Dynamic model of oxygen transport for transcutaneous PO2 analysis

A dynamic model of oxygen transport through the outer skin layers and a polarographie sensor was developed for the analysis of transcutaneous oxygen tension (tePO₂). It provides a basis for quantifying the factors that determine the relationship between tePO₂ and arterial oxygen tension (P _aO₂). Model simulations show the importance of stratum papillare blood flow: viable epidermis and stratum papillare metabolic oxygen consumption: the oxygen permeability of the skin relative to that of the sensor membrane and electrolyte: and temperature and the oxyhemoglobin dissociation curve. These simulations were consistent with experimental data obtained by using microcathode transcutaneous oxygen sensors, which were placed on the skin of 10 healthy adults. Furthermore, the model indieates that accurate evaluation of arterial oxygen tension by using transcutaneous measurements requires continuous estimation of skin perfusion. On the basis of tcPO₂ measurements made during arterial occlusion, simulations indicate that quantitative evaluation of the metabolic oxygen consumption of the viable skin tissues is possible only when the oxygen permeabilities of the skin and sensor are known.     

International Federation of Clinical Chemistry (IFCC). Scientific Division. Committee on pH, Blood Gases and Electrolytes. Guidelines for transcutaneous PO2 and PCO2 measurement

This document provides guidelines in the terminology, methodology, and in the interpretation of data obtained from the use of skin (transcutaneous) P02 and PCO2 electrodes. The transcutaneous technique has found special application in newborn infants. The causes of analytical bias with respect to arterial blood gas values and imprecision obtained with transcutaneous P02 and PCO2 electrodes are reviewed. Electrode temperatures above 44 degrees C should not be used routinely, and, at a measuring temperature of 44 degrees C, the measuring site should be changed at least every 4 hours to avoid skin burning.     

Analytical performances of the i-STAT portable clinical analyzer for the measurement of PO2 and PCO2

Aware of some limitations on blood gas results, we performed an extensive evaluation before introducing i-STATs in our hospitals. Three i-STATs were tested in parallel with an ABL-520, on three types of cartridges (EG7, EG6 and EG3), using tonometered whole blood (9 gas levels, n = 720) and aqueous QC solutions (3 levels, n = 600). Reference systems were the theoretical calculated values from gas composition used for tonometry and results given by the ABL-520, respectively. On aqueous controls dispersion intervals reached 10-20 mmHg for both analytes for inter-lot as well as intra-lot data. PO2 values on blood showed marked dispersion: 5 mmHg (CV = 2 to 7%) at clinically critical levels. PCO2 showed several (10%) major outliers: mV recording of the PCO2 electrode allowed to incriminate a pre-humidification problem (due to incorrect shipping conditions). Once outliers have been discarted, there still was a 5 mmHg non negligible residual dispersion (CV = 3 to 5%). i-STAT analytical performances for blood gases which are the analytes whose determination at the bed-side is potentially the most useful, do not match capabilities of classical laboratory instruments. Thus even though the i-STAT approach represents a seducive solution for the STAT problem, for the moment, it's use cannot be recommended in a hospital environment where classical instruments can be made available.     

Pedal blood flow and transcutaneous PO2 in normal subjects and in patients suffering from severe arterial occlusive disease

Pedal transcutaneous partial pressure of oxygen (tcPO2) has been measured by polarographic method, heating the skin at 44 degrees C. In 50 normal subjects, mean tcPO2 measured 54.5 +/- 7 mmHg. Among 43 patients suffering from chronic ischaemia of lower limbs, mean tcPO2 measured 40.8 +/- 8 mmHg in patients with from claudication and 16.1 +/- 15 mmHg in patients suffering from rest pain and/or gangrene. The variability of repeated tcPO2 measurements, expressed as 1 SD of the mean, was 4.5 mmHg in normal subjects and 2.9 mmHg in patients. The relationships between pedal subcutaneous blood flow measured in xenon-133 clearance method and pedal tcPO2 have been studied in nine normal subjects and in five patients suffering from severe chronic ischaemia of lower limbs (rest pain and/or gangrene). There was a positive correlation between blood flow and tcPO2 in normal subjects (r = 0.77, P less than 0.001). In patients suffering from severe ischaemia, there was no correlation between these two parameters, but measured blood flow was sometimes very high in areas where tcPO2 was low. It is likely that 133Xe clearance method considerably overestimates local blood flow in these patients, because there is considerably less fat in subcutaneous tissue of chronic severely ischaemic areas. Thus, partition coefficient should be determined in each patient. However, tcPO2 may constitute an index of nutritional circulation, while 133Xe clearance actually measures total subcutaneous blood flow.     

Performance limiting causes in single use unit systems for the measurement of PO2 and PCO2 in blood

Having had the opportunity of testing two of the three commercially available single use unit systems (i-STAT and OPTI-1), we have listed a series of performance limiting factors which they share, even though, their detectors being based on different methodologies, they have specific causes of dysfunction. PO2 detectors face the worse problems for different reasons. Temperature control is critical: the cassette or at last the detectors, the internal solution and the specimen have to be heated to 37 C, thermostabilisation to one tenth of degree centigrade is a must. Gas equilibration between liquid phases, the outside ambiance and the plastic material can raise problems: a decrease of the external barometric pressure alters the result, an absence of oxygen buffer capacity can induce modifications of the PO2 in the solutions. The transition of the detectors from a dry to a wet stage can provoke changes of the final result in case of accidental pre-humidication of the cassette. The presence of a single internal solution generates problems linked to a single point calibrage procedure which is a source of variability. The general quality of the cassette can only be evaluated by numerous internal electronic checks: increasing the severity of the controls improves quality but increases the number of rejected cassettes. External quality control through classical procedures are not applicable and finally the quality of the cassettes depends primarily on rigorous manufacturing conditions.     

Transcutaneous measurement of PO2 and PCO2 in the dermis at the site of the tuberculin reaction in healthy human subjects

The respiration of the skin at the site of a delayed hypersensitivity reaction in tuberculin skin tests was studied by transcutaneous measurement of dermal oxygen and carbon dioxide tensions in normal individuals who had been immunized with BCG: six reactions were strong positives, four were weak positives, and four without induration were regarded as negative. The tcPO2 fell over the first 2 days of the reaction and remained low for the next 2 days: the severity of the changes was greater in the 'strong' reactions than in the 'weak' reactions. The tcPCO2 showed a reciprocal rise over the first 2 days and, although still high, tended to recover over the fourth day. These results indicate that local hypoxia and hypercapnia are prominent features of the positive tuberculin test, probably as a consequence of the respiration of the infiltrating lymphocytes and monocytes. It is likely that similar respiratory changes occur in those chronic inflammatory diseases where delayed hypersensitivity reactions make a contribution to the pathogenesis of the lesion.     

Evaluation of a new re-usable electrode for continuous monitoring of blood PO2 during open-heart surgery

In 46 patients undergoing cardiac surgery blood PO2 was continuously monitored with an in-line oxygen electrode during cardiopulmonary bypass and readings were compared with intermittent measurements of samples on a blood gas analyser. In 35 patients in whom arterial PO2 was monitored, there were rapid changes in PO2 and the mean difference between the in-line electrode and the blood gas analyser was 6.63 kPa (p less than 0.001). Venous PO2 was monitored in 11 patients and, in contrast to the arterial readings, the venous PO2 was relatively stable. The mean difference between the in-line electrode readings and the blood gas analyser was 0.21 kPa (p less than 0.05). The advantages of continuous monitoring of blood PO2 are described and the relative merits of mixed venous PO2 and mixed venous oxygen saturation measurements are discussed.     

Study of transcutaneous and intraluminal measurement of gastric electrical activity in humans

Several authors have reported measurements of the human electrogastrogram (EGG), both with intraluminal electrodes and with improved noninvasive techniques. These methods provide information about the existence and frequency of gastric electrical activity (GEA) which may aid in diagnosing tachygastria. However, none of these methods have provided information regarding the direction and velocity of propagation of GEA. Such information could help isolates such abnormalities as gastro-oesophageal and antral reflux and gastric stasis. To ascertain the reliability of the extracorporal measurements, a new intraluminal electrode assembly was developed. This probe measures GEA through the mucosa at several locations simultaneously; yielding frequency, direction of propagation and velocity of the electrical activity.     

Reliability of the compensation comparison stray-light measurement method

The compensation comparison (CC) method is a psychophysical technique to measure retinal stray light. It uses a two alternative forced choice (2AFC) measurement paradigm. The 25 binary (0 and 1) responses resulting from the 2AFC test are analyzed using maximum likelihood estimates. The likelihood function is used to give two quantities: the most likely stray-light level of the eye under investigation, and the accuracy of this estimate [called expected standard deviation (ESD)]. The CC method is used in 2422 subjects of the GLARE study. Each eye is tested twice to allow analysis of measurement repeatability. Furthermore, the large amount of responses is used to evaluate the shape of the psychometric function, for which a mathematical model is used. The shape of the psychometric function found by averaging the 0 and 1 responses fit well to the model function. Data sorted according to ESD show differences in the shape of the psychometric function between good and bad observers. These different shapes for the psychometric function are used to reanalyze the data, but the stray-light results remain virtually identical. ESD proves to be an efficient tool to detect unreliable measurements. In clinical practice, ESD may be used to decide whether to repeat a measurement.     

Reliability of the cardiac output measurement with the indirect Fick-principle for CO2 during exercise

Cardiac output measurements were performed during 50 exercise tests in 16 normal subjects employing the indirect Fick principle for CO₂. During sub-maximal steady state exercise the plateau CO₂ tension ( ) was estimated with a rebreathing procedure. The mixed venous CO₂ tension ( ) was calculated by subtracting the alveolocapillary CO₂ tension difference from the . Compared with data from the literature the most valid calculation of the cardiac output was obtained by using the . Cardiac output values, calculated via the turned out to be too low.The reproducibility was tested by repitition of 18 exercise tests at least after 5 days. The relative standard error of a single observation was 4.1% for the cardiac output, which was found to be as good as that of invasive measurements.     

Reliability of measurement of oxygen uptake by a portable telemetric system

Summary The purpose of the present study was to check the reliability of measurements of oxygen uptake (VO₂) using a newly developed portable telemetry system. This system (K2) consisted of a face mask, a flow meter, a gas analyser with a transmitter, and a receiver. The total mass for the subject to carry was about 850 g. Three experiments were carried out, firstly to check the reliability and reproducibility of the flow meter and the K2 gas analyser, secondly to check the accuracy of K2 by comparing it with the Douglas bag method (DB), and thirdly to apply K2 to sports activities. In the first experiment, the flow meter was highly accurate up to 180 l·min^–1 with good reproducibility. The measurement error of the gas analyser was less than 2%. In the second experiment, there was no significant difference in the calculated ventilation between K2 and DB. The VO₂ showed no significant difference between K2 and DB with some exceptions. In the third experiment, we succeeded in the measurement of VO₂ during rowing on water. The measurement of VO₂ during running and playing soccer was also possible. It would seem that the present system could well be a powerful tool in the field measurement of VO₂ during various sports activities.     

Electrical characteristic of the titanium mesh electrode for transcutaneous intrabody communication to monitor implantable artificial organs

We have developed a tissue-inducing electrode using titanium mesh to obtain mechanically and electrically stable contact with the tissue for a new transcutaneous communication system using the human body as a conductive medium. In this study, we investigated the electrical properties of the titanium mesh electrode by measuring electrode–tissue interface resistance in vivo. The titanium mesh electrode (Hi-Lex Co., Zellez, Hyogo, Japan) consisted of titanium fibers (diameter of 50 μm), and it has an average pore size of 200 μm and 87 % porosity. The titanium mesh electrode has a diameter of 5 mm and thickness of 1.5 mm. Three titanium mesh electrodes were implanted separately into the dorsal region of the rat. We measured the electrode–electrode impedance using an LCR meter for 12 weeks, and we calculated the tissue resistivity and electrode–tissue interface resistance. The electrode–tissue interface resistance of the titanium mesh electrode decreased slightly until the third POD and then continuously increased to 75 Ω. The electrode–tissue interface resistance of the titanium mesh electrode is stable and it has lower electrode–tissue interface resistance than that of a titanium disk electrode. The extracted titanium mesh electrode after 12 weeks implantation was fixed in 10 % buffered formalin solution and stained with hematoxylin-eosin. Light microscopic observation showed that the titanium mesh electrode was filled with connective tissue, inflammatory cells and fibroblasts with some capillaries in the pores of the titanium mesh. The results indicate that the titanium mesh electrode is a promising electrode for the new transcutaneous communication system.     

Non-invasive measurement of hepatic oxygenation by an oxygen electrode in human orthotopic liver transplantation

Precise evaluation of graft reperfusion is difficult in clinical liver transplantation. The oxygen electrode (OE) is a novel technique to detect blood flow indirectly by measuring the quantity of oxygen which can diffuse from the hepatic tissue to the surface electrode. Application of the surface OE does not influence the liver blood flow or parenchymal perfusion. Adequate graft oxygenation is essential to the outcome of organ transplantation and has not previously been analysed intra-operatively in liver transplant recipients. The OE was applied to the surface of the graft intra-operatively in 22 human liver grafts after restoring portal vein and hepatic artery inflow. OE readings were compared with liver blood flow using an electromagnetic flowmeter (EMF). Intra-operative haemodynamics and donor organ parameters known to influence graft function were correlated with the OE readings. There was a significant correlation (r=0.89; p<0.001, n=14) between tissue oxygenation using the OE and total liver blood flow measured by EMF. The tissue oxygenation measurements were reproducible with a coefficient of variation of 5%. The hepatic tissue oxygenation increased significantly from baseline following venous reperfusion of the graft (282+/-23 vs 3107+/-288 (+/-SE) nA, p<0.001). Hepatic arterial revascularisation resulted in a significant (p<0.001) increase of 41+/-7% in liver oxygen perfusion. There was significant negative correlation (r=0.80, p<0.001, n=22) between cold ischaemic time and graft tissue oxygenation. The OE provides a reliable, cheap and non-invasive method of monitoring liver graft oxygenation and perfusion during transplantation.     

PO2-profiles in hippocampal slices of the guinea pig

Summary For a detailed analysis of the oxygen supply of hippocampal slices, tissue PO₂ (P_{t,o 2}) was recorded polarographically in the neural layers of thick and thin slice preparations from the guinea pig. The experiments showed that the P_t,o₂-gradients were extremely steep in the outer zones of vital slices. In an air equilibrated salt solution the surface PO₂ was reduced to less than 50% within ca. 25 m. Minimum values were measured at a depth of ca. 150 m. A rise of temperature lowered the oxygen supply in the deeper layers of the excised tissue. An elevation of the surface PO₂ hardly improved P_{t,o 2} in the deep structures, because the O₂-consumption of the hippocampal slices increased with rising PO₂.Supported by DFG (Sp 108/6)Dedicated to Professor Dr. H. Caspers on the occasion of his 60th birthday