acid base imbalances

acidosis

pH below 7.35

Too acidic

Increased H⁺

CO₂ = ACID

alkalosis

pH above 7.45

Too alkaline

Decreased H⁺

• HCO₃⁻ = BASE

normal ABG values

Measurement	Normal range	Represents
pH	7.35–7.45	Overall balance
PaCO₂	35–45 mmHg	Lungs
HCO₃⁻	22–26 mEq/L	Metabolic/kidneys
PaO₂	80–100 mmHg	Oxygenation

Why does CO₂ matter?

CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid)

• Increased CO₂ → increased acid → decreased pH.

• Decreased CO₂ → decreased acid → increased pH.

The normal bicarbonate-to-carbonic acid ratio is 20:1.

BUFFER SYSTEM

• First line of defense.

• Works immediately to minimize pH changes.

• Converts strong acids into weaker acids or binds acids until they can be eliminated.

• Main buffer: carbonic acid–bicarbonate.

• Other buffers: phosphate, proteins, and hemoglobin.

Memory trick: Buffers = first responders.

⭐️ fastest

RESPIRATORY SYSTEM

The lungs regulate pH by controlling CO₂.

Breathing	CO₂	pH
Increased respirations	↓	↑
Decreased respirations	↑	↓

• Hyperventilation = blows off CO₂.

• Hypoventilation = retains CO₂.

• Controlled by the respiratory center in the medulla.

Memory trick: LUNGS control CO₂.

⭐️ minutes - hours

RENAL SYSTEM

The kidneys are the slowest but most powerful regulator.

They:

• Reabsorb bicarbonate (HCO₃⁻).

• Excrete hydrogen ions (H⁺).

• Form ammonium (NH₄⁺).

During acidosis: Retain bicarbonate and excrete hydrogen ions.

During alkalosis: Reverse these processes.

• Normal urine pH is around 6.

• Urine pH may range from 4–8 during compensation.

Memory trick: KIDNEYS control bicarbonate and H⁺.

⭐️ hours-days

Respiratory Acidosis

HYPOVENTILATION = RETAINING CO₂

What happens?

The patient is not breathing enough → CO₂ builds up → carbonic acid increases → pH decreases.

Common causes:

• COPD

• Pneumonia

• Atelectasis

• Sedative or opioid overdose

• Pulmonary edema

• Respiratory muscle weakness

ABG findings:

pH	PaCO₂	HCO₃⁻
↓	↑	Normal or ↑ with compensation

Compensation: Kidneys retain HCO₃⁻ and excrete H⁺. This takes time (potentially 24+ hours).

Clinical example:

A patient with COPD has shallow respirations and becomes confused and lethargic. They are retaining CO₂, which causes respiratory acidosis.

Remember: Respiratory ACIDOSIS = CO₂ ACCUMULATION.

Respiratory Alkalosis

HYPERVENTILATION = BLOWING OFF CO₂

What happens?

The patient breathes too fast → CO₂ decreases → carbonic acid decreases → pH increases.

Common causes:

• Anxiety

• Pain

• Fever

• Exercise

• Hypoxemia

• Excessive mechanical ventilation

• CNS injury affecting the respiratory center

ABG findings:

pH	PaCO₂	HCO₃⁻
↑	↓	Normal or ↓ with compensation

Compensation: Kidneys excrete HCO₃⁻.

Clinical example:

A patient is having a panic attack and breathing rapidly. They are blowing off too much CO₂, causing respiratory alkalosis.

Remember: Respiratory ALKALOSIS = TOO MUCH CO₂ LEAVING.

Metabolic Acidosis

TOO MUCH ACID OR LOSS OF BICARBONATE

What happens?

The body accumulates acid or loses bicarbonate → pH decreases.

Common causes:

• Diabetic ketoacidosis (DKA)

• Lactic acidosis from shock

• Renal failure

• Diarrhea

• Starvation

ABG findings:

pH	PaCO₂	HCO₃⁻
↓	Normal or ↓ with compensation	↓

Compensation: Lungs increase respirations to blow off CO₂.

HIGH-YIELD NCLEX

Kussmaul respirations:

• Deep, rapid breathing.

• The body is trying to eliminate CO₂.

• Commonly associated with DKA.

Clinical example:

A patient with DKA develops deep, rapid respirations. Their body is trying to compensate for metabolic acidosis by removing CO₂.

Remember: Metabolic ACIDOSIS = LOW BICARBONATE.

Metabolic Alkalosis

LOSS OF ACID OR GAIN OF BICARBONATE

What happens?

The body loses too much acid or gains bicarbonate → pH increases.

Common causes:

• Vomiting

• NG suctioning

• Diuretic therapy

• Excess bicarbonate intake

ABG findings:

pH	PaCO₂	HCO₃⁻
↑	Normal or ↑ with compensation	↑

Compensation: Respiratory rate decreases to retain CO₂. This compensation is limited by the risk of hypoxemia.

Nursing considerations:

• Monitor potassium (hypokalemia).

• Monitor calcium (hypocalcemia).

• Watch for muscle weakness and cardiac rhythm changes.

Clinical example:

A patient has been vomiting repeatedly for several days. They lose stomach acid, causing metabolic alkalosis.

Remember: Metabolic ALKALOSIS = HIGH BICARBONATE

ROME: Interpreting ABGs

ROME stands for:

• Respiratory = Opposite

• Metabolic = Equal

Disorder	pH	CO₂ / HCO₃⁻
Respiratory acidosis	↓	CO₂ ↑
Respiratory alkalosis	↑	CO₂ ↓
Metabolic acidosis	↓	HCO₃⁻ ↓
Metabolic alkalosis	↑	HCO₃⁻ ↑

How to interpret any ABG:

• Look at pH. Is the patient acidotic or alkalotic?

• Look at CO₂. Is it abnormal, and does it explain the pH?

• Look at HCO₃⁻. Is it abnormal, and does it explain the pH?

• Use ROME to identify the primary problem.

• Look at the other system to determine whether compensation is occurring.

compensation

Compensation is when the body attempts to correct an acid–base imbalance using the opposite regulatory system.

Primary problem	Compensating system
Respiratory	Kidneys
Metabolic	Lungs

Remember:

• Respiratory problem → kidneys help.

• Metabolic problem → lungs help.

Compensation tries to bring the pH back toward normal. It does not necessarily fix the underlying cause.

Assessment Findings

Subjective data — what should the nurse ask?

• History of kidney, lung, or cardiac disease.

• Diabetes or endocrine disorders.

• Medications: diuretics, corticosteroids, electrolyte supplements, antacids.

• Vomiting, diarrhea, NG drainage, excessive sweating.

• Diet and activity changes.

• Numbness, tingling, confusion, or weakness.

Objective data — what should the nurse assess?

• Vital signs and respiratory pattern.

• Blood pressure and pulse irregularities.

• Muscle weakness and tremors.

• Tetany.

• Confusion and seizures.

• ABGs and serum electrolytes.

• Urine pH.

• BUN and creatinine.

• Osmolality.

⭐️NURSING SAFETY ALERT

Always pay attention to potassium levels and ECG changes. Acid–base imbalances can affect potassium and increase the risk of cardiac complications.

nursing management

1. Assess and treat the underlying cause.

   • Example: insulin for DKA as prescribed.

   • Support ventilation for respiratory problems.

2. Monitor ABGs and electrolyte trends.

3. Monitor vital signs and cardiac rhythm.

4. Administer prescribed IV fluids, electrolytes, or bicarbonate cautiously.

5. Implement safety measures for weakness or altered mental status.

6. Use seizure precautions when indicated.

7. Use smart infusion pumps safely.

8. Educate patients about medications, dietary restrictions, and symptoms to report.

disorders at a glance

Respiratory acidosis	Hypoventilation / COPD / CO₂ retention
Respiratory alkalosis	Hyperventilation / anxiety / CO₂ loss
Metabolic acidosis	DKA / diarrhea / shock / low HCO₃⁻
Metabolic alkalosis	Vomiting / NG suction / high HCO₃⁻

memory tricks

• CO₂ = acid.

• Bicarbonate = base.

• ROME = Respiratory Opposite, Metabolic Equal.

• Kussmaul = deep, rapid breathing in metabolic acidosis.

• Vomiting = loss of stomach acid = metabolic alkalosis.

• COPD with CO₂ retention = respiratory acidosis.

• Anxiety with hyperventilation = respiratory alkalosis.

• Respiratory disorders are compensated by kidneys; metabolic disorders by lungs.