Chapter 1: Acid-Base Disorders

Web Chapter 1

Acid-Base Disorders

Acid-base disorders are often encountered in critical care and outpatient settings in association with several conditions. A clear understanding of metabolic-respiratory interactions and a systematic approach aimed at identifying the separate components of acid-base disorders not only serve as diagnostic tools, but also help in formulating therapeutic interventions. For example, abnormal acid-base balance may be harmful in part because of the patient’s response to the abnormality, as when a spontaneously breathing patient with metabolic acidosis attempts to compensate by increasing minute ventilation. Such a response may lead to respiratory muscle fatigue, with respiratory failure or diversion of blood flow from vital organs to the respiratory muscles, and eventually result in organ injury. Thus it is important to understand both the causes of acid-base disorders and the limitations of various treatment strategies.

Blood pH and bicarbonate concentration can change secondary to alterations in carbon dioxide tension (PCO2), strong ion difference (SID), or total plasma concentration of nonvolatile weak buffers (Atot). Respiratory acid-base disorders occur whenever there is a primary change in PCO2, whereas metabolic acid-base disorders occur whenever SID or Atot are changing primarily. Changes in SID or Atot can be identified clinically by alterations in imageconcentration or base excess (BE). The SID is the difference between all strong cations and all strong anions. Strong ions are fully dissociated at physiologic pH and therefore exert no buffering effect. However, strong ions do exert an electrical effect because the sum of completely dissociated cations does not equal the sum of completely dissociated anions. Because strong ions do not participate in chemical reactions in plasma at physiologic pH, they act as a collective positive unit of charge, the SID. The quantitatively most important strong ions in plasma are Na+, K+, Ca2+, Mg2+, Cl, lactate, β-hydroxybutyrate, acetoacetate, and image. The influence of strong ions on pH and image concentration can always be summarized in terms of the SID. Changes in SID of a magnitude capable of altering acid-base balance usually occur as a result of increasing concentrations of Na+, Cl, image, or organic anions or decreasing concentrations of Na+ or Cl. An increase in SID (by decreasing Cl or increasing Na+) causes a strong ion (metabolic) alkalosis, whereas a decrease in SID (by decreasing Na+ or increasing Cl, image, or organic anions) causes a strong ion (metabolic) acidosis. The main nonvolatile plasma buffers that constitute Atot act as weak acids at physiologic pH (e.g., phosphate, imidazole [histidine] groups on plasma proteins). An increase in the total concentration of phosphate leads to Atot (metabolic) acidosis, whereas a decrease in albumin concentration causes Atot (metabolic) alkalosis.

Stepwise Approach

A routine methodical approach to interpretation of blood gas data facilitates the clinician’s approach to the patient. The first step is a careful history to search for clues that may lead the clinician to suspect the presence of acid-base disorders, followed by a complete physical examination.

Calculate the Expected Compensation

Any alteration in acid-base equilibrium sets into motion a compensatory response by either the lungs or the kidneys. The compensatory response attempts to return the ratio between PCO2 and image to normal and thereby minimize the pH change. A primary increase or decrease in one component is associated with a predictable compensatory change in the same direction in the other component (Web Table 1-1). Adaptive changes in plasma image in respiratory disorders occur in two phases: acute and chronic. In respiratory acidosis, the first phase represents titration of nonbicarbonate buffers, whereas in respiratory alkalosis, the first phase represents release of H+ from nonbicarbonate buffers within cells. This response is completed within 15 minutes. The second phase reflects renal adaptation and consists of increased net acid excretion and increased image reabsorption (decreased Cl reabsorption) in respiratory acidosis and decreased net acid excretion in respiratory alkalosis. Adaptive respiratory response to metabolic disorders begins immediately and is complete within hours. Some guidelines for use of compensatory rules from Web Table 1-1 are presented in Web Box 1-1.

The definition of a simple acid-base disturbance includes both the primary process causing changes in PCO2 or image and the compensatory mechanisms affecting these measurements. Lack of appropriate compensation is evidence of a mixed acid-base disorder. Compensation is said to be inappropriate if a patient’s PCO2 differs from expected PCO2 by more than 2 mm Hg in a primary metabolic process or if a patient’s image differs from the expected image by more than 2 mEq/L in a respiratory acid-base disorder.

Calculate Gaps and Gradients

Calculating the various gaps and gradients can be useful in evaluation of acid-base disorders (Web Box 1-2).

Strong Ion Gap and Anion Gap

Increases in the anion gap (AG) and strong ion gap (SIG) are associated with increases in concentration of unmeasured anions, both strong (e.g., lactate, acetoacetate, β-hydroxybutyrate, strong anions of renal failure) and weak (e.g., phosphate). The AG also is used to differentiate between hyperchloremic (normal AG) and high-AG metabolic acidoses. The AG in normal dogs and cats is mostly a result of the net negative charge of proteins and thus is heavily influenced by protein concentration, especially albumin. At plasma pH of 7.4 in dogs, each decrease of 1 g/dl in albumin concentration is associated with a decrease of 4.1 mEq/L in the AG (Constable and Stämpfli, 2005). The SIG is not affected by changes in albumin concentration, and an increase in unmeasured strong anions is suspected whenever SIG is less than −5 mEq/L. The SIG has not been clinically tested in dogs and cats, but its derivation is sound, and it is superior to the AG for detecting increases in unmeasured strong anions in other species.

Chloride Gap

Chloride is the most important extracellular strong anion. Increases in chloride lead to metabolic acidosis by decreasing SID, whereas decreases in chloride cause metabolic alkalosis by increasing SID. Therefore plasma Cl and image have a tendency to change in opposite directions in hypochloremic alkalosis and hyperchloremic acidosis. The contribution of Cl to changes in BE and image can be estimated by calculating the chloride gap (see Web Box 2-2). Chloride gap values greater than 4 mEq/L are associated with hypochloremic alkalosis, whereas values less than −4 mEq/L are associated with hyperchloremic acidosis. Whenever sodium concentration is normal, the difference between the sodium and chloride concentrations ([Na+] − [Cl]) can be used. Normally, [Na+] − [Cl] is approximately 36 mEq/L in dogs and cats. Values greater than 40 mEq/L are an indication of hypochloremic alkalosis, whereas values less than 32 mEq/L are associated with hyperchloremic acidosis.

Alveolar-Arterial Oxygen Gradient

Frequently, patients with respiratory acidosis or alkalosis also are hypoxemic. When determining management options, it is important to differentiate between hypoxia from primary lung disease (e.g., ventilation-perfusion mismatching) and alveolar hypoventilation by calculating the alveolar-arterial oxygen gradient, or (A – a) O2 gradient. Values less than 15 mm Hg generally are considered normal. If the (A − a) O2 gradient is increased, a component of the hypoxemia results from ventilation-perfusion mismatching, although it may be increased in some patients with extrapulmonary disorders. Clinically, a normal gradient excludes pulmonary disease and suggests some form of central alveolar hypoventilation or an abnormality of the chest wall or inspiratory muscles. To increase the specificity of the test to diagnose ventilation-perfusion mismatch, only patients with (A − a) O2 gradient values of more than 25 mm Hg should be considered abnormal (Johnson and de Morais, 2012). These patients are likely to have primary pulmonary disease, but extrapulmonary disorders cannot be completely ruled out.

Respiratory Acid-Base Disorders

Disorders of PCO2

Respiratory acid-base disorders are those abnormalities in acid-base equilibrium initiated by a change in PCO2. The PCO2 is regulated by respiration: a primary increase in PCO2 acidifies body fluids and initiates the acid-base disturbance called respiratory acidosis, whereas a decrease in PCO2 alkalinizes body fluids and is known as respiratory alkalosis.

Respiratory Alkalosis

Respiratory alkalosis, or primary hypocapnia, is characterized by decreased PCO2, increased pH, and a compensatory decrease in image concentration in the blood. Respiratory alkalosis occurs whenever the magnitude of alveolar ventilation exceeds that required to eliminate the CO2 produced by metabolic processes in the tissues. Common causes of respiratory alkalosis include stimulation of peripheral chemoreceptors by hypoxemia, primary pulmonary disease, direct activation of the brainstem respiratory centers, overzealous mechanical ventilation, and situations that cause pain, anxiety, or fear (Web Box 1-3). It is difficult to attribute specific clinical signs to respiratory alkalosis in the dog and cat. The clinical signs usually are caused by the underlying disease process and not by the respiratory alkalosis itself. However, in humans, headache, light-headedness, confusion, paresthesias of the extremities, tightness of the chest, and numbness around the mouth have been reported in acute respiratory alkalosis. If the pH exceeds 7.6 in respiratory alkalosis, neurologic, cardiopulmonary, and metabolic consequences may arise. Such a pH only can be achieved in acute respiratory alkalosis before renal compensation ensues. Alkalemia results in arteriolar vasoconstriction that can decrease cerebral and myocardial perfusion. In addition, hyperventilation (PCO2 < 25 mm Hg) causes decreased cerebral blood flow, potentially resulting in clinical signs such as confusion and seizures. Treatment of respiratory alkalosis should be directed at relieving the underlying cause of the hypocapnia; no other treatment is effective. Respiratory alkalosis severe enough to cause clinical consequences for the animal is uncommon. Hypocapnia itself is not a major threat to the well-being of the patient. Thus the underlying disease responsible for hypocapnia should receive primary therapeutic attention.

Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 1: Acid-Base Disorders

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