By Shawna Renga, NRP
The application of waveform capnography in prehospital care has expanded far beyond its most ubiquitous use as the gold standard for endotracheal tube placement confirmation. Capnography as a tool during cardiopulmonary resuscitation is now considered an integral part of monitoring the quality of chest compressions and detecting a return of spontaneous circulation. Additional applications during cardiac arrest management include using capnography to influence decisions to terminate resuscitation, as well as identifying those patients who may have a higher probability of achieving return of spontaneous circulation by evaluating end tidal carbon dioxide measurements at the onset of resuscitation.
Capnography review
Capnography measurements are achieved by shining an infrared light through a stream of air in an endotracheal tube or oxygen delivery device onto a sensor. By measuring the amount of light that hits the sensor, capnography devices are able to determine the partial pressure of carbon dioxide present in the stream of air.
In the pre-hospital setting, capnography capabilities have been largely integrated into portable cardiac monitors, where capnography readings are displayed as a waveform or capnogram. These devices also provide a measurement of the end-tidal CO2 reading (ETCO2), or the amount of carbon dioxide in air at the end of the exhaled breath.
Analysis of both the waveform and the ETCO2 measurement provides important information on the respiratory, circulatory and metabolic status of the patient. The shape of the waveform reflects the emptying pattern of the alveoli and provides information on the ventilation-perfusion ratio.
It has also been shown that end tidal carbon dioxide values have a strong correlation with cardiac output [1]. The combination of these factors makes ETCO2 monitoring a valuable addition to cardiac arrest management protocols.
Capnography as a tool to predict outcomes and guide care
For patients who present in cardiac arrest, early measurement of ETCO2 during resuscitation may be a reliable predictor of the likelihood of achieving a sustained return of spontaneous circulation [2]. However, there are many factors that influence capnography values in the cardiac arrest patient.
Cardiac arrests with a respiratory cause generally have much higher ETCO2 readings than those of cardiac origin. The initiation of bystander CPR, and the quality of bystander compressions also strongly influences ETCO2 values at the onset of capnography monitoring. ETCO2 values of less than 10 mm Hg throughout resuscitation appear to have a strong correlation with mortality, while readings consistently higher than 10 mm Hg correlate with ROSC and positive neurologic outcomes [3].
Trending ETCO2 values provides additional information on the likelihood of achieving ROSC. When ETCO2 values remain consistently above 10 mm Hg for the first 20 min of resuscitation the probability of achieving ROSC is high.
Capnography readings that consistently fall during resuscitation, as is common in asystolic and pulseless electrical activity arrest, have been shown to be predictive of death [5]. Patients presenting in asystole or PEA have been shown to have very low likelihood of achieving a return of spontaneous circulation, regardless of the initial ETCO2 values.
Although evidence suggests the prognostic value of capnography is significant, it should not be used as an independently reliable predictor of cardiac arrest outcome. Rather, ETCO2 values should be viewed in context with the cause of arrest, presenting rhythm, presence of bystander CPR, and total patient down time prior to EMS arrival when determining whether or not to terminate resuscitation.
Capnography may also help identify patients for whom extracorporeal life support (ECLS) may be appropriate. Extracorporeal techniques require vascular access, specialized equipment and allow blood to bypass the heart entirely. This allows for the restoration of perfusion and may provide additional time for resuscitation and reversal of the original cause of arrest. Although very new in the management of out of hospital cardiac arrest, the decision to initiate ECLS or ECMO may be guided by ETCO2 levels. ETCO2 readings of less than 10 mm Hg suggest that ECLS may not be beneficial to the long term survival of the patient [5].
Capnography and airway management
Despite many studies on the subject, there is little to no high quality data on which airway management approach is optimal during cardiac arrest management [6]. In the presence of a well trained and experienced provider, a properly inserted endotracheal tube allows for the most accurate monitoring and management of the airway and ventilation of a patient in cardiac arrest. Waveform capnography is a highly reliable method of confirming and monitoring the placement of an endotracheal tube.
In the prehospital environment there are many factors, including provider experience and austere environments, which make successful endotracheal intubation difficult. Supraglottic airway devices offer many of the advantages of an endotracheal tube with regards to airway protection and capnography monitoring, and can usually be inserted without an interruption in chest compressions.
The BLS approach of providing ventilations using two-person bag-mask device technique can also be utilized, with adaptors allowing for both ETCO2 readings and continuous waveform capnography. The most effective approach to airway management during cardiac arrest is variable and dependent upon provider experience, local protocol and the equipment available.
Ensuring high-quality chest compressions
The use of continuous waveform capnography as a tool for monitoring chest compressions during CPR is now considered standard practice for prehospital ALS providers. As previously stated, ETCO2 values are reflective of both cardiac output and pulmonary blood flow. Normal ETCO2 values of 35-45 mm Hg reflect both adequate cardiac output and adequate pulmonary blood flow ion a healthy patient. During chest compressions, ETCO2 values will be comparably much lower, as even high quality compressions generate minimal cardiac output. While the increase in ETCO2 values generated by compressions of appropriate rate and depth may seem insignificant, even slight increases in cardiac output are critical to the positive outcome of a patient in cardiac arrest.
Continuously monitoring ETCO2 with waveform capnography allows for real-time evaluation and adjustment of chest compressions. Chest compression rate and depth should be adjusted to consistently achieve ETCO2 values of greater than 10 mm Hg whenever possible [6]. Waveform capnography is most accurately monitored via an endotracheal tube; however capnography can also be used with a supraglottic airway or bag-mask device.
A fall in ETCO2 readings may indicate a variety of issues, including provider fatigue, sustained asystole or PEA, and issues with airway management or ventilations. Monitoring continuous waveform capnography and adjusting the elements of cardiac arrest management allows for quick changes of technique and improve the chance of a positive outcome.
ROSC and post-resuscitation care
Continuous waveform capnography can be useful in detecting ROSC without interrupting chest compressions for a pulse or rhythm check. The increase in cardiac output caused by the restoration of a normal heart rhythm is reflected by an immediate rise in ETCO2 readings [7]. More specifically, an abrupt rise of 10 mm Hg or greater is a specific indicator that spontaneous circulation has been restored and post resuscitation care can begin.
Patients who achieve ROSC will have complications secondary to resuscitation. Pulmonary injury and aspiration are common, as is hypoxic brain injury. ETCO2 levels are one of many factors that must be considered when providing post resuscitation care. There is limited evidence on what target ETCO2 levels should be post resuscitation. However there is some evidence that suggests capnography values outside of the normal range may predict negative long-term outcomes. Therefore it is currently considered appropriate to support ventilations in such a way as to maintain normal ETCO2 levels in patients post cardiac arrest.
The use of continuous waveform capnography and ETCO2 monitoring provides valuable insight into the metabolic state of a patient in cardiac arrest. Using capnography to guide care during resuscitation and after ROSC allows the EMS providers to give patients the greatest chance of survival after an out-of-hospital cardiac arrest.
References
1. Kodali BS. Capnography outside the operating rooms. Anesthesiology
2013;118:192-201.
2. Wang, A. Y., Huang, C. H., Chang, W. T., Tsai, M. S., Wang, C. H., & Chen, W. J. (2016). Initial end-tidal CO2 partial pressure predicts outcomes of in-hospital cardiac arrest. The American Journal of Emergency Medicine.
3. Touma, O., & Davies, M. (2013). The prognostic value of end tidal carbon dioxide during cardiac arrest: a systematic review. Resuscitation, 84(11), 1470-1479.
4. Akinci, E., Ramadan, H., Yuzbasioglu, Y., & Coskun, F. (2014). Comparison of end-tidal carbon dioxide levels with cardiopulmonary resuscitation success presented to emergency department with cardiopulmonary arrest. Pakistan journal of medical sciences, 30(1), 16.
5. Kodali, B. S., & Urman, R. D. (2014). Capnography during cardiopulmonary resuscitation: current evidence and future directions. Journal of emergencies, trauma, and shock, 7(4), 332.
6. Link, M. S., Berkow, L. C., Kudenchuk, P. J., Halperin, H. R., Hess, E. P., Moitra, V. K., ... & White, R. D. (2015). Part 7: adult advanced cardiovascular life support 2015 american heart association guidelines update for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation, 132(18 suppl 2), S444-S464.
7. Pokorná, M., Nečas, E., Kratochvíl, J., Skřipský, R., Andrlík, M., & Franěk, O. (2010). A sudden increase in partial pressure end-tidal carbon dioxide (P ET CO 2) at the moment of return of spontaneous circulation. The Journal of emergency medicine, 38(5), 614-621.
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