9,12 The mainstream (in-line) capnometer incorporates an infrared light source and detector into an airway adapter, which is generally attached in a breathing circuit near a patient face mask, laryngeal mask airway, or endotracheal tube. Phase IV (the inspiratory downstroke) is the beginning of the next inspiration.Ĭapnometers can measure airway carbon dioxide in two ways. The end of the plateau is the end-tidal Pco 2 (Petco 2), which represents the Pco 2 of the last alveolar gas exhaled at the airway opening. The alveolar plateau normally has a gentle positive slope. Phase III (the alveolar plateau) represents most of the exhalation from the alveolar compartment. Phase II (the expiratory upstroke) is the transition between anatomical dead space, which does not participate in gas exchange, and carbon-dioxide–containing alveolar gas from the respiratory bronchioles and alveoli. The presence of carbon dioxide in inspired gas (carbon dioxide rebreathing) will raise the inspiratory baseline. Phase I (the inspiratory baseline) is inspiration, which normally is devoid of carbon dioxide. The normal capnogram 9,10 is shown in Figure 2, page 28. 9 Capnography is the measurement, at the airway opening, of airway gas Pco 2 as a function of time over each respiratory cycle, with results displayed graphically. 8,9 Accordingly, the ability to detect hypercarbia or apnea will alert the clinician to the presence of respiratory compromise, allowing for intervention before the development of hypoxemia.Ĭarbon dioxide is produced by metabolism in body tissues, is transported in blood through venous return to the respiratory system, and is eliminated by cyclical expiration from the lungs. Depression of central respiratory drive will result in decreased respiratory rate with or without decreased tidal volume both cause hypoventilation, increased alveolar Pco 2 (Paco 2), and increased end-tidal Pco 2 (Petco 2). An obstructed airway will result in the absence of the expiratory capnogram. 2,6,7,8 In contrast, capnography, the breath-by-breath graphical measurement of the partial pressure of carbon dioxide (Pco 2) at the airway opening, 9,10 may provide the earliest warning of impending respiratory insufficiency 7,11 (Figure 1, page 26). Respiration can cease for several minutes, however, before oxygen stores in the lung are consumed (especially during the breathing of supplemental oxygen) and before Spo 2 actually begins to decrease. Pulse oximetry measures arterial blood oxygen saturation (Spo 2), generally using a digit or ear sensor. The pulse oximeter is the most commonly used monitor in the ambulatory setting to detect hypoxia. 1-4 Early detection of hypoventilation or apnea is critical to the prevention of the complications of hypoxia. The administration of sedative/analgesic drugs may cause respiratory depression in the ambulatory setting, and the ensuing hypoxia is a common complication. Capnography appears to be the most effective monitoring aid for detecting respiratory compromise in patients during mild sedation.įor patients undergoing ambulatory procedures, monitoring of ventilatory status is fundamental.
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