Pulse Oximetry and Capnography

Pulse oximetry and capnography represent two of the most important safety monitoring technologies in anesthesia, providing continuous, non-invasive assessment of oxygenation and ventilation that has dramatically improved patient safety. These technologies exemplify how engineering advances can be successfully integrated into clinical practice to provide actionable information that prevents complications and improves outcomes. Understanding the principles, capabilities, and limitations of these monitors is essential for their effective use in clinical practice.

Pulse oximetry operates on the principle of spectrophotometric analysis, using light-emitting diodes that transmit red and infrared light through tissue to photodetectors that measure light absorption. Oxygenated and deoxygenated hemoglobin absorb these wavelengths differently, allowing calculation of oxygen saturation through mathematical algorithms that account for pulsatile changes in light absorption caused by arterial blood flow. The technology requires adequate peripheral perfusion and pulsatile flow to function accurately, explaining why readings may be unreliable in patients with severe hypotension, hypothermia, or peripheral vascular disease.

Modern pulse oximeters provide remarkably accurate measurements of oxygen saturation in the range of 70-100%, with accuracy typically within 2-3% of actual values measured by arterial blood gas analysis. The technology has several important limitations that clinicians must understand for proper interpretation. Pulse oximetry measures oxygen saturation of hemoglobin but provides no information about oxygen content, which also depends on hemoglobin concentration, or about adequacy of oxygen delivery, which depends on cardiac output and tissue perfusion. Additionally, certain conditions can interfere with accuracy, including carbon monoxide poisoning, methemoglobinemia, severe anemia, and motion artifacts.

Capnography measures and displays the concentration of carbon dioxide in respiratory gases, providing both quantitative information about end-tidal CO2 concentration and qualitative waveform information about the respiratory cycle. The capnogram waveform consists of four phases: inspiration (when CO2 should be zero), early expiration (dead space gas), late expiration (alveolar gas), and the transition back to inspiration. This waveform provides valuable information about ventilation adequacy, equipment function, and physiological status that extends well beyond simple CO2 measurement.

The clinical applications of capnography extend far beyond monitoring ventilation adequacy to include confirmation of proper endotracheal tube placement, detection of equipment disconnection or malfunction, assessment of circulation status, and monitoring of metabolic changes. The capnography waveform can reveal problems like bronchospasm (upsloping expiratory phase), rebreathing (elevated inspiratory baseline), or cardiac oscillations (small fluctuations synchronous with heartbeat). Changes in end-tidal CO2 values can indicate alterations in ventilation, circulation, or metabolism, making capnography an essential monitor for comprehensive patient assessment during anesthesia.