Capnography Vs. Pulse Oximetry

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In this topic, we’ll look at a key difference between EtCO₂ monitoring by a capnograph (Figure 1) and SpO₂ monitoring by a pulse oximeter (Figure 1).

Figure 1. Capnograph and Pulse Oximeter

Capnography provides a real-time assessment of a patient’s ventilatory status. In contrast, pulse oximetry lags behind and may give a false sense of security. For example, if a patient holds their breath, capnography will show immediate apnea—a flat or absent waveform. Pulse oximetry will continue to display a normal saturation for several minutes due to the delay in detecting hypoxia. Refer to the example in Figure 2.

Figure 2. Capnography Vs. Pulse Oximetry

The strip shows an ECG Lead II along the top, SpO₂ in the middle, and a capnography EtCO₂ waveform along the bottom. While the pulse oximeter continues to indicate adequate oxygenation in the SpO₂ wave, the capnography wave clearly shows the absence of ventilation. This demonstrates the importance of using both tools.

Relying solely on pulse oximetry is like driving by only looking in the rearview mirror. It tells you where the patient was, not where they are going. Capnography, on the other hand, is like looking through the windshield—it lets you anticipate and respond to changes before they become critical.

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In this topic, we’ll look at a key difference between EtCO₂ monitoring by a capnograph (pictured in Figure 1) and SpO₂ monitoring by a pulse oximeter (also pictured in Figure 1). Capnography provides a real-time assessment of a patient’s ventilatory status. In contrast, pulse oximetry lags behind and may give a false sense of security. For example, if a patient holds their breath, capnography will show immediate apnea—a flat or absent waveform while pulse oximetry will continue to display a normal saturation for several minutes due to the delay in detecting hypoxia. Refer to the ECG strip in Figure 2. The strip shows an ECG Lead II along the top, SpO₂ in the middle, and a capnography EtCO₂ waveform along the bottom. While the pulse oximeter continues to indicate adequate oxygenation in the SpO₂ wave, the capnography wave clearly shows the absence of ventilation. This demonstrates the importance of using both tools. Relying solely on pulse oximetry is like driving by only looking in the rearview mirror. It tells you where the patient was, not where they are going. Capnography, on the other hand, is like looking through the windshield—it lets you anticipate and respond to changes before they become critical.

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