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

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 VI/delta PCO2) divided by resting ventilation Sr = (VI''--VI')/(VI'.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 (VCO2') = 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 VI = 1, and its CO2 slope was designated directly measured normalized CO2 slope (Sx), permitting error to be calculated as Sr/Sx.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrode and cuvette for rapid continuous CO2 tension and pH measurement in blood

An electrode and cuvette system has been developed for the continuous and rapid measurement of either blood CO2 tension or pH. The CO2 electrode consists of a 1.5-mm-diameter flat-tip glass pH electrode covered by a film of carbonic anhydrase solution, over which a 25-micron-thick dimethyl silicone membrane is attached. Porous ceramic filled with 20% polyacrylamide, equilibrated with a salt solution, serves as a salt bridge between a Ag-AgCl reference electrode and the pH electrode surface. The electrode is housed in a four-port cuvette assembly. Blood from a vessel of interest is delivered to the cuvette by means of an occlusive roller pump. The cuvette maintains the electrode and blood at a constant temperature and directs a continuous jet of blood against the electrode surface. The cuvette also allows for easy and frequent calibration of the electrode with either gas or liquid standards. The 90% response time of the CO2 electrode is 3.0 s for liquids and 1.3 s for gases. Removal of the dimethyl silicone membrane and carbonic anhydrase film yields a pH electrode that can continuously measure blood pH with a 90% response time of 1.6 s.

Transcutaneous and capillary pCO2 and pO2 measurements in healthy adults.

Transcutaneous carbon dioxide and oxygen tensions (tc-pCO2 and tc-pO2) were measured in seven healthy adult volunteers during hyperventilation in atmospheric air and during CO2 inhalation. Three skin sensors were applied to each subject: an O2 electrode, a CO2 electrode, and a combined O2-CO2 electrode, each heated to 44 degrees C. We observed close correlation between tc-pCO2 and capillary-pCO2, the relation being close to that calculated from the anaerobic temperature coefficient of pCO2 in blood. For O2, on the other hand, the relationship between transcutaneous and capillary values appeared more complex. Electrode drift during in-vivo monitoring was greater for pCO2 (up to 12%) than for pO2 (up to 7%), but generally we observed no differences in drift between the combined and the single electrodes. We conclude that tc-pCO2 measured with a single or a combined electrode reliably predicts capillary-pCO2 in healthy adults and that changes are rapidly observed. Our conclusions regarding tc-pO2 values are less definite because of uncertain interpretation of the capillary-pO2 values.

Determination of oxalate by continuons analysis using photochemical oxidation by iron(III) and a CO2 electrode detector

Oxalate concentrations were measured by photochemical oxidation with iron(III) to carbon dioxide. Technicon AutoAnalyzer components were used to automate the sampling and mixing steps, and the carbon dioxide produced in the radiation step was measured with a CO2 electrode. Variables in the System were evaluated and the optimum conditions identified. A theoretical study was made of the effect on ionic species concentrations of variations in iron(III), acid, and counter ion concentrations. A linear calibration curve could be obtained for oxalate concentrations in the range of 2 × 10−4 to 0.02 M; by modifying conditions linearity could be achieved at both higher and lower concentration ranges. Several other acids are partially oxidized under optimum conditions for oxalate. Tartrate, citrate, and malonate introduce errors of 2% or less at concentrations equivalent to oxalate; malate and pyruvate cause errors on the order of 10%. Several small carboxylic acids were found to interfere with the CO2 electrode.

Measurement of blood O2 and CO2 concentrations using PO2 and PCO2 electrodes

Accurate dilution of a small blood volume with a carbon monoxide-saturated solution allows measurement of the whole blood O2 concentration as an increase in O2 tension in the solution. We have improved the method by simplifying both equipment and procedure. We also suggest an additional step in which the mixture is acidified, thereby allowing the measurement of CO2 concentration in the same solution with a CO2 electrode. The accuracy of both the O2 and CO2 determinations compares favorably with that obtained with other micromethods.

Effect of temperature on rate of CO2 uptake by human red cell suspensions

The time course of carbon dioxide uptake by oxygenated supensions of human red cells was followed using a CO2 electrode in a Hartridge-Roughton continuous-flow rapid-reaction apparatus. Measurements were made at several temperatures from 1i to 42 degrees C, with the initial PCO2 in the reacting mixture from 40 to 60 mmHg. The initial part of the uptake curve is presumably rate limited by the intracellular hydration of CO2 with reaction-velocity constants in cell water from 280 to 960 S-1 at 42 degrees C and an activation energy of 2.4 kcal mol-1. The later stages of CO2 uptake were much slower, with half-times from greater than 1.5 s at 12 degrees C to 0.7 s at 42 degrees C, and were presumably rate limited by the chloride-bicarbonate shift and H+ interchanges. The results indicate that despite the acceleration of the hydration reaction in cell water by a factor of 5,000 at 37 degrees C and 3,800 at 42 degrees C, the later part of the exchange is too slow to permit blood to come intoC02, equilibrium with actively exercising muscles during its passage through the capillary bed.