Transcutaneous Pco2 to Monitor Noninvasive Mechanical Ventilation in Adults: Discussion

In: Mechanical Ventilation

21 Apr 2014

By contrast, in some other studies, the limits of agreement are such that measurement of TcPco2 would appear unfit for clinical purposes such as monitoring NPPV. In some cases, it seems to be related to a particular capnograph. However, other factors such as calibration before all measurements, frequent change of membranes, allowing sufficient time for stabilization of TcPco2 signals, and adequate cutaneous perfusion are important in ensuring sufficient precision of transcutaneous measurements. TcPco2 measurements are reported to be adversely affected by cutaneous vasoconstriction due to low cardiac output or vasoconstricting agents, hypothermia, and elevated cutaneous vascular resistance due to hypovolemic or cardiogenic hypotension. A cardiac index below 1.5 L/min can dramat ically increase the (TcPco2—PaC02) gradient. However, Palmisano and Severinghaus did not find any significant effect of use of vasopressors on bias, limits of agreement, or regression slope. Similarly we found that low-dose dopamine did not alter measurement of TcPco2. Finally, it is interesting to note that the error of TcPco2 is the same in fat and lean subjects, and is unrelated to body mass index. buy asthma inhalers online

A drawback of the method is the necessity to change the skin site after 4 h to avoid skin burning— and to recalibrate the capnograph. One might consider decreasing the temperature to allow a longer time at the same site. However, temperatures of approximately 43°C are required to maintain adequate perfusion at the skin site: lower sensor temperatures have been associated with longer response times and would not be desirable. The limit of 4 h at 43°C must be considered as a rule of safety, but other investigators have reported no skin burn after periods of 6 to 8 h at sensor temperatures of 43 to 44°C. Furthermore, although we found no significant drift of TcPco2 vs PaC02 over a 4-h period, one case did show a (PaC02—TcPco2) difference of 6 mm Hg after 4 h of continuous recording. Others have reported, over periods of 6 to 7 h, a drift of 0.1 to 0.8 mm Hg/h.
The aim of our study was to evaluate if a TcPco2 capnograph that appears suitable for measurements under steady-state conditions would also be suitable for monitoring noninvasive, intermittent ventilation. We found that when a ventilatory event was induced by initiating or interrupting NPPV in hypercapnic patients, a change in TcPco2 was detected within <60 s, and that the trend accurately reflected the change in PaC02. Moreover, by 3 min, at least half of the amplitude of the change in PaC02 had been displayed by TcPco2 recording. An estimation of the lag time averaged 5±3 min (range, 1 to 9 min).


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