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Introduction

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Objectives

  1. Describe the principle of operation of pulse oximetry, capnography, and transcutaneous blood gas monitoring.

  2. Discuss the appropriate use and limitations of pulse oximetry, capnography, and transcutaneous blood gas monitoring.

  3. Describe the normal capnogram.

  4. Discuss the relationship between noninvasive monitors of blood gases and arterial blood gases.

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Noninvasive monitoring of respiratory function is common for mechanically ventilated patients. This is particularly the case with pulse oximetry, which is now available as part of the bedside monitoring system in most critical care units. Although pulse oximetry has become a standard of care during mechanical ventilation, it is important to recognize that there are few, if any, outcome studies to demonstrate the effectiveness of this monitor. Much of the success of pulse oximetry is related to its ease of use, compared to the capnograph and transcutaneous monitors. Capnography is commonly used in the operating room and is popular in some critical care units, while transcutaneous monitoring is used less commonly.

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Pulse Oximetry

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Principle of Operation

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Pulse oximetry passes two wavelengths of light (usually 660 nm and 940 nm) through a pulsating vascular bed and determines oxygen saturation (Spo2) from the ratio of the amplitudes of the plethysmographic waveforms. New pulse oximetry technology uses eight wavelengths of light to measure Spco (oximetric estimate of carboxyhemoglobin), Spmet (oximetric estimate of methemoglobin), and Sphb (oximetric estimate of hemoglobin concentration).

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A variety of oximeter probes are available in disposable and nondisposable designs, and they include finger probes, toe probes, ear probes, nasal probes, and foot probes. Most pulse oximeters provide a display of the plethysmographic waveform. Inspection of this waveform allows the user to detect artifacts such as that which occurs with motion and low perfusion. Because pulse oximeters evaluate each arterial pulse, they display heart rate as well as oxygen saturation. The saturation reading should be questioned if the oximeter heart rate differs considerably from the actual heart rate, but good agreement between the pulse oximeter heart rate and the actual heart rate does not guarantee a correct Spo2 reading.

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At saturations greater than 70%, the accuracy of pulse oximetry is about ±4% to 5%. To appreciate the implications of these accuracy limits, one must consider the oxyhemoglobin dissociation curve. If the pulse oximeter displays an Spo2 of 95%, the true saturation could be as low as 90% or as high as 100%. If the true saturation is 90%, the Pao2 will be about 60 mm Hg. However, if the true saturation is 100%, the Pao2 might be very high (≥ 150 mm Hg). Below 70%, the accuracy of pulse oximetry is worse, but the clinical importance of this is questionable. When using Spo2, one must understand the relationship between So2 and Po2. However, because ...

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