Validation of a maskless CO2-response test for sleep and infant studies

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Abstract
The Hazinski method is an indirect, noninvasive, and maskless CO2-response test useful in infants or during sleep. It measures the classic CO2-response slope (i.e., .DELTA..ovrhdot.VI/.DELTA.PCO2) divided by resting ventilation Sr = (.ovrhdot.VI - .ovrhdot.VI'')/(.ovrhdot.VI'' .cntdot. .DELTA.PCO2) between low ('')- and high (")-inspired CO2 as the fractional increase of alveolar ventilation per Torr rise of PCO2. In steady states when CO2 excretion (.ovrhdot.VCO2'') = .ovrhdot.VCO2", Hazinski CO2- response slope (Sr) may be computed from the alveolar exchange equation as Sr = (PACO2'' - PICO2'')/(PACO2" - PICO2") where PICO2 is inspired PCO2. To avoid use of a mask or mouthpiece, the subject breathes from a hood in which CO2 is mixed with inspired air and a transcutaneous CO2 electrode is used to estimate alveolar PCO2 (PACO2). To test the validity of this method, we compared the slopes measured simultaneously by the Hazinski and standard steady-state methods using a pneumotachograph, mask, and end-tidal, arterial, and four transcutaneous PCO2 samples in 15-min steady-state challenges at PICO2 23.5 .+-. 4.5 and 37 .+-. 4.1 Torr. Sr was computed using PACO2 and arterial PCO2 (PaCO2) as well as with the four skin PCO2 (PsCO2) values. After correction for apparatus dead space, the standard method was normalized to resting .ovrhdot.VI = 1, and its CO2 slope was designated directly measured normalized CO2 slope (Sx), permitting error to be calculated as Sr/Sx. In 10 normal adults Sr/Sx was 1.12 .+-. 0.38 with PACO2 and 1.20 .+-. 0.44 with PaCO2 neither being significantly different from one. In six subjects, the four PsCO2 electrode sensitivities or "tracking fractions" (.DELTA.PsCO2/.DELTA.PaCO2) were 0.94 .+-. 0.15, 1.01 .+-. 0.09, 1.04 .+-. 0.07, and 0.91 .+-. 0.11. Sr/Sx for the four electrodes averaged 1.34 .+-. 0.39, 0.99 .+-. 0.23, 0.97 .+-. 0.39, and 1.49 .+-. 0.54. We conclude that CO2 slope values with this method vary by about .+-. 40% from standard steady-state responses, whether one uses PaCO2, PaCO2, PACO2, or PsCO2 in the computation. When transcutaneous PCO2 electrodes are used, accuracy is dependent on their stability and accuracy in tracking the PCO2 rise. Special calibration may be needed for transcutaneous CO2 electrodes to obtain optimal tracking to PaCO2.