History of blood gas analysis. VI. Oximetry

Oximetry, the measurement of hemoglobin oxygen saturation in either blood or tissue, depends on the Lambert-Beer relationship between light transmission and optical density. Shortly after Bunsen and Kirchhoff invented the spectrometer in 1860, the oxygen transport function of hemoglobin was demonstrated by Stokes and Hoppe-Seyler, who showed color changes produced by aeration of hemoglobin solutions. In 1932 in Göttingen, Germany, Nicolai optically recorded the in vivo oxygen consumption of a hand after circulatory occlusion. Kramer showed that the Lambert-Beer law applied to hemoglobin solutions and approximately to whole blood, and measured saturation by the transmission of red light through unopened arteries. Matthes in Leipzig, Germany, built the first apparatus to measure ear oxygen saturation and introduced a second wavelength (green or infrared) insensitive to saturation to compensate for blood volume and tissue pigments. Millikan built a light-weight ear "oximeter" during World War II to train pilots for military aviation. Wood added a pneumatic cuff to obtain a bloodless zero. Brinkman and Zijlstra in Groningen, The Netherlands, showed that red light reflected from the forehead could be used to measure oxygen saturation. Zijlstra initiated cuvette and catheter reflection oximetry. Instrumentation Laboratory used multiple wavelengths to measure blood carboxyhemoglobin and methemoglobin is cuvette oximeters. Shaw devised an eight-wavelength ear oximeter. Nakajima and co-workers invented the pulse oximeter, which avoids the need for calibration with only two wavelengths by responding only to the pulsatile changes in transmitted red and infrared light. Lübbers developed catheter tip and cuvette fiberoptic sensors for oxygen tension, carbon dioxide tension, and pH.

An in vivo 31P NMR study of cerebral hypoxic hypoxia in rats

Twenty minutes of hypoxic hypoxia in five anesthetized rats reversibly reduced cerebral PCr and pH while ATP stayed constant. Complete metabolic and neurologic recovery occurred after oxygen was restored. Careful control of physiological parameters resulted in metabolite changes that were the same, within errors, in each animal.

History of blood gas analysis. V. Oxygen measurement

The first biologic use of a platinum cathode for oxygen monitoring was reported in 1938 by Blinks and Skow, who was studying photosynthesis. Their report led to the tissue oxygen studies of Davies, Brink, and Bronk. Clark, by covering cathode and anode with a polyethylene membrane, changed the polarographic cathode from a sensor of oxygen availability by diffusion to a measure of oxygen tension (PO2) in the solution and thereby facilitated an enormous expansion of the study of the respiratory physiology of blood oxygen after 1956. Clark's electrode led to the development of the present commercial blood gas systems that measure pH, carbon dioxide tension (PCO2), and PO2 and calculate many derived variables. Variations on Clark's electrode were designed for in vivo catheter-tip recording; gas phase oxygen monitoring; determining oxygen content of blood by releasing hemoglobin-bound oxygen and measuring PO2; and determining oxygen consumption in cell cultures (thus replacing Warburg manometry). By reducing the cathode diameter, Staub and others eliminated the need for stirring the blood samples. Concurrent research with amperometric or polarographic oxygen measurement led Hersch to develop the means of determining oxygen content by coulometry in large cells that consumed all the injected oxygen. Methods of applying noninsulating, but protein impermeable, membranes to cathodes and of recessing cathodes into glass permitted measurement of PO2 in tissues and fluids with microelectrodes.

Cerebral intracellular ADP concentrations during hypercarbia: an in vivo 31P nuclear magnetic resonance study in rats

Qualitatively different responses of ADP levels have previously been observed in the brain during hypercarbia. One investigation has found that cerebral ADP stayed constant during hypercarbia in rats that were anesthetized with halothane, while another observed that ADP decreased during supercarbia in rats that received no supplemental anesthesia. This article reports an in vivo 31P nuclear magnetic resonance study to test the hypothesis that halothane anesthesia accounts for the discrepant observations. Isoflurane anesthesia was also studied in a second group of rats to see if a different general anesthetic agent would cause the same effects that halothane causes. The two groups of five rats underwent dual episodes of hypercarbia that were separated by a 45-min recovery period. General anesthesia, either 0.5% halothane or 1.0% isoflurane, was administered during the first episode but not during the second. Hypercarbia during halothane anesthesia caused the measured phosphocreatine (PCr) to decrease by 40%, while the calculated change in ADP was 10%, in agreement with the former investigation. In contrast, hypercarbia during either isoflurane anesthesia or no anesthesia caused a decrease of only 10% in PCr, which meant that the calculated decrease in ADP was 60%, in agreement with the results of the second investigation. We conclude that during hypercarbia, clinical concentrations of halothane, unlike clinical concentrations of isoflurane, interfere with the regulation of ATP metabolism.

History of blood gas analysis. IV. Leland Clark's oxygen electrode

The electrochemical reduction of oxygen was discovered by Heinrich Danneel and Walter Nernst in 1897. Polarography using dropping mercury was discovered accidentally by Jaroslav Heyrovsky in Prague in 1922. This method produced the first measured oxygen tension values in plasma and blood in the 1940s. Brink, Davies, and Bronk implanted platinum electrodes in tissue to study oxygen supply, or availability, from about 1940, but these bare electrodes became poisoned when immersed in blood. Leland Clark sealed a platinum cathode in glass and covered it first with cellophane; he then tested silastic and polyethylene membranes. In 1954 Clark conceived and constructed the first membrane-covered oxygen electrode having both the anode and cathode behind a nonconductive polyethylene membrane. The limited permeability of polyethylene to oxygen reduced depletion of oxygen from the sample, making possible quantitative measurements of oxygen tension in blood, solutions, or gases. This invention led to the introduction of modern blood gas apparatus.

History of blood gas analysis. III. Carbon dioxide tension

The measurement of carbon dioxide tension (Pco2) owes its development to the 1952 polio epidemics in Copenhagen and the United States, during which artificial ventilation was first widely and effectively used and it was necessary to assess its effectiveness. Pco2 had been determined by various "bubble methods" in which carbon dioxide (CO2) was measured in gas equilibrated with blood at body temperature, or by one of two methods using the manometric apparatus of Van Slyke: interpolation on a plot of CO2 content versus equilibration gas Pco2 or use of the Henderson-Hasselbalch equation to calculate Pco2 from pH and plasma CO2 content. In 1954 Richard Stow described a CO2 electrode--a new concept--using a rubber membrane permeable to CO2 to separate a wet pH and reference electrode from the blood sample. This was the first membrane electrode, a device now used in hundreds of different ways. Severinghaus developed Stow's electrode, stabilizing it with a bicarbonate-salt solution and a spacer. The CO2 electrode concept had occurred to Gesell in 1925, but for measurement of gas only, and to Gertz and Loeschcke, who were unaware of the Stow-Severinghaus electrode, in 1958. The development of the CO2 electrode terminated the use of bubble methods, the Van Slyke methods, and the Astrup technique and at the same time reinforced the Astrup-Siggaard-Andersen acid-base analytic theory.

Cerebral intracellular changes during supercarbia: an in vivo 31P nuclear magnetic resonance study in rats

31P nuclear magnetic resonance (NMR) spectroscopy was used noninvasively to measure in vivo changes in intracellular pH and intracellular phosphate metabolites in the brains of rats during supercarbia (PaCO2 greater than or equal to 400 mm Hg). Five intubated rats were mechanically ventilated with inspired gas mixtures containing 70% CO2 and 30% O2. Supercarbia in the rat was observed to cause a greater reduction in cerebral intracellular pH (pHi) and increase in PCO2 than observed in other experiments with rats after 15 min of global ischemia. Complete neurologic and metabolic recovery was observed in these animals, despite and average decrease in pHi of 0.63 +/- 0.02 pH unit during supercarbia episodes that raised PaCO2 to 490 +/- 80 mm Hg. No change was observed in cerebral intracellular ATP and only a 25% decrease was detected in phosphocreatine. The concentration of free cerebral intracellular ADP, which can be calculated if one assumes that the creatine kinase reaction is in equilibrium, decreased to approximately one-third of its control value. The calculated threefold decrease in the concentration of free ADP and twofold increase in the cytosolic phosphorylation potential suggest that there is increased intracellular oxygenation during supercarbia. Because a more than fourfold increase in intracellular hydrogen ion concentration was tolerated without apparent clinical injury, we conclude that so long as adequate tissue oxygenation and perfusion are maintained, a severe decrease in intracellular pH need not induce or indicate brain injury.

History of blood gas analysis. II. pH and acid-base balance measurements

Electrometric measurement of the hydrogen ion concentration was discovered by Wilhelm Ostwald in Leipzig about 1890 and described thermodynamically by his student Walther Nernst, using the van't Hoff concept of osmotic pressure as a kind of gas pressure, and the Arrhenius concept of ionization of acids, both of which had been formalized in 1887. Hasselbalch, after adapting the pH nomenclature of Sørensen to the carbonic-acid mass equation of Henderson, made the first actual blood pH measurements (with a hydrogen electrode) and proposed that metabolic acid-base imbalance be quantified as the "reduced" pH of blood after equilibration to a carbon dioxide tension (PCO2) of 40 mm Hg. This good idea, coming 40 years before simple blood pH measurements at 37 degrees C became widely available, was never adopted. Instead, Van Slyke developed a concept of acid-base chemistry that depended on measuring plasma CO2 content with his manometric apparatus, a standard method until the 1960s, when it was displaced by the three-electrode method of blood gas analysis. The 1952 polio epidemic in Copenhagen stimulated Astrup to develop a glass electrode in which pH could be measured in blood at 37 degrees C before and after equilibration with known PCO2. He introduced the interpolative measurement of PCO2 and bicarbonate level (later base excess) using only pH measurements and, with Siggaard-Andersen, developed clinical acid-base chemistry. Controversy arose when blood base excess was noted to be altered by acute changes in PCO2 and when abnormalities of base excess were called metabolic acidosis or alkalosis, even when they represented compensation for respiratory abnormalities in PCO2. In the 1970s it became clear that "in-vivo" or "extracellular fluid" base excess (measured at an average extracellular fluid hemoglobin concentration of 5 g) eliminated the error caused by acute changes in PCO2. Base excess is now almost universally used as the index of nonrespiratory acid-base imbalance.

History of blood gas analysis. I. The development of electrochemistry

In 1982 Poul Astrup, in writing a history of acid base balance and blood gases, invited me to contribute a chapter about the modern period, from 1950 to the present. Astrup's book is scheduled for publication at the end of 1985 by Radiometer Company of Copenhagen; it will be distributed by Munksgaard (Blackwell). The story of blood gas analysis since 1950 is vast: there are some 420 references to methodology and closely related physiology. This "modern" history will appear in the Journal of Clinical Monitoring as a series of essays. This first essay centers on electrochemistry, the basis of modern blood gas analysis, and accordingly examines its roots in more detail. The 17th and 18th century exploration of electricity and gas laws led to the development of thermodynamic electrochemistry in 1887 through the collaborative efforts of van't Hoff, Arrhenius, Ostwald, and Nernst. The importance of the hydrogen ion in biology and in the body's buffering mechanisms was worked out by Henderson, Van Slyke, Barcroft, and many others in the first quarter of this century. The glass electrode became available after 1925, but practical blood pH measurement was introduced in the 1950s by Astrup and Siggaard Andersen. Succeeding essays will concern micro pH methods and base excess analysis, the discoveries of Stow's CO2 electrode and Clark's O2 electrode, the development of oximetry, and related physiology.

Estimation of ventilatory response to carbon dioxide in newborn infants using skin surface blood gas electrodes

Using only skin surface blood gas measurements, we calculated the ventilatory response to inhaled carbon dioxide from changes in skin surface PCO2 (PSCO2). This new method is based on the fact that if CO2 elimination is nearly constant, the change in alveolar ventilation from one steady state level to another is inversely proportional to the change in PSCO2. From this we derived a ventilatory ratio (VR) for 0%, 2%, and 4% CO2 breathing. A ventilatory response slope is then calculated from the three VR values, and is similar to a standard CO2 response slope. We serially studied 20 infants (28 to 40 weeks gestation) 2 to 9 weeks of age. Ten infants had serious apnea, ten did not. The infants breathed each test gas for 8 to 10 minutes during quiet sleep with skin surface electrodes attached. Infants with apnea were studied before and after apneic spells resolved. We found that apneic infants had a significantly reduced VR slope compared with that in the nonapneic infants, regardless of age. When apnea disappeared, the ventilatory ratio slope always increased into the range measured in nonapneic infants. In nonapneic infants the ventilatory ratio slope significantly increased with postnatal age. We conclude that infants with serious apnea have a reduced ventilatory response to CO2 and that the resolution of apnea is associated with the development of a normal CO2 response.

Ventrolateral medullary surface blood flow determined by hydrogen clearance

The H2 clearance technique was used to determine the blood flow of the postulated respiratory chemosensitive areas near the ventrolateral surface of the medulla. In 12 pentobarbital sodium-anesthetized cats, flow (mean +/- SD) was measured from 25-micron Teflon-coated platinum wire electrodes implanted to a depth of 0.3-0.7 mm. Flow (in ml X min-1 X 100 g-1, n = 35) was 52.8 +/- 28.5 in hypocapnia [arterial CO2 partial pressure (PaCO2) = 21.8 +/- 1.6 Torr], 57.8 +/- 27.5 in normocapnia (PaCO2 = 31.9 +/- 2.2 Torr), and 75.0 +/- 31.7 in hypercapnia (PaCO2 = 44.5 +/- 3.0 Torr). Flow determined from 15 electrodes in adjacent pyramidal tracts (white matter) was less at all levels of CO2; 22.9 +/- 12.3 in hypocapnia, 29.1 +/- 15.9 in normocapnia, and 33.9 +/- 13.9 in hypercapnia. In hypoxia [arterial O2 partial pressure (PaO2) = 39.9 +/- 6.3 Torr] ventrolateral surface flow rose to 87.9 +/- 47.6, and adjacent white matter flow was 35.8 +/- 15.6. These results indicate that flow in the postulated central chemoreceptor areas exceeds that of white matter and is sensitive to variations in PaCO2 and PaO2.

A fiber optic PCO2 sensor

The theory, construction and performance of a catheter tip optical PCO2 probe is described. The sensor, called the Opticap, is made with plastic fiber optics. One fiber carries light to the sensitive tip which is a silicone rubber tube 0.6 mm dia. X 1.0 mm long filled with a phenol red-KHCO3 solution. Ambient PCO2 controls the pH of the solution which influences the optical transmittance of the phenol red. A second fiber carries the transmitted signal to a receiver; the resulting electrical signal is linearly related to the PCO2 over the range of 2.7 to 10.7 kPa. The probe was tested as a tissue PCO2 sensor on the cerebral cortex of the cat and as an arterial PCO2 sensor. Drift over one day's use was 0.6 KPa or less and individual probes have been used as long as 12 weeks.

A noninvasive in vivo method of assessing the kinetics of halothane metabolism in humans

The authors describe a noninvasive method of estimating the kinetic constants that characterize metabolism of inhaled anesthetics in humans. Ten healthy male volunteers breathed subanesthetic concentrations of halothane and isoflurane in a fixed inspired ratio of 20:1. Isoflurane served as a marker that identified changes in uptake in nonmetabolizing depots. Each study progressed through nine 30-min levels (numbered 0-8). At each level, inspired concentrations of both halothane and isoflurane were doubled, and alveolar concentrations and uptakes were determined. Clearance (uptake/alveolar concentration) of isoflurane remained constant over a range of concentrations of 0.00006 to 0.008%. In contrast, clearance of halothane decreased as the alveolar concentration increased from 0.0007 to 0.13%. On this basis, the authors assumed that the clearance of halothane was a combination of linear clearance to depots and saturable metabolism, the former proportional to the clearance of isoflurane, and the latter attributable to a Michaelis-Menten process. Applying such a model to halothane, they estimated the mean (+/- SE) Vmax (the composite maximum rate of metabolism) to be 0.79 +/- 0.09 ml . min-1 . individual-1, and the Km (the composite concentration at which half-saturation of enzymes occurs) to be 0.029 +/- 0.003%. This model provides a significantly better data fit than that provided by two simpler submodels, one of which assumes that all clearance is linear, and the other of which allows a part of clearance to be saturable but ignores the isoflurane marker data. The value of 0.029% for Km indicates that a wide range of clinical anesthetic concentrations will produce similar rates of metabolism; that metabolism will proceed at near maximum rates during the first several minutes of recovery; and that most metabolism probably occurs after, rather than during, anesthesia.

Transcutaneous analysis of arterial PCO2

Commercially available skin surface PCO2 sensors, when properly maintained, calibrated, and applied, report arterial PCO2 over a wide range of values and in virtually all clinical conditions to an accuracy of +/- 3 torr. Inappropriate mathematical treatment of in vivo skin surface-arterial PCO2 comparisons has led to controversy regarding the precise relationship between these variables. The proper method of calibration involves applying a temperature correction factor of 4.5%/degrees C to the calibration gas setting, and subtracting 4 torr by offsetting zero. For analysis of accuracy, the resulting corrected values should be used to determine the mean and standard deviation of the skin surface:arterial PCO2 ratio. Tests of correlation as a function of PaCO2 require deliberate wide variation of PCO2 within each subject of a test group. Skin surface PCO2 monitors record blood gas tensions continuously and noninvasively, and they can be used to study cardiorespiratory function in normal subjects, in whom arterial blood sampling would be difficult to justify--two distinct advantages of the devices.

No effect of naloxone on hypoxia-induced ventilatory depression in adults

The ventilatory response to acute isocapnic hypoxia is prompt but is not maintained at its peak. Within 10 min, it begins to fall, and by 30 min has reached an approximately steady level, usually still above control. We used naloxone to test in four men the hypothesis that this fade is hypoxic depression mediated by endogenous opioid peptides, e.g, endorphins. Breath by breath minute ventilation was recorded during a hyperoxic control period (FIO2 = 0.3) to establish control alveolar PCO2. After 15 min. of isocapnic hypoxia (end-tidal PO2 = 45 Torr), naloxone injection (1.2 or 10 mg, iv) failed to alter the slow decrement of ventilation. Hypoxic ventilatory depression appears not to be mediated by endorphins in adults.

Cerebral oxygen availability and blood flow during middle cerebral artery occlusion: effects of pentobarbital

To determine whether barbiturate administration can improve oxygenation, oxygen availability (aO2) and local cortical blood flow (ICBF) were measured in cats before and during middle cerebral artery occlusion (MCAO) using 10 platinum electrodes distributed over the cortex. Halothane/N2O anesthesia was used during the surgical preparation and N2O with a relaxant thereafter. After 15 to 30 min of MCAO, 50 mg/kg of pentobarbital was infused slowly. Measured from electrodes in severely ischemic cortex, aO2 increased if the blood pressure was maintained with dopamine. Control animals in which no pentobarbital was given showed no change in aO2 over the same period of time. In areas of cortex not affected by middle cerebral artery occlusion the aO2 did not change from control values despite a decrease in blood flow from 72.7 +/- 49.8 to 48.9 +/- 26.7 ml/min/100 g. Thus, pentobarbital appears to decrease ICBF and metabolism proportionally in well perfused cortex so that aO2 remains constant, while improving the flow to metabolism ratio in poorly perfused cortex so that aO2 rises.

A combined transcutaneous PO2-PCO2 electrode with electrochemical HCO3- stabilization

Combined transcutaneous PO2-PCO2 electrodes are described in which the interaction between the two electrodes due to OH- production at the O2 cathode has been eliminated. An anode of either anodized aluminum or platinum has been driven at a current equal to cathode current to force stoichiometric consumption of OH- at its rate of production. The AgCl reference electrode operates at zero current. O2 sensitivity was not significantly altered by electrolyte pH variation from 6.7 to 9.0 with variations by PCO2. These electrodes have been found stable both with and without spacers, and with electrolytes dissolved in 50-100% ethylene glycol. In 22 anesthetized patients, with electrode temperature of 43 degrees C (s refers to skin surface, a to arterial blood); PsO2 = 0.52PaO2 + 15 (range 54-300) (r = 0.66; Sy . x = 29.6; n = 46); and PsCO2 = 1.39PaCO2 + 2.1 (range 24-98) (r = 0.99; Sy . x = 2.28; n = 48).

Multipatient anesthetic mass spectrometry: rapid analysis of data stored in long catheters

A centrally located mass spectrometer sequentially samples airway gases from ten anesthetized patients through 30 m long, 1.07 mm, ID, nylon catheters and three way solenoid valves. End-tidal and inspired concentrations of O2, N2, CO2, N2O, and halothane, enflurane, or isoflurane are displayed on a computer terminal screen in each OR with trend plots. While a gas sample from one room is being analyzed, all other catheters are slowly sampled in order to continuously store 20-s concentration profiles ready for analysis. The stored gas sample is analyzed at twice the rate it was sampled. The computer switches catheters after one breath has been validated from two comparable end-tidal PCO2 values. Large flow changes produced by switching from one catheter to the next require regulation of the pump pressure in the mass spectrometer. This method reduces the time required to sample each room to 6.96 s (4-10 rooms). Catheter transit slows the response to a step increase in concentration by about 0.13 s (from 10 per cent-90 per cent) and prolongs the transit time through the catheter for a volatile anesthetic by about 0.04 s more than N2. The monitoring facility is used in each room for an average of 5.5 h/day. Two years of experience suggest that it can facilitate detection of faulty technique and equipment, reduce cost of anesthetic agents by encouraging use of closed systems, increase patient safety, aid research and teaching, and diminish exposure of OR personnel to anesthetics. Inherent problems have resulted in an inoperative time of less than 2 per cent.

Relationship of CSF pH, O2, and CO2 responses in metabolic acidosis and alkalosis in humans

The effect of induced metabolic acidosis (48 h of NH4Cl ingestion, BE - 10.6 +/- 1.1) and alkalosis (43 h of NaHCO3- ingestion BE 8.8 +/- 1.6) on arterial and lumber CSF pH, Pco2, and HCO3- and ventilatory responses to CO2 and to hypoxia was assessed in five healthy men. In acidosis lumbar CSF pH rose 0.033 +/- 0.02 (P less than 0.05). In alkalosis CSF pH was unchanged. Ventilatory response lines to CO2 at high O2 were displaced to the left in acidosis (9.0 +/- 1.4 Torr) and to the right in alkalosis (4.5 +/- 1.5 Torr) with no change in slope. The ventilatory response to hypoxia (delta V40) was increased in acidosis (P less than 0.05) and it was decreased in four subjects in alkalosis (P, not significant). We conclude that the altered ventilatory drives of steady-state metabolic imbalance are mediated by peripheral chemoreceptors, and in acidosis the medullary respiratory chemoreceptor drive is decreased.

Decreased exercise hyperpnea in patients with bilateral carotid chemoreceptor resection

Exercise hyperpnea was compared in 5 asthmatics 25 yr after bilateral carotid body resection (BR), 4 others 19 yr after unilateral resection (UR), and 12 controls (C) matched for age and pulmonary flow limitation. In the BR group, ventilation rose less with exercise, mostly because BR experienced less tachypnea. End-tidal PCO2 rose 5.8 +/- 3.2 (P less than 0.05) to 46 Torr at 50 W. In UR and C the same load did not increase PETCO2 significantly (+2.1 and +1.4 Torr, respectively). Arterial-end-tidal PCO2 differences before and 15--45 s postexercise were insignificant in all three groups. Heart rate and blood pressure rose equally in the three groups, suggesting that the ventilatory effects were not secondary to blood flow differences and disclosing no evidence of baroreceptor denervation during glomectomy.