C.E.W. Hahn

Electrochemical improvement of the performance of Po2 electrodes

Rotating ring-disc electrode studies have indicated that relatively large quantities of hydrogen peroxide ion, HO2-, are produced when oxygen is reduced at a platinum or gold polarographic electrode surface. The electrochemical reduction processes are improved and the quantity of HO2- is reduced by using alkaline buffer electrolytes (pH 10 to 11) and by polarising the electrode at voltages more negative than -0.9 V. The presence of HO2- in the electrolyte has been shown to be the cause of excessively long time response in both blood-gas and respiratory polarographic PO2 electrodes; electrode alinearity on micro-blood-gas PO2 electrodes has also been shown to be due to the absence of a plateau on the polarogram of electrodes when used with conventional electrolytes. The use of a high pH buffer and high negative voltage results in a long, flat plateau and a marked improvement in both electrode linearity and response time. This two-fold improvement in electrode performance holds true for both platinum and gold polarographic respiratory gas and blood-gas PO2 electrodes.

Properties of blood oxygen transport in the turtle Pseudemys Scripta and the Tortoise Testudo Graeca: Effects of temperature, Co2 and pH

Properties of oxygen-haemoglobin binding have been investigated in the aquatic turtle Psuedemys scripta and the terrestrial tortoise Testudo graeca. Haematocrit (30-35%) and haemoglobin concentration (12-14 g/100 ml blood) were similar in both species. P50 at physiological levels of PCO2 (20-25 mm Hg) was 21 mm Hg in Pseudemys, compared with 23 mm Hg in Testudo. The Bohr shift of the blood of both the turtle and the tortoise was almost identical at -0.28. The heat of oxygenation, deltaH, reflecting the temperature sensitivity of O2-Hb affinity, was -10.55 in Pseudemys and -8.12 kcal/mol in Testudo. These data on whole blood do not support previous generalizations in the literature suggesting marked differences in oxygen-haemoglobin binding between aquatic and terrestrial chelonian reptiles.

The development of new microelectrode gas sensors: an odyssey. Part 1. O2 and CO2 reduction at unshielded gold microdisc electrodes

Abstract In this paper we describe the reduction of O 2 and CO 2 , when both molecules are always present and the balance gas is N 2 , at unshielded gold microdisc electrodes in dimethyl sulphoxide (DMSO) in a standard reaction cell. We show that both gases can be reduced in the presence of each other, in the range 3–15 vol.% CO 2 and 10–30 vol.% O 2 , with minimal cross-interference from “feedforward” and “feedback” reaction products from the individual O 2 and CO 2 reduction processes. Well-separated O 2 and CO 2 reduction processes were obtained with gold microdisc electrodes when the polarizing voltage was swept typically at 0.1–0.5 V s −1 , and steady state limiting currents were measured which were proportional to the O 2 and CO 2 concentrations.

A tidal breathing model of the forced inspired inert gas sinewave technique.

We have shown previously that it is possible to assess the cardio-respiratory function using sinusoidally oscillating inert gas forcing signals of nitrous oxide and argon (Hahn et al., 1993). This method uses an extension of a mathematical model of respiratory gas exchange introduced by Zwart et al. (1976), which assumed continuous ventilation. We investigate the effects of this assumption by developing a mathematical model using a single alveolar compartment and incorporating tidal ventilation, which must be solved using numerical methods. We compare simulated results from the tidal model with those from the continuous model, as the governing ventilatory and cardiac parameters are varied. The mathematical model is designed to be the basis of an on-line, non-invasive, cardio-respiratory measurement method, and will only be useful if the associated parameter recovery techniques are both reliable and robust. We demonstrate, in the presence of simulated measurement errors, that: (a) accurate recovery of the ventilatory parameters end-tidal volume, VA, and airways series dead-space, VD, are possible using the tidal breathing model; and (b) that a robust technique for recovery of pulmonary blood flow, QP, can be obtained using the more familiar continuous ventilation model.

A Cylindrical-Core Fiber-Optic Oxygen Sensor Based on Fluorescence Quenching of a Platinum Complex Immobilized in a Polymer Matrix

A miniature (200 μm in diameter) cylindrical-core fiber-optic oxygen sensor has been developed for measuring rapid change in oxygen partial pressure (pO2). The fiber-optic sensing element is based on a cylindrical-core waveguide structure formed by coating a thin medical grade polymer sensing film that contains immobilized Pt(II) complexes on silica optical fiber. The performance such as sensitivity and time response of the fiber-optic oxygen sensors were evaluated using luminescence intensity measurement. To determine accurately the response time of the fiber optic oxygen sensors, a test chamber was used to provide rapid changes in the partial pressure of oxygen. The result showed that the time response (time-constant, τ) of this cylindrical-core fiber optic oxygen sensor is less than 50 ms. To our knowledge, this is the fastest such sensor of this size covering the full dynamic range of pO2 from 0 to 100 kPa.

The effect of dyshemoglobins on pulse oximetry: Part I, theoretical approach and part II, experimental results using an in vitro test system

Pulse oximeters are known to be inaccurate in the presence of elevated concentrations of carboxyhemoglobin and methemoglobin. This paper attempts to alleviate some of the confusion that exists between fractional and functional saturation, and to clarify the comparison of each with SpO2. A series of theoretical relationships between pulse oximeter reading (SpO2) and actual oxygen saturation (both fractional and functional) is derived using simple absorption theory. The theoretical relationships are checked using an experimental in vitro test system. This consists of a blood circuit containing a model finger, capable of simulating the pulsatile transmission signals through a real finger. Theoretical predictions and experimental results are compared and are found to agree well in the presence of carboxyhemoglobin, but less well with methemoglobin. Possible reasons are discussed.Pulse oximeters are known to be inaccurate in the presence of elevated concentrations of carboxyhemoglobin and methemoglobin. This paper attempts to alleviate some of the confusion that exists between fractional and functional saturation, and to clarify the comparison of each with SpO2. A series of theoretical relationships between pulse oximeter reading (SpO2) and actual oxygen saturation (both fractional and functional) is derived using simple absorption theory. The theoretical relationships are checked using an experimental in vitro test system. This consists of a blood circuit containing a model finger, capable of simulating the pulsatile transmission signals through a real finger. Theoretical predictions and experimental results are compared and are found to agree well in the presence of carboxyhemoglobin, but less well with methemoglobin. Possible reasons are discussed.

The development of new microelectrode gas sensors: an odyssey. Part 2. O2 and CO2 reduction at membrane-covered gold microdisc electrodes

Abstract In a previous paper we described the reduction of O 2 and CO 2 , in the presence of each other, at unshielded microdisc electrodes in an aprotic solvent with minimal cross-interference between the competing O 2 and CO 2 reduction reactions. In this new work, we describe a practical Clark-type membrane-covered sensor for the simultaneous measurement of O 2 and CO 2 . The sensor comprises a gold disc microelectrode, housed in a PTFE holder and covered with a PTFE membrane, with dimethylsulphoxide (DMSO) as the solvent and with a silver quasi-reference electrode. The polarizing voltage is swept from 0 to −2.4 V vs. Ag at a typical sweep rate of 0.25 or 0.5 V s −1 and the O 2 and CO 2 concentrations are determined from the limiting currents of the well separated O 2 and CO 2 reduction waves. We demonstrate that the addition of up to 10% v/v H 2 O to the solvent does not appear to compromise the ability of the sensor to analyse O 2 and CO 2 simultaneously.

A fibre optic oxygen sensor that detects rapid PO2 changes under simulated conditions of cyclical atelectasis in vitro

Highlights • Real time detection of cyclical atelectasis is fundamental for individualised mechanical-ventilation therapy in ARDS.• Intra-arterial oxygen sensors could be used to detect the breath-by-breath oscillations in PO2 during cyclical atelectasis.• The fidelity with which oxygen sensors can detect these arterial PO2 oscillations depends on the sensors’ speed of response.• We present a system for testing fast-response fibre optic oxygen sensors under simulated conditions of cyclical atelectasis.• We show that a prototype fibre optic oxygen sensor, compatible with clinical use, can detect rapid PO2 changes in vitro.

O2 and N2O analysis with a single intravascular catheter electrode

The simultaneous measurement of O2 and N2O in liquid, using a single polarographic catheter electrode, is described. It is shown that commercial PO2 intravascular electrodes, with silver cathodes, produce separate and distinct polarograms for O2 and N2O, and that these electrodes can be used for the measurement of both PO2 and PN2O.

A tidal ventilation model for oxygenation in respiratory failure

We develop tidal-ventilation pulmonary gas-exchange equations that allow pulmonary shunt to have different values during expiration and inspiration, in accordance with lung collapse and recruitment during lung dysfunction (Am. J. Respir. Crit. Care Med. 158 (1998) 1636). Their solutions are tested against published animal data from intravascular oxygen tension and saturation sensors. These equations provide one explanation for (i) observed physiological phenomena, such as within-breath fluctuations in arterial oxygen saturation and blood-gas tension; and (ii) conventional (time averaged) blood-gas sample oxygen tensions. We suggest that tidal-ventilation models are needed to describe within-breath fluctuations in arterial oxygen saturation and blood-gas tension in acute respiratory distress syndrome (ARDS) subjects. Both the amplitude of these oxygen saturation and tension fluctuations, and the mean oxygen blood-gas values, are affected by physiological variables such as inspired oxygen concentration, lung volume, and the inspiratory:expiratory (I:E) ratio, as well as by changes in pulmonary shunt during the respiratory cycle.