When fluid therapy is required, it is important to know, in addition to the electrolyte status, the acid–base balance of the patient. In this chapter, acid–base disturbances will first be evaluated based on the traditional approach, before being analyzed according to a nontraditional approach (the Stewart approach), for a better understanding of how some electrolytes and ions can influence the acid–base balance.
One interpretation, however, does not exclude the other; indeed, both complement each other to provide an overview that allows an evaluation of the greatest number of components of the acid–base balance.
Under normal conditions, the body produces more acidic chemical species (about 15,000–20,000 mEq) than buffer systems (about 2500 mEq); it must therefore be able to quickly eliminate or buffer excess acids.
The body has a better ability to correct acidosis than alkalosis, and thus tolerates metabolic acidosis better than metabolic alkalosis.
A dog running in a field during a walk may have some degree of metabolic acidosis, but this will be corrected promptly; however, if the effort, because of its intensity and duration, were to exceed the normal compensation capacity, acidosis could become incompatible with life or create acid–base disorders with a negative impact on several of the animal’s vital functions.
It is the clinician’s task to identify as early as possible severe uncompensated acid–base alterations that could compromise the patient’s vital functions.
Carbon dioxide (CO2) acts as an acid in the body because carbonic anhydrase, a ubiquitous enzyme, catalyzes a reaction that converts CO2 and water into carbonic acid (H2CO3). With an increase in partial pressure of carbon dioxide (PaCO2), the carbonic acid equation (Equation 1) will shift to the right and the hydrogen ion concentration will increase:
CO2, in addition to being transformed into H2CO3, is mainly eliminated through alveolar ventilation. If alveolar ventilation is compromised (e.g., due to pneumothorax or positive pressure ventilation with high levels of CO2), this mechanism becomes ineffective and the reaction will shift to the right. Alveolar ventilation can change the CO2 concentration within 1–5 minutes; for example, if a patient has an increased respiratory rate as a result of fear or pain during blood sampling, the blood gas analysis (ABG) may indicate a false respiratory alkalosis (decreased CO2).
While the respiratory system can correct the pH through alveolar ventilation within a few minutes, renal adaptation, which is characterized by bicarbonate reabsorption (80–90% of which occurs in the proximal tubule) and excretion of ammonium and phosphate ions and water, needs a few hours to begin and is completed within 2–5 days.
According to the law of mass action, the elements of the chemical species indicated inEquation 1 (Box 2.1) must remain in equilibrium on both sides of the equation. For example, if HCO3− increases, as occurs when sodium bicarbonate is administered, the reaction will tend to sh
