Acid Base Balance PDF

Chapter 9 Acid Base Balance ACID--BASE BALANCE OVERVIEW Acid--base balance refers to the homeostasis of the hydrogen ion (H+) concentration in body fluids. The body fluid's concentration of hydrogen ions is expressed as the body's pH level (Fig. 9.1). Body fluids are further classified as acids o...

Chapter 9 Acid Base Balance ACID--BASE BALANCE OVERVIEW Acid--base balance refers to the homeostasis of the hydrogen ion (H+) concentration in body fluids. The body fluid's concentration of hydrogen ions is expressed as the body's pH level (Fig. 9.1). Body fluids are further classified as acids or bases according to their hydrogen ion concentration. \*\*\*\*\*\*\*\*\*\*\*\*Hydrogen Ion Concentration The hydrogen ion concentration of body fluids is vital to acid--base balance. The hydrogen ion concentration is measured in terms of the pH. The pH numerical value is inversely proportional to the number of hydrogen ions in the body fluid: ↑ Hydrogen ion concentration, ↓ pH (acid) ↓ Hydrogen ion concentration, ↑ pH (base) The normal arterial blood pH range is 7.35 to 7.45. A pH level below 6.8 or above 7.8 is considered incompatible with life because cell function becomes seriously impaired. \*\*\*\*\*\*\*\*\*\*\*Acids Acids are known as hydrogen ion donors. An acidic solution has more hydrogen ions and a pH of less than 7.35. The excess of hydrogen ions can be a result of overproduction of acids that causes the release of hydrogen ions or an under-elimination of acids that causes retention of hydrogen ions. Because hydrogen ions are positively charged ions, hydrogen ion excess in the blood causes imbalances in other electrolytes, such as calcium, potassium, and sodium. These imbalances can lead to disorders of the cardiac, central nervous, neuromuscular, and respiratory systems. \*\*\*\*\*\*\*\*\*\*\*\*\*Bases Bases are known as hydrogen ion acceptors. A base, or alkaline, solution has fewer hydrogen ions and a pH of greater than 7.45. Alkalosis can result from either an overproduction of base or an under-elimination of base. The most common base is bicarbonate, and the systems affected are the cardiovascular, muscular, and nervous systems. \*\*\*\*\*\*\*\*\*\*\*\*Oxygenation Oxygenation is measured using arterial blood gas (ABG) sampling. The PaO2 measures the partial pressure of oxygen in arterial blood and is the most important factor in determining how oxygen binds to hemoglobin. If the PaO2 is high, more oxygen is able to bind with hemoglobin; if the PaO2 is low, less oxygen is able to bind with hemoglobin. The SaO2 measures the oxygen-carrying capacity of hemoglobin and can be assessed through the use of pulse oximetry. Primarily, SaO2 is used to evaluate respiratory function and is not used to evaluate acid--base balance. Higher than normal PaO2 and SaO2 values are associated with unnecessarily high levels of supplemental oxygen administration. In contrast, lower than normal PaO2 and SaO2 values indicate hypoxemia (Table 9.1). \*\*\*\*\*\*\*\*\*\*\*\*\*\*Carbon Dioxide Carbon dioxide (CO2) is the natural by-product of cellular metabolism. Carbon dioxide is measured by the PaCO2 levels, which indicate the partial pressure of CO2 in the arterial blood. The PaCO2 is used to evaluate the respiratory component of acid--base balance. Primarily regulated by the ventilatory function of the lungs, normal arterial CO2 levels are 35 to 45 mm Hg. PaCO2 greater than 45 mm Hg is related to hypoventilation or excessive CO2 retention, and therefore acidosis. PaCO2 less than 35 mm Hg is related to hyperventilation or excessive CO2 exhalation, and therefore alkalosis. \*\*\*\*\*\*\*\*\*\*\*\*Bicarbonate Bicarbonate helps the body regulate pH and accomplishes this by its ability to accept a hydrogen ion (H+). Regulated by the kidneys, bicarbonate is measured by the arterial blood HCO3-- concentration and is used to evaluate the metabolic component of acid--base balance. Normal HCO3-- values are between 22 and 26 mEq/L. REGULATION OF ACID--BASE BALANCE To maintain the pH of the extracellular fluid within the narrow range, the body has three regulating mechanisms: chemical buffers, respiratory buffers, and renal buffers. \*\*\*\*\*\*\*\*\*\*\*\*Chemical Buffers The first line of defense against a change in the body's pH is chemical buffers. Chemical buffers include bicarbonate, phosphate, and proteins. Bicarbonate buffers are mainly responsible for buffering blood and interstitial fluid. They rely on a series of chemical reactions in which pairs of weak acids and bases combine with stronger acids and bases to weaken them. These chemical reactions are assisted by the kidneys and lungs. Phosphate buffers are found in the intracellular fluids as bicarbonates. They control small fluctuations in pH and respond quickly. They prove especially effective in the renal tubules, where phosphates exist in greater concentration. Protein buffers are the most abundant buffers in the body. They work both inside and outside of cells: hemoglobin inside the cell and albumin and globulins outside the cell. Protein buffers work by binding with acids and bases to neutralize them. Respiratory Buffers Acid--base disturbances resulting from primary alterations in the PaCO2 are regulated by respiratory buffers. Any retention of CO2 or increase in the body's concentration of CO2 produces an increase in hydrogen ions through the generation of carbonic acid (H2CO3). This lowers the pH and thus promotes the development of an acidotic state, which is observed in conditions such as chronic obstructive pulmonary disease (COPD). While in the acidotic state, chemoreceptors in the brain respond to the increased CO2 levels by stimulating the respiratory system to increase the respiratory rate and depth in an effort to excrete CO2 through the lungs (Table 9.2). Conversely, decreases in CO2 concentration result in a decrease in hydrogen ions. This leads to a pH rise, and an alkalotic state results, as seen in conditions such as hyperventilation. CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3-- Carbon dioxide + water ⇔ carbonic acid ⇔ hydrogen + bicarbonate \*\*\*\*\*\*\*\*\*\*\*\*\*Renal Buffers The renal buffers are the most effective, yet the slowest-acting, buffering system and buffer by regulating bicarbonate levels. If the body is in a state of alkalosis, the kidneys excrete HCO3-- and reabsorb hydrogen ions (H+). This results in urine becoming more alkaline, a drop in the blood bicarbonate levels, and a decrease in pH. If the body is in a state of acidosis, the kidneys excrete hydrogen ions (H+) and reabsorb HCO3--. This results in urine becoming more acidic, bicarbonate levels increasing, and the pH increasing (Table 9.3; Fig. 9.2). This response to acid--base imbalances begins within hours but requires several days to be marginally effective. This is in comparison to the faster changes brought about through the increases or decreases in the respiratory rate. RESPIRATORY AND RENAL COMPENSATION Compensation describes physiological responses to an acid--base imbalance in an attempt to normalize pH. If the problem is of respiratory origin, the kidneys work to correct. If the problem is of renal origin, the lungs work to correct. Compensation may be affected by age-related changes to the respiratory or renal systems (see Geriatric/Gerontological Considerations). \*\*\*\*\*\*\*\*\*\*\*\*\*Respiratory Compensation If the primary alteration in acid--base balance has resulted from a metabolic disorder, the respiratory system may compensate by retaining or removing CO2 while minimizing a change in the pH. The lungs may take as little as 5 to 15 minutes to begin compensation. The respiratory system responds to metabolic-based pH imbalances (Table 9.4) by Metabolic acidosis: increase in respiratory rate and depth Metabolic alkalosis: decrease in respiratory rate and depth \*\*\*\*\*\*\*\*\*\*\*\*\*Renal Compensation If the primary alteration in acid--base balance results from a respiratory disorder, the renal system may compensate by excreting or retaining hydrogen and bicarbonate. The renal system may take as long as 24 hours to correct a respiratory-induced problem because there are three major renal mechanisms that work to compensate the disorder: tubular kidney movement of bicarbonate, kidney tubule formation of acids, and the formation of ammonium from amino acid catabolism. The renal system responds to respiratory-based pH imbalances (Table 9.5) \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*Steps to Determine Acid--Base Compensation Does the pH range indicate acidosis or alkalosis? If pH is within the normal range, in which direction does it trend? Has the PaCO2 or HCO3-- changed to account for the acidosis or alkalosis? Has the opposite system worked to correct and shift back toward a normal pH? Types of Compensation \*\*\*\*\*\*\*\*\*\*\*\*The three types of compensation are uncompensated, partially compensated, and fully compensated: Uncompensated: pH is abnormal, and either the PaCO2 or the HCO3-- is also abnormal. There is no indication that the opposite system has tried to correct the imbalance (Table 9.6). Partially compensated: pH is abnormal, and both PaCO2 and HCO3-- are also abnormal (Table 9.7). This indicates that the opposite system has attempted to correct for the other but has not been completely successful. Fully compensated: pH is normal, and both the PaCO2 and HCO3-- are abnormal (Table 9.8). The normal pH indicates that one system has been able to compensate for the other. Maintaining a homeostatic pH environment is essential for normal body functioning. To achieve this goal, the body must constantly monitor its hydrogen ion concentration. When there is an increase or decrease in this balance, the body must use the blood bicarbonate, proteins, and phosphate buffer body fluids to compensate. There does come a time in the disease process when these buffers can no longer maintain adequate concentrations of hydrogen ions, and outside resources must be used to help the body compensate. \*\*\*\*\*\*\*\*\*\*\*\*\*\*Obtaining an Arterial Blood Gas Most ABG samples are collected by a respiratory therapist or specially trained registered nurse (see institution's policy). Collection from the femoral artery, however, is usually performed only by a physician or advanced practice registered nurse (APRN). Before a radial puncture is performed, an Allen test should be conducted (Box 9.1). This test is performed to ensure adequate collateral circulation in the event there is hemorrhage or thrombosis of the radial artery that is punctured. Arterial Blood Gas Assessment There are six parameters included in an ABG measurement: pH PaCO2 HCO3-- Base excess PaO2 SaO2 Blood acidity is reflected by pH levels, and the normal range is 7.35 to 7.45. A pH greater than 7.45 indicates alkalosis; a pH less than 7.35 indicates acidosis. If the level is normal or borderline, the body may be attempting to compensate for a slightly abnormal or chronic acid--base imbalance. Compensation is the way in which the body responds to an acid--base imbalance in an attempt to normalize the pH of the blood. \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*ARTERIAL BLOOD GAS INTERPRETATION Arterial blood gas analysis describes the set of values that are assessed to determine an individual's ability to maintain normal acid--base balance. Arterial blood gases are obtained from arteries such as the radial, brachial, or femoral and can also be obtained through an indwelling arterial line that may be present in one of these arteries. Arterial lines also provide continuous blood pressure (BP) monitoring and allow for arterial blood sampling. A patient with an arterial line must be cared for in the intensive care unit (ICU) or a monitored unit because of possible circulatory impairment to the affected limb, risk of hemorrhage, and the need for continuous arterial line monitoring. The PaCO2 level reflects the partial pressure of CO2 in arterial blood and is adjusted by changes in the rate and depth of respiratory ventilation. The normal range is 35 to 45 mm Hg. A PaCO2 greater than 45 mm Hg indicates hypoventilation or excessive CO2 retention, as in COPD, drug overdose, or acute respiratory failure, and therefore respiratory acidosis. A PaCO2 less than 35 mm Hg indicates hyperventilation or excessive CO2 exhalation, and therefore respiratory alkalosis. Respiratory alkalosis may occur with an anxiety attack, pulmonary edema, hepatic failure, or inappropriate mechanical ventilator settings. The HCO3-- level reflects the arterial blood's HCO3-- concentration and is the renal component of acid--base regulation. The normal range is 22 to 26 mEq/L. An HCO3-- less than 22 mEq/L indicates acidosis and may occur with chronic alcohol abuse, starvation, or acute renal failure. An HCO3-- greater than 26 mEq/L indicates alkalosis and may be the result of gastric drainage, diuretic use, or severe potassium depletion. An anion gap calculation may be used to determine the cause of metabolic acidosis and is a way to monitor a person's response to treatments. The calculation of the anion gap is based on the difference between major cations (sodium and potassium) and primary anions (chloride and bicarbonate). The normal reference range for the anion gap is 8 to 16 mEq/L. Base excess reflects the level of HCO3-- and other bases such as proteins and hemoglobin, and the normal range is --2 to +2. A base excess less than --2 indicates an acid excess. A base excess greater than +2 indicates an acid deficit. The PaO2 level reflects the partial pressure of oxygen in arterial blood (Table 9.9). The normal range is 80 to 95 mm Hg. A PaO2 level less than 80 mm Hg indicates hypoxemia. The SaO2 level reflects hemoglobin's oxygen-carrying capacity. A normal value is anything greater than 95%. This value is not used to evaluate acid--base balance. \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*Steps for Interpreting Arterial Blood Gas Step 1: Review the pH result. Is it normal (7.35--7.45), acidosis (\7.45)? Step 2: Review the PaCO2 result. Is it normal (35--45 mm Hg), acidosis (\>45 mm Hg), or alkalosis (\