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Introduction ⬇

Blood pH is the measure of the hydrogen ion concentration in the blood. The sources of hydrogen ions are volatile acids, which can vary between a liquid and a gaseous state, and nonvolatile acids, which cannot be volatilized but remain fixed (e.g., dietary acids, lactic acids, ketoacids).

Blood pH indicates the body’s chemical balance and represents a ratio of acids to bases. It is also an indicator of the degree to which the body is adjusting to dysfunctions by means of its buffering systems. It is one of the best ways to determine whether the body is too acidic or too alkaline and is an indicator of the patient’s metabolic and respiratory status. The acid–base balance in the extracellular fluid is extremely delicate and intricate and must be kept within the very narrow range of 7.35–7.45 (slightly alkaline). Values <7.35 indicate an acid state, whereas pH values >7.45 indicate an alkaline state.

Procedure ⬆ ⬇

  1. Obtain an arterial (or venous if ordered) blood sample. (See Chapter 2 for venous and arterial blood collection.)

  2. Two methods may be used to determine the pH:

    1. Direct method: A small amount of blood is placed into a blood gas machine or analyzer.

    2. Indirect method: Solve the Henderson–Hasselbalch equation for the pH of a buffer system. In this equation, pK is the negative logarithm of the acid dissociation constant (the pH at which the associated and unassociated forms of an acid exist in equal concentrations). [A−] is the concentration of the ionized form (in this case HCO3−, the major blood base), and [HA] is the concentration of the free acid (in this case H2CO3, the major blood acid) in milliequivalents per liter.

Clinical Implications ⬆ ⬇

  1. Generally speaking, the pH is decreased in acidemia (acidosis) because of increased formation of acids, and pH is increased in alkalemia (alkalosis) because of a loss of acids.

  2. When interpreting an acid–base abnormality, certain steps should be followed:

    1. Check the pH to determine whether an acid or an alkaline state exists.

    2. Check the PCO2 to determine whether a respiratory acidosis or alkalosis is present. (PCO2 is the pulmonary component.)

    3. Check the HCO3− concentration to determine whether a metabolic acidosis or alkalosis is present. (HCO3− is the kidney component.)

  3. See Table 14.4 for a more complete explanation of the changes occurring in acute and chronic respiratory and metabolic acidosis and alkalosis.

  4. Metabolic acidemia (acidosis) occurs in:

    1. Acute kidney injury

    2. Ketoacidosis in diabetes and starvation

    3. Lactic acidosis

    4. Strenuous exercise

    5. Severe diarrhea

  5. Metabolic alkalemia (alkalosis) occurs in:

    1. Hypokalemia

    2. Hypochloremia

    3. Gastric suction or vomiting

    4. Massive doses of steroids

    5. Sodium bicarbonate administration

    6. Aspirin intoxication

  6. Respiratory alkalemia (alkalosis) occurs in:

    1. Acute pulmonary disease

    2. Myocardial infarction

    3. Chronic and acute heart failure

    4. Adult CF

    5. Third trimester of pregnancy and during labor and delivery

    6. Anxiety, neuroses, psychoses

    7. Pain

    8. Central nervous system diseases

    9. Anemia

    10. Carbon monoxide poisoning

    11. Acute pulmonary embolus

    12. Shock

  7. Respiratory acidemia (acidosis) occurs in:

    1. Acute or chronic respiratory failure

    2. Ventilatory failure

    3. Neuromuscular depression

    4. Obesity

    5. Pulmonary edema

    6. Cardiopulmonary arrest

Clinical Alert

  1. Ventilatory failure is a medical emergency. Aggressive and supportive measures must be taken immediately.

  2. Rate and depth of respirations may give a clue to blood pH.

    1. Acidosis usually increases respirations; this is the body’s way of adjusting once the state is established.

    2. Alkalosis usually decreases respirations; this is the body’s way of adjusting once the state is established.

  3. Respiratory alkalosis may reflect hyperventilation in response to treatment for hypoxemia; however, correction of hypoxemia is essential.

  4. Metabolic alkalosis, which is compensated through hypoventilation, may produce hypoxemia.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose, benefits, and risks of invasive blood sampling.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed.

  2. Assess, monitor, and intervene appropriately for metabolic and respiratory acidosis and alkalosis (see Table 14.4).

  3. Frequently observe the arterial puncture site for bleeding (see Chapter 2). Be prepared to initiate proper interventions in the event of life-threatening situations.

  4. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

A number of drugs may alter the components of acid–base balance. see Appendix E.

Reference Values ⬆

Normal

  • Arterial pH: 7.35–7.45

  • Venous pH: 7.31–7.41