Arterial blood gas (ABG) - Metabolic alkalosis: Nursing

An elderly client is brought to the emergency department from a nursing home. The nurse from the nursing home reported that the client has been experiencing nausea, vomiting, and diarrhea for the past few days. On assessment, the client is confused and has a slow and shallow respiratory rate. Based on these findings, the health care provider suspects metabolic alkalosis secondary to dehydration, so an arterial blood gas is ordered to assess for changes in the acid-base balance.

Alright, arterial blood gas, or ABG for short, is a test used to measure the acid-base components and pressure of gasses in the arterial blood. Normal ABG values for healthy adults are a pH ranging from 7.35 to 7.45, bicarbonate, or HCO3- ranging from 21 to 28 mEq/L; carbon dioxide or PaCO2 ranging from 35 to 45 mm Hg; PaO2 ranging from 80 to 100 mm Hg, and SaO2 should be more than 95%.

Now, metabolic alkalosis is a condition where the pH is increased, which can occur due to an excessive loss of acid, or hydrogen ions; or a gain of base, or bicarbonate ions. A common way for this to occur is through the gastrointestinal system. First, too much acid can be lost from excessive vomiting or prolonged gastric suctioning. In this situation, the amount of base remains the same, but the amount of acid is reduced, resulting in a relative base excess. Metabolic alkalosis can also be caused by an increased intake of base, like an excessive oral intake of bicarbonate-containing antacids. When this happens, the amount of acid remains the same, but the amount of base increases, resulting in an absolute base excess.

Another way metabolic alkalosis can happen is through the renal system. One common cause is when there’s too much of the hormone aldosterone. Now, excess aldosterone can be the result of an adrenal tumor that secretes excess aldosterone. It can also happen when the renin-angiotensin-aldosterone mechanism is triggered by volume depletion from loop or thiazide diuretic use, or from vomiting and diarrhea.

Whatever the cause, this excess aldosterone causes the distal convoluted tubule of the kidney to dump out hydrogen ions and reabsorb more bicarbonate ions. The result is that the urine becomes more acidic and the blood becomes more basic. Finally hypokalemia can contribute to metabolic alkalosis. Low levels of potassium in the extracellular fluid causes potassium to move from inside cells and out to the extracellular fluid. This prompts hydrogen ions to move into the cell, which makes the blood less acidic.

Now, as the pH continues to increase and move out of normal range, the body will attempt to correct the imbalance, a process called compensation. With metabolic alkalosis, the respiratory system begins the process of compensation when the chemoreceptors, that are located in the walls of the carotid arteries and in the wall of the aortic arch, start firing less often when the pH rises, and that notifies the respiratory center in the brainstem to decrease the respiratory rate and depth of breathing.