Our results therefore suggest that the apparatus would appear suitable for monitoring NPPV, with some important limitations that the clinician must bear in mind: there is a distinct possibility of occasional errant TcPco2 values: Figure 3, for instance, shows two values overestimating PaC02 by 8 to 12 mm Hg; the high correlation depicted between TcPco2 and PaC02 values was obtained after recalibration for each individual measurement—a 10-min procedure that may appear time-consuming in clinical practice. Furthermore, although TcPco2 did accurately detect acute changes in PaC02 in five of six cases, case 4 (Fig 5) showed an initial drop in TcPco2 without any concomitant change in PaC02. Substantial changes in TcPco2 should probably be confirmed by arterial blood gas determination, once a new steady state is reached. Also, the slow response time, although inferior to 1 min in our study, prevents TcPco2 from detecting short, transient changes in PaC02 that can be associated with brief apneas or hypopneas. This point has been well illustrated by Lanigan and coworkers, with healthy subjects breathing a hypercapnic gas mixture that will induce almost instantaneous changes on PaC02. The lag time of various TcPco2 devices in these studies ranged from 31 to 56 s. Recording TcPco2 would appear therefore unsuitable for monitoring patients with obstructive sleep apnea, for example. buy yasmin online
In patients with NPPV, however, the correction of long-lasting periods of hypoventilation or hyperventilation, particularly during sleep, represents a major goal. Recently, three groups of investigators have used TcPco2 monitoring to estimate the mean level of PaC02 during the night. Naughton and colleagues found that treatment of Cheynes-Stockes breathing with nasal continuous positive airway pressure increased mean nocturnal TcPco2, and decreased the number of apneas, underscoring the role of hypocapnia in the pathogenesis of periodic breathing. In a different setting, Piper and Sullivan found that treatment with nasal continuous positive airway pressure in patients with combined obstructive sleep apneas and hypoventilation resulted in gradual decrease of mean nocturnal TcPco2. Finally, Meecham-Jones and coworkers found that treatment with nocturnal nasal pressure support significantly decreased mean TcPco2 in selected patients with COPD, and that this change was associated with improved daytime PaC02.
Our results suggest that TcPco2 monitoring may give more detailed information than just the computation of mean nocturnal PaC02 as reported by others. The responses that we observed after initiating or interrupting NPPV show that the method can identify ventilatory events, as far as they last >1 min, and may permit us to undertake the necessary adjustments in ventilatory support. This might be particularly important in patients who receive combined NPPV and 02 supplementation, in whom monitoring by pulse oximetry alone has limited value.
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