Journal of clinical monitoring
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Review Historical Article
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. ⋯ 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.
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A pulse oximeter was compared with an ear oximeter for measurement of arterial hemoglobin saturation within the range of 70 to 100% in 11 healthy volunteer subjects. Two hundred seventy-seven pooled data points were obtained, and analysis was performed by means of linear regression. The accuracy was 3% (95% confidence limits). ⋯ The pulse oximeter was easy to use because, unlike the ear oximeter, it required no time-consuming instrument calibration or site preparation. In addition, the delays involved in taking an invasive sample, transporting it to the blood gas laboratory, and waiting for the results were eliminated. Saturation values were continually available, and placement and use of the pulse oximeter sensor caused no discomfort to the volunteer subjects.
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Comparative Study
Comparison of radial and femoral arterial blood pressures in children after cardiopulmonary bypass.
We compared radial and femoral arterial blood pressures in 29 patients, ranging in age from 1.25 to 17 years, during and after cardiopulmonary bypass for repair of congenital heart disease. Radial mean arterial pressure (MAP) was more than 10% lower than femoral MAP in 17 patients (58%), and in 7 of these patients (24%) radial MAP was more than 20% lower than femoral MAP. ⋯ We found no correlation between femoral-minus-radial pressure difference and postoperative course. These data demonstrate that radial arterial pressure may be misleadingly low in children undergoing operation for correction of congenital cardiac defects.