NCLIP

Acid-Base Balance Principles

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Acid-Base Balance Principles

Key Points

  • Normal arterial blood pH is 7.35 to 7.45, and values outside this range indicate acid-base imbalance.
  • Carbon dioxide (PaCO2) represents the respiratory acid component, with a normal range of 35 to 45 mm Hg.
  • Bicarbonate (HCO3-) represents the metabolic base component, with a typical range of 22 to 29 mmol/L.
  • Severe or prolonged imbalance can cause major electrolyte shifts, including relative hypocalcemia in alkalemia and hyperkalemia in acidosis.

Pathophysiology

Acid-base balance reflects the relationship between hydrogen ion concentration and two major regulators: respiratory elimination of carbon dioxide and renal regulation of bicarbonate. The respiratory system influences PaCO2 through ventilation, while the kidneys adjust bicarbonate reabsorption or excretion to help restore balance.

Arterial blood pH below 7.35 indicates acidosis, and pH above 7.45 indicates alkalosis. Even when imbalance starts as mild, persistent deviation disrupts enzyme activity and cellular function. This is why early nursing recognition and trend interpretation are central to preventing deterioration.

Classification

  • Respiratory component: PaCO2 acts as the blood acid marker and is primarily regulated by the respiratory-system.
  • Metabolic component: HCO3- acts as the major base marker and is primarily regulated by the renal-system.
  • Acidemia pattern: Blood pH less than 7.35.
  • Alkalemia pattern: Blood pH greater than 7.45.

Nursing Assessment

NCLEX Focus

Prioritize recognition of whether pH is acidotic or alkalotic first, then identify whether the dominant driver is PaCO2 or HCO3-.

  • Review arterial-blood-gas results with attention to pH, PaCO2, and HCO3- as a linked pattern.
  • Confirm sample source because venous-blood-gas and capillary samples can appear more acidotic than arterial samples.
  • Trend respiratory status for clues of PaCO2 dysregulation and ventilation change.
  • Trend renal function and fluid status for clues of bicarbonate handling impairment.
  • Monitor for neuromuscular and cardiac effects that may signal associated electrolyte movement.

Nursing Interventions

  • Escalate abnormal pH trends early and communicate whether findings suggest respiratory, metabolic, or mixed disturbance.
  • Support oxygenation and ventilation strategies when PaCO2 abnormalities are present.
  • Support renal-focused management and fluid-electrolyte correction when bicarbonate abnormalities are present.
  • Reassess serial blood gases to confirm response after interventions.
  • Educate patients with chronic lung-disease or kidney-disease about increased risk for acid-base instability.

Severe Imbalance Risk

Uncorrected acid-base imbalance can progress to life-threatening physiologic instability and requires urgent reassessment.

Pharmacology

This source section emphasizes physiologic regulation and interpretation rather than specific medication regimens.

Clinical Judgment Application

Clinical Scenario

A patient with chronic pulmonary disease develops worsening ventilation and later shows blood gas changes outside the normal pH range.

Recognize Cues: pH is abnormal and PaCO2 is outside the expected 35 to 45 mm Hg range. Analyze Cues: Respiratory contribution to acid-base imbalance is likely dominant. Prioritize Hypotheses: Immediate priority is preventing progression of acidemia and secondary complications. Generate Solutions: Optimize ventilation support, repeat blood gas monitoring, and evaluate renal compensation status. Take Action: Escalate findings and implement ordered respiratory interventions promptly. Evaluate Outcomes: pH trends back toward 7.35 to 7.45 with stabilization of respiratory and metabolic indicators.

Self-Check

  1. What is the first interpretation step when reviewing an ABG for acid-base status?
  2. Why does sample source (arterial vs venous) matter during pH interpretation?
  3. Which findings suggest severe progression rather than a mild transient imbalance?

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