Porth's Essentials of Pathophysiology, 4e

198

Integrative Body Functions

U N I T 2

Consequently, excessive loss of HCO 3 – occurs with severe diarrhea; small-bowel, pancreatic, or biliary fis- tula drainage; ileostomy drainage; and intestinal suc- tion. In diarrhea of microbial origin, HCO 3 – is also secreted into the bowel as a means of neutralizing the metabolic acids produced by the microorganisms caus- ing the diarrhea. Hyperchloremic Acidosis. Hyperchloremic acidosis occurs when Cl – levels are increased. 67 Because Cl – and HCO 3 – are exchangeable anions, the serum HCO 3 – decreaseswhen there is an increase inCl – .Hyperchloremic acidosis can occur as the result of abnormal absorption of Cl – by the kidneys or as a result of treatment with chloride-containing medications (i.e., sodium chloride, amino acid–chloride hyperalimentation solutions, and ammonium chloride). With hyperchloremic acidosis, the anion gap remains within the normal range, while serum Cl – levels are increased and HCO 3 – levels are decreased. Manifestations. Metabolic acidosis is characterized by a decrease in serum pH (<7.35) and HCO 3 – levels (<24 mEq/dL[24 mmol/L]) due to H + gain or HCO 3 – loss. The manifestations of metabolic acidosis fall into three categories: signs and symptoms of the disorder causing the acidosis, changes in body function due to recruit- ment of compensatory mechanisms, and alterations in

cardiovascular, neurologic, and musculoskeletal func- tion resulting from the decreased pH 65–66 (Table 8-10). Metabolic acidosis is seldom a primary disorder; it usually develops during the course of another disease. The manifestations of metabolic acidosis frequently are superimposed on the symptoms of the contributing health problem. With diabetic ketoacidosis, which is a common cause of metabolic acidosis, there is an increase in blood and urine glucose and a characteristic smell of ketones to the breath. 67 In metabolic acidosis that accompanies chronic kidney disease, blood urea nitro- gen levels are elevated and other tests of renal function yield abnormal results. Manifestations related to respiratory and renal compensatory mechanisms usually occur early in the course of metabolic acidosis. 65,66 In situations of acute metabolic acidosis, the respiratory system compen- sates for a decrease in pH by increasing ventilation to reduce PCO 2 ; this is accomplished through deep and rapid respirations. There may be complaints of difficulty breathing or dyspnea with exertion; with severe acidosis, dyspnea may be present even at rest. Respiratory compensation for acute metabolic acido- sis tends to be somewhat greater than for chronic aci- dosis. When kidney function is normal, H + excretion increases promptly in response to acidosis, and the urine becomes more acid.

TABLE 8-10 Manifestations of Metabolic Acidosis and Alkalosis Metabolic Acidosis Metabolic Alkalosis

LaboratoryTests pH decreased

LaboratoryTests

pH increased

Bicarbonate (primary) decreased

Bicarbonate (primary) increased

PCO 2

(compensatory) decreased

PCO 2

(compensatory) increased

Signs of Compensation

Signs of Compensation

Increased respirations (rate and depth) Hyperkalemia

Decreased respirations (rate and depth) with various degrees of hypoxia and respiratory acidosis

Acid urine Increased ammonia in urine Gastrointestinal Effects Anorexia Nausea and vomiting Abdominal pain Nervous System Effects

Nervous System Effects Hyperactive reflexes

Weakness Lethargy Confusion

Tetany

Confusion

Stupor

Seizures

Coma Depression of vital functions Cardiovascular Effects Peripheral vasodilation Decreased cardiac output Cardiac arrhythmias Skin Warm and flushed Skeletal System Effects Bone disease (chronic acidosis)

Cardiovascular Effects

Hypotension

Cardiac arrhythmias