Normal end tidal co2 waveform8/7/2023 ![]() ![]() (b) The concentration of CO2 in exhaled gas depends only on the body's CO2 production and the volume of gas exhaled per minute (the minute ventilation).(a) The amount of CO2 clearance in the lungs will always equal the CO2 production by the body.In an equilibrium state, the amount of CO2 produced by the body tissues must be cleared by the lungs.Two general principles can guide us here: #1) at steady state, cardiac output does not affect etCO2 For patients with severe lung disease, the gap can be much greater. In patients with normal lungs, the gap is typically ~3-10 mm. Any increase in dead space (e.g., due to severely injured lungs), will widen the gap. (2) The gap between the etCO2 and the PaCO2 is a reflection of the amount of dead space.□ For the purpose of everyday critical care practice, it's reasonable to assume that the PaCO2 is above the etCO2. Extraordinarily rarely, etCO2 can be slightly higher than arterial CO2 in pregnant patients with otherwise normal lungs. (1) Arterial CO2 should be higher than etCO2.This leads to two foundational principles of etCO2:.#3) By the time gas reaches the endotracheal tube, the end-tidal CO2 concentration will be lower than the arterial CO2 tension.#2) As gas flows from this alveolus out of the lung, it will be diluted by dead space gas that will have a lower CO2 concentration (since this dead space gas doesn't absorb CO2 from the blood).#1) Within the high-functioning alveolus, the CO2 pressure will be equal to the arterial CO2 pressure.The best way to conceptualize this is to imagine gas flowing from a high-functioning alveolus into the ventilator.The relationship between etCO2 and arterial CO2 (PaCO2) The amount of alveolar dead space may change over time, as lung disease improves or deteriorates. For example, increased dead space is seen in pulmonary embolism, in pneumonia or other parenchymal lung diseases such as aspiration, or in obstructive lung diseases such as asthma. Alveolar dead space may be increased in most types of lung disease (reflecting dysfunction at the alveolar, vascular, or airway level).Consequently, a strategy of high-frequency, low-tidal volume breaths will tend to achieve less CO2 clearance for any specific total minute ventilation. However, if the patient is ventilated with small tidal volumes, then a greater fraction of each breath will be wasted on dead space ventilation. Anatomic and instrument dead space are generally fixed.Alveolar dead space – dysfunctional alveoli which receive ventilation but don't exchange CO2 well (or at all).Instrument dead space – for intubated patients, this includes any tubing interposed between the patient's airway and the site of fresh gas insufflation.Anatomic dead space (e.g., trachea and large bronchi) – with each breath gas enters these areas, but doesn't participate in gas exchange.Dead space refers to inhaled gas that doesn't participate in carbon dioxide clearance. ![]() If the etCO2 curve doesn't reach a plateau, then the numeric value is less reliable. Both the numeric value and the shape of the etCO2 tracing are important.Waveform capnography should be monitored in all intubated patients and displayed on the monitor (as above).etCO2 is a measurement of the partial pressure of CO2 in gas expired at the end of exhalation (when exhaled gas will most closely resemble the alveolar CO2 concentration).Understanding how to interpret etCO2 waveforms.Understanding changes in etCO2 within the context of other data (especially trends in minute ventilation).Paying attention to etCO2 values (e.g., noting them daily in reviews of the patient, along with other vital signs).Within the next decade, continuous waveform capnography will likely become a universal standard of care across all well-resourced intensive care units.Īs the use of waveform capnography expands, we need to be thoughtful about integrating this into our practice. Capnography is increasingly recommended both to confirm endotracheal tube insertion and to subsequently monitor the patency and effectiveness of ventilation throughout the duration of intubation. Capnography was pioneered in the operating room, but the safety implications for all critically ill patients are clear (the standard of safety monitoring in the ICU shouldn't be lower than in the operating room). Failure to use waveform capnography contributed to >70% of ICU-related airway deaths in the NAP4 audit. Waveform capnography is emerging as a standard monitoring tool to improve safety among intubated patients. Introduction – an emerging standard of care ![]()
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