Acid-Base Imbalance

NORMAL VALUES AND DEFINITIONS

• pH normal range: $7.35 \le pH \le 7.45$

• Hydrogen ion concentration regulates acidity

• Buffer systems prevent rapid pH changes; buffering does not replace the need for organ function

• Acidemia/alkalemia defined by pH outside normal range

• Base Excess (BE): metabolic parameter; normal range $\pm 2\text{ mEq/L}$; typically ~50 mEq/L of buffer available

• Anion gap: $\text{Na}^+ - (\text{HCO}_3^- + \text{Cl}^-)$; normal ~12; helps identify unmeasured anions in metabolic acidosis

• pH of various body fluids (essential context):

  • ECF pH: $7.35-7.45$

  • ICF pH: $6.9-7.2$

  • Gastric fluid: $1.0-2.0$

  • Intestinal fluid: $6.6-7.6$

  • Bile: $5.0-6.0$

  • Urine: $6.0$

BUFFERING SYSTEMS

• Bicarbonate-carbonic acid buffer (primary)

• Phosphate buffer: important intracellular buffering

• Hemoglobin-oxyhemoglobin buffer: maintains pH between arterial and venous blood

• Protein buffers: intracellular/extracellular proteins

ACID-BASE REGULATION MECHANISMS

• Respiratory system

  • Regulates via CO2 elimination; rapid response (minutes)

  • Acidosis triggers increased ventilation to blow off CO2; alkalosis triggers decreased ventilation

• Renal system

  • Regulates via Na+ and HCO3- handling, H+ secretion, ammonia production; slower (hours to days)

• Buffers provide immediate pH stabilization, but restoration of normal pH requires intact respiratory and renal function

• Correction vs compensation

  • Compensation: physiologic adjustments to minimize pH change but does not fully normalize pH

  • Correction: resolution of the underlying disorder (normal pH and values)

ABG ANALYSIS AND DIAGNOSIS

• ABG components: pH, PaCO2, HCO3^-, Base Excess (BE), PaO2, O2 saturation

• Stepwise interpretation:

  1. Determine acidemia or alkalemia from $pH$

  2. Identify primary disorder by looking at PaCO2$(respiratory) vs$ HCO3 (metabolic)

  3. Assess compensation: if the non-primary value is abnormal in the direction expected for compensation

  4. Decide between complete compensation (pH normal) or partial compensation (pH abnormal)

• Rome mnemonic:

  • Respiratory disturbances are opposite to pH (alkalosis vs acidosis) – "Opposite"

  • Metabolic disturbances are equal to pH changes (alkalosis vs acidosis) – "Equal"

• Decision tree (conceptual):

  • If pH < 7.35 → acidemia

  • If PaCO2 high → respiratory acidosis; if PaCO2 low → respiratory alkalosis

  • If HCO3- low → metabolic acidosis; if HCO3- high → metabolic alkalosis

TYPICAL ACID-BASE DISTURBANCES

• RESPIRATORY ACIDOSIS

  • pH < 7.35 and PaCO2 > 45 mmHg

  • Etiology: hypoventilation or impaired gas exchange (e.g., COPD, pneumonia, overdose, chest injuries)

  • Manifestations: headache, confusion, tachycardia, lethargy

  • Interventions: assess ventilation, ABGs, improve ventilation, treat underlying cause

• RESPIRATORY ALKALOSIS

  • pH > 7.45 and PaCO2 < 35 mmHg

  • Etiology: hyperventilation (anxiety, pain, fever, mechanical ventilation)

  • Manifestations: rapid deep breathing, dizziness, paresthesias, tetany

  • Interventions: slow breathing, adjust ventilator, emotional support

• METABOLIC ACIDOSIS

  • pH < 7.35 and HCO3- < 24 mEq/L

  • Etiology: lactic acidosis, ketoacidosis (DKA), renal failure, diarrhea, shock

  • Manifestations: headache, weakness, tachycardia, hypotension, altered mental status

  • Interventions: treat underlying cause, fluid resuscitation, bicarbonate in selected cases

• METABOLIC ALKALOSIS

  • pH > 7.45 and HCO3- > 28 mEq/L

  • Etiology: vomiting, NG suction, diuretics, bicarbonate ingestion, K+ deficit

  • Manifestations: dizziness, tingling, tetany, hypoventilation, confusion

  • Interventions: correct fluid/electrolyte imbalances, stop causative losses, monitor electrolytes

POTASSIUM SHIFTS IN ACID-BASE BALANCE

• Acidosis: K+ shifts from ICF to ECF → hyperkalemia tendency

• Alkalosis: K+ shifts from ECF to ICF → hypokalemia tendency

• Insulin administration and blood glucose levels influence cellular K+ distribution

• Overall effect: K+ levels are sensitive to acid-base status and require monitoring during disturbances

COMPENSATION VS CORRECTION

• Compensation is a physiological adjustment to minimize pH change without correcting the underlying disorder

• Complete compensation: pH within normal range, but PaCO2 and HCO3- are not (they are reciprocally abnormal)

• Partial compensation: pH not within normal range even after compensation

• Correction implies resolution of the underlying disorder leading to normalization of pH, PaCO2, and HCO3-

OXYGENATION AND VENTILATION ESSENTIALS

• Oxygenation: ability to on-board O2

• Hypoxia: decreased PO2 → anaerobic metabolism (potential metabolic problem)

• Ventilation: ability to off-load CO2

• Increased CO2 due to ventilatory failure is a respiratory problem

GERIATRIC CONSIDERATIONS

• Age-related changes: reduced respiratory function and alveolar surface area, decreased gas exchange, higher risk of CO2 retention

• Diminished response to hypoxia and hypercapnia

• Acid-base imbalances easier to misinterpret and harder to correct in elderly due to comorbidities

QUICK REFERENCE VALUES AND MOLECULES

• Key diagnostic cues:

  • pH and PaCO2 alignment indicate respiratory component

  • pH and HCO3- alignment indicate metabolic component

  • BE and anion gap provide additional context for metabolic disturbances