![]() 01).Ĭhopin et al 10 also found a larger difference between maximum P ETCO 2 and P aCO 2 in patients with pulmonary embolism (12 mm Hg) versus patients without (1 mm Hg). 001), and the arterial-to-end-tidal difference was 4 ± 4 mm Hg ( P <. Maximum P ETCO 2 was the most accurate indicator of P aCO 2 (r = 0.77, P <. P ETCO 2 varied widely from breath to breath, and two thirds of the time the P ETCO 2 of spontaneous breaths was greater than that of ventilator breaths. 9 They recorded P aCO 2 and P ETCO 2 in 25 patients after cardiotomy and being weaned with intermittent mandatory ventilation. Maximum P ETCO 2 was also found more accurate by Weinger et al. 1), which indicates that maximum P ETCO 2 better approximates P aCO 2 than does averaged P ETCO 2. The maximum P ETCO 2 had a higher correlation coefficient (r = 0.65 vs 0.57) in all periods (see Fig. 11– 14 Our maximum P ETCO 2 values were closer to P aCO 2 by about 4 mm Hg, compared to the averaged P ETCO 2. Table 1 shows that the difference between averaged P ETCO 2 and P aCO 2 was around 10 mm Hg, a value reported in several previous studies. We measured averaged P ETCO 2, maximum P ETCO 2, and P aCO 2 contemporaneously. We inferred that the use of maximum P ETCO 2 during periods of 2 min or 5 min, as determined by the SmartCare system for averaging its parameters, could be a better indicator of alveolar ventilation than the averaged P ETCO 2 during the same period. 10 A longer exhalation may therefore better reflect P ACO 2 among spontaneous breaths. Allowing a prolonged or complete exhalation (compared to a normal breath) could increase P ETCO 2. P ACO 2 and P ETCO 2 can thus be different, owing to the patient's ventilation and pulmonary condition (eg, restrictive or obstructive lung disease). ![]() 7 So P aCO 2 tends to be higher than P ACO 2, mainly because of ventilation-perfusion discrepancies. If V̇/Q̇ is high, the P ACO 2 will be closer to the inspired CO 2. ![]() If this ratio is normal, the P ACO 2 will be close to P aCO 2. If the alveolar ventilation-perfusion ratio (V̇/Q̇) is low, the P ACO 2 will be close to venous pressure. CO 2 exhalation from alveoli depends on alveolar ventilation. The P ACO 2 depends on CO 2 production by tissues and venous blood flow content. P ETCO 2 is supposed to represent alveolar P CO 2 (P ACO 2), which is determined by the speeds at which CO 2 is filling alveoli and being emptied from alveoli. Relationship Between P ETCO 2 and P aCO 2 On the one hand, this could improve the accuracy of P ETCO 2 as a proxy for P aCO 2, and on the other hand it could improve SmartCare's classifications of the patient's ventilatory status by using a more reliable P ETCO 2 value. We assessed the use of the maximum P ETCO 2 value instead of the averaged P ETCO 2 value over 2 min or 5 min. 9 Such a value may better reflect alveolar P CO 2 and may thus be closer to P aCO 2. 7, 8 A spontaneously breathing patient may intermittently have higher P ETCO 2 values than the averaged value during prolonged exhalations. P ETCO 2 is known to frequently underestimate P aCO 2 because of ventilation-perfusion mismatching and dead-space effect. P ETCO 2 is not a main control parameter in SmartCare, but can be used in situations such as a low respiratory rate to help differentiate between, for instance, central hypoventilation leading to hypercapnia versus hyperventilation with hypocapnia. 5, 6 SmartCare averages P ETCO 2, respiratory rate, and V T over 2-min or 5-min periods, classifies the patient's ventilatory status, and adjusts the pressure-support level accordingly. ![]() The SmartCare automated ventilation and weaning system (Dräger, Lübeck, Germany) uses P ETCO 2 as a safety parameter, in addition to respiratory rate and tidal volume (V T), to automatically control the pressure-support level. 1, 2 Although P ETCO 2 does not perfectly reflect arterial CO 2 measured from an arterial blood sample (P aCO 2), 3, 4 capnometry allows continuous monitoring of alveolar ventilation in intubated patients. Monitoring of end-tidal partial pressure of CO 2 (P ETCO 2) has applications in emergency medicine, anesthesia, and intensive care. ![]()
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