| Author: Fawn O. Workman
|
| Author: Fawn O. Workman
|
The chemical reactions that sustain life depend on a delicate balance—or homeostasis—between acids and bases in the body. Even a slight imbalance can profoundly affect metabolism and essential body functions. Several conditions, such as infection or trauma, and medications can affect acid-base balance. However, to understand this balance, you need to understand some basic chemistry.
Understanding acids and bases requires an understanding of pH, a calculation based on the percentage of hydrogen ions in a solution as well as the amount of acids and bases.
Acids consist of molecules that can give up, or donate, hydrogen ions to other molecules. Carbonic acid is an acid that occurs naturally in the body. Bases consist of molecules that can accept hydrogen ions; bicarbonate is one example of a base.
A solution that contains more base than acid has fewer hydrogen ions, so it has a higher pH. A solution with a pH above 7 is a base, or alkaline. A solution that contains more acid than base has more hydrogen ions, so it has a lower pH. A solution with a pH below 7 is an acid, or acidotic.
Getting your PhD in pH
You can assess a patient's acid-base balance if you know the pH of the patient's blood. Because arterial blood is usually used to measure pH, this discussion focuses on arterial samples.
Arterial blood is normally slightly alkaline, ranging from 7.35 to 7.45. A pH level within that range represents a balance between the percentage of hydrogen ions and bicarbonate ions. Generally, pH is maintained in a ratio of 20 parts bicarbonate to 1 part carbonic acid. A pH below 6.8 or above 7.8 is usually fatal. (See Understanding normal pH.)
Too low
Under certain conditions, the pH of arterial blood may deviate significantly from its normal narrow range. If the blood's hydrogen ion concentration increases or bicarbonate level decreases, pH may decrease. In either case, a decrease in pH below 7.35 signals acidosis. (See Understanding acidosis.)
Too high
If the blood's bicarbonate level increases or hydrogen ion concentration decreases—the opposite effect of a low pH—pH may increase. In either case, an increase in pH above 7.45 signals alkalosis. Even small changes to CO2 and HCO3 levels can alter the pH balance and prompt attempts to maintain homeostasis (Harris, 2022). (See Understanding alkalosis.)
Regulating acids and bases
A person's well-being depends on the person's ability to maintain a normal pH. A deviation in pH can compromise essential body processes, including electrolyte balance, activity of critical enzymes, muscle contraction, and basic cellular function. The body normally maintains pH within a narrow range by carefully balancing acidic and alkaline elements. When one aspect of that balancing act breaks down, the body can't maintain a healthy pH as easily, and problems arise.
The big three
The body regulates acids and bases to avoid potentially serious consequences. Therefore, when pH rises or falls, three regulatory systems come into play:
Regulation system 1: Buffers
The body maintains a healthy pH in part through chemical buffers, substances that minimize changes in pH by combining with excess acids or bases. Chemical buffers in the blood, intracellular fluid, and interstitial fluid serve as the body's most efficient pH-balancing weapon. The main chemical buffers are bicarbonate, phosphate, and protein.
Bring on the bicarbonate
The bicarbonate buffer system is the body's primary buffer system. It's mainly responsible for buffering blood and interstitial fluid. This system relies on a series of chemical reactions in which pairs of weak acids and bases (such as carbonic acid and bicarbonate) combine with stronger acids (such as hydrochloric acid) and bases to weaken them.
Decreasing the strength of potentially damaging acids and bases reduces the danger those chemicals pose to pH balance. The kidneys assist the bicarbonate buffer system by regulating production of bicarbonate. The lungs assist by regulating the production of carbonic acid, which results from combining carbon dioxide and water.
Feeling better with phosphate
Like the bicarbonate buffer system, the phosphate buffer system depends on a series of chemical reactions to minimize pH changes. Phosphate buffers react with either acids or bases to form compounds that slightly alter pH, which can provide extremely effective buffering. This system proves especially effective in renal tubules, where phosphates exist in greater concentrations.
Plenty of protein
Protein buffers, the most plentiful buffers in the body, work inside and outside cells. They're made up of hemoglobin as well as other proteins. Behaving chemically like bicarbonate buffers, protein buffers bind with acids and bases to neutralize them. In red blood cells, for instance, hemoglobin combines with hydrogen ions to act as a buffer.
Regulation system 2: Respiration
The respiratory system serves as the second line of defense against acid-base imbalances. The lungs regulate blood levels of carbon dioxide, a gas that combines with water to form carbonic acid. Increased levels of carbonic acid lead to a decrease in pH.
Chemoreceptors in the medulla of the brain sense those pH changes and vary the rate and depth of breathing to compensate (Alexander et al., 2014). Breathing faster or deeper eliminates more carbon dioxide from the lungs. The more carbon dioxide that is lost, the less carbonic acid that is made and, as a result, pH rises. The body detects that pH change and reduces carbon dioxide excretion by breathing slower or less deeply. (See Carbon dioxide and hyperventilation.)
Check for success
To assess the effectiveness of ventilation, look at the partial pressure of carbon dioxide in arterial blood (PaCO2). A normal PaCO2 level in the body is 35 to 45 mm Hg. PaCO2 values reflect carbon dioxide levels in the blood. As those levels increase, so does PaCO2.
Although the respiratory system responds to pH changes within minutes, it can restore normal pH only temporarily. Renal compensation is a much slower but still important process to adjust pH balance (Willis, 2018).
Regulation system 3: Kidneys
The kidneys serve as yet another mechanism for maintaining acid-base balance in the body. They can reabsorb acids and bases or excrete them into urine. They can also produce bicarbonate to replenish lost supplies. Such adjustments to pH can take the kidneys hours or days to complete (Cho, 2016). As with other acid-base regulatory systems, the effec tiveness of the kidneys changes with age. (See Acid-base balance across the life span.)
The kidneys also have a part in the regulation of the bicarbonate level, which is a reflection of the metabolic component of acid-base balance. Normally, the bicarbonate level is reported with arterial blood gas (ABG) results. The normal bicarbonate level is 22 to 26 mEq/L.
The kidneys keep working
If the blood contains too much acid or not enough base, pH drops and the kidneys reabsorb sodium bicarbonate. The kidneys also excrete hydrogen along with phosphate or ammonia. Although urine tends to be acidic because the body usually produces slightly more acids than bases, in such situations, urine becomes more acidic than normal.
The reabsorption of bicarbonate and the increased excretion of hydrogen causes more bicarbonate to be formed in the renal tubules and eventually retained in the body. The bicarbonate level in the blood then rises to a more normal level, increasing pH.
Ups and downs of acids and bases
If the blood contains more base and less acid, pH rises. The kidneys compensate by excreting bicarbonate and retaining more hydrogen ions. As a result, urine becomes more alkaline and blood bicarbonate level drops. Conversely, if the blood contains less bicarbonate and more acid, pH drops.
Altogether now
The body responds to acid-base imbalances by activating compensa tory mechanisms that minimize pH changes. Returning the pH to a normal or near-normal level mainly involves changes in the component— metabolic or respiratory—not primarily affected by the imbalance.
If the body compensates only partially for an imbalance, pH remains outside the normal range. If the body compensates fully or completely, pH returns to normal.
Respiratory helps metabolic . . .
If metabolic disturbance is the primary cause of an acid-base imbalance, the lungs compensate in one of two ways. When a lack of bicarbonate causes acidosis, the lungs increase the rate of breathing, which blows off carbon dioxide and helps raise the pH to normal. When an excess of bicarbonate causes alkalosis, the lungs decrease the rate of breathing, which retains carbon dioxide and helps lower pH.
. . . And vice versa
If the respiratory system disturbs the acid-base balance, the kidneys compensate by altering levels of bicarbonate and hydrogen ions. When PaCO2 is high (a state of acidosis), the kidneys retain bicarbonate and excrete more acid to raise the pH. When PaCO2 is low (a state of alkalosis), the kidneys excrete bicarbonate and hold on to more acid to lower the pH.
Memory JoggerRemember, PaCO2 and pH move in opposite directions. If PaCO2 rises, then pH falls, and vice versa.
A number of tests are used to diagnose acid-base disturbances. Here's a look at the most commonly used tests.
An ABG analysis is a diagnostic test in which a sample of blood obtained from an arterial puncture can be used to assess the effectiveness of breathing and overall acid-base balance. In addition to helping you identify problems with oxygenation and acid-base imbalances, the test can help you monitor a patient's response to treatment. (See Taking an ABG sample.)
Keep in mind that ABG analysis should be used only in conjunction with a full patient assessment. Only by assessing all information can you gain a clear picture of what's happening.
An ABG analysis involves several separate test results, only three of which relate to acid-base balance: pH, PaCO2, and bicarbonate level. The normal ranges for adults are:
The ABCs of ABGs
Recall that pH is a measure of the hydrogen ion concentration of blood; PaCO2 is a measure of the partial pressure of carbon dioxide in arterial blood, which indicates the effectiveness of breathing. PaCO2 levels move in the opposite direction of pH levels. Bicarbonate, which moves in the same direction of pH, represents the metabolic component of the body's acid-base balance.
Other information routinely reported with ABG results includes partial pressure of oxygen dissolved in arterial blood (PaO2) and arterial oxygen saturation (SaO2). The normal PaO2 range is 80 to 100 mm Hg; however, PaO2 varies with age. After age 60 years, the PaO2 may drop below 80 mm Hg without signs and symptoms of hypoxia. The normal SaO2 range is 95% to 100%.
Interpreting ABG results
When interpreting results from an ABG analysis, follow a consistent sequence to analyze the information. Here's one step-by-step process you can use. (See Quick look at ABG results.)
Step 1: Check the pH
First, check the pH level. This figure forms the basis for understanding most other figures.
If pH is abnormal, determine whether it reflects acidosis (below 7.35) or alkalosis (above 7.45). Then figure out whether the cause is respiratory or metabolic.
Step 2: Determine the PaCO2
Remember that the PaCO2 level provides information about the respiratory component of acid-base balance.
If PaCO2 is abnormal, determine whether it's low (less than 35 mm Hg) or high (greater than 45 mm Hg). Then determine whether the abnormal result corresponds with a change in pH. For example, if the pH is high, you would expect the PaCO2 to be low (hypocapnia), indicating that the problem is respiratory alkalosis. Respiratory alkalosis is caused by hyperventilation, mechanical overventilation, pregnancy, stroke, high altitudes, and septicemia. Conversely, if the pH is low, you would expect the PaCO2 to be high (hypercapnia), indicating that the problem is respiratory acidosis caused by hypoventilation. Causes of respiratory acidosis may be acute or chronic and are linked to chronic diseases such as chronic bronchitis, asthma, pneumonia, and airway obstruction.
Step 3: Watch the bicarbonate
Next, examine the bicarbonate level. This value provides information about the metabolic aspect of acid-base balance.
If the bicarbonate level is abnormal, determine whether it's low (less than 22 mEq/L) or high (greater than 26 mEq/L). Then determine whether the abnormal result corresponds with the change in pH. For example, if pH is high, you would expect the bicarbonate level to be high, indicating that the problem is metabolic alkalosis. Causes of metabolic alkalosis include the use of diuretics, vomiting, hyperaldosteronism, excessive use of alkaline medications such as antacids, and Cushing syndrome. Conversely, if pH is low, you would expect the bicarbonate level to be low, indicating that the problem is metabolic acidosis. Causes of metabolic acidosis include diabetic ketoacidosis, lactic acidosis, and severe diarrhea that lead to a loss of bicarbonate.
Memory JoggerRemember, bicar bonate and pH increase or decrease together. When one rises or falls, so does the other.
Step 4: Look for compensation
Sometimes you'll see a change in both the PaCO2 and the bicarbonate level. One value indicates the primary source of the pH change; the other, the body's effort to compensate for the disturbance.
Complete compensation occurs when the body's ability to compensate is so effective that pH falls within the normal range. Partial compensation, on the other hand, occurs when pH remains outside the normal range.
Compensation involves opposites. For instance, if results indicate primary metabolic acidosis, compensation will come in the form of respiratory alkalosis. For example, the following ABG results indicate metabolic acidosis with compensatory respiratory alkalosis:
The low pH indicates acidosis. However, the PaCO2 is low, which normally leads to alkalosis, and the bicarbonate level is low, which normally leads to acidosis. The bicarbonate level, then, more closely corresponds with the pH, making the primary cause of the problem metabolic. The resultant decrease in PaCO2 reflects partial respiratory compensation.
Normal values for pH, PaCO2, and bicarbonate would indicate that the patient's acid-base balance is normal.
Step 5: Determine PaO2 and SaO2
Last, check PaO2 and SaO2, which yield information about the patient's oxygenation status. If the values are abnormal, determine whether they're high (PaO2 greater than 100 mm Hg) or low (PaO2 less than 80 mm Hg and SaO2 less than 95%).
Remember that PaO2 reflects the body's ability to pick up oxygen from the lungs. A low PaO2 represents hypoxemia and can cause hyperventilation. The PaO2 value also indicates when to make adjustments in the concentration of oxygen being administered to a patient. (See Inaccurate ABG results.)
Anion gap
You may also come across a test result called the anion gap. (See Crossing the great anion gap.) Earlier chapters discuss how the strength of cations (positively charged ions) and anions (negatively charged ions) must be equal in the blood to maintain a proper balance of electrical charges. The anion gap result helps you differentiate among various acidotic conditions.
Identifying the gap
The anion gap refers to the relationship among the body's cations and anions. Sodium accounts for more than 90% of the circulating cations. Chloride and bicarbonate together account for 85% of the counterbalancing anions. (Potassium is generally omitted because it occurs in such low, stable amounts.)
The gap between the two measurements represents the anions not routinely measured, including sulfates, phosphates, proteins, and organic acids such as lactic acid and ketone acids. Because these anions aren't measured in routine laboratory tests, the anion gap is a way of determining their presence.
Gazing into the gap
An increase in the anion gap that's greater than 14 mEq/L indicates an increase in the percentage of one or more unmeasured anions in the bloodstream. Increases can occur with acidotic conditions characterized by higher than normal amounts of organic acids. Such conditions include lactic acidosis and ketoacidosis.
The anion gap remains normal for certain other conditions, including hyperchloremic acidosis, renal tubular acidosis, and severe bicarbonate-wasting conditions, such as biliary or pancreatic fistulas and poorly functioning ileal loops.
A decreased anion gap is rare but may occur with hypermagnesemia and paraprotein anemia states, such as multiple myeloma and Waldenström macroglobulinemia.
If you answered all six questions correctly, congratulations! You did a great job covering all the bases (and acids)!
If you answered four or five correctly, great! You certainly didn't hydrogen bomb!
If you answered fewer than four correctly, don't worry! It's never too late to get your PhD in pH!
References
Alexander, M., Corrigan, A. M., Gorski, L. A., & Phillips, L. (2014). Core curriculum for infusion nursing (4th ed.). Lippincott Williams & Wilkins.
Alfano, G., Fontana, F., Mori, G., Giaroni, F., Ferrari, A., Giovanella, S., Ligabue, G., Ascione, E., Cazzato, S., Ballestri, M., Di Gaetano, M., Meschiari, M., Menozzi, M., Milic, J., Andrea, B., Franceschini, E., Cuomo, G., Magistroni, R., Mussini, C.,..., Pinti, M. (2021). Acid base disorders in patients with COVID-19. International Urology and Nephrology, 54(2), 405–410. https://doi.org/10.1007/s11255-021-02855-1
Willis, L. (Ed.) (2018). Chapter 6: Disruptions in homeostasis. In Lippincott certification review: Medical-surgical nursing (6th ed., pp. 59–88). Wolters Kluwer.
Cho, K. C. (2016). Electrolyte & acid-base disorders. In M. A. Papadakis, S. J. McPhee, & M. W. Rabow (Eds.), Current medical diagnosis and treatment 2017 (56th ed., pp. 901–912). McGraw-Hill Education.
Harris, D. (2022). Essential critical care skills 6: arterial blood gas analysis. Nursing Times [online], 118(4). https://www.nursingtimes.net/clinical-archive/critical-care/essential-critical-care-skills-6-arterial-blood-gas-analysis-28-03-2022/
In this chapter, you'll learn: