1 Acid–Base Balance and ABG Interpretation

Acid–base physiology and terminology

Blood pH reflects the balance between acids and bases. The bicarbonate–carbonic acid buffer system is regulated chiefly by the lungs and kidneys: the lungs adjust carbon dioxide through ventilation, while the kidneys regulate bicarbonate and acid excretion. Respiratory responses can occur quickly; renal compensation takes longer.

Acidemia and alkalemia describe measured blood pH below or above the reference range. Acidosis and Alkalosis describe physiologic processes that tend to lower or raise pH. A patient can have an acidifying or alkalinizing process even when compensation keeps measured pH within its reference range.

Reference values and what they indicate

Typical adult reference values are:

• $\mathrm{pH}$: $7.35\text{–}7.45$, indicating overall acidity or alkalinity.

• $\mathrm{PaCO_2}$: $35\text{–}45\,\mathrm{mm\,Hg}$, indicating the respiratory component; carbon dioxide acts as an acid.

• $\mathrm{HCO_3^-}$: $22\text{–}26\,\mathrm{mEq/L}$, indicating the metabolic or renal component; bicarbonate acts as a base.

• $\mathrm{PaO_2}$: about $80\text{–}100\,\mathrm{mm\,Hg}$, indicating arterial oxygenation.

Ranges vary somewhat by laboratory, age, clinical context, and altitude. Interpret oxygenation in context, including the patient’s oxygen delivery and clinical condition.

A systematic approach to ABG interpretation

Use a consistent sequence when interpreting an ABG:

1. Check the patient and sample. Assess the patient first, confirm that the sample is arterial, and review oxygen or ventilator settings and the clinical context.

2. Read the pH. A value below $7.35$ indicates acidemia; above $7.45$ indicates alkalemia. If the value is within range, note whether it is below or above $7.40$ as a clue—not proof—of the underlying direction.

3. Check carbon dioxide. High $\mathrm{PaCO_2}$ tends to lower pH; low $\mathrm{PaCO_2}$ tends to raise it.

4. Check bicarbonate. Low $\mathrm{HCO_3^-}$ tends to lower pH; high $\mathrm{HCO_3^-}$ tends to raise it.

5. Identify the primary process. In a simple disorder, the value that changes in the direction that explains the pH points to the primary disturbance. The ROME memory aid means Respiratory Opposite: pH and $\mathrm{PaCO_2}$ move in opposite directions; Metabolic Equal: pH and $\mathrm{HCO_3^-}$ move in the same direction.

6. Assess compensation. Determine whether the other system is responding and whether the response is within the expected range. Compensation does not normally overshoot; an unexpected value suggests a mixed disorder or another clinical issue.

7. Assess oxygenation and the whole clinical picture. Review $\mathrm{PaO_2}$, oxygen support, electrolytes, renal function, and likely cause. An ABG alone does not establish the diagnosis.

The four primary acid–base disorders

The four primary disorders are distinguished by the pH direction and the primary change in carbon dioxide or bicarbonate:

• Respiratory acidosis: pH tends to be low, with $\mathrm{PaCO_2}$ rising. Example causes include hypoventilation, respiratory depression, severe obstructive lung disease, or impaired ventilation.

• Respiratory alkalosis: pH tends to be high, with $\mathrm{PaCO_2}$ falling. Example causes include hyperventilation associated with pain, anxiety, hypoxemia, fever, or other illness.

• Metabolic acidosis: pH tends to be low, with $\mathrm{HCO_3^-}$ falling. Example causes include diarrhea, ketoacidosis, lactic acidosis, renal dysfunction, or some toxins.

• Metabolic alkalosis: pH tends to be high, with $\mathrm{HCO_3^-}$ rising. Example causes include vomiting, gastric suction, diuretics, or other causes of acid or chloride loss or alkali gain.

These causes are examples, not diagnoses. Investigate the patient’s history, medications, laboratory results, and current condition.

Compensation and mixed disorders

The lungs compensate for metabolic disorders by changing ventilation. The kidneys compensate for respiratory disorders by adjusting bicarbonate and acid handling. Respiratory compensation for a metabolic disturbance can begin quickly; renal compensation for a respiratory disturbance develops more slowly. Compensation moves pH toward its reference range but does not remove the underlying cause.

Use expected compensation estimates to check whether the response fits the primary disorder:

• Metabolic acidosis: Winter’s formula estimates expected $\mathrm{PaCO_2}$:

  $$\mathrm{Expected\ PaCO_2} = (1.5 \times \mathrm{HCO_3^-}) + 8 \pm 2\,\mathrm{mm\,Hg}$$

  A measured $\mathrm{PaCO_2}$ outside the expected range suggests an additional respiratory disorder.

• Metabolic alkalosis: $\mathrm{PaCO_2}$ usually rises about $0.6\text{–}0.75\,\mathrm{mm\,Hg}$ for each $1\,\mathrm{mEq/L}$ rise in $\mathrm{HCO_3^-}$ above $24\,\mathrm{mEq/L}$. Compensation is limited.

• Respiratory acidosis: For each $10\,\mathrm{mm\,Hg}$ rise in $\mathrm{PaCO_2}$ above $40\,\mathrm{mm\,Hg}$, $\mathrm{HCO_3^-}$ rises about $1\text{–}2\,\mathrm{mEq/L}$ in an acute process and $3\text{–}4\,\mathrm{mEq/L}$ in a chronic process.

• Respiratory alkalosis: For each $10\,\mathrm{mm\,Hg}$ fall in $\mathrm{PaCO_2}$ below $40\,\mathrm{mm\,Hg}$, $\mathrm{HCO_3^-}$ falls about $1\text{–}2\,\mathrm{mEq/L}$ acutely and $4\text{–}5\,\mathrm{mEq/L}$ chronically.

These are clinical estimates, not substitutes for judgment or laboratory reference ranges. If values do not fit expected compensation, consider a mixed disorder, timing, and the patient’s clinical condition.

Clinical manifestations and nursing priorities

Acid–base disturbances can affect neurologic, respiratory, cardiovascular, and muscular function. Manifestations are variable and nonspecific, so assess trends and the cause rather than diagnosing from a symptom alone. Changes in ventilation, alertness, heart rhythm, strength, or sensation warrant attention in context.

Nursing priorities include:

• Assess airway, breathing, circulation, vital signs, mental status, and oxygenation. Escalate promptly for respiratory distress, declining consciousness, hemodynamic instability, or dangerous rhythm changes.

• Verify the ABG result and compare it with the patient’s presentation, oxygen support, previous results, electrolytes, and relevant history.

• Support prescribed oxygenation and ventilation, monitor the response, and report deterioration. For respiratory acidosis, assess for inadequate ventilation and contributing factors such as sedating medications. For respiratory alkalosis, assess for pain, fever, hypoxemia, and other underlying causes.

• For suspected metabolic causes, assess fluid status, gastrointestinal losses, renal function, glucose, medications, and relevant laboratory results. Report significant findings and carry out prescribed treatment.

• Collaborate with the clinical team to treat the underlying cause, then reassess symptoms and relevant results after interventions. Do not attempt to correct an ABG number in isolation.

Worked interpretation examples

These cases illustrate how to identify a primary process, assess compensation, and connect findings to care.

• Acidemia with elevated carbon dioxide: With $\mathrm{pH}=7.28$, $\mathrm{PaCO_2}=52\,\mathrm{mm\,Hg}$, and $\mathrm{HCO_3^-}=24\,\mathrm{mEq/L}$, the primary disorder is respiratory acidosis. There is no evident metabolic compensation in these values: the pH indicates acidemia, elevated $\mathrm{PaCO_2}$ moves in the acidifying direction, and bicarbonate is within the typical reference range.

• Metabolic acidosis with expected respiratory response: With $\mathrm{pH}=7.30$, $\mathrm{PaCO_2}=30\,\mathrm{mm\,Hg}$, and $\mathrm{HCO_3^-}=15\,\mathrm{mEq/L}$, low pH and low bicarbonate indicate primary metabolic acidosis. Winter’s formula predicts $\mathrm{PaCO_2}=(1.5\times15)+8=30.5\pm2\,\mathrm{mm\,Hg}$, so the measured value is within the expected range.

• Respiratory alkalosis: With $\mathrm{pH}=7.50$, $\mathrm{PaCO_2}=30\,\mathrm{mm\,Hg}$, and $\mathrm{HCO_3^-}=23\,\mathrm{mEq/L}$, the primary disorder is respiratory alkalosis, with little or no evident metabolic compensation. The pH indicates alkalemia, low carbon dioxide moves in the alkalinizing direction, and bicarbonate is near the typical range.

• Possible mixed disorder despite normal pH: With $\mathrm{pH}=7.40$, $\mathrm{PaCO_2}=20\,\mathrm{mm\,Hg}$, and $\mathrm{HCO_3^-}=12\,\mathrm{mEq/L}$, do not assume fully compensated metabolic acidosis. Winter’s formula predicts $\mathrm{PaCO_2}=(1.5\times12)+8=26\pm2\,\mathrm{mm\,Hg}$. The measured value of $20\,\mathrm{mm\,Hg}$ is lower than expected, suggesting additional respiratory alkalosis. A normal pH does not rule out serious acid–base disorders.

• Acidemia with worsening ventilation: If a patient has acidemia, elevated $\mathrm{PaCO_2}$, increasing drowsiness, and shallow respirations, promptly assess and support airway and ventilation, monitor oxygenation and level of consciousness, and escalate care according to the patient’s condition and local protocol. These findings may indicate worsening ventilatory failure; immediate assessment and escalation take priority over treating the laboratory value in isolation, and the cause also requires evaluation.

• Repeated vomiting: Metabolic alkalosis is a concern because loss of gastric acid can raise bicarbonate relative to acid. Assess the amount and duration of losses, hydration and volume status, medications, and relevant electrolyte and renal results; report findings and carry out prescribed care. Assessment of the cause and associated fluid or electrolyte problems guides care.