Waveform Capnography (End Tidal CO2 (EtCO2)) and Pulse Oximetry (SpO2) Interpretation

Detection of ETCO2 is the most reliable way to continuously monitor ventilation and therefore confirm placement of an advanced airway (the only exception is during CPR when ETCO2 may be undetectable). Waveform capnography is the preferred method to detect ETCO2, and with the development of small, portable devices, is the recommended technique even in austere field environments. Inexpensive colorimetric CO2 detectors are available, however the color change method may be very difficult to visualize with poor lighting or night vision devices. Waveform capnography measures the end-tidal carbon dioxide that passes through the device as the patient exhales in real time since it is placed directly in-line with the endotracheal tube. ETCO2 may also be attached to a face mask to verify normal and spontaneous breaths if an advanced airway has not been placed. With most portable, field capnographs, a number in mmHg will be appear on the display, which indicates the value of the CO2 in the exhaled breath and can be an immediate confirmation of correct tube placement. If the airway was placed correctly, and the patient is ventilating normally, the capnograph should read between 35-45 mmHg. Some other examples include:

- ETCO2 = 0: the tube is not transmitting any CO2: disconnected, tube placed in wrong position or has become dislodged. This may also occur if the patient is dead and there is no gas exchange.
- ETCO2 <35: Hyperventilation. The most common cause is over-bagging the patient, but may also indicate pain or anxiety. The only indication for “induced” hyperventilation is severe traumatic brain injury with signs of acute herniation, GOAL = 30-35 (no less than 30)
- ETCO2 >45: Retaining CO2, ineffective ventilation may indicate oversedation, primary lung problem, brain injury, worsening obstructive disease (asthma). If the trend is rising, this is an indicator of need for active ventilation assistance (BVM or mechanical ventilator)

During CPR (as an indicator of effectiveness of chest compressions and return of circulation):

- ETCO2 < 10: there is no return of CO2 to the lungs (no effective circulation). If CPR is initiated, it is ineffective
- ETCO2 = 10-20: EFFECTIVE CPR
- ETCO2 = 40 OR GREATER: You may see an abnormally high CO2 reading immediately after return of spontaneous circulation (ROSC), for instance after a successful defibrillation or return of effective cardiac activity.

Monitor that can provide waveform capnography can provide much more insight into a patient’s ventilation and oxygenation status. A quick reference to the most common waveforms is helpful to understanding the status of a patient.

Pulse oximetry is also of use in monitoring the oxygenation status of a patient. It can be an indirect measure of oxygen delivery to the tissues, and overall pulmonary function. Pulse oximetry monitors oxygenation by measuring absorbance differences between oxyhemoglobin and deoxyhemoglobin through the use of an infrared light. Pulse oximetry, however, has some important key limitations. A pulse oximetry reading indirectly reflects the patient’s central (pulmonary) oxygenation status by measuring the peripheral oxygenation. This means that any intervention that addresses oxygenation in the lungs may not be detected by the pulse oximeter until 30-90 seconds after the intervention. Additionally, if the patient is suffering from carbon monoxide or some other forms of poisoning, the pulse oximetry may read inaccurately. Also, a strong peripheral pulse and warm extremity are required to perfuse the capillary beds of the extremities and allow the pulse oximeter to obtain a valid measurement, therefore it may be difficult to measure in cold or hypotensive patients.