Important Points - Chapter 7 : Metabolic Alkalosis

• Metabolic alkalosis is an abnormal primary process causing a decrease in fixed acids in the blood. Buffering results in an increase in plasma bicarbonate level.
• An acute metabolic alkalosis will NOT persist long as the normal kidney rapidly increases bicarbonate excretion from the body
• A metabolic alkalosis requires BOTH an initiating process and a maintaining process. Without an abnormal process maintaining it, the alkalosis will rapidly correct as the kidney pours out HCO3 in the urine.
• The maintaining process causing persistence of the elevated plasma bicarbonate level works by impairing renal bicarbonate excretion. The four factors which are involved in maintaining the disorder are:
  • chloride depletion
  • reduced GFR
  • potassium depletion
  • ECF volume depletion
• The initiating cause in most cases is loss of gastric acid (eg vomiting) or diuretic use. Chloride depletion is the abnormality that impairs renal bicarbonate excretion.
• All these patients (>90% of clinical cases) require chloride replacement (usually as saline solution) before they can be corrected
• Rare causes include various adrenocortical excess syndromes.
• Hypokalaemia is the most common associated electrolyte abnormality and can be life-threatening itself
• Spot urinary chloride levels can be useful in differentiating the cause in those cases where vomiting or diuretic use are uncertain
• The compensatory response is hypoventilation but there is variation in the degree of this. Oxygen therapy should be used in most hospital patients.
• Remember: Correction usually requires replacement of chloride usually in association with fluid and potassium. In rare severe cases, hydrochloric acid infusion or use of acetazolamide may be used but there are risks