Ole Siggaard-Andersen

Measured and derived quantities with modern pH and blood gas equipment: Calculation algorithms with 54 equations

Modern pH and blood gas equipment, combining electrochemical and optical methods, measures pH, Pco2, po2, hemoglobin concentration, oxygen saturation, carboxy-hemoglobin, and methemoglobin. Measurements in arterial and mixed venous blood and in expired air (including measurement of expiratory ventilation) allow calculation of a series of derived quantities: concentrations of bicarbonate, total carbon dioxide, base excess, standard bicarbonate, total oxygen, 2,3-diphosphoglycerate; p50, arteriovenous CO2 and O2 differences, pulmonary shunting, ventilation-perfusion ratio, dead space ventilation, CO2 production, O2 consumption, respiratory quotient, energy metabolism, and cardiac output. Accurate measurement of carboxyhemoglobin even gives an opportunity of measuring blood volume.Equations are reviewed for these calculations including the Henderson-Hasselbalch equation for plasma and whole blood, the Van Slyke equation, the ODC-equation (hyperbolic tangent function), alveolar air equation, Bohr equation, an...

Evaluation of a new semiautomatic electrode system for simultaneous measurement of ionized calcium and pH

The instrument (ICA 1 Radiometer, Copenhagen) measures the activity of calcium ion and hydrogen ion and displays the estimated concentration of free calcium ion (cCa2+) and pH at 37 degrees C in 110 microliter of whole blood or serum. The cCa2+ at pH 7.40 is calculated by means of a fixed relationship between cCa2+ and pH. The results are indicated on a display approximately 1 min after sample introduction. The measuring cycle plus automatic rinsing takes 3 min. The apparatus is equipped with a number of control functions. The sensitivity of the calcium electrode showed a fall from 100% to 92% over 20 weeks. The cell potential of the calcium cell showed a slow drift of--0.02 mV per day over a 20-week period while the pH cell which employs the same reference electrode showed no drift. Interference of Na+, K+, Li+, Mg2+ and H+ on the calcium measurement in whole blood is estimated to be at most +0.1%. The effect of erythrocytes on the static liquid junction potential is minimized by a special extrapolation procedure. The analytical standard deviation for cCa2+ in serum was 0.008 mmol/l within series, 0.017 mmol/l between days. The mean value for cCa2+ (at pH 7.4) in serum for 53 healthy adults was 1.249 mmol/l with a standard deviation of 0.036 mmol/l. cCa2+ in capillary blood at the actual pH from 20 healthy volunteers gave a mean value of 1.215 +/- 0.047 mmol/l (+/- 2SD) with pH 7.428 +/- 0.031 (+/- 2 SD). The slopes delta lgcCa2+/delta pH measured after equilibration at two different PCO2 values in venous blood (n = 20) and in serum (n = 104) gave a mean value of -0.221 +/- 0.040 (+/- 2 SD). The semi-automatic combined Ca2+ - pH electrode system makes the measurement of ionized calcium for clinical use a reliable and accurate analysis.

Direct reading glucose electrodes detect the molality of glucose in plasma and whole blood.

It is the activity that determines the direction of chemical processes, transport, etc. and thus provides the clinically more relevant information. Direct reading glucose electrodes consume glucose at a rate proportional to the glucose activity in the sample. The activity equals the molality (mmol glucose per kg water), so results from direct reading glucose electrodes must differ from the conventionally measured glucose concentration. This was observed in 159 whole blood samples which gave higher results from a direct reading glucose electrode than by our conventional method (y = 1.21x - 0.37 mmol/l). However, adjustment for the different water concentration due to salt, plasma proteins, and hemoglobin occupying space, gave results equal to the concentrations (y = 1.00x - 0.28 mmol/l, r = 0.997). Furthermore, results for samples with constant glucose concentration and varying albumin concentration correlated with the albumin concentration (r = 0.989), but not after adjustment for water concentration (r = 0.037, n.s.).

Classes of tissue hypoxia

We identify eight causes of tissue hypoxia, falling into three classes, A, B, and C, depending upon the effect on the critical mixed venous pO2 and the optimal oxygen consumption rate. The critical mixed venous pO2 is the value above which the oxygen consumption rate is optimal and independent of the mixed venous pO2 and below which the oxygen consumption rate decreases towards zero. Class A hypoxia: primary decrease in mixed venous pO2. Causes: 1) ischaemic hypoxia (decrease in cardiac output), 2) low‐extractivity hypoxia (decrease in oxygen extraction tension, p8). Class B hypoxia: primary increase in critical mixed venous pO2. Causes: 1) shunt hypoxia (increased a‐v shunting), 2) dysperfusion hypoxia (increased diffusion length from erythrocytes to mitochondria and/or decreased total capillary endothelial diffusion area, e. g., tissue oedema, microembolism), 3) histotoxic hypoxia (inhibition of the cytochrome chain). Class C hypoxia: primary increase in optimal oxygen consumption rate. Causes: 1) uncoupling hypoxia (uncoupling of the ATP formation associated with O2 reduction), 2) hypermetabolic hypoxia (increased energy metabolism, e. g., due to hyperthermia).

Diode-array spectrophotometry for simultaneous measurement of hemoglobin pigments.

A prototype oxygen saturation meter was used to measure the concentrations of deoxygenated hemoglobin (rHb), oxyhemoglobin (HbO2), carboxyhemoglobin (HbCO), methemoglobin (MetHb), and sulfhemoglobin (SHb) in 35 microliter blood. Simultaneous absorbance measurements at 535, 560, 577, 622, 636, and 670 nm permitted the composition of any hemoglobin pigment mixture to be determined more accurately, precisely and easily than before. The inclusion of 670 nm, where the hemoglobin pigments have low absorption coefficients, allowed correction for turbidity.

Guidelines for routine measurement of blood hemoglobin oxygen affinity: International Federation of Clinical Chemistry (IFCC)

Two methods for the routine determination of blood hemoglobin oxygen affinity are described. Both methods use whole blood and do not require special equipment, tonometry or special gas mixtures. The first method consists of a one-point determination of p50, and requires only 200 μL to 400 μL of whole blood, therefore making it suitable for the pediatric population. The second method uses multiple points, thereby establishing both the shape and position of the hemoglobin oxygen equilibrium curve between 10 and 99 % oxygen saturation. Interpretation of p50 is discussed in relation to evaluation of patients with hemoglobinopathies and as a parameter in estimating availability of oxygen to the tissues.

Evaluation of the Gas-STAT® fluorescence sensors for continuous measurement of pH, pCO2 and pO2 during cardiopulmonary bypass and hypothermia

Continuous measurement of pH, pCO2 and pO2 during extracorporeal circulation has become feasible using disposable fluorescence sensors (optodes). We have evaluated a commercial system: Gas-STAT (American Bentley) by reference to in-vitro measurements on discrete samples using conventional electrochemical sensors (BMS-3, Radiometer). The Gas-STAT measures at the actual temperature of the blood in the extracorporeal circuit. The reference measurements were performed at two fixed temperatures of 25 and 37 °C with interpolation of the values to the actual temperature of the Gas-STAT.10 patients undergoing coronary artery bypass grafting during hypothermic extracorporeal circulation with hemodilution were monitored in the venous as well as the arterial line with the Gas-STAT with 6–9 samplings of arterial and venous blood from each patient, a total of 136 samples.The comparisons revealed a large scatter which was due partly to inter-optode partly to intra-optode variation and partly to a memory effect which re...

pH effect on the COHb absorption spectrum: Importance for calibration of the OSM3 and measurement of circulating hemoglobin and blood volume

An easy method to measure blood volume is clinically needed. We used carbon monoxide (CO) and the OSM3 to measure circulating hemoglobin and blood volume with the indicator dilution principle. 50 mL of CO was administered into a closed rebreathing system and taken up via the lungs, and the amount of hemoglobin in the blood was calculated from the increase in carboxyhemoglobin fraction after 10 min. Blood volume was calculated by division with the concentration of hemoglobin. We observed that the absorption spectrum of carboxyhemoglobin (COHb) depends on pH and pCO2, which must be controlled when very accurate spectrophotometry is necessary. The bias is 3% COHb per pH unit during calibration of the OSM3, which may be permissible for patients with CO poisoning, but not for the present purpose. With this in mind the method is very accurate, precise and simple.

Haemoglobin oxygen saturation and related quantities: definitions, symbols and clinical use

(1990). Haemoglobin oxygen saturation and related quantities: definitions, symbols and clinical use. Scandinavian Journal of Clinical and Laboratory Investigation: Vol. 50, No. 4, pp. 455-459.

Changes in plasma ionized calcium and magnesium in blood donors after donation of 450 mL blood. Effects of hernodilution and Donnan equilibrium

The plasma concentration of ionized calcium and ionized magnesium in 26 blood donors decreased 0.01 mmol/L during blood donation. The changes could be explained by admixture of interstitial fluid. About 162 mL or 36% of the donated blood was replaced by interstitial fluid during blood donation. From the changes in concentration and hematocrit we could estimate the composition of the added fluid. The concentration of protein was much lower than in plasma. The concentration of protein-bound and free cations was also lower, in accord with the Donnan theory. We conclude that blood donors immediately after blood donation are unsuited as a reference population for proteins and ions.