Acid/Base Balance

Arterial blood gas (ABG) analysis provides vital clinical information regarding a patient’s oxygenation status, ventilation, and acid/base balance. Just like with any lab values, when these levels shift outside the normal reference range, they indicate underlying physiological disruption and disease processes.

An ABG panel measures five primary components from an arterial blood sample:

• Blood Acidity (\(\text{pH}\)): Indicates overall hydrogen ion concentration in arterial blood.

• Partial Pressure of Oxygen (\(\text{PaO}_2\)): Measures dissolved oxygen in arterial blood.

• Partial Pressure of Carbon Dioxide (\(\text{PaCO}_2\)): Measures dissolved carbon dioxide, reflecting respiratory ventilation.

• Bicarbonate (\(\text{HCO}_3^-\)): Measures metabolic base concentration, reflecting renal regulation.

• Oxygen Saturation (\(\text{SaO}_2\)): Represents the percentage of hemoglobin saturated with oxygen.

Standard Reference Ranges

ABG Component	Normal Reference Range	Primary System Regulated
\(\text{pH}\)	7.35 – 7.45	Systemic Acid-Base Balance
\(\text{PaO}_2\)	75 – 100 mmHg	Respiratory (Oxygenation)
\(\text{PaCO}_2\)	35 – 45 mmHg	Respiratory (Ventilation)
\(\text{HCO}_3^-\)	22 – 26 mEq/L	Renal / Metabolic
\(\text{SaO}_2\)	94% – 100%	Tissue Oxygenation

Deciphering Acid-Base Imbalances

The body tightly regulates blood \(\text{pH}\) between 7.35 and 7.45.

• Acidosis: A blood \(\text{pH}\) less than 7.35.

• Alkalosis: A blood \(\text{pH}\) greater than 7.45.

To determine whether the primary imbalance is respiratory or metabolic, evaluate the \(\text{PaCO}_2\) and \(\text{HCO}_3^-\) levels relative to the \(\text{pH}\):

• \(\text{PaCO}_2\) (Respiratory Marker): Controlled by lungs. Carbon dioxide forms carbonic acid in blood; elevated \(\text{PaCO}_2\) lowers \(\text{pH}\) (acidic), while decreased \(\text{PaCO}_2\) raises \(\text{pH}\) (alkaline).

• \(\text{HCO}_3^-\) (Metabolic Marker): Controlled by kidneys. Bicarbonate is a base; decreased \(\text{HCO}_3^-\) lowers \(\text{pH}\) (acidic), while elevated \(\text{HCO}_3^-\) raises \(\text{pH}\) (alkaline).

Primary Types of Acid-Base Disturbances

1. Respiratory Acidosis (\(\text{pH} < 7.35\), \(\text{PaCO}_2 > 45\text{ mmHg}\))

Occurs when the respiratory system fails to adequately eliminate \(\text{CO}_2\) (hypoventilation), leading to carbonic acid buildup.

• Common Causes: Opioid or sedative overdose, Chronic Obstructive Pulmonary Disease (COPD), severe asthma exacerbations, chest wall injury, or neuromuscular disorders (e.g., Guillain-Barré, ALS).

2. Respiratory Alkalosis (\(\text{pH} > 7.45\), \(\text{PaCO}_2 < 35\text{ mmHg}\))

Occurs when hyperventilation causes the lungs to blow off excessive amounts of \(\text{CO}_2\).

• Common Causes: Severe anxiety or panic attacks, hyperventilation due to pain, high fever, pulmonary embolism, early sepsis, or excessive mechanical ventilation.

3. Metabolic Acidosis (\(\text{pH} < 7.35\), \(\text{HCO}_3^- < 22\text{ mEq/L}\))

Occurs from either excessive acid accumulation or significant loss of bicarbonate.

• Common Causes: Diabetic Ketoacidosis (DKA), renal failure, lactic acidosis, severe diarrhea (loss of alkaline fluids), or salicylate toxicity.

• Serum Anion Gap: Metabolic acidosis is often further evaluated using the anion gap:

\[\text{Anion Gap} = [\text{Na}^+] - ([\text{Cl}^-] + [\text{HCO}_3^-])\]

An elevated anion gap (\(>12\text{ mEq/L}\)) points to added organic acids (e.g., DKA, lactic acid), while a normal anion gap points to direct bicarbonate loss (e.g., diarrhea).

4. Metabolic Alkalosis (\(\text{pH} > 7.45\), \(\text{HCO}_3^- > 26\text{ mEq/L}\))

Occurs from excessive loss of hydrogen ions (acid) or accumulation of bicarbonate.

• Common Causes: Severe vomiting, nasogastric (NG) tube suctioning, diuretic overuse (e.g., loop diuretics), or excessive ingestion of antacids (alkali).

How Can I Remember This? The ROME Mnemonic

A reliable bedside tool for interpreting primary imbalances is the ROME mnemonic, which stands for Respiratory Opposite, Metabolic Equal. It compares the directional movement of \(\text{pH}\) against \(\text{PaCO}_2\) or \(\text{HCO}_3^-\).

Respiratory Opposite

In respiratory disorders, \(\text{pH}\) and \(\text{PaCO}_2\) move in opposite directions:

• Respiratory Acidosis: \(\text{pH} \downarrow\) and \(\text{PaCO}_2 \uparrow\)

• Respiratory Alkalosis: \(\text{pH} \uparrow\) and \(\text{PaCO}_2 \downarrow\)

Metabolic Equal

In metabolic disorders, \(\text{pH}\) and \(\text{HCO}_3^-\) move in the same (equal) direction:

• Metabolic Acidosis: \(\text{pH} \downarrow\) and \(\text{HCO}_3^- \downarrow\)

• Metabolic Alkalosis: \(\text{pH} \uparrow\) and \(\text{HCO}_3^- \uparrow\)

Physiological Compensation

When an acid-base disturbance occurs, the body tries to restore physiological balance through secondary compensation:

• Uncompensated: \(\text{pH}\) is abnormal. One indicator (\(\text{PaCO}_2\) or \(\text{HCO}_3^-\)) is abnormal, while the other remains within normal reference limits.

• Partially Compensated: \(\text{pH}\) is still abnormal, but both \(\text{PaCO}_2\) and \(\text{HCO}_3^-\) are abnormal as the secondary system works to fix the imbalance.

• Fully Compensated: \(\text{pH}\) has returned to the normal range (\(7.35–7.45\)), though both \(\text{PaCO}_2\) and \(\text{HCO}_3^-\) remain abnormal.

Clinical Interpretation Tip: If \(\text{pH}\) is within normal range but both \(\text{PaCO}_2\) and \(\text{HCO}_3^-\) are abnormal (Full Compensation), look at which side of 7.40 the \(\text{pH}\) falls on. A \(\text{pH}\) of 7.36 leans acidotic (pointing to a primary acidosis), whereas a \(\text{pH}\) of 7.44 leans alkalotic (pointing to a primary alkalosis).

6-Step ABG Interpretation Method

1. Check the \(\text{pH}\): Is it acidotic (\(<7.35\)), normal (\(7.35–7.45\)), or alkalotic (\(>7.45\))?

2. Check the \(\text{PaCO}_2\): Is it high (\(>45\text{ mmHg}\)), normal (\(35–45\text{ mmHg}\)), or low (\(<35\text{ mmHg}\))?

3. Check the \(\text{HCO}_3^-\): Is it low (\(<22\text{ mEq/L}\)), normal (\(22–26\text{ mEq/L}\)), or high (\(>26\text{ mEq/L}\))?

4. Apply ROME: Match which parameter aligns with the direction of the \(\text{pH}\) shift.

5. Determine Compensation: Evaluate if the opposing parameter has shifted to correct the \(\text{pH}\).

6. Assess Oxygenation: Look at \(\text{PaO}_2\) (\(75–100\text{ mmHg}\)) and \(\text{SaO}_2\) (\(94–100\%\)) to identify hypoxemia.