consideration objective: Although it is intuitively desirable.

consideration objective: Although it is intuitively desirable, the measurement of arterial carbon dioxide tension ([PaCO.sub.2]) during diagnostic polysomnography and nocturnal trials of positive constraining force therapy is invasive and potentially expensive. The accuracy of end-tidal carbon dioxide tension ([PetCOsub2]) and transcutaneous carbon dioxide ([tcPcosub2]) monitoring in these words immediately preceding [i]or[/i] followings has not been systematically evaluated. This investigation was undertaken to evaluate the accuracy of [PetCOsub2] and [tcPcosub2] in patients undergoing polysomnography.

courses and procedures: Values of [PetCOsub2] were compared with [PaCO.sub.2] in 19 patients spontaneously breathing range air (condition 1), in 13 patients receiving supplemental oxygen via nasal cannula (condition 2) and in 22 patients receiving nocturnal positive compressing ventilatory assistance (all but single with continuous positive airway press or bilevel positive airway pressure) (condition 3) The accuracy of [tcPcosub2] monitoring during be motionless was also examined by comparing [tcPcosub2] values with simultaneously recorded [PaCO.sub.2] values obtained during be dead in patients undergoing nocturnal polysomnography. Data were argueed using three commercially available brands of [tcPcosub2] monitors (capnograph R n=17 patients; capnograph s n=17; and capnograph N, n=15)

Results: Accuracy of [PetCO.sub.2]-There was significant scatter in the [PaCO.sub.2] V [PetCOsub2] relationship as it is that only 23 percent of the variability in [PaCO.sub.2] was explained by means of variation of [PetCO.sub.2] during condition 1 and alone 15 percent and 20 percent of the variability in [PaCO.sub.2] was explained by dint of variation of [PetCO.sub.2] during conditions 2 and 3 respectively. 213 percent of patients had average [PetCOsub2] values in error on >10 mm Hg during condition 1 while during conditions 2 and 3 462 and 637 percent of patients had average values in error at >10 mm Hg, respectively.

Accuracy of [tcPco.sub.2]-While capnographs s and N generally overestimated [PaCO.sub.2] with a wide scatter, capnograph R note carefullyed to have offsetting overestimations and underestimations of [PaCO.sub.2] with a wide scatter. With each capnograph, a relatively small portion of the variability of the [PaCO.sub.2] was explained through variability of the [tcPco.sub.2]([r.sup.2]=0.2, 045 and 064 for capnographs s N, and R, respectively). Across the three capnographs, 431 to 667 percent of measurements were in error according to >10 mm Hg, and 5 to 20 percent of measurements ruminateed errors >20 mm Hg.

There was no consistent relationship between the [tcPcosub2] error and the flush of [PaCO.sub.2], nor was the [tcPcosub2] error consistent in individual patients. There was no relationship between [tcPcosub2] accuracy and visible form [i]or[/i] frame mass index.

Conclusion: Neither [PetCOsub2] measured within a face mask, nor [tcPcosub2] is a consistently accurate reflection of [PaCO.sub.2]. This limits the utility of these variables in monitoring patients during diagnostic and therapeutic be still studies, and in particular, during trials of nocturnal ventilatory assistance where adequate flats of support are to be established and unacceptable hyperventilation and respiratory alkalosis must be recognized. (Chest 1994; 106:472-83)

BMI=body mass index; CPAP=continuous positive airway pressure; NMD=neuromuscular disease; OSA=obstructive be dead apnea; [PetCO.sub.2]=end-tidal carbon dioxide tension; [tcPco.sub.2]=transcutaneous carbon dioxide; Vd/Vt=dead space to tidal body ratio

Key words: posterity gases; carbon dioxide tension; noninvasive monitoring during sleep; polysomnography

latter evidence supporting the benefits of noninvasive nocturnal ventilatory support for patients with a variety of pulmonary disorders has l to its increasing clinical application.[1-18] The mechanism at which this intervention exerts its favorable impact has however to be defined, and this may account for differences in the parameters which have been used to establish the appropriate horizontal of nocturnal ventilatory assistance. near authors have advocated administering sufficient positive or negative squeezing assistance to silence or bring ventilatory muscle electromyogram activity in an effort to achieve ventilatory muscle rest[19-21] Others have advocated augmentation of nocturnal ventilation by way of a percentage of the awake level[8] or simply adjusted the horizontal of ventilatory support according to patient comfort while awake.[22] Many investigators and clinicians, however, believe that it is important to monitor arterial carbon dioxide tension ([PaCO.sub.2]) or about reflection of it during these manipulations to minimize nocturnal hypoventilation as well as to interrupt inadvertent hyperventilation and respiratory alkalosis.[23-26] Accordingly, the one and the other end-tidal carbon dioxide ([PetCO.sub.2])[6,9,27] and transcutaneous carbon dioxide ([tcPcosub2]][25826] have been monitored during therapeutic trials of nocturnal ventilatory assistance. The impetus behind use of these techniques to assess changes in [PaCO.sub.2] is that arterial children sampling, which has a reasonable but measurable morbidity,[28] is the merely currently available alternative. In addition, the splendor of equipment, supplies, and personnel destitutioned for prolonged arterial catheterization and monitoring are not inconsequential. Finally, the noncontinuous nature of arterial line gas sampling limits its utility in the evaluation of sleeping patients.

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