Journal of clinical monitoring
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When we ask, what renders essential a particular monitoring approach during routine anesthesia for a healthy patient, perplexing questions, rather than satisfying answers, are raised. I have examined these questions with the help of three lenses that focus on the relationship between the outcome of anesthesia and the detection, and thus correction, of abnormalities during anesthesia. The first lens looks at whether the monitoring modalities accepted by anesthesiologists as "minimal" and "essential" have been scientifically proven to affect outcome from routine anesthesia. ⋯ The third lens looks at whether there are nonclinical influences on monitoring practice. This lens views the gap between recognizing monitoring possibilities and adopting them clinically; it also views geographic differences in monitoring, as well as social pressures exerted through legal proceedings. Finally, currently recognized essential monitors such as blood pressure measurement, electrocardiography, and oxygen analysis are mentioned, and candidates for inclusion in the list of essential monitors, such as oximeters, capnographs, and the automated record, are discussed.
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Development of the flow-directed pulmonary artery catheter in combination with reflective fiberoptic oximetry techniques allows the clinician to continuously measure mixed venous oxygen saturation (SvO2). A brief review of the determinants of oxygen balance, the Fick principle, and the technology of continuous SvO2 monitoring is preliminary to a debate between two clinicians on the usefulness of SvO2 monitoring. One clinician highly recommends use of the flow-directed pulmonary artery catheter in patients who require pulmonary artery catheterization. ⋯ Major mistakes in patient management could follow from overreliance upon either absolute SvO2 measurements or analysis of trends over time. Use of the SvO2 monitor has not been proven cost-effective and may actually increase monitoring costs. Both clinicians agree that continuous SvO2 monitoring is valuable in many clinical circumstances, provided the limitations of the measurement are understood.
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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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To confirm the clinical applicability of a commercial pulse oximeter, we compared arterial hemoglobin saturation values determined by in-vitro oximetry and pulse oximetry in 15 critically ill children. One hundred ninety-two paired hemoglobin saturations were determined by both noninvasive pulse oximetry and direct measurement of arterial blood samples. ⋯ Pulse oximetry was found to be safe and less cumbersome than other methods of monitoring arterial oxygen content. Overall, pulse oximetry was precise and provided a clinically satisfactory noninvasive method for continuously monitoring arterial hemoglobin saturation in critically ill children.