Acid-base Imbalances Student Notes PDF

2/22/24, 2:22 PM Realizeit for Student Introduction Acid–base disturbances are commonly encountered in clinical practice, especially in critical-care units. Identification of the specific acid–base imbalance is important in ascertaining the underlying cause of the disorder and determining appropriat...

2/22/24, 2:22 PM Realizeit for Student Introduction Acid–base disturbances are commonly encountered in clinical practice, especially in critical-care units. Identification of the specific acid–base imbalance is important in ascertaining the underlying cause of the disorder and determining appropriate treatment. Plasma pH is an indicator of hydrogen ion (H+) concentration and measures the acidity or alkalinity of the blood. Homeostatic mechanisms keep pH within a normal range (7.35 to 7.45). These mechanisms consist of buffer systems, the kidneys, and the lungs. The H+ concentration is extremely important: The greater the concentration, the more acidic the solution and the lower the pH. The lower the H+ concentration, the more alkaline the solution and the higher the pH (Larkin & Zimmanck, 2015; Norris, 2019). Buffer systems prevent major changes in the pH of body fluids by removing or releasing H+; they can act quickly to prevent excessive changes in H+ concentration. Hydrogen ions are buffered by both intracellular and extracellular buffers. The body’s major extracellular buffer system is the bicarbonate–carbonic acid buffer system, which is assessed when ABGs are measured. Normally, there are 20 parts of bicarbonate (HCO3−) to 1 part of carbonic acid (H2CO3). If this ratio is altered, the pH will change. It is the ratio of HCO3− to H2CO3 that is important in maintaining pH, not absolute values (Larkin & Zimmanck, 2015; Norris, 2019).. Acute and Chronic Metabolic Acidosis (Base Bicarbonate Deficit) Metabolic acidosis is a common clinical disturbance characterized by a low pH due to an increased H+ concentration and a low plasma bicarbonate concentration. Metabolic acidosis can occur by a gain of hydrogen ions or a loss of bicarbonate ions in the bloodstream. It can be divided clinically into two forms, according to the values of the serum anion gap: high anion gap metabolic acidosis and normal anion gap metabolic acidosis. The anion gap refers to the difference between the sum of all measured https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zRrdbUNNMKTSTb3II3pEltD4s%2fPbBdKcWtJFoZ4TUro%2b%… 1/4 2/22/24, 2:22 PM Realizeit for Student positively charged electrolytes (cations) and the sum of all negatively charged electrolytes (anions) in blood. Because the sum of measured cations is typically greater than the sum of measured anions in the bloodstream, there normally exists a disparity with predominance of cations; this is referred to as the anion gap. The anion gap reflects unmeasured anions (phosphates, sulfates, and proteins) in plasma that replace bicarbonate in metabolic acidosis. Measuring the anion gap is necessary when analyzing conditions of metabolic acidosis as it can help determine the cause of the acidosis (Emmett & Szerlip, 2018). The anion gap can be calculated by either of the following equations: Potassium is often omitted from the equation because of its low level in the plasma; therefore, the second equation is used more often than the first (Theodore, 2019). The normal value for an anion gap is 8 to 12 mEq/L (8 to 12 mmol/L) without potassium in the equation. If potassium is included in the equation, the normal value for the anion gap is 12 to 16 mEq/L (12 to 16 mmol/L). The unmeasured anions in the serum normally account for less than 16 mEq/L of the anion production. Metabolic acidotic conditions can be differentiated according to the anion gap; there is either a normal anion gap or high anion gap. A person diagnosed with metabolic acidosis is determined to have normal anion gap metabolic acidosis if the anion gap is within this normal range (8 to 12 mEq/L). An anion gap greater than 16 mEq (16 mmol/L) suggests excessive accumulation of unmeasured anions and would indicate high anion gap metabolic acidosis. An anion gap occurs because not all electrolytes are measured. More anions are left unmeasured than cations (Emmett & Szerlip, 2018). Pathophysiology Normal anion gap metabolic acidosis results from the direct loss of bicarbonate, as in diarrhea, lower intestinal fistulas, ureterostomies, use of diuretics, early renal insufficiency, excessive administration of chloride, and the administration of parenteral nutrition without bicarbonate or bicarbonate-producing solutes (e.g., lactate) (Emmett & Szerlip, 2018). https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zRrdbUNNMKTSTb3II3pEltD4s%2fPbBdKcWtJFoZ4TUro%2b%… 2/4 2/22/24, 2:22 PM Realizeit for Student High anion gap metabolic acidosis occurs when there is an excessive accumulation of acids. High anion gap occurs in lactic acidosis, salicylate poisoning (acetylsalicylic acid), renal failure, methanol, ethylene or propylene glycol toxicity, DKA, and ketoacidosis that occurs with starvation. The high amount of hydrogen ions due to the acids present are neutralized and buffered by HCO3− causing the bicarbonate concentration to fall and become exhausted. Other anions in the bloodstream are called upon to neutralize the high acid in the blood. In all of these instances, abnormally high levels of anions are used to neutralize the H+, which increases the anion gap above normal limits (high anion gap) (Emmett & Szerlip, 2019). Clinical Manifestations Signs and symptoms of metabolic acidosis vary with the severity of the acidosis but include headache, confusion, drowsiness, increased respiratory rate and depth, nausea, and vomiting. Peripheral vasodilation and decreased cardiac output occur when the pH drops to less than 7. Additional physical assessment findings include decreased blood pressure, cold and clammy skin, arrhythmias, and shock. Chronic metabolic acidosis is primarily caused by chronic kidney disease because dysfunctional kidneys do not excrete acid (Emmett & Szerlip, 2018; Kovesdy, 2018). Assessment and Diagnostic Findings ABG measurements are used in the diagnosis of acid–base imbalances such as metabolic acidosis. Expected ABG changes include a low bicarbonate level (less than 22 mEq/L) and a low blood pH (less than 7.35). The cardinal feature of metabolic acidosis is a decrease in the serum bicarbonate level. In conditions of acidosis there is elevated H+ and the sodium–potassium cellular pump brings H+ into the cells in place of K+. Therefore, high K+ accumulates in the bloodstream in metabolic acidosis as a result of the shift of potassium out of the cells (Theodore, 2019). Later, when the https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zRrdbUNNMKTSTb3II3pEltD4s%2fPbBdKcWtJFoZ4TUro%2b%… 3/4 2/22/24, 2:22 PM Realizeit for Student acidosis is corrected and pH normalized, the cellular pump causes potassium to move back into the cells and hypokalemia may occur. Blood levels of potassium need to be closely monitored. ECG monitoring is recommended as changes of potassium in the bloodstream can cause arrhythmias (Palmer & Clegg, 2016a). In metabolic acidosis, the lungs compensate for the high H+ through hyperventilation to decrease the CO2 level, which in turn reduces H+ (see carbonic acid equation). Calculation of the anion gap is helpful in determining the cause of metabolic acidosis. There are certain conditions that cause high anion gap metabolic acidosis and others that cause normal anion gap metabolic acidosis. Medical Management Treatment is directed at correcting the metabolic imbalance. If the problem results from excessive intake of chloride, treatment is aimed at eliminating the source of the chloride. When necessary, bicarbonate is given; however, the administration of sodium bicarbonate during cardiac arrest can result in paradoxical intracellular acidosis. Hyperkalemia may occur with acidosis and hypokalemia with reversal of the acidosis and subsequent movement of potassium back into the cells. Therefore, the serum potassium level is monitored closely, and hypokalemia is corrected as acidosis is reversed (Mount, 2017c). In chronic metabolic acidosis, low serum calcium levels are treated before the chronic metabolic acidosis is treated to avoid tetany resulting from an increase in pH and a decrease in ionized calcium. Alkalizing agents may be given. Treatment modalities may also include hemodialysis or peritoneal dialysis (Goltzman, 2019b). https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zRrdbUNNMKTSTb3II3pEltD4s%2fPbBdKcWtJFoZ4TUro%2b%… 4/4