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Overview of Acid-Base Balance

8 min read

References:

  1. Saunders Comprehensive Review for the NCLEX-RN Examination, 9th Edition, ISBN 978-032-37-9530-2, by Linda Anne Silvestri, Angela E. Silvestri, and Jessica Grimm (Ch. 8, pp. 106-115)

Hydrogen ions are what determine the pH of the body. The pH scale extends from 1 to 14, where 7 is considered as the middle ground; “neutral”. Acids (which contain hydrogen ions) decrease pH, while Bases (which have no hydrogen ions) increase pH. The pH of body fluid is normally between 7.35 to 7.45. 2. Acids contain ions, and are produced as end products of metabolism. They “give” hydrogen ions to neutralize or decrease the strength of an acid. 3. Bases have no ions, and “accept” ions from acids to neutralize or decrease the strength of a base or to form a weaker acid. The normal serum level of bicarbonate (HCO3-) are 21 to 28 mEq/L (21 to 28 mmol/L).

Regulatory Systems

  1. Buffers: fast-acting regulatory systems that provide immediate protection against changes in hydrogen ion concentration in the extracellular fluid. They function only to keep the pH with the narrow limits of stability when too much acid or base is released into the system. They absorb or release hydrogen ions as necessary. These function as a transport mechanism that carries hydrogen ions to the lungs. The body’s buffers are consumable; limited. Once used, the body is less able to handle further stress until they are replaced.
    • Hemoglobin System: the use of a chloride shift, where chloride moves in and out of cells in response to oxygen levels in the blood. There is an exchange between chloride inside the cell, and bicarbonate outside the cell and vice versa.
    • Plasma Protein System: functioning with the liver to vary the amount of hydrogen ions in the chemical structure of plasma proteins. These proteins have the ability to attract or release hydrogen ions.
    • Carbonic Acid-Bicarbonate System: the primary buffer system; it maintains a pH of 7.4 with a ratio of 20:1 between bicarbonate (HCO3-) and carbonic acid (H2CO3).
      • Carbonic acid concentration is controlled by the excretion of CO2 by the lungs; changes in CO2 is reflected in the rate and depth of respirations.
      • Bicarbonate concentration is controlled by the kidneys; they selectively retain or excrete bicarbonate in response to bodily needs.
    • Phosphate Buffer System: a system present in cells and body fluids, especially active in the kidneys. It acts like bicarbonate and neutralizes excess hydrogen ions.
  2. Lungs: the second defense of the body, interacting with the buffer system to maintain acid-base balance. The lungs can only act upon excess hydron ions carried by carbonic acid. Other carriers are handled by the kidneys.
    • During acidosis, respiratory rate and depth increase in an attempt to exhale acids: carbonic acid created by the neutralizing action of bicarbonate is carried to the lungs, and is reduced to CO2 and water, and exhaled.
    • During alkalosis, respiratory rate and depth decrease, retaining CO2 and increasing carbonic acid, reducing the strength of excess bicarbonate.
    • This correction takes between 10 to 30 seconds to complete.
    • The lungs can hold hydrogen ions until the deficit is corrected, or can inactive hydrogen ions, changing the ions to water molecules to be exhaled along with CO2.
  3. Kidneys: a more inclusive corrective response to acid-base disturbances than other corrective mechanisms, but much more slowly. Compensation requires a few hours to several days, but is more thorough and selective than other regulators, such as the buffers and lungs.
    • In acidosis, hydrogen ions are secreted into the tubules and combine with buffers to be excreted in the urine.
    • In alkalosis, excess bicarbonate ions move into the tubules and combine with sodium to be excreted in the urine.
    • Bicarbonate is selectively regulated in the kidneys. (a) The kidneys restore bicarbonate by excreting hydrogen ions and retaining bicarbonate ions. (2) Excess hydrogen ions are excreted in the urine in the form of phosphoric acid. (3) The alteration of certain amino acids in the renal tubules results in a diffusion of ammonia into the kidneys, which combine with excess hydrogen ions and is excreted in the urine.
  4. Potassium (K+) plays an exchange role in acid-base balance. Moving hydrogen ions into and from cells changes the potassium level (potassium movement across cell membranes is facilitated by transcellular shifting in response to acid-base patterns) i.e. potassium levels change to compensate for hydrogen ion level changes.
    • In acidosis, the body moves hydrogen ions into the cells, moving potassium out of cells. Serum potassium levels increase.
    • In acidosis, cells release hydrogen ions into the blood to increase acidity, forcing potassium back into the cells. Serum potassium levels decrease.

Arterial Blood Gases

Sample Collection

  1. Obtain vital signs.
  2. Determine if the client has an arterial line in place.
  3. Perform Allen’s test to determine the presence of collateral circulation; note that in most facilities, a respiratory therapist performs the sample collection.
  4. Assess factors that may affect the accuracy of the results, such as changes in the O2 settings, suctioning within the past 20 minutes, and the client’s activities.
  5. Provide emotional support to the client.
  6. Assist with the specimen draw; prepare a heparinized syringe (if not already packaged). After obtaining a specimen, prevent any air from entering the syringe, because it may alter the blood gas analysis.
  7. Apply pressure immediately to the puncture site following the blood draw; maintain pressure for 5 minutes or for 10 minutes if the client is taking an anticoagulant to decrease the risk of hematoma. Reassess the radial pulse after removing the pressure.
  8. Appropriately label the specimen and transport it on ice to the laboratory.
  9. On the laboratory form, record the client’s temperature and the type of supplemental oxygen that the client is receiving.

Allen’s Test

Explain the Allen’s test to the client, including its purpose of assessing collateral circulation. 10. Pressure is applied over the ulnar and radial arteries simultaneously. 11. The client is asked to open and close the hands repeatedly. 12. Pressure is released from the ulnar artery while compressing the radial artery. 13. The color of the extremity distal to the pressure point is assessed. If flow through the ulnar artery is good, flushing will be seen immediately. The Allen test is then positive, and the radial artery can be used for puncture. 14. If the test is negative (no flushing seen), the test is repeated on the other arm. If both arms are negative, another artery is selected for puncture. 15. Findings are documented.

Arterial Blood Gas Analysis

Laboratory TestNormal RangeNormal Range (SI)
Arterial pH7.35 to 7.457.35 to 7.45
PaCO235 to 45 mm Hg35 to 45 mm Hg
Bicarbonate21 to 28 mEq/L21 to 28 mmol/L
PaO280 to 100 mm Hg80 to 100 mm Hg
Venous pH7.31 to 7.417.31 to 7.41
PvO240 to 50 mm Hg40 to 50 mm Hg

WARNING

The indicator for respiratory function is PaCO2. The indicator for metabolic function is HCO3-. The following steps do not consider a mixed imbalance, where both indicators correspond with the findings for respiratory and metabolic types. PaO2 is usually normal, except for with accompanying conditions and respiratory acidosis, where it is usually decreased.

  1. pH: is it increased (alkalosis) or decreased (acidosis)?
  2. PaCO2: if it has an opposite relationship with pH, it is a respiratory imbalance (high CO2 lowers pH, while low CO2 increases pH). If it does not, move to step 3.
  3. HCO3-: if it corresponds with pH, the condition is a metabolic imbalance.
  4. Compensation:
    • If pH is within normal ranges (7.35 to 7.45), full compensation has occurred. Acidosis or alkalosis depend on where the pH leans towards, despite being in the normal range.
    • If pH and both indicators are abnormal, partial compensation has occurred.
    • If pH is abnormal and only one of the indicators are abnormal, no compensation has occurred.

Respiratory Acid-Base Imbalance

Remember that the indicator for respiratory function is PaCO2. In a respiratory imbalance, you will find an opposite relationship between the pH and the PaCO2 i.e., elevated PaCO2 results in decreased pH (respiratory acidosis), and decreased PaCO2 results in elevated pH (respiratory alkalosis).

Respiratory acidosispHHCO3-PaCO2K+
Fully CompensatedNormalIncreasedIncreasedIncreased
Partially CompensatedDecreasedIncreasedIncreasedIncreased
UncompensatedDecreasedNormalIncreasedIncreased
Respiratory alkalosispHHCO3-PaCO2K+
Fully CompensatedNormalDecreasedDecreasedDecreased
Partially CompensatedIncreasedDecreasedDecreasedDecreased
UncompensatedIncreasedNormalDecreasedDecreased

Metabolic Acid-Base Imbalance

Remember that the indicator for respiratory function is HCO3-. In a respiratory imbalance, you will find a corresponding relationship between the pH and the HCO3- i.e., decreased HCO3- results in decreased pH (metabolic acidosis), and elevated HCO3- results in increased pH (metabolic alkalosis).

Metabolic acidosispHHCO3-PaCO2K+
Fully CompensatedNormalDecreasedDecreasedIncreased
Partially CompensatedDecreasedDecreasedDecreasedIncreased
UncompensatedDecreasedDecreasedNormalIncreased
Metabolic alkalosispHHCO3-PaCO2K+
Fully CompensatedNormalIncreasedIncreasedDecreased
Partially CompensatedIncreasedIncreasedIncreasedDecreased
UncompensatedIncreasedIncreasedNormalDecreased

Mixed Acid-Base Disorders

These occur when two or more disorders are present at the same time. The pH will depend on the type and severity of the disorders involved, including any compensatory mechanisms at work (e.g., respiratory acidosis combined with metabolic acidosis will result in a greater decrease in pH than either imbalance occurring alone).

  • Example: Mixed alkalosis can occur if a client begins to hyperventilate due to postoperative pain (respiratory alkalosis) and is also losing acid due to gastric suctioning (metabolic alkalosis).
  • These imbalances will feature both the inversion of PaCO2 and the correspondence of HCO3- to the pH imbalance.

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