A free online resource for Intensive Care Medicine.
An unofficial Fellowship Exam (CICM Part 2) preparation resource.
Deranged Physiologyis a slowly growing archive of discussions and study notes relevant (or if not relevant, then at least interesting) to the practice of intensive care medicine. The content provides an introduction to the fundamental themes in intensive care: mechanical ventilation, vasopressors, electrolyte management, hemodynamic monitoring, dialysis, and so forth. Attention is directed at equipment in intensive care, and there are attempts to revisit interesting pharmacology and physiology. The aim of this resource is to supplement the bedside teaching of senior staff, and to consolidate resources for intensive care trainees in the initial stages of their training.
This is a suggested pattern for the interpretation of ABGs, as presented in T.J. Morgan's chapter for Oh's Manual (Chapter 92, "Acid base balance and disorders"). This method of interpretation should probably be used to answer all those ABG-related CICM Fellowship SAQs. The original version of this sequence on page 943 (7th ed.); a summary is available in one of the Required Reading chapters.
This page acts as a footnote to the "Boston vs. Copenhagen" chapter from Acid-Base Physiology by Kerry Brandis. His chapter explores the epistemology of acid-base interpretation systems by means of which we might be able to determine whether a patient has a single or mixed acid base disorder; i.e. whether there is a purely metabolic or a purely respiratory disturbance, or some mixture of the two. As it happens, there are two well-accepted systems for doing this, each with its own merits and demerits. These are the Boston and Copenhagen methods of acid-base interpretation.
Standard base excess is the concentration of titrable base when the blood is titrated back to a normal plasma pH of 7.40, at a normal pCO2 ( 40 mmHg) and 37° C, at the actual oxygen saturation, AND at an "anaemic" haemoglobin concentration, to account for the buffering of extravascular fluid by haemoglobin. It is reported as cBase(Ecf), to reflect the fact that the entirety of the extracellular fluid is under investigation here. In summary, it is the actual base excess adjusted to a Hb level of around 50g/L.
Actual base excess is the concentration of titrable base when the blood is titrated back to a normal plasma pH of 7.40, at a normal pCO2 ( 40 mmHg) and 37° C, at the actual oxygen saturation. It is reported as cBase(B)c. This base excess represents the metabolic contribution to the change in base excess. In essence, this is what the base excess should be if all the non-metabolic influences were corrected.
Standard bicarbonate is the concentration of bicarbonate in the plasma from blood which is equilibrated with a normal PaCO2 (40 mmHg) and a normal pO2 (over 100 mmHg) at a normal temperature (37°C). Usually, it is obtained by solving the Henderson-Hasselbalch equation to get a bicarbonate value when the pH is known and PaCO2 is 40mmHg. This bicarbonate level represents the metabolic contribution to the change in bicarbonate. It was introduced in 1957 by Jorgensen and Astrup. In essence, this is what the bicarbonate should be if all the non-metabolic influences were corrected. It answer the question, "how much would my patient's bicarbonate be if I were ventilating them properly?".
Actual bicarbonate is the concentration of hydrogen carbonate in the plasma. The ABG machine usually reports this as cHCO3-(P). It is a derived variable. One is interested in the bicarbonate value because it is the most important extracellular fluid buffer, accounting for 75% of total buffering in metabolic acid-base disturbances (the rest being performed by blood proteins, such as haemoglobin).
The alpha-stat hypothesis suggests we always interpret our blood gases as corrected to the same temperature (normal body temperature) irrespective of what the body temperature actually is. The pH-stat hypothesis instead recommends that we always correct the temperature to the core body temperature. Each approach has its merits and demerits. In order to maintain one's appearance as an intelligent interpreter of blood gas data, one should decide on which approach to use, and come up with some well-articulated arguments as to why one is using it.
The reaction which creates H3O+ and OH- is an endothermic reaction. Therefore, as heat is removed from the system by cooling, the reaction is driven to the left (i.e. in the direction of reassociation) - thereby reducing the concentration of H3O+ and raising the pH. Increasing the heat in the system lowers the pH. In short, the pH of any given solution will change in a fairly linear association with temperature.
Sometimes, one does not wish to be bogged down in elaborate discussions of definitions, or in the minutae of historical developments surrounding a specific concept. Sometimes one only has a ten-minute viva session, or a paragraph in a short answer question to devote to an explanation of pH measurement. For these situations, a brief summary of ABG machine function is available in the "Required Reading" section of the CICM Fellowship Exam Preparation chapters. That, in fact, is enough. The rambling digression on this page certainly does not represent the level of familiarity with ABG analysis expected from the ICU trainee in Australia.
The history of the glass electrode enjoys a thorough and well-developed exploration in a 2011 article by Fritz Scholz, from which much of this information is derived. Generally speaking, the explanation of a concept like this probably benefits from an exploration of its origin, so that the trains of thought can be followed from early observations all the way to the modern era (so that one can understand how the current state of the art ABG analytic technology performs its basic functions).
The modern concept of pH can be defined as a number expressing the acidity or alkalinity of a solution as the logarithm of the reciprocal of hydrogen ion concentration or hydrogen ion activity. This is not the definition. One might search far and wide for a simple lay definition, and one would meet no interesting or informative obstacles for many miles. To uncover a proper definition one must descend into the atmosphere-of-Venus-like environment of Pure and Applied Chemistry. There, one may finally discover the 2002 IUPAC definition, as well as the IUPAC-specified international standard for the procedures of pH measurement.
The original concept of pH was developed by Søren Peder Lauritz Sørensen, and involved the concentration rather than the activity of hydrogen ions. This was the result of earlier work by Svante Arrhenius whose 1884 definition of an acid was "something that dissociates in solution to produce hydrogen ions". In order to precisely measure the hydrogen ion concentration without resorting to colour-change tests, Sørensen devised an experiment in which the concentration gradient of ions could be related to the electric gradient between electrodes in an electrochemical cell.
Since Arrhenius started being taught in schools, and since most people started going to school, the concentration of hydrogen ions became a household term, synonymous with pH. Even as Sørensen revised his definition in terms of activity rather than concentration, people were taking up the notion that in every glass of water, some 107 protons and hydroxyl ions were lurking, free and naked, ready to tear molecules to shreds. Plainly, this is insane.