Chest
Volume 102, Issue 5, November 1992, Pages 1512-1515
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Accuracy of Capnography in Nonintubated Surgical Patients

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Previous studies have reported mixed results when correlating etCO2 and PaCO2 in mechanically ventilated patients with underlying respiratory disease. However, the utility and accuracy of capnography in nonintubated patients, without chronic pulmonary disease, has received little attention. We studied 25 nonintubated surgical patients to (1) examine the correlation between PaCO2 and etCO2 and (2) describe the relationship between dead space (Vd/Vt), venous admixture and P(a-et)CO2. End tidal CO2 was lower than PaCO2 by an average of 3.6 mm Hg. Regression analysis found a close correlation between dead space and the P(a-et)CO2 gradient (r = 0.77, p<0.001), while venous admixture was of lesser importance (r = 0.47). Capnographic estimates of PaCO2 can be useful for continuously monitoring the respiratory status of nonintubated spontaneously breathing patients weaned from mechanical ventilation. This may be of particular value in trauma victims and in selected surgical patients without underlying respiratory disease in whom other injuries require continued critical care.

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PATIENTS AND METHODS

After obtaining Human Subjects Committee approval, 25 patients admitted to the surgical/trauma intensive care unit for perioperative care were studied. There were 15 men and 10 women between the ages of 18 and 71 years. Only those with a pulmonary artery flotation catheter previously inserted for cardiovascular or hemodynamic management, or both, were included. All were extubated and breathing spontaneously with or without supplemental oxygen.

Measurements on an individual patient were completed

RESULTS

Twelve study patients were weaned from mechanical ventilation and extubated within 24 h prior to entry into the protocol; the remaining 13 were admitted to the surgical ICU for either preoperative monitoring or for potential intubation and ventilatory support following trauma. Individual values of PaCO2, PetCO2, P(a-et)CO2, FIo2, Vd/Vt, and Qs/Qt are listed in Table 1.

Simultaneous PaCO2 and PetCO2 values are graphed in Figure 1. A correlation between the two was observed (r = 0.61, p<0.001;

DISCUSSION

In patients with normal lungs, ventilation and perfusion are well matched (V/Q≈0.8). The partial pressure of CO2 at the end of expiration (PetCO2) is almost equal to that of the blood leaving the pulmonary circulation. However, when aberrations occur in the Va/Q distribution, PetCO2 may not accurately reflect PaCO2. This is particularly true in mechanically ventilated patients with underlying respiratory disease in whom ventilation of some lung units may be high relative to the amount of blood

REFERENCES (12)

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Manuscript received December 16, 1991; revision accepted April 16.

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