Accuracy of End-Tidal and Transcutaneous Pco₂ Monitoring During Sleep

¹ From the Division of Pulmonary, Allergy, and Critical Care Medicine, Department of Medicine, University of Pittsburgh School of Medicine; Department of Medicine, Department of Veterans Affairs Medical Center, Oakland, Pa; and the Department of Biostatistics, Graduate School of Public Health, University of Pittsburgh, Pittsburgh

Study objective: Although it is intuitively desirable, the measurement of arterial carbon dioxide tension (PaCO₂) during diagnostic polysomnography and nocturnal trials of positive pressure therapy is invasive and potentially expensive. The accuracy of end-tidal carbon dioxide tension (PETCO₂) and transcutaneous carbon dioxide (tcPco₂) monitoring in these contexts has not been systematically evaluated. This investigation was undertaken to evaluate the accuracy of PETCO₂ and tcPco₂ in patients undergoing polysomnography.

Methods and procedures: Values of PETCO₂ were compared with PaCO₂ in 19 patients spontaneously breathing room air (condition 1), in 13 patients receiving supplemental oxygen via nasal cannula (condition 2), and in 22 patients receiving nocturnal positive pressure ventilatory assistance (all but one with continuous positive airway pressure or bilevel positive airway pressure) (condition 3). The accuracy of tcPco₂ monitoring during sleep was also examined by comparing tcPco₂ values with simultaneously recorded PaCO₂ values obtained during sleep in patients undergoing nocturnal polysomnography. Data were collected using three commercially available brands of tcPco₂ monitors (capnograph R, n=17 patients; capnograph S, n=17; and capnograph N, n=15).

Results: Accuracy of PETCO₂-There was significant scatter in the PaCO₂ vs PETCO₂ relationship such that only 23 percent of the variability in PaCO₂ was explained by variation of PETCO₂ during condition 1 and only 15 percent and 20 percent of the variability in PaCO₂ was explained by variation of PETCO₂ during conditions 2 and 3, respectively. 21.3 percent of patients had average PETCO₂ values in error by >10 mm Hg during condition 1, while during conditions 2 and 3, 46.2 and 63.7 percent of patients had average values in error by >10 mm Hg, respectively.

Accuracy of tcPco₂-While capnographs S and N generally overestimated PaCO₂ with a wide scatter, capnograph R tended to have offsetting overestimations and underestimations of PaCO₂ with a wide scatter. With each capnograph, a relatively small portion of the variability of the PaCO₂ was explained by variability of the tcPco₂ (r²=0.2, 0.45 and 0.64 for capnographs S, N, and R, respectively). Across the three capnographs, 43.1 to 66.7 percent of measurements were in error by >10 mm Hg, and 5 to 20 percent of measurements reflected errors >20 mm Hg. There was no consistent relationship between the tcPco₂ error and the level of PaCO₂, nor was the tcPco₂ error consistent in individual patients. There was no relationship between tcPco₂ accuracy and body mass index.

Conclusion: Neither PETCO₂, measured within a face mask, nor tcPco₂ is a consistently accurate reflection of PaCO₂. This limits the utility of these variables in monitoring patients during diagnostic and therapeutic sleep studies, and in particular, during trials of nocturnal ventilatory assistance where adequate levels of support are to be established and unacceptable hyperventilation and respiratory alkalosis must be recognized.

Key Words: blood gases • carbon dioxide tension • noninvasive monitoring during sleep • polysomnography

Submitted on May 17, 1993
Accepted on January 14, 1994

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