![]() ![]() Oxygen saturation is the percent of Hemoglobin (Hgb) binding sites in the blood that are carrying oxygen. Pulse oximetry was a non-invasive way of measuring oxygen saturation. Pulse oximetry revolutionized anesthetic safety. We were essentially using our ears in place of the plethysmograph waveform that pulse oximetry would eventually provide. Low blood pressure brought muffled, faint heart tones independent of heart rate. As the patient got lighter or if the blood pressure rose, heart tones got louder and sharper. An attentive anesthesiologist could use changes in the loudness or crispness of the heart tones to alert him or her to changes in cardiac output. To provide an indirect indicator of perfusion, we used a precordial stethoscope attached to an earpiece to continuously listen to heart sounds. ![]() Frequent blood gas determinations, or the need to monitor continuous perfusion pressures, often necessitated placement of an arterial line. If we wanted to determine PaO2, or PaCO2, we needed to draw a blood gas. Pulse oximetry and capnography were not yet in clinical use. When I started my anesthesia training in 1980, we monitored the patient with a manual blood pressure cuff, EKG, pulse, and temperature. On the other hand, if you breathe a high concentration of oxygen without changing your respiratory rate, your arterial oxygen content (PaO2) will greatly increase. Under normal circumstances, hyperventilation with room air will lower your arterial carbon dioxide content (PaCO2) significantly, but not change your oxygen levels much at all. Oxygenation is simply the addition of oxygen to the body. Ventilation exchanges air between the lungs and the atmosphere so that oxygen can be absorbed and carbon dioxide can be eliminated. Ventilation and oxygenation are different. It can be an essential tool in ensuring optimal, high quality chest compressions during cardiac resuscitation. But ETCO2 can also provide valuable information on the adequacy of cardiac perfusion. We routinely use ETCO2 to provide information on ventilation. Like pulse oximetry before it alerting us to changes in oxygenation, end-tidal CO2 monitoring, or ETCO2, is rapidly becoming an additional vital sign. ![]()
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