The Radiometer ABL300 blood gas analyzer

A modern blood gas analyzer such as the ABL300 directly accepts samples from syringes or capillary tubing. The ABL300 measures pH, carbon dioxide tension, and oxygen tension on 85-µl samples by using specific electrodes, and estimates hemoglobin concentration from the optical density of a nonhemolyzed sample. From these it calculates bicarbonate, standard bicarbonate, total carbon dioxide, base excess, standard base excess, and oxygen saturation and content. The electrodes are automatically calibrated by pumping bicarbonate/phosphate buffer solutions, gas-equilibrated within the thermostatic chamber, into the measuring cuvettes. Samples are preheated in metal tubing before being pumped automatically into the measuring chamber. An open liquid junction to a half-saturated potassium chloride solution is renewed with each sample to complete the pH circuit. The electrodes are housed in a 37°C circulated-air thermostat, which also contains the equilibrators for the calibration solutions. Two known carbon dioxide mixtures are made from pure carbon dioxide by dilution with room air, which is compressed continuously, avoiding the use of premixed calibration gases. The computer determines the derived variables and can correct pH, carbon dioxide tension, and oxygen tension values to a user-defined body temperature. Corrections are made for predicted discrepancy between the measured and true value caused by the small sample size. The corrections depend on the difference in gas tension between sample and rinse solution.     

The Radiometer ABL300 blood gas analyzer

A modern blood gas analyzer such as the ABL300 directly accepts samples from syringes or capillary tubing. The ABL300 measures pH, carbon dioxide tension, and oxygen tension on 85-μl samples by using specific electrodes, and estimates hemoglobin concentration from the optical density of a nonhemolyzed sample. From these it calculates bicarbonate, standard bicarbonate, total carbon dioxide, base excess, standard base excess, and oxygen saturation and content. The electrodes are automatically calibrated by pumping bicarbonate/phosphate buffer solutions, gas-equilibrated within the thermostatic chamber, into the measuring cuvettes. Samples are preheated in metal tubing before being pumped automatically into the measuring chamber. An open liquid junction to a half-saturated potassium chloride solution is renewed with each sample to complete the pH circuit. The electrodes are housed in a 37°C circulated-air thermostat, which also contains the equilibrators for the calibration solutions. Two known carbon dioxide mixtures are made from pure carbon dioxide by dilution with room air, which is compressed continuously, avoiding the use of premixed calibration gases. The computer determines the derived variables and can correct pH, carbon dioxide tension, and oxygen tension values to a user-defined body temperature. Corrections are made for predicted discrepancy between the measured and true value caused by the small sample size. The corrections depend on the difference in gas tension between sample and rinse solution.     

Dedicated monitoring of anesthetic and respiratory gases By Raman scattering

The monitoring of respiratory and anesthetic gases in the operating room is important for patient safety. This study measured the accuracy and response time of a multiplegas monitoring instrument that uses Raman light scattering. Measurements of oxygen, carbon dioxide, nitrogen, nitrous oxide, halothane, enflurane, and isoflurane concentrations were compared with a gas mixer standard and with measurements made with an infrared anesthetic agent analyzer. Correlation coefficients were all greater than 0.999, and probable errors were less than 0.43 vol% for the gases and less than 0.03 vol% for the volatile anesthetics. Response time was 67 ms with a sample flow rate of 150 ml/min. There was some signal overlap between nitrogen and nitrous oxide and between the volatile anesthetic agents. Such overlap can be compensated for by linear matrix analysis. The Raman instrument promises a monitoring capability equivalent to the mass spectrometer and should prove attractive for the monitoring of respiratory and anesthetic gases in the operating room.     

Evaluation of long sampling tubes for remote monitoring by mass spectrometry

The long sampling tubes required for remote mass spectrometry alter the sampling system’s performance characterized by sample flow, residence time, and 10 to 90% response time. We searched for an easy-to-handle tube with (1) a length of 30 m, (2) sample flow less than 50 ml · min^-1, and (3) residence and response times approaching those predicted by our mathematical model. We tested tubes of various geometries and various commercially available materials by using them as inlet catheters for a quadrupole mass spectrometer (Centronic 200 MGA, Centronic Ltd, Craydon, UK). We measured their responses at 0 to 10% (on transients) and 10 to 0% (off transients) step changes in gas concentration for nitrogen, argon, nitrous oxide, oxygen, and carbon dioxide and 0 to 3% and 3 to 0% for halothane, enflurane, and isoflurane. With 5 polyethylene tubes, halothane response times were up to 38 times longer than predicted. One 30-m polyethylene tube combined a 158-ms response time for nitrogen and argon with a 2,205-ms response time for halothane. Teflon, polyvinyl chloride, and stainless steel also proved to be unsuitable because of unacceptable signal distortion: the carbon dioxide response time for a 30-m Teflon tube was 2,600 ms. A glass tube showed the least signal distortion but was hard to handle. Our requirements were fulfilled by a 29.77-m tube made from nylon with a 1.00-mm inside diameter to which a 0.23-m length of nylon with a 0.25-mm inside diameter was added at the patient end. It offers (1) sample flow equals 46 ml · min^-1, (2) residence time equals 11.1 seconds, and (3) response times approaching our theoretical predictions, that is, 159, 164, 180, 159, 188, 302, 298, and 300 ms (means of on and off transients) for nitrogen, argon, nitrous oxide, oxygen, carbon dioxide, halothane, enflurane, and isoflurane, respectively. This tube allows the accurate monitoring of breathing frequencies up to 25 and 50 breaths/min for volatile agents and gases, respectively.     

Laboratory evaluation of the vital signs (ICOR) piezoelectric anesthetic agent analyzer

The Vital Signs (ICOR) anesthetic agent analyzer, which measures anesthetic vapor concentration by a piezoelectric crystal technique, was evaluated by using standard-calibration gases to measure the accuracy, response time, gas interference, and water vapor dependence of the analyzer. The accuracy for the measurement of vapor concentration was better than 0.08 vol%. The reproducibility of repeated measures averaged 0.003 vol%. The offsets caused by other gases were 0.02 vol% for water vapor, 0.08 vol% for 70% nitrous oxide, and less than 0.01 vol% for oxygen and carbon dioxide. Response time (10 to 90%) was 475 ms. The agent analyzer may be well suited for monitoring volatile agent concentrations during anesthesia.     

Laboratory evaluation of the vital signs (ICOR) piezoelectric anesthetic agent analyzer

The Vital Signs (ICOR) anesthetic agent analyzer, which measures anesthetic vapor concentration by a piezoelectric crystal technique, was evaluated by using standard-calibration gases to measure the accuracy, response time, gas interference, and water vapor dependence of the analyzer. The accuracy for the measurement of vapor concentration was better than 0.08 vol%. The reproducibility of repeated measures averaged 0.003 vol%. The offsets caused by other gases were 0.02 vol% for water vapor, 0.08 vol% for 70% nitrous oxide, and less than 0.01 vol% for oxygen and carbon dioxide. Response time (10 to 90%) was 475 ms. The agent analyzer may be well suited for monitoring volatile agent concentrations during anesthesia.     

A new method for measurement of gas exchange during anaesthesia using an extractable marker gas

A new method for the measurement of pulmonary gas exchange during inhalational anaesthesia is described which measures fresh gas and exhaust gas flows using carbon dioxide as an extractable marker gas. The theoretical precision of the method was compared by Monte Carlo modelling with other approaches which use marker gas dilution. A system was constructed for automated measurement of uptake of oxygen, nitrous oxide, volatile anaesthetic agent and elimination of carbon dioxide by an anaesthetized patient. The accuracy and precision of the method was tested in vitro on a lung gas exchange simulator, by comparison with simultaneous measurements made using nitrogen as marker gas and the Haldane transformation. Good agreement was obtained for measurement of simulated uptake or elimination of all gases studied over a physiologically realistic range of values. Mean bias for oxygen and nitrous oxide uptake was 0.003 l min(-1), for isoflurane 0.0001 l min(-1) and for carbon dioxide 0.001 l min(-1). Limits of agreement lay within 10% of the mean uptake rate for nitrous oxide, within 5% for oxygen and isoflurane and within 1% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas exchange in an anaesthetic breathing system with any inspired gas mixture.     

Erroneous mass spectrometer readings caused by desflurane and sevoflurane

Objective. Medical mass spectrometers are configured to detect and measure specific respiratory and anesthetic gases. Unrecognized gases entering these systems may cause erroneous readings. We determined how the Advantage 1100 (Perkin-Elmer, now Marquette Gas Systems, Milwaukee, WI) and PPG-SARA (PPG Biomedical Systems, Lenexa, KS) systems that were not configured to measure desflurane or sevoflurane respond to increasing concentrations of these new potent volatile anesthetic agents.Methods. Desflurane 0% to 18% in 3% increments or sevoflurane 0% to 7% in 1% increments in 5-L/min oxygen was delivered to the Advantage and PPG-SARA mass spectrometry systems. For each concentration of each agent, the displayed gas analysis readings and uncompensated collector plate voltages were recorded.Results. The Advantage 1100 system read both desflurane and sevoflurane mainly as enflurane and, to a lesser extent, as carbon dioxide and isoflurane. For enflurane(E) readings <9.9%, the approximate relationships are: %Desflurane=1.6E; %Sevoflurane=0.3E. These formulas do not apply if E >9.9% because of saturation of the summation bus. PPG-SARA read desflurane mainly as isoflurane(I) and, to a lesser extent, as nitrous oxide. PPG-SARA read sevoflurane mainly as enflurane(E) and, to a lesser extent, as nitrous oxide and halothane. The approximate relationships are: %Desflurane=1.11 (for I < 9%); %Sevoflurane=2.1E.Conclusions. Advantage 1100 and PPG-SARA systems not configured for desflurane or sevoflurane display erroneous anesthetic agent readings when these new agents are sampled. Advantage 1100 also displays falsely elevated carbon dioxide readings when desflurane is sampled.     

Monitoring of the inspired and end-tidal oxygen,carbon dioxide,and nitrous oxide concentrations: Clinical applications during anesthesia and recovery

Respiratory oxygen, carbon dioxide, and nitrous oxide concentrations were recorded in 20 patients breath-by-breath during general anesthesia and early recovery, using the Cardiocap multiparameter monitor. Several approved maneuvers were performed to demonstrate the usefulness of endtidal oxygen measurement. Oxygrams provided by the fast paramagnetic oxygen sensor confirmed the capnometric information in the diagnosis of hypoventilation, apnea, and disconnections. In one patient, the alarm for inspiratory oxygen concentration, set at 18%, appeared to prevent alveolar hypoxia and low arterial saturation from occurring when oxygen instead of nitrous oxide was turned off. Low end-tidal oxygen levels revealed inadequate fresh gas oxygen supplementation while low flow circuits were closed. During manual hypoventilation at the end of anesthesia, the inspiratory-expiratory oxygen difference increased almost twofold while end-tidal carbon dioxide increased by only 30%. Changes in nitrous oxide concentration often complemented oxygen-related information obtained in our observations. In the recovery room, a decrease in end-tidal oxygen concentration preceded low pulse oximetry readings. Therefore, it is suggested that all three gases should be monitored continuously to prevent mishaps related to insufficient ventilation and inappropriate gas concentrations during anesthesia and immediate recovery.     

Capnography for detection of endobronchial migration of an endotracheal tube

A patient is described in whom migration of an endotracheal tube into the right main bronchus was suspected when end-tidal carbon dioxide suddenly decreased from 28 to 22 mm Hg. Acute changes with migration of the endotracheal tube into the main bronchus were also studied in an animal experimental model. End-tidal carbon dioxide decreased and tracheal (inflation) pressure increased, with no change in tidal volume. Arterial blood gases showed time-dependent decreases in pH and oxygen tension and an increase in carbon dioxide tension.     

Evaluation of three transportable multigas anesthetic monitors: The Bruel & Kjaer Anesthetic Gas Monitor 1304, the Datex Capnomac Ultima,and the Nellcor N-2500

We compared the performance of three newly developed anesthetic agent (AA) monitors: the Bruel &; Kjaer Anesthetic Agent Monitor 1304 (BK 1304), the Datex Capnomac Ultima (ULTIMA), and the Nellcor N-2500 (N-2500). The following were investigated: the linearity and accuracy in measuring AAs, oxygen, carbon dioxide, and nitrous oxide; the linearity and accuracy during warm-up time; the effect of increasing respiratory rate on the accuracy; the consequences of a difference between monitored and delivered AA and of delivering a mixture of AAs; and, finally, the effect of water vapor and alcohol. For all three monitors we found that the accuracy in determining the respiratory and anesthetic gases was sufficient for clinical use (the N-2500 does not measure oxygen). Because of the calibration mixture supplied with the device, however, the ULTIMA recorded values that were 10 to 12% (relative) less than the AA that was present. The BK 1304 had greater accuracy at higher respiratory rates than did the other two monitors, probably favoring its use in pediatric anesthesia. The N-2500 will detect which agent (isoflurane, enflurane, or halothane) is being used, alone or in a mixture. With the two other monitors the user must define which agent is given. In some situations a difference between this and the one actually delivered can theoretically lead to an overdose of AA, with the ULTIMA up to a 14.9 minimal alveolar concentration (MAC) overdose. No interference from alcohol or water vapor in the expired air was found. When the units millimeters of mercury (mm Hg) and kilopascal (kPa) were chosen, the ULTIMA displayed the values in standard temperature pressure dry (STPD) instead of body temperature pressure saturated (BTPS) conditions. Following power-up, some time lapsed before the monitors accurately displayed all variables, shortest with the BK 1304 and ULTIMA (15 min) and longest with the N-2500 (40 min).     

Evaluation of a single-room, dedicated mass spectrometer

A single-room dedicated mass spectrometer can be used to measure carbon dioxide, halogenated anesthetic agents, nitrous oxide, nitrogen, and oxygen. This device challenges the multiplexed mass spectrometer, a current standard in measurement. This study compared the single-room dedicated mass spectrometer with a conventional mass spectrometer that is normally used in a multiplexed setting. In this study, a single-room dedicated Ohmeda 6000 Mini-Mass Spectrometer and the Perkin-Elmer MGA-1100 mass spectrometer were calibrated with the same reference gases and both devices sampled various concentrations of dry gases. Regression lines and intercepts were plotted and showed excellent correlation between the two devices. The intraclass correlation test of Lee, Koh, and Ong, showed the devices to be equivalent with regard to the ability to determine various gas concentrations. Various advantages of a single-room dedicated mass spectrometer are discussed.     

Refractive indices for volatile anesthetic gases: Equipment and method for calibrating vaporizers and monitors

Objective. The objective of our study was to establish the refractive indices and the virial coefficients of the volatile anesthetic vapors. These indices and coefficients will allow refractometry to be used by manufacturers to produce accurate calibration, without requiring expensive high-precision calibration gases.Methods. We used a precision refractometer to measure the refractive indices for five volatile anesthetic vapors. We prepared our calibration gases by mixing a gravimetrically calibrated amount of liquid agent with a constant gas flow.Results. The refractive indices for the volatile anesthetic vapors are 1,603.2 for halothane, 1,540.4 for enflurane, 1,563.3 for isoflurance, 1,538.3 for sevoflurane, and 1,211.7 for desflurane. The maximum theoretical error in our measurements, due to all sensors and all uncertainty in our measurement of apparatus and physical constants, is ±0.56% of the reading (±0.70% for desflurane).Conclusions. If refractometry replaced calibration gases in cylinders, as a calibration standard, manufacturers might avoid errors that now occur because calibration gases manufactured by numerous companies seem to differ. We propose that our values serve as an interim database.     

Evaluation of two oxygen analyzers by computerized data acquisition and processing

Monitoring of inspired oxygen concentration during anesthesia with nitrous oxide is becoming accepted as essential. This type of monitoring demands accurate monitors that respond rapidly. We evaluated two such devices for their response patterns to rapid changes in oxygen concentration, a galvanic or “fuel cell” unit and a polarographic device. Data were stored after analog-to-digital conversion. The response patterns to stepwise changes in nitrous oxide and oxygen mixtures were recorded at flow rates ranging from 2 to 10 L/min. Both units responded accurately to all changes in the absolute oxygen concentration; the polarographic unit was, on average, twice as fast. Responsiveness to nitrous oxide was low (<0.4% at 100% nitrous oxide), and the stability of the signals was good. The 90% response time (T₉₀) was consistent for any stepwise increase or decrease in oxygen concentration between 0, 21, 33, 50, and 100%. After a step change from 0 to 100% oxygen at a gas flow rate of 10 L/min, the T₉₀ was 5.8 seconds in the polarographic device and 11.4 seconds in the galvanic device (p<0.01). After a decrease from 100 to 0% oxygen, the T₉₀ was 0.6 second longer in both monitors. Comparing flow rates of 2 L/min with 10 L/min, the T₉₀ was delayed by 1.1 and 2.3 seconds for an increase, and by 1.4 and 2.9 seconds for a decrease in oxygen concentration. Experimental data suggest that both sensors respond adequately during routine clinical use. The faster response of the polarographic device is probably of limited clinical relevance, but it may aid in calibration.     

Effects of anesthetic agents on the drift of a transcutaneous oxygen tension sensor

Both halothane and nitrous oxide can be reduced at the cathode of a polarographic oxygen electrode, causing the electrode current to drift upward and report falsely high oxygen tension. Because transcutaneous oxygen tension is measured by a heated oxygen electrode, there is a potential for significant upward drift of these values. To examine the clinical significance of this drift, the following study was performed. Transcutaneous oxygen tension sensors were calibrated at oxygen tensions of 0 mm Hg and 157 mm Hg (room air) just before clinical use during anesthesia. This calibration was rechecked immediately upon removal of the sensor from the patient at the end of the anesthesia. The predominant anesthetic agent used and the duration of monitoring were noted from the record. Data were collected from 208 patients representing a total of 463.6 hours of anesthesia. The patients were divided into five groups based on anesthetic administered: halothane, enflurane, isoflurane, nitrous oxide-narcotic, and local/regional. The mean zero point recalibration value was 0.4 mm Hg or less for all agents except halothane, for which it was 1.8 ± 3.2 mm Hg. This halothane drift was significantly greater than that for the other agents (P<0.01). Room air recalibration was not significantly different in any of the five groups, varying from 160 ± 4.9 mm Hg for halothane to 157 ± 4.9 mm Hg for enflurane. All these drift values are within the manufacturer’s specifications. We conclude that the drift of the transcutaneous oxygen tension sensor due to anesthetic agents is not clinically significant. However, caution should be exercised when halothane is used during an extremely long period of anesthesia.     

Blood-gas analyzer calibration and quality control using a precision gas-mixing instrument.

We describe a new instrument that performs on-site mixing of oxygen (O2), carbon dioxide (CO2), and nitrogen (N2) to create compositions that can replace gases from standard premixed cylinders. This instrument yields accurate and predictable gas mixtures that can be used for two-point gas calibration of blood gas/pH analyzers or for liquid tonometry of either an aqueous buffer or blood used as quality-control material on blood-gas electrodes. The desired mixture of O2, CO2, and N2 is produced by microprocessor control of the sequential open-times on three solenoid valves that meter these pure gases through a common small-bore orifice. Any combination of O2 and CO2 can be chosen by dialing the front panel thumbwheels and pressing a button. Gas chromatographic evaluation of this gas-mixing instrument demonstrates its accuracy and precision to be better than +/- 0.1% absolute full scale for O2, CO2, and N2, making this instrument calibration and tonometry.     

Detection of venous air embolism in dogs by emission spectrometry

Emission spectrometers provide alternative, relatively inexpensive methods for detecting the concentration of respiratory gas nitrogen. Mass spectrometers are accepted as reliable monitors of end-tidal nitrogen for detection of venous air embolisms. We evaluated an inexpensive emission spectrometer for detecting changes in nitrogen levels and compared it with a mass spectrometer for detecting increased endtidal nitrogen levels in dogs with venous air embolisms. During in vitro gas flow studies (helium; oxygen; helium/ oxygen mixtures; or 70% nitrous oxide/30% oxygen with 0, 1, 2, or 3% isoflurane), air boluses (0.01 to 5.0 ml) were injected into a gas flow circuit and outlet nitrogen levels were measured by a Collins 21232 emission spectrometer. Responses were greater after each bolus when helium rather than oxygen was the major diluent gas. During in vivo studies, 5 dogs were anesthetized, ventilated, denitrogenated, and given venous air embolisms (0.1, 0.5, and 1.0 ml. kg^-1) during oxygen and then during Heliox (20% oxygen:80% helium) breathing. End-tidal nitrogen increased approximately two-fold after venous air embolisms given during Heliox as compared with oxygen ventilation. In all 0.1-ml. kg^-1 venous air embolisms end-tidal nitrogen increased when the emission spectrometer was used, but venous air embolisms less than 1.0 ml. kg^-1 were not consistently detected by mass spectrometry. Emission spectrometry can be used to detect increased end-tidal nitrogen levels indicative of venous air embolism and may be a more sensitive detector than mass spectrometry. Its sensitivity and relatively low cost (one-eighth of a magnetic fixed-sector mass spectrometer) make it a great potential monitor for both clinical detection of venous air embolism and air embolism research.     

Therapeutic gases for neonatal and pediatric respiratory care

Though oxygen is the most frequently administered gas in respiratory care, the use of other specialty gases has become common practice in neonatal and pediatric intensive care and emergency departments across the United States. This report reviews the literature and evidence regarding 4 such specialty gases: heliox (helium-oxygen mixture), nitric oxide, hypoxic gas (ie, < 21% oxygen), and carbon dioxide. Because heliox is less dense than air or nitrogen, it offers less resistance and turbulence as an inhaled gas and therefore decreases the pressure and work of breathing necessary to ventilate the lung, which assists in the management of conditions that involve airway obstruction. Inhaled nitric oxide is a selective pulmonary vasodilator and during the last 2 decades research has focused on its potential value for treating disorders that involve pulmonary vasoconstriction. Hypoxic gas and carbon dioxide are used in the management of infants suffering hypoplastic left heart syndrome (a congenital heart defect), to equilibrate the pulmonary vascular resistance with the systemic vascular resistance, which is necessary to assure adequate oxygenation and tissue perfusion. Balancing the systemic and pulmonary vascular resistances requires increasing pulmonary vascular resistance and decreasing pulmonary blood flow; hypoxic gas does this by maintaining blood oxygen saturation at around 70%, whereas carbon dioxide does so by increasing P(aCO2) to the range of 45-50 mm Hg.     

Therapeutic Antioxidant Medical Gas

Medical gases are pharmaceutical gaseous molecules which offer solutions to medical needs and include traditional gases, such as oxygen and nitrous oxide, as well as gases with recently discovered roles as biological messenger molecules, such as carbon monoxide, nitric oxide and hydrogen sulphide. Medical gas therapy is a relatively unexplored field of medicine; however, a recent increasing in the number of publications on medical gas therapies clearly indicate that there are significant opportunities for use of gases as therapeutic tools for a variety of disease conditions. In this article, we review the recent advances in research on medical gases with antioxidant properties and discuss their clinical applications and therapeutic properties.     

An evaluation of nitrous oxide analgesia during transcutaneous pacing

Transcutaneous cardiac pacing (TCP) is a promising prehospital intervention, but there are little data available regarding protocols to improve patient tolerance to TCP. A 50:50 nitrous oxide:oxygen analgesic mixture also is a commonly employed prehospital intervention. In this randomized, prospective study, we compared the discomfort experienced by 18 healthy subjects when paced in two trials at the capture threshold: one following breathing of a 50:50 nitrous oxide:oxygen mixture; and the second only breathing room air. Discomfort was rated on an analog scale from 1 (minimal discomfort) to 10 (severe pain). Of the 18 subjects, 15 (83%) reported that nitrous oxide improved the tolerance to pacing at capture threshold. The median pain scores at capture threshold in the nitrous oxide and room air group were 3.8 and 5.0 respectively (P less than .05). Nine of the subjects tolerated TCP for the maximum allotted time of 30 seconds in each trial; six tolerated TCP for a longer time period in the nitrous oxide trial; three tolerated TCP longer in the room air trial. These data suggest that inhalation of 50:50 nitrous oxide:oxygen mixture may improve tolerance to TCP in the conscious patient.