Simple acid-base disorders

Because C02 is a volatile acid, it can rapidly be changed by the respiratory system. If a respiratory acid-base disturbance is present for minutes to hours it is considered an acute disorder while if it is present for days or longer it is considered a chronic disorder. By definition, the metabolic machinery that regulates HC03 results in slow changes in serum bicarbonate and all metabolic disorders are chronic. This means that there are six simple acid-base disorders as outlined in Table 25-1.2... 

The amount of compensation (metabolic or respiratory) can be reliably predicted based on the degree of derangement in the primary disorder. Table 25-1 outlines the simple acid-base disorders and provides formulas for calculating the... 

Arterial blood gases are the primary tools for evaluation of therapeutic outcome. They should be monitored closely to ensure resolution of simple acid-base disorders without deterioration to mixed disorders due to compensatory mechanisms. For example, arterial blood gases should be obtained every 2 to 4 hours during the acute phase of respiratory acidosis and then every 12 to 24 hours as acidosis improves. 

Classification and Characteristics of Simple Acid-Base Disorders ... 

Metabolic alkalosis is a simple acid-base disorder that presents as al-kalemia (increased arterial pH) with an increase in plasma bicarbonate. It is an extremely common entity in hospitalized patients with acid-base disturbances. Under normal circumstances, the kidney is readily able to excrete an alkali load. Thus evaluation of patients with metabolic alkalosis must consider two separate issues (1) the initial process that generates the metabolic alkalosis and (2) alterations in renal function that maintain the alkalemic state. °... 

Respiratory and metabolic derangements can occur in isolation or in combination. If a patient has an isolated primary acid-base disorder that is not accompanied by another primary acid-base disorder, a simple (uncomplicated) disorder... 

The causes of acid-base disorders, resulting laboratory values, and compensatory responses are discussed here in the traditional categorization of these disorders. However, it is often difficult to remember which disorders fall into which categories, so it is common for mnemonic devices or tables to be used to facilitate description of these disorders. A useful and more logical approach is to reaUze tliat an acidosis can only occur as a result of one (or a combination) of three mechanisms (1) increased addition of acid, (2) decreased elimination of acid, and (3) increased loss of base. Similariy, alkalosis occurs only by (1) increased addition of base, (2) decreased ehmination of base, and (3) increased loss of acid. Dufour has illustrated this simple concept by depicting the body as a two-tank vat, one of acid and one of base, with inputs and outputs for each vat (Figure 46-13). In the normal setting, these inputs and outputs are balanced an acid-base disorder then involves a perturbation in the input or output of these body reservoirs, as discussed in the next section. 

Narins RG, Emmett M. Simple and mixed acid-base disorders A practical approach. Medicine (Baltimore) 1980 59 161-187. 

Care must be taken in the interpretation of the blood gas results in these patients. Knowledge of the clinical picture is essential. Theoretically, the limits of the compensatory rcspon.ses in simple primary acid-base disorders are known (Fig. 5). When compensation apparently falls outside ofthe.se expected limits, it is likely that a second acid-base disorder is present. 

Figure 46-13 Simple depiction of the body as a two-vat system of acid and base. At equilibrium input and output from each vat are equal. (From Dufour DR. Add-base disorders. In Dufour DR, Christenson RH, eds Professional practice in clinical chemistry A review. Washington DC AACC Press, 1995 604-35.)...

Acid-base disturbances frequently coexist with two or more simple disorders (Table 39-2). In these settings, blood pH is either severely depressed (e.g., a patient with metabolic acidosis and respiratory acidosis) or normal. Both plasma HCOj and pH may be within normal limits when metabolic alkalosis and metabolic ketoacidosis coexist, as in a patient with diabetic ketoacidosis who is vomiting. In this situation, an elevated anion gap may be the initial abnormality that can be detected in the underlying mixed acid-base disturbance. 

Nevertheless, all of this amazing functionality displayed by natural proteins seems to be based on a simple fact a complex and completely defined primary structure. In living cells, protein biosynthesis is carried out with an absolute control of the amino acid sequence, from the first amino acid to the last with a complete absence of randomness. In fact, the need for this absolute control is dramatically clear in some genetic disorders in which the lack or a substitution of a single amino acid in the whole protein leads to a complete loss of the original function, which can have dramatic consequences in some cases such as falciform anemia (sickle cell anemia), phenylketonuria, and cystic fibrosis [7]. 

A further group of branched-chain acids differ from those already discussed in that they are produced from isoprenoid precursors and not by the normal de novo pathway or some simple modification of it. These acids are based on the diterpene phytol derived from chlorophyll and the more common acids are phytanic (C20), pristanic (C19) and a Cie homologue. These molecules contain chiral centres and the natural forms are not always the same enantiomer. These acids occur widely, but at low levels, in the lipids of land and aquatic animals and have been identified in ancient sediments. In animals they occur in tissue, milk and blood lipids and are present as trace components of butter fat. The tissue lipids of humans suffering from the rare neurological disorder Refsum s disease contain up to 50% of phytanic acid because of an inability to metabolize this unusual branched-chain acid (Section 12.6). 

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