![]() During disease the difference between PaCO 2 and ETCO 2, called the “PaCO 2 – ETCO 2 gradient”, can grow and be a quantitative indicator of the severity of the pathology. ETCO 2 can never be higher than PaCO 2, but the reverse can be true. ![]() In healthy patients, ETCO 2 and PaCO 2 values can be used virtually interchangeably as there are only a few pressure points of difference. Partial pressure of arterial carbon dioxide (PaCO 2 ) is the measurement of CO 2 pressure in arterial blood that is obtained through lab values. However, one study suggests that in patients with normal ETCO 2 values for their baseline, an ETCO 2 of approximately 70-80 mmHg 1 is an indicator that their respiratory distress is severe enough for ventilatory failure to be eminent. There is no absolute ETCO 2 value that indicates when respiratory distress has become severe enough to cause ventilatory failure, as it depends on the patient’s baseline ETCO 2 levels and the respiratory pathology involved. While a rise in ETCO 2 does not directly cause ventilatory failure, it is an indicator to the severity of the respiratory issue. Several factors can change a person’s ETCO 2, such as ventilatory factors and circulatory/metabolic status (circulator/metabolic capnography changes are discussed in other articles).Īs respiratory distress worsens, ventilations become less effective leading to decreased gas exchange, hypercapnia and a rise in ETCO 2. ETCO 2 is normally 35-45 mmHg in a healthy individual. The actual unit of ETCO 2 is a measurement of pressure (NOT parts!) reported in millimeters of mercury (mmHg). Nonetheless, pulse oximetry still has value as long as the provider understands the technological limitations and uses it in conjunction with capnography to complete the clinical picture.Įnd tidal carbon dioxide (ETCO 2 ) is the technical term for the amount of CO 2 measured in exhaled air. This is the biggest reason for central pulse oximetry monitoring on critical patients, particularly pediatric patients. Also, there are significant delays in measuring pulse oximetry in the finger from what is going on centrally in the core. However, a patient with carbon monoxide poisoning is obviously NOT oxygenated.Īdditional problems with pulse oximetry are related to intermittent readings, usually secondary to artifact caused by low perfusion, cold fingers, long finger nails and/or nail polish, patient movement, your partner hitting a curb at 90 mph, etc. Therefore, a patient with carbon monoxide poisoning will read 100% on their pulse oximetry as carbon monoxide’s higher affinity causes the Hbg molecule to coil tightly and appear very red (hence the textbook “cherry red” appearance of postmortem carbon monoxide patients). ![]() This becomes crucial to understand in the context of carbon monoxide poisoning.Ĭarbon monoxide is 200 times more attracted to hemoglobin than oxygen. An important technological limitation to know regarding pulse oximetry is that it cannot decipher what is bound to the Hgb molecule. The more red/blue the Hgb the higher/lower the saturation percentage estimated by pulse oximetry. Hemoglobin is a helix-type molecule that coils tighter and appears redder as it becomes saturated and uncoils and appears bluer as it desaturates. Pulse oximetry measures the color of the hemoglobin (Hgb) molecule and then uses mathematic algorithms to estimate the percent of Hgb saturation. Capnography is a direct measurement of ventilation and, indirectly, circulatory and metabolic status. While quantitative waveform capnography and pulse oximetry are both infrared technologies and may seem like similar measurements, they assess very different things.
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