory and circulatory performance instanteously. It allows us to notice trends that might be potentially life threatening, if not corrected, as soon as they start. Respiratory depression as a result of over sedation will show as an abnormally high or low ETCO2 long before a falling oxyhemoglobin (pulse ox) can be detected especially in patients receiving supplemental oxygen. Because of this fact, capnography is strongly recommended in all patients undergoing procedural sedation receiving supplemental oxygen.
In conclusion, capnography is a valuable tool in providing instantaneous information about ventilation. It is versatile in monitoring both the intubated and non-intubated patient as well as the conscious and unconscious patient. There are two ways we can collect ETCO2, one is mainstream and the other is side stream. Colorimetric detectors are qualitative and report a range value where as capnometry is quantitative and give a specific ETCO2 value. Normal ETCO2 levels or values can range between 35 mm Hg to 45 mm Hg. Hyperventilation will drop values and hypoventilation will raise values. A capnogram consists of a 4-phase waveform and is valuable in the evaluation of ventilation quality and effort. ETCO2 monitoring can be performed on patients in cardiac arrest to determine the adequacy of chest compressions and the return of spontaneous circulation (ROSC). During cardiac arrest, ETCO2 reflects pulmonary blood flow and can be used as a gauge of the effectiveness of cardiac compressions. As effective CPR leads to a higher cardiac output, ETCO2 will rise. ETCO2 is also the earliest indicator of the ROSC. When the heart restarts, the dramatic increase in cardiac output, and resulting increase in perfusion, leads to a rapid increase in ETCO2. Capnography can rapidly detect common adverse events associated with procedural sedation more efficiently and rapidly than pulse ox. Limitations of capnography include difficulty interpreting results if the patient does not have a pure ventilation, perfusion or metabolic problem.
*References available upon request.