Mixed acid base disturbances: a clinical approach

Fluid and electrolyte balance in critical patients

Recognition and appropriate management of fluid and electrolyte disorders in critical patients is extremely important. In many cases, these secondary problems are more complicated and more serious than the initiating disease process. A severely ill diabetic patient, for example, is more likely to die from dehydration, hyperosmolality, metabolic acidosis, hypokalemia or hypophosphatemia than from hyperglycemia or lack of insulin therapy. Proper fluid therapy and treatment of electrolyte abnormalities make a major difference in the survival rate of critically ill animals.

Mixed acid-base disorders

Mixed acid-base disturbances are combinations of two or more primary acid-base disturbances. Mixed acid-base disturbances may be suspected on the basis of findings obtained from the medical history, physical examination, serum electrolytes and chemistries, and anion gap. The history, physical examination, and serum biochemical profile may reveal disease processes commonly associated with acid-base disturbances. Changes in serum total CO2, serum potassium and chloride concentrations, or increased anion gap may provide clues to the existence of acid-base disorders. Blood gas analysis is usually required to confirm mixed acid-base disorders. To identify mixed acid-base disorders, blood gas analysis is used to identify primary acid-base disturbance and determine if an appropriate compensatory response has developed. Inappropriate compensatory responses (inadequate or excessive) are evidence of a mixed respiratory and metabolic disorder. The anion gap is also of value in detecting mixed acid-base disturbances. In high anion gap metabolic acidosis, the change in the anion gap should approximate the change in serum bicarbonate. Absence of this relationship should prompt consideration of a mixed metabolic acid-base disorder. Finding an elevated anion gap, regardless of serum bicarbonate concentration, suggests metabolic acidosis. In some instances, elevated anion gap is the only evidence of metabolic acidosis. In patients with hyperchloremic metabolic acidosis, increases in the serum chloride concentration should approximate the reduction in the serum bicarbonate concentration. Significant alterations from this relationship also indicate that a mixed metabolic disorder may be present. In treatment of mixed acid-base disorders, careful consideration should be given to the potential impact of therapeutically altering one acid-base disorder without correcting others.

Disturbances in acid-base metabolism. Recognition in the office setting

Determination of the appropriateness of the compensatory response is essential in evaluation of any acid-base disturbance. Adequate compensation supports the presence of a single acid-base event. Inadequate compensation may indicate that a second primary acid-base event is present or, in some circumstances, that time has been inadequate for maximal physiologic compensation. When acid-base disturbances are mixed, treatment of one disorder may unmask the full pH-altering potential of the second disorder. Close monitoring of patients at risk for the development of acid-base disorders permits detection of problems at a time when intervention may prevent serious complications. Relatively inexpensive and easy-to-use chemical analytic systems to assist early diagnosis are now present in many office-based laboratories.

[Practical approach to complex acid-base disorders using a slide rule]

Diagnosis of mixed acid-base disturbances is often difficult. Nowadays it depends on biochemical and statistical interpretation, coupled with clinical data. The acid-base slide-rule is a useful tool to carry out this five step procedure, which it simplifies, giving rapidly at the patient's bed-side an objective support for the diagnosis of acid-base disturbances.

Ascites, renal abnormalities, and electrolyte and acid-base disorders associated with liver disease

Ascites and renal dysfunction are often associated with decreased liver function and reflect the complex abnormalities of water, protein, electrolyte, and acid-base metabolism that may complicate severe liver disease. This article discusses the pathophysiology and management of ascites, polydipsia and polyuria, decreased renal function, and acid-base and electrolyte alterations that can complicate liver disease.

Extreme metabolic alkalosis with fludrocortisone therapy

We present an unusual case of extreme metabolic alkalosis resulting from severe hypokalaemia caused by unmonitored fludrocortisone therapy. Biochemical aspects of the disorder are discussed, as is the successful treatment with diuretics and potassium replacement. Some dangers of this therapy and necessary precautions are emphasized.

Clinical approach to patients with acid-base disorders

Simple relationships that hold during compensation for metabolic or respiratory acid-base disturbances have been emphasized. When considered by the physician, they should allow for the interpretation and diagnosis of both simple and mixed disturbances. Since the correct diagnosis of an acid-base disturbance is central to initiation of correct therapy, these considerations are not merely academic exercises but are essential steps to follow in careful management. Table 6 summarizes the stepwise approach to diagnosis of an acid-base disorder. Application of these principles in a routine fashion combined with a careful history and physical examination often results in the successful diagnosis of complicated disturbances. Understanding of the pathophysiologic derangements in each specific disorder is necessary for full appreciation of acid-base balance in disease.