[Diffusion error and specific consumption of the Pt electrode during pO2 measurements in the steady state]

Po2measurements in the microcirculation using phosphorescence quenching microscopy at high magnification

In phosphorescence quenching microscopy (PQM), the multiple excitation of a reference volume produces the integration of oxygen consumption artifacts caused by individual flashes. We analyzed the performance of two types of PQM instruments to explain reported data on Po2in the microcirculation. The combination of a large excitation area (LEA) and high flash rate produces a large oxygen photoconsumption artifact manifested differently in stationary and flowing fluids. A LEA instrument strongly depresses Po2in a motionless tissue, but less in flowing blood, creating an apparent transmural Po2drop in arterioles. The proposed model explains the mechanisms responsible for producing apparent transmural and longitudinal Po2gradients in arterioles, a Po2rise in venules, a hypothetical high respiration rate in the arteriolar wall and mesenteric tissue, a low Po2in lymphatic microvessels, and both low and uniform tissue Po2. This alternative explanation for reported paradoxical results of Po2distribution in the microcirculation obviates the need to revise the dominant role of capillaries in oxygen transport to tissue. Finding a way to eliminate the photoconsumption artifact is crucial for accurate microscopic oxygen measurements in microvascular networks and tissue. The PQM technique that employs a small excitation area (SEA) together with a low flash rate was specially designed to avoid accumulated oxygen photoconsumption in flowing blood and lymph. The related scanning SEA instrument provides artifact-free Po2measurements in stationary tissue and motionless fluids. Thus the SEA technique significantly improves the accuracy of microscopic Po2measurements in the microcirculation using the PQM.

Theoretical simulation of oxygen tension measurement in tissues using a microelectrode: I. The response function of the electrode

The aim of this article is to determine the correlation between the actual oxygen distribution in tissues and the distribution of oxygen measured by microelectrodes. This correlation is determined by the response function of the electrode, which depends on the oxygen consumed by the electrode. In tissue it is necessary to consider the gradients resulting from cellular respiration. A computer program has been used to simulate the vascular structure of various tissues and also the measurements of oxygen tension using a polarographic electrode. The electrode absorption process is described using a theoretical model. The gradient of oxygen in tissue is described by a mathematical model that takes into consideration both diffusion and cellular consumption of oxygen. We have compared the results obtained using the response function of the electrode and some simplifications of it. The results of these comparisons show that there are some differences in the 'observed' distributions of the oxygen tension in tissues predicted using different formulae for the electrode response function. Also, there are considerable differences between the input oxygen distribution and the measured values in all cases. All the results of the simulations of the oxygen tension 'observed' by a 12 microm polarographic electrode, using different response functions of the electrode, show that the electrode averages the values from many cells. Care should be taken in using a simplification for the response function of the electrode, especially if the results are going to be used as input values in modelling the tumour response to new treatments and/or as a basis of selecting patients for treatments. A computer simulation of measurement of oxygen tensions in regions of steep pO2 gradients shows that extremely high and extremely low pO2 values will not be detected.

Measurements of oxygen tension in the rat kidney after contrast media using an oxygen microelectrode with a guard cathode

The oxygen tension (PO2) in the rat kidney was studied by modified Clark microelectrodes. Changes in PO2 were measured in the renal cortex and outer medulla after intravenous injections of the X-ray contrast medium (CM) diatrizoate, 370 mg iodine/mg body weight. Injection of diatrizoate caused a slight fall in PO2 in the renal cortex (from 42 +/- 4 to 38 +/- 4 mm Hg). In the medulla PO2 decreased significantly (from 34 +/- 6 to 20 +/- 4 mm Hg). Ringer's solution did not induce any changes.

Evaluation of brain tissue O2 supply based on results of PO2 measurements with needle and surface microelectrodes

Tissue PO2 distribution was measured in rat cerebral cortex during arterial normoxia and arterial hypoxia. The study was designed to examine the relationship between the PO2 histograms determined with surface electrodes in the superficial cortical cells and with needle electrodes in the cortical laminae below. Under normoxic as well as under hypoxic conditions the PO2 distributions of the compared brain regions were in close agreement. The results indicate that PO2 measurements in the brain cortex with surface microelectrodes give a correct view over the tissue oxygenation in cortical regions up to 500 microns below the measuring field.

Transcutaneous hydrogen clearance--a new non-invasive technique for assessing blood flow in human skin

A new technique for the non-invasive assessment of skin blood flow is described using a modified, commercially available transcutaneous oxygen electrode. Initially, experiments were carried out on normal volunteers at skin temperatures of 40 degrees C and 44 degrees C. Transcutaneous H2 (tcpH2) clearances were measured after the subject had breathed a mixture of 2% H2 in air until a saturation value in the skin had been reached. tcpH2 clearance results were compared with laser Doppler flux values measured simultaneously on adjacent sites and at the same temperatures under control and reactive hyperaemic conditions. Good reproducibility for the tcpH2 technique and significant correlation (p < 0.002) between it and laser Doppler flux values at the same temperatures was demonstrated. In further experiments, (I125) 4-iodoantipyrine clearances (IAP) were carried out immediately prior to and at the same sites as tcpH2 clearances in critically ischaemic lower limbs. The correlation between the methods and the differences between tcpH2 values observed in normal and critically ischaemic skin were highly significant (p < 0.002).

Spatial variation of aortic wall oxygen diffusion coefficient from transient polarographic measurements

Polarographic current transients following a voltage step (turn-on transient) were measured with bare cathodes (25 microns diameter) and shallowly recessed oxygen microelectrodes (< 5 microns diameter). Except for the initial part of the current transient, the experimental measurements were in excellent agreement with simple models in the literature, which predict an inverse relationship with square root of t. Turn-on transients were measured in aqueous solutions with known physical properties, and in aortic wall tissue from three different species (n = 6 rabbits, n = 3 dogs, and n = 1 miniature pig). Oxygen diffusion coefficients (D) were determined in vitro by comparing time constants measured by the same microelectrode in saline and in strips of aortic wall tissue at 37 degrees C. On the inner side (endothelium and intima) of the aorta, D averaged (+/- S.E.) 7.0 (+/- 0.8) x 10(-6) cm2/s in 6 rabbits, 6.4 (+/- 1.0) x 10(-6) cm2/s in 3 dogs, and was 4.6 x 10(-6) cm2/s in the pig. On the adventitial side, D was 9.5 x 10(-6) cm2/s in 1 rabbit, 11.4 (+/- 1.2) x 10(-6) cm2/s in 3 dogs, and 8.1 x 10(-6) cm2/s in the pig. For every aortic strip on which D was measured from both sides, D for the inner wall was always lower, overall by a little more than one third (p < 0.001). The lower D on the endothelial side may limit oxygen transport to the vascular wall and play a role in atherogenesis.

Use of an oxygen multiwire surface electrode for the direct measurement of the distribution of alveolar PO2 at different inspired oxygen concentrations

Experiments were performed in six anaesthetised, mechanically ventilated rabbits. After thoracotomy, a multiwire surface PO2 electrode was placed at the pleural surface by means of a special holder. The local distribution of PO2 was investigated at different values of inspired oxygen concentration (FIO2). The data were analysed in order to establish (a) whether the PO2 values measured at the lung surface reflect intra-alveolar PO2 and (b) that the distribution of these values represent real physiological differences between alveoli. The mean PO2 values were related to the calculated mean alveolar PO2 (PAO2) values (r = 0.97) indicating that it is possible to study PAO2 by local measurements. It can therefore be concluded that the technique used allows the study of the influence of different FIO2 values on the distribution of PAO2. Each local PAO2 value is the result of the corresponding local ventilation/perfusion ratio (VA/Q) therefore the distribution of PAO2 values represents the distribution of VA/Q. An increasing FIO2 causes an increasing heterogeneity of PAO2 and thus VA/Q leading to a concomitant increase in the alveolo-arterial oxygen difference. At the moment it is not possible to decide if this heterogeneity is primarily caused by heterogeneities in perfusion or ventilation. The results could throw fresh light on the effect of different ventilation procedures.

Relating measuring signals from PO2 electrodes to tissue PO2: a theoretical study

Organ surface PO2 measurement by oxygen sensitive electrodes has proved to be an efficient tool for monitoring changes in tissue oxygenation status. A parameter giving more direct information is the PO2 distribution within tissue cells which, however, can only be assessed by more invasive methods. The present study addresses the problem of relating electrode measured surface PO2 to muscle cell PO2. To that end, the magnitude of tissue volumes the PO2 in which electrodes are sensitive to, has been reassessed. It turned out that the measuring signal of current membrane covered PO2 electrodes is only sensitive to the PO2 within the muscle surface and not any deeper, thus rendering the measured PO2 an area weighted average over the surface PO2 rather than a spatial average. The consequences of this finding are illustrated in an example of a maximally working muscle. Under the assumption that there are capillaries located on the muscle surface or immediately beneath it, the PO2 predicted to be measured by a surface electrode is almost four times the average muscle fiber PO2. However, PO2 values actually measured by surface PO2 electrodes are even higher calling for further explanation, in which superficial blood vessels and surface fascia may play an important role. Quantitative information on vascular morphology near muscle surfaces is needed in order to more definitely determine the importance of mechanisms discussed, for experimental results.

The contribution of reoxygenation to ischemic brain damage

This study examined the hypothesis that the level of postischemic reperfusion affects the severity of the resulting neuronal necrosis. In rats, tissue PO2% was monitored as an index of flow (reoxygenation) at four cortical sites by chronically implanted platinum electrodes. Twenty minutes of total global cerebral ischemia was followed by 30 min of reoxygenation. The level of reoxygenation was controlled to maintain the PO2 nearly constant at one or more of the cortical electrodes. Tissue from within 400 microns of each of 19 electrode sites among seven rats was evaluated histologically. There was a positive correlation between reoxygenation level and severity of neuronal damage. Perineuronal lucent halo formation, probably representing astrocyte foot process swelling, was negatively correlated with reoxygenation level. This study demonstrates that ischemic neuronal damage was aggravated by increased reoxygenation but that perineuronal swelling, as evidenced by halo formation, was somewhat ameliorated.

Tumor tissue oxygenation as evaluated by computerized-pO2-histography

A computerized pO2 measurement system with a novel electrode motion pattern (Sigma-pO2-histography) was evaluated in vitro and in vivo. The system was found to be reliable in 0.9% saline and 10% hydroxyethylene starch solution and in fresh donor blood. Marked deviations were found in lipid and hemoglobin solutions and in fluorocarbon emulsions. Histograms obtained in rat liver, mouse muscle, and subcutis were similar to previously reported distributions. Direct comparison between Sigma-Eppendorf and self-constructed Whalen-type electrodes in hypoxic tumors gave similar results. A large series of measurements indicated that hypoxic and anoxic tissue areas were frequently found both in isografted rodent and in xenografted human tumors. The extent of oxygen deprivation depended on the cell line studied, tumor size, implantation site, the vascularity, and the actual tissue perfusion. Pentobarbital anesthesia redistributed the tumor oxygenation without affecting the median pO2 value. Tumors growing in a pre-irradiated bed were less oxygenated than those at untreated sites. Hyperthermia at therapeutically relevant temperatures reduced pO2 levels in adequately oxygenated tumors whereas little change was detected in poorly oxygenated tumors. First measurements in tumors in patients revealed marked inter- and intratumor heterogeneity. It is concluded that this novel technique is suitable for routine measurements of tissue oxygenation of solid tumors in situ.

A multiwire hydrogen electrode for in vivo use

A multiwire surface electrode is described for measuring the partial pressure of hydrogen gas within extremely small volumes. The purpose was to record hydrogen clearance curves in vivo in order to analyse capillary blood flow. A method for improving the sensitivity and stability of the Clark-type polarographic sensor is presented. The in vitro and in vivo properties were investigated and are critically compared with the characteristics predicted from various models for the polarographic measurements of gases. The high stability and low drift of the electrode together with its small catchment volume (a hemisphere of radius 32 microns) meant that it could be reliably used for accurate, reproducible local measurements of hydrogen clearance curves in vivo. The experiments also demonstrated that the electrode could be used most successfully for the measurement of capillary blood flow even in the heart and contracting skeletal muscle.

Oxygen supply of the blood-free perfused guinea-pig brain in normo- and hypothermia measured by the local distribution of oxygen pressure

The O2 supply of the blood-free perfused brain cortex of the guinea pig was investigated by measuring polarographically the local distribution of tissue PO2 at 18 degrees C, 24 degrees C, and 37 degrees C. The perfusion was performed in situ, using a medium equilibrated by a gas mixture of 95% O2 and 5% CO2. Papaverine was added to prevent vasoconstriction during hypothermia. To avoid measuring artefacts thin micro electrodes with a small sharpened tip of ca. 4 microns in diameter were used and a special puncturing technique was applied. The experimental results indicate the presence of a large variation of local tissue PO2. Local mean PO2 increased up to a depth of 1000 microns, reached a plateau, and then decreased towards 3000 microns. This demonstrates that the O2 supply changes in dependence of the distance of the brain surface. This may partly be caused by the special vascularization pattern of the brain cortex. As it follows from the PO2 histograms, at 24 degrees C the tissue layer between 0-2000 microns (layer I) was well supplied with oxygen, whereas at the same time the layer between 2001-3000 microns (layer II) was hypoxic. At 37 degrees C, both layers were hypoxic, but layer III showed the more pronounced tissue hypoxia. To obtain a sufficient oxygen supply the temperature had to be reduced below 24 degrees C to sufficiently decrease tissue O2 consumption: at 18 degrees C, there was no sign of hypoxia any more. In comparison with the PO2 histogram of the tissue the PO2 histogram of the pial surface was shifted to higher PO2 values.(ABSTRACT TRUNCATED AT 250 WORDS)

Staining PO2 measurement sites in the rat brain cortex and quantitative morphometry of the surrounding capillaries

The exact positions of microelectrodes used to measure the PO2 in the cerebral cortex of the rat were determined by staining the tissue with Alcian Blue. The measurement sites were subsequently located under a light microscope and correlated with the capillary and cellular arrangement of the cortex. The microelectrodes used for the PO2 measurements were made of gold glass fibers; the Alcian Blue was injected hydrostatically through a micropipette attached to the PO2 microelectrode. The sites where dye had been deposited were seen under a light microscope as green blue spots about 100 micrometers in diameter. The capillaries were visualized by silver nitrate perfusion. Differences between the local PO2 values in the neo- and the archeocortex were found. In the neocortex the mean PO2 was 31 mm Hg, capillary volume 1.6%, capillary surface area 980/mm2, capillary length 13.5/mm; whereas in the archeocortex these values where 21 mm Hg, 0.9%, 820/mm2 and 9.4/mm respectively. These data indicate a relationship between the microcirculatory transport system and the local oxygen tension and provide further evidence that the mean PO2 level tends to decrease when moving from the surface into the archeocortex.

Data acquisition and information processing in MDO oxygen electrode measurement of tissue oxygen pressure

In 1956, CLARK presented the principle for the modern, membrane-covered, oxygen electrode, comprising the anode and the cathode within the same unit. A further development was the MDO (Mehrdraht Dortmund Oberfläche) oxygen electrode presented by LUBBERS & KESSLER during the 1960s. This electrode comprises eight separate measuring points for collecting statistical samples to calculate tissue oxygen pressure fields. In order to perform studies on humans and to enable fast presentation of measurement results, a system was developed including a PDP 11/03 D computer, which fulfils patient safety demands. The computer program included corrections for electrode drift and temperature influences, as well as statistical calculations including mean, standard deviation, skewness and kurtosis. Also included was the two-sample Kolmogorov-Smirnov test for comparison of tissue oxygen distributions. Experience from studies on humans has shown that the measurement system enables us to collect and read important patient data directly at the bedside in the intensive care ward.

Permeability and diffusion coefficient of oxygen in membranes for oxygen electrodes

Oxygen diffusion coefficient and permeability in thin membranes of low-density polythene and laminated polytetrafluorethylene (PTFE) are measured over the temperature range of about 20 to 30 degrees C. The diffusion coefficient is estimated from the time response of a large area platinum electrode covered with the membrane material and exposed to a sudden change in PO2. outside the membrane. The electrode is operated at a very low polarising voltage so that consumption of oxygen at the cathode surface is made negligible. This method is considered to be more reliable than that based on operating the electrode at more normal polarising voltages at which the boundary condition for oxygen tension at the cathode surface is often ill-defined. Oxygen permeability is estimated from the flux of oxygen across an area of membrane separating a nitrogen gas stream from one containing oxygen, the oxygen being measured with a platinum/zirconium oxide reversible cell. Diffusion coefficient in the membranes is very temperature dependent, whereas oxygen solubility is only weakly so. The diffusion coefficient at 25 degrees C is 0.28 +/- 0.06 . 10(-6) and 1.0 +/- 0.2 . 10(-6) cm2 . sec-1 for PTFE and polythene respectively. The corresponding values for permeability are 0.80 +/- 0.08 . 10(-7) and 0.43 +/- 0.04 . 10(-7) ml O2 atm-1 . cm-1 . sec-1.

[Local pO2- and pH2-measurements with needle electrodes for the examination of the hydrogen maintenance and microcirculation of the cochleae (author's transl)]

The local partial pressure of oxygen (pO2) and the rate of hydrogen elimination were measured in the three scalae of the basal turn of 28 Guinea-pig cochleae under conditions of normoxia, hyperoxia and hypercapnia and with acoustical stimulation with the needle electrodes developed by Baumgärtl and Lübbers. In the scala tympani a pO2 decrease from the round window toward Corti's organ was registered and pO2 values of over 100 mm Hg were measured near the membrane of the round window and of 10-40 mm Hg near the basilar membrane depending on how deeply the electrode penetrated into the scala tympani. The pO2 profiles were changed or reversed when the animal breathed a mixture of 95% oxygen and 5% carbon dioxide and when the round window membrane was covered with agar-agar or paraffine and exteriorly flooded with nitrogen. Acoustical stimulation with a white noise of 85 dB caused a considerable pO2 drop in the perilymph of the scala tympani while in the endolymph of the scala media we observe only a slight decrease. Intravenous application of dextran of low viscosity leads to a pO2 increase when the original oxygen value in the scala tympani was low. The half-life of hydrogen in the scala tympani amounts to about 4 min. The results permit the conclusion that, in the area of the cochlear basis, Corti's organ receives its oxygen supply via the capillary system as well as via the membrane of the round window.

A transcutaneous PO2 electrode for continuous monitoring of fetal state during delivery

A transcutaneous PO2 electrode has been designed and constructed for the continuous measurement of fetal oxygen tension during labor and delivery. The electrode, tested in vitro, has a low oxygen consumption, a short response time, 10 sec, and a sufficient stability, 5% over 3 hr. The in vivo performance of the electrode, tested in neonates, shows a good correlation between transcutaneous PO2 and PO2 measured in samples of arterial blood. The continous registration of fetal tcPO2 during labor and delivery is exemplified.

Oxygen fields induced by recessed and needle oxygen microelectrodes in homogeneous media

Computer simulations of the operation of both the recessed and conically-shaped needle type microelectrodes were developed. The accurate portrayal of their geometries was greatly facilitated by the use of specialized three dimensional orthogonal coordinate systems with coordinate surfaces coincident with the geometry of the problem. From the simulations, calculations were made of the induced PO2 fields, oxygen sensitivities, stirring artifacts, measurement errors, and time constants.

The oxygen pressure histogram in the left ventricular myocardium of the dog

Myocardial PO2 in its relation to arterial and coronary venous PO2 was measured in anesthetized open-chest dogs by means of polarographic methods. At normal arterial O2 supply, there exists a broad PO2 spectrum in the myocardium, ranging from about 0 Torr to arterial values (mean 19.3 Torr, median 14 Torr). 60% of the values are below the mean coronary venous PO2 of 18.8 Torr, the maximum is in the class of 0 to 5 Torr. Neither the shape of the histogram nor the mean coronary venous PO2 are significantly changed under moderate arterial hypoxemia (mean arterial PO2 63.7 Torr). Also increases in arterial PO2 to more than 300 Torr cause no distinct change in coronary venous PO2, whereas myocardial PO2 shows heterogeneous reactions. Myocardial as well as coronary venous PO2 seem to be well regulated over a wide range of arterial PO2.