Conventional Blood Gas Analysis Research Articles

The Development of the Acoustic Capnometer

We developed an acoustic capnometer to estimate the partial pressures of arterial carbon dioxide from expired air in the pulmonary rehabilitation devices for patients with respiratory failure. Because partial pressures of carbon dioxide reduce the velocity of sound propagating through expired air, we developed an acoustic capnometer. The present study proposes a unique method based on the measurement of acoustic velocity in expired air, thus eliminating the use of a specific carbon dioxide sensor. The current method can fabricate the capnometer at an appreciably low cost, enabling the device to be used for various rehabilitation purposes. The acoustic capnometer comprises a cylindrical small sample cavity, an ultrasonic transmitter-receiver system, electronic circuits for velocity determination, and a microprocessor for data processing. To validate the device, 12 patients with pulmonary disease were enrolled, and end-tidal partial pressures of carbon dioxide obtained from the current device were compared to the carbon dioxide tension measured by conventional arterial blood gas analysis. The results show a linear relationship in the region of interest (40–60 torr). Therefore, the proposed device facilitates the estimation arterial carbon dioxide partial pressures without sampling blood.

Feasibility of photoacoustic imaging for the non‐invasive quality management of stored blood bags

During the in vitro storage of red blood cells (RBCs), unfavourable changes (storage lesions) cause a rapid consumption of intracellular diphosphoglycerate. The latter deregulates the oxygen-haemoglobin binding potential, subsequently increasing oxygen saturation (SO2 ) and membrane degradation, transforming RBCs from biconcave discs to rigid spherical bodies (spheroechinocytes). Current laboratory techniques invasively extract RBC samples to assess the quality of red cell concentrate (RCC) units. Optical technologies could provide a means of assessing quality non-invasively. A photoacoustic (PA) imaging technique was developed for acquiring the SO2 of blood bags non-invasively. Seven RCC units were monitored every 3-5days until expiry (6weeks). Measurements were validated against a conventional blood gas analyzer (BGA). Using an image flow cytometry assay, morphological profile trends were compared against the SO2 trends during blood bag storage. A strong correlation (r2 ≥0·95) was found when comparing temporal data between PA and BGA SO2 measurements. Inter-sample PA variability was found to be similar to that produced by BGA (±0·8%). A strong correlation was found to exist between the temporal changes in SO2 and relative spheroechinocyte population (0·79≤r2 ≤0·97). This study suggests that PA imaging can non-invasively track the SO2 of stored RBCs non-invasively. By longitudinally monitoring the change in SO2 , it is possible to infer the effects of the storage lesion on RBC morphology. This non-invasive monitoring technique allows for the assessment of blood bags, without compromising sterility pre-transfusion.

Accuracy and trending ability of hemoglobin measurement by the Pulse CO-Oximeter during vascular surgery.

Real time information of Hb concentration can guide a tailored patient blood management. The study investigates the accuracy, precision and trending ability of the Pulse CO-Oximeter (SpHb) and blood gas analyzer in measuring the Hb concentration, compared to hematological analysis, in surgery at high risk of hemorrhage. We performed an observational study, involving 48 patients undergoing abdominal aortic open surgery. The primary endpoints of the study were to compare the accuracy in measuring the Hb concentration using non-invasive method (Masimo rainbow SET® Radical 7 Pulse CO-Oximetry™) compared to the values provided by invasive conventional blood gas analyzer and hematological analysis. The secondary endpoint was to compare the differences between the baseline and the final value of the Hb after surgery (Δ-values), as well as the trending ability. Bias (precision) for the SpHb was 1.63g/dL (± 0.05) with 95% limits of agreement between 0.85 and + 2.4g/dL, while for the blood gas analyzer was 0.69g/dL (± 0.04) with 95% limits of agreement between 0.07 and 1.3g/dL. Δ-values values were not statistically different from the reference values of ΔHb obtained with the hematological analysis. Trending ability was good for both Pulse CO-Oximeter and blood gas analysis. Our results have shown that the SpHb is not precise enough to replace an invasive approach, but the trending ability of SpHb is accurate and may provide important information on the changes in hemoglobin concentration to guide blood management.

Evaluation of the i-STAT Blood Gas Analysis System in Cardiovascular Surgery

The aim of this study was toinvestigate the compatibility of the parameters measured with the i-STAT blood gas analyser and the conventional blood gas analyser Rapid Point 500 (Siemens Healthcare Diagnostics, USA) in patients who underwent cardiovascular surgery. This clinical study included fifty patients undergoing coronary artery bypass surgery. Fifty whole blood samples were portioned and measured on the i-STAT and RP500 laboratory analyzers. The compatibility between pH, pCO2, pO2, Hb, Na+, K+, iCa2+ and glucose values was investigated.There was a good correlation of the i-STAT analyser with the RP500 analyser, with the exception Hb and Na+. Also all parameters except for Hb and ionized calcium were found to be within acceptable range in terms of clinical decision limits. It is very important that the point-of-care devices give accurate results as well as quick results. For this reason, we absolutely think that the point of care devices should be subjected to external and internal quality control programs, users should be trained regularly and feedback studies should be done.

Evaluation of a novel point of care blood gas analyser

IntroductionPoint-of-care testing is becoming increasingly relevant to the practice of anaesthesia and critical care medicine with the recent development of new devices. We performed a prospective observational study to evaluate a novel, in-line, patient dedicated, point of care blood gas analysis device called the Proxima. The sampling volume is flushed into the patient after analysis resulting in no loss of blood during analysis. The device was also subjected it to extreme physiological conditions, specifically deep hypothermic circulatory arrest.MethodsEight patients who were undergoing pulmonary endarterectomy were studied, and had between 7 and 11 samples taken at set intervals were analysed both with Proxima and a conventional blood gas analyser for seven variables (pH, pCO2, pO2, HCO3-, base excess [BE], K+ and haematocrit [Hct]). All patients underwent a process of cooling to 19°C on cardiopulmonary bypass, and subsequent rewarming to normothermia.ResultsSee tableDiscussionIn summary, we have demonstrated that the Proxima device is effective in measuring pH, pO2, pCO2 and [K+], when compared with a conventional, laboratory blood gas analyser. Derived measures calculated by the device ([HCO3-] and (BE) differed substantially from those produced by the benchtop analyser. The advantages of this device is its dedicated nature, quicker analysis and the reduction of sampling anaemia.(1)

Abstract 83: Effect of Therapeutic Hypothermia on Oxygenation Index in Rat Model of Cardiac Arrest

Introduction: Therapeutic hypothermia improves the outcomes of cerebral function after resuscitation from cardiac arrest (CA). The effect of therapeutic hypothermia on post resuscitation pulmonary function, however, is less known. In the present study, we investigated the effect of therapeutic hypothermia on oxygenation index, a sensitive index of pulmonary function,in a rat model of cardiac arrest and resuscitation. Hypothesis: We hypothesize that during therapeutic mild hypothermia in a rat model of cardiac arrest, the pulmonary function following resuscitation is less impaired when compare to normothermia. Methods: Twenty-one male Sprague-Dawley rats were randomized into three groups: 1) control group (control, n=5): the normothermic rats only received anesthesia and the surgical procedure as the other groups without ventricular fibrillation (VF); 2) normothermia group (NT, n=7): the normothermic rats were subjected to induced VF for 8 mins followed by 8 mins of cardiopulmonary resuscitation (CPR); 3) Mild hypothermia group (HT, n=9): the rats were subjected to induced VF for 8 mins followed by 8 mins of CPR. Mild hypothermia of 33±0.5°C was started 5 mins after return of spontaneous circulation (ROSC) and maintained for 8 hrs. The oxygenation indexes were measured at baseline, 2, 4 or 8 hours after ROSC with a conventional blood gas analyzer (PHOX plus L; Nova Biomedical Corporation, Waltham, MA, USA). Results: Compared to the control group, the oxygenation indexes of both the NT and HT groups were significantly decreased at 2 hrs after ROSC. However, more significant reduction in oxygenation index was observed in the NT group (Figure). Conclusions: There is an early pulmonary dysfunction after successful resuscitation from cardiac arrest. Hypothermia reduces the impairment of pulmonary function.

Valoración precoz de la anemia en el paciente con hemorragia digestiva aguda: concordancia entre gasometría venosa y laboratorio convencional

Background and objectiveEvaluation of patients with acute gastrointestinal bleeding (AGB) requires early clinical evaluation and analysis. The aim of this study is to evaluate early concordance of hemoglobin (Hb) and hematocrit (HTC) levels determined by conventional venous blood gas analysis (VBG) and by conventional Laboratory in Emergencies (LAB). Patients and methodsObservational and prospective study of patients admitted in the Gastrointestinal Haemorrhage Unit with both high and low AGB. Demographic and clinical variables and simultaneous venous blood samples were obtained to determine Hb and HTC by VBG and LAB. Concordance in both methods was analysed by intra-class correlation coefficient (ICC) and Bland-Altman analysis. ResultsOne hundred and thirty-two patients were included: 87 (65.9%) males, average age 66.8 years. VBG overestimated Hb in 0.49g/dl (95% confidence interval: 0.21-0.76) with respect to LAB. Concordance was very high in Hb (ICC 0.931) and high in HTC (0.899), with the Bland-Altman graphs showing both concordance and overestimation of Hb levels determined by VBG. In 19 patients (14.39%), Hb by VBG exceeded in more than 1g/dL the final determination obtained by LAB. ConclusionsEarly determination of Hb and HTC in patients with AGB by VBG provides reliable results in the initial evaluation of anaemia. VBG systematically overestimates Hb values by less than 0.5g/dl, and therefore clinical and hemodynamic evaluation of the bleeding patient should prevail over analytical results.

Continuous Intra-Arterial Blood pH Monitoring by a Fiber-Optic Fluorosensor

Continuous intra-arterial blood pH monitoring is highly desirable in clinical practice. However, devices with appreciable accuracy are still not commercially available to date. In this study, we present a fiber-optic fluorosensor that can be used to continuously and accurately measure blood pH changes. The pH sensor is developed based on a proton-sensitive fluorescence dye, N-allyl-4-(4'-methyl-piperazinyl)-1,8-naphthalimide, which is bonded covalently to an optical fiber through heat polymerization. Fluorescence intensity was recorded after the sensor was exposed to different pH buffer solutions or intra-arterial blood in rabbits. Fluorescence intensity with emission peak at 510 nm decreased immediately as the blood pH increased. Linear and reproducible responses were observed when pH ranges from 6.8 to 8.0 with resolution of 0.03 pH units. The correlation coefficient between the pH sensor and the conventional blood gas analyzer was 0.93 in vivo ( n=75, p<0.001) with a bias and precision of -0.02 ± 0.08 pH units. The pH sensor was stable during measurement for at least 72 h. The pH sensor is not sensitive to fluctuations of various ions' concentrations and plasma osmosis at pathophysiological limits, suggesting that it is useful for the continuous measurement of blood pH at various clinical settings.

The accuracy of non‐invasive carbon dioxide monitoring: A clinical evaluation of two transcutaneous systems

We determined the accuracy of two transcutaneous carbon dioxide monitoring systems (SenTec Digital Monitor with V-Sign Sensor and TOSCA 500 with TOSCA Sensor 92) for the measurement of single values and trends in the arterial partial pressure of carbon dioxide in 122 adult patients during major surgery and in 50 adult patients in the intensive care unit. One or several paired measurements were performed in each patient. The first measurement was used to determine the accuracy of a single value of transcutaneous carbon dioxide; the difference between the first and the last measurements was used to analyse the accuracy and to track trends. We defined a 95% limit of agreement of <or=1 kPa as being clinically useful. There was insufficient agreement between transcutaneous carbon dioxide partial pressure values derived from the two systems and arterial carbon dioxide values for both single values and trends as defined by our suggested limit of agreement. We conclude that these systems cannot replace conventional blood gas analysis in the clinical setting studied.

Methemoglobinemia-induced Cardio-respiratory Failure Secondary to Topical Anesthesia

Acute toxic methemoglobinemia is an infrequent complication of the use of topical anesthetics, most notably benzocaine. The clinical picture is characterized by sudden development of tissue hypoxia without underlying cardiac or respiratory dysfunction, and deceptively normal oxygen saturation on conventional arterial blood gas analysis. This condition may be rapidly fatal and management depends upon prompt recognition, confirmation of clinical suspicion using cooximetry of arterial blood, and quick institution of therapy. We describe the first reported case of cardio-respiratory failure associated with acute toxic methemoglobinemia, which was initially misconstrued as an evolving acute coronary event and rapidly responded to methylene blue therapy.

The Implications of Arterial Po2 Oscillations for Conventional Arterial Blood Gas Analysis

In a surfactant-depletion model of lung injury, tidal recruitment of atelectasis and changes in shunt fraction lead to large Pao2 oscillations. We investigated the effect of these oscillations on conventional arterial blood gas (ABG) results using different sampling techniques in ventilated rabbits. In each rabbit, 5 different ventilator settings were studied, 2 before saline lavage injury and 3 after lavage injury. Ventilator settings were altered according to 5 different goals for the amplitude and mean value of brachiocephalic Pao2 oscillations, as guided by a fast responding intraarterial probe. ABG collection was timed to obtain the sample at the peak or trough of the Pao2 oscillations, or over several respiratory cycles. Before lung injury, oscillations were small and sample timing did not influence Pao2. After saline lavage, when Po2 fluctuations measured by the indwelling arterial Po2 probe confirmed tidal recruitment, Pao2 by ABG was significantly higher at peak (295 +/- 130 mm Hg) compared with trough (74 +/- 15 mm Hg) or mean (125 +/- 75 mm Hg). In early, mild lung injury after saline lavage, Pao2 can vary markedly during the respiratory cycle. When atelectasis is recruited with each breath, interpretation of changes in shunt fraction, based on conventional ABG analysis, should account for potentially large respiratory variations in arterial Po2.

Significance of continuous blood gas monitoring in cardiac surgery with cardiopulmonary bypass

Significance of continuous blood gas monitoring in cardiac surgery with cardiopulmonary bypass

Reduction in Red Blood Cell Transfusions Using a Bedside Analyzer in Extremely Low Birth Weight Infants

Preterm infants typically experience heavy phlebotomy losses from frequent laboratory testing in the first few weeks of life. This results in anemia, requiring red blood cell (RBC) transfusions. We recently introduced a bedside point-of-care (POC) blood gas analyzer (iSTAT, Princeton, NJ) that requires a smaller volume of blood to replace conventional Radiometer blood gas and electrolyte analysis used by our neonatal intensive care unit (NICU). The smaller volume of blood required for sampling (100 vs 300-500 microl), provided an opportunity to assess if a decrease in phlebotomy loss occurred and, if so, to determine if this resulted in decreased transfusions administered to extremely low birth weight (ELBW) infants. We hypothesized that the use of the POC iSTAT analyzer that measures pH, PCO(2), PO(2), hemoglobin, hematocrit, serum sodium, serum potassium and ionized calcium would result in a significant decrease in the number and volume of RBC transfusions in the first 2 weeks of life. A retrospective chart review was conducted of all inborn premature infants with birth weights less than 1000 g admitted to the NICU that survived for 2 weeks of age during two separate 1-year periods. Blood gas analysis was performed by conventional laboratory methods during the first period (designated Pre-POC testing) and by the iSTAT POC device during the second period (designated post-POC testing). Data collected for individual infants included the number of RBC transfusions, volume of RBCs transfused, and the number and kind of blood testing done. There was no effort to change either the RBC transfusion criteria applied or blood testing practices. The mean (+/-SD) number of RBC transfusions administered in the first 2 weeks after birth was 5.7+/-3.74 (n=46) in the pre-POC testing period to 3.1+/-2.07 (n=34) in the post-POC testing period (p<0.001), a 46% reduction. The mean volume of RBC transfusions decreased by 43% with use of the POC analyzer, that is, from 78.4+/-51.6 ml/kg in the pre-POC testing group to 44.4+/-32.9 ml/kg in the Post-POC testing group (p<0.002). There was no difference between the two periods in the total number of laboratory blood tests done. Use of a bedside blood gas analyzer is associated with clinically important reductions in RBC transfusions in the ELBW infant during the first two weeks of life.

Investigation of the effects of naratriptan, rizatriptan, and sumatriptan on jugular venous oxygen saturation in anesthetized pigs: implications for their mechanism of acute antimigraine action.

The effects of naratriptan, rizatriptan, and sumatriptan on arteriovenous oxygen saturation difference and carotid hemodynamics were compared in the anesthetized pig. Oxygen and carbon dioxide partial pressures in systemic arterial and jugular venous blood as well as hemoglobin oxygen saturation were determined by conventional blood gas analysis. Vehicle (n = 19) or naratriptan, rizatriptan, or sumatriptan (0.63, 2.5, 10, 40, 160, 630, and 2,500 microg/kg i.v.; n = 7/group) were infused cumulatively. In naratriptan-, rizatriptan-, and sumatriptan-treated animals, jugular venous oxygen saturation decreased dose dependently (geometric mean ED50 values of 3.1, 17.9, and 16.0 microg/kg, respectively) concomitantly with increases in carotid vascular resistance. Rizatriptan significantly and dose dependently, from 160 microg/kg, increased PvCO2 (P < 0.05 versus vehicle). Naratriptan and sumatriptan also tended to increase PvCO2 albeit nonstatistically significantly. All three triptans consistently evoked quantitatively similar carotid vasoconstriction, whereas decreases in jugular venous oxygen saturation (VOS) and increases in PvCO2 had different magnitudes and occurred only in around one-half of the animals studied. Maximal variations in PvCO2 were found to correlate highly with those in PvO2 (P = 0.002), but maximal variations in carotid resistance failed to correlate with those in PvCO2 (P = 0.76) or PvO2 (P = 0.28). The results demonstrate that the triptans investigated robustly produced carotid vasoconstriction, but elicited less consistent decreases in VOS and increases in jugular PvCO2, possibly suggestive of distinct mechanisms. Collectively, the data suggest that triptan-induced increases in arteriovenous oxygen saturation difference and carbon dioxide partial pressure in venous blood draining the head are class effects.

Donitriptan selectively decreases jugular venous oxygen saturation in the anesthetized pig: further insights into its mechanism of action relevant to headache relief.

The effects of donitriptan on systemic arterial-jugular venous oxygen saturation difference were evaluated in pentobarbitone-anesthetized pigs. Oxygen and carbon dioxide partial pressures in systemic arterial and jugular venous blood as well as hemoglobin oxygen saturation were determined by conventional blood gas analysis. Vehicle (40% polyethyleneglycol in saline, n = 9) or donitriptan (0.01, 0.04, 0.16, 0.63, 2.5, 10, and 40 microg/kg, n = 7) were cumulatively infused over 15 min/dose. The involvement of 5-hydroxytryptamine(1B) (5-HT(1B)) receptors was assessed in the presence of the 5-HT(1B/1D) receptor antagonist, GR 127935. Donitriptan decreased markedly and dose dependently jugular venous oxygen saturation [ED(50) 0.5 (0.3-1.1) microg/kg], in parallel with increases in carotid vascular resistance [ED(50) 0.9 (0.7-1.1) microg/kg]. Since arterial oxygen saturation and partial pressure remained unchanged, donitriptan significantly increased arteriovenous oxygen saturation difference from 0.63 microg/kg (maximal variation: 57 +/- 18%, P < 0.05 compared with vehicle). Unexpectedly, donitriptan from 2.5 microg/kg induced marked and significant increases in carbon dioxide partial pressure (pVCO(2)) in venous blood (maximal increase 18.8 +/- 5.7%; P < 0.05 compared with vehicle). Pretreatment with GR 127935 (0.63 mg/kg, n = 5) abolished the fall in venous oxygen saturation and the increase in carotid vascular resistance and reduced the increases in pVCO(2) induced by donitriptan. The results demonstrate that donitriptan, via 5-HT(1B) receptor activation, decreases the oxygen saturation of venous blood draining the head, concomitantly with cranial vasoconstriction. Since donitriptan also increased pVCO(2), an effect upon cerebral oxygen consumption and metabolism is suggested in addition to cranial vasoconstriction, which may be relevant to its headache-relieving effects.

Accuracy of a Portable Blood Gas Analyzer Incorporating Optodes for Canine Blood

The accuracy of a portable blood gas analyzer (OPTI 1) was evaluated using canine blood and aqueous control solutions. Sixty‐four arterial blood samples were collected from 11 anesthetized dogs and were analyzed for pH, partial pressure of carbon dioxide (Pco2), partial pressure of oxygen (Po2), and bicarbonate concentration ([HCO3‐]) values by the OPTI 1 and a conventional blood gas analyzer (GASTAT 3). The conventional analyzer was considered as a standard against which the OPTI 1 was evaluated. Comparison of OPTI 1 results with those of GASTAT 3 by linear regression analysis revealed a high degree of correlation with the GASTAT 3 (r= .90‐.91). The mean ± SD of the differences between OPTI 1 and GASTAT 3 values was ‐0.008 ± 0.017 for pH, ‐0.88 ± 3.33 mm Hg for Pco2, 3.71 ± 6.98 mm Hg for Po2, and ‐0.34 ± 1.45 mEq/L for [HCO3‐]. No statistically significant difference was found between the OPTI 1 and the GASTAT 3. Agreement between these 2 methods is within clinically acceptable ranges for pH, Pco2, Po2, and [HCO3‐]. The coefficients of variation for measured pH, Pco2, and Po2 values of 3 aqueous control solutions (acidic, normal, and alkalotic) analyzed by the OPTI 1 ranged from 0.047 to 0.072% for pH, 0.78 to 1.81% for Pco2, and 0.73 to 2.77% for Po2. The OPTI 1 is concluded to provide canine blood gas analysis with an accuracy that is comparable with that of conventional benchtop blood gas analyzers.

Accuracy of a portable blood gas analyzer incorporating optodes for canine blood.

The accuracy of a portable blood gas analyzer (OPTI 1) was evaluated using canine blood and aqueous control solutions. Sixty-four arterial blood samples were collected from 11 anesthetized dogs and were analyzed for pH, partial pressure of carbon dioxide (PCO2) partial pressure of oxygen (PO2), and bicarbonate concentration ([HCO3-]) values by the OPTI 1 and a conventional blood gas analyzer (GASTAT 3). The conventional analyzer was considered as a standard against which the OPTI 1 was evaluated. Comparison of OPTI 1 results with those of GASTAT 3 by linear regression analysis revealed a high degree of correlation with the GASTAT 3 (r = .90-.91). The mean +/- SD of the differences between OPTI 1 and GASTAT 3 values was -0.008 +/- 0.017 for pH, -0.88 +/- 3.33 mm Hg for PCO2, 3.71 +/- 6.98 mm Hg for PO2, and -0.34 +/- 1.45 mEq/L for [HCO3-]. No statistically significant difference was found between the OPTI 1 and the GASTAT 3. Agreement between these 2 methods is within clinically acceptable ranges for pH, PCO2, PO2, and [HCO3-]. The coefficients of variation for measured pH, PCO2, and PO2 values of 3 aqueous control solutions (acidic, normal, and alkalotic) analyzed by the OPTI 1 ranged from 0.047 to 0.072% for pH, 0.78 to 1.81% for PCO2, and 0.73 to 2.77% for PO2. The OPTI 1 is concluded to provide canine blood gas analysis with an accuracy that is comparable with that of conventional benchtop blood gas analyzers.

Research publications involving proprietary products: science, news, or advertising?

Commercial interests in health care have begun to affect the contents of some medical research reports in a way that could interfere with the essential function of medical journals. Recently I was asked to review a manuscript submitted for publication to Pediatrics , investigating the application of a proprietary clinical scoring system in a pediatric population. Basic information was withheld because of its proprietary nature, such as the identification of clinical variables, values of normal age-related ranges, as well as the equations relating patient characteristics to outcome. The missing information made a potentially useful contribution uninterpretable. The difficulties encountered in evaluating this incomplete report typify a problem that journals increasingly will face as they deal with articles having commercial importance. Many journal editors have not yet stated a position on the practice of withholding proprietary information from articles submitted for publication. The fact that commercial interests arise in an article on health services research indicates the economic importance of this growing field. Health services research deals with effectiveness, costs, and efficiency of health care. In order to interpret observations of health outcomes, objective methods are needed to characterize the patient population involved. For example, a clinical scoring system measuring severity of illness might assist in the interpretation of differences in clinical outcomes between two health care organizations. Using clinical scoring systems, the investigator may be able to attribute differing outcomes … Address correspondence to: Robert K. Kanter, MD, Department of Pediatrics, SUNY Health Science Center, 750 East Adams St, Syracuse, NY 13210.

Catheter-type sensor for potentiometric monitoring of oxygen, pH and carbon dioxide

The fabrication and analytical performance of a catheter-type electrode suitable for potentiometric monitoring of P O 2 , pH and P CO 2 in flowing blood is described. The catheter electrode is based on impregnating a single segment of dual-lumen gas permeable silicone rubber tubing with the proton ionophore tridodecylamine to impart H + permselectivity to both inner and outer walls of the tubing. One lumen is filled with an unbuffered bicarbonate solution and the other lumen is filled with a strong buffer. By inserting Ag/AgCl reference electrode wires in each lumen and a cobalt electrode in the buffered lumen, simultaneous potentiometric detection of P O 2 , pH and P CO 2 is achieved. The response of cobalt electrode to P O 2 arises from a steady-state mixed potential owing to slow oxidation of cobalt and simultaneous reduction of oxygen on the surface of the cobalt electrode. The response towards P CO 2 is completely analogous to the response mechanism of a conventional Severinghaus P CO 2 sensor (i.e., change in pH of the bicarbonate solution). Continuous measurements of P O 2 , pH and P CO 2 during 4–5 h blood pump studies using the catheter electrodes correlate well with conventional bench-top blood gas analyzer (P O 2 : r 2 = 0·992; pH: r 2 = 0·940; P CO 2 : r 2 = 0·993).

Carbon dioxide and oxygen partial pressure measurements in the cerebrospinal fluid in a conventional blood gas analyzer: analysis of bias and precision

Cerebrospinal fluid (CSF) gas tension measurements have been used as a marker of cerebral oxygenation in animal models and in human studies. Discrepancies in the measurement of PCO 2 and PO 2 in non-blood solutions by standard blood gas analyzers have been described. The CSF is a physiological non-blood fluid with an ability to carry only dissolved O 2 or CO 2. The aim of this experiment was to determine the bias and precision of CSF pCO 2 and pO 2 measurements of a contemporary blood gas analyzer and a continuous gas sensor. CSF from human patients was tonometered to known PCO 2 and PO 2 using standard calibration gases in a bubble tonometer. Following equilibration, a continuous measurement of the partial pressure of gases in the CSF in the tonometer solution was made using a calibrated Paratrend 7 gas sensor (Biomedical Sensors, Bucks., UK) inserted into the equilibration chamber of the tonometer and continuous CSF pH, PCO 2 and PO 2 measurements recorded at 1-min intervals. After equilibration, a 3-ml CSF sample was aspirated anaerobically from the tonometer and analysed in duplicate using an ABL 620 (Radiometer, Copenhagen) blood gas analyser. The measured pCO 2 and pO 2 from the ABL 620 blood gas analyser and the Paratrend 7 sensor were paired with the expected values to calculate bias and precision. Small offsets in PCO 2 and large offsets in PO 2 measurement were seen with the ABL 620 blood gas analyser. This study brings into question clinical decisions based on CSF PO 2 measurements. The implications for calibrating CSF sensors against CSF gas measurements are discussed.