Acid-Base Balance Regulation and ABG Interpretation

pH Balance and Arterial Blood Ranges

• Arterial pH Balance:

  • Normal Arterial Range: $7.35 - 7.45$

  • Acidosis: Occurs when pH falls below $7.35$

  • Alkalosis: Occurs when pH rises above $7.45$

• Venous pH Balance:

  • Normal Venous Range: $7.30 - 7.40$

• Physiologic and Midpoint Ranges:

  • Midpoint of the pH scale: $7$

  • Physiologic range (limit of life): $6.90 - 7.80$

Acids and Bases: Definitions and Examples

• Acids:

  • Definition: Substances that release Hydrogen ions ($H^+$).

  • Examples: Hydrochloric acid ($HCl$) and Carbonic acid ($H_2CO_3$).

  • Classification: Can be categorized as strong or weak acids.

• Bases:

  • Definition: Substances that accept Hydrogen ions ($H^+$).

  • Examples: Sodium hydroxide ($NaOH$) and Bicarbonate ($HCO_3^-$).

  • Classification: Can be categorized as strong or weak bases.

The pH Scale and Specific pH Conditions

• The pH scale ranges from $1$ to $14$, where $1-6$ is acidic, $7$ is neutral, and $8-14$ is basic.

• Examples of pH Conditions:

  • pH $2$: Gastric juices

  • pH $4$: Tomato juice

  • pH $5$: Human urine

  • pH $7$: Pure water (Neutral)

  • pH $7.4$: Human blood

  • pH $10$: Hand soap

  • pH $12$: Household bleach

Regulatory Mechanisms of Acid-Base Balance

• The body utilizes three primary systems to regulate $H^+$ and $HCO_3^-$ levels:

  1. Chemical Buffer System: The first line of defense; acts almost immediately.

  2. Respiratory System: Acts within minutes.

  3. Renal System: Most powerful, but takes hours to days to act.

Chemical Buffer Systems

• Carbonic Acid-Bicarbonate Buffer System:

  • Primary first line of defense.

  • Chemical reaction: $H_2CO_3 \rightarrow HCO_3^- + H^+$

• Phosphate Buffer System:

  • Efficiency: Approximately $1/6$ as effective as the carbonic acid-bicarbonate system.

  • Limitation: Not considered an effective buffer system for blood plasma.

  • Location: Highly effective as a buffer system in the urine and intracellular fluid ($ICF$).

• Protein Buffer System:

  • Location: Found in both plasma and within cells.

  • Potency: Accounts for $75\%$ of the total buffering power of body fluids.

  • Components: Includes intracellular proteins and amphoteric molecules.

  • Key Example: Hemoglobin.

Henderson-Hasselbalch Equation

• Used to determine pH based on the ratio of bicarbonate to dissolved carbon dioxide.

• Formula:

  • $pH = pK + \log \frac{[HCO_3^-]}{[CO_2]}$

• Constants and Specifics:

  • $pK$ for Carbonic Acid: $6.1$

  • Note: $pK$ varies depending on the specific acid.

• Example Calculation for Human Blood:

  • Normal ratio of bicarbonate to carbon dioxide is $20:1$.

  • $\log(20) = 1.3$

  • $pH = 6.1 + 1.3 = 7.4$

Respiratory System and the Role of Ventilation

• Buffering Power: The respiratory system possesses twice the buffering power of all combined chemical buffering systems.

• Equilibrium Reaction:

  • $H_2O + CO_2 \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$

• Normal Ventilation (Eucapnia):

  • $CO_2$ production: $200\,mL/min$

  • $CO_2$ elimination: $200\,mL/min$

  • Alveolar Ventilation ($V_A$): $4\,L/min$

  • $PACO_2$ (Alveolar): $40\,mm\,Hg$

  • Plasma $PaCO_2$: $40\,mm\,Hg$

  • Concentrations: $CO_2$ ($1.2\,mmol/L$); $400$ molecules of $CO_2$ / $400$ molecules of $H_2O$ for every $1$ molecule of $H^+$.

• Hypoventilation (Hypercapnia):

  • $V_A$: Decreased to $2\,L/min$

  • $PACO_2$: Elevated to $80\,mm\,Hg$

  • Plasma $PaCO_2$: Elevated to $80\,mm\,Hg$

  • Concentrations: $CO_2$ ($2.4\,mmol/L$); $800$ molecules of $CO_2$ / $800$ molecules of $H_2O$ for every $2$ molecules of $H^+$.

• Hyperventilation (Hypocapnia):

  • $V_A$: Increased to $8\,L/min$

  • $PACO_2$: Decreased to $20\,mm\,Hg$

  • Plasma $PaCO_2$: Decreased to $20\,mm\,Hg$

  • Concentrations: $CO_2$ ($0.6\,mmol/L$); $200$ molecules of $CO_2$ / $200$ molecules of $H_2O$ for every $0.5$ molecules of $H^+$.

The Renal System

• Mechanism of Regulation:

  • Rids the body of "fixed acids" (acids that cannot be exhaled by the lungs).

  • Regulates levels of alkaline substances (primarily bicarbonate) within the blood.

Arterial Blood Gas (ABG) Analysis and Parameters

• Introduction:

  • The most basic test used to evaluate lung function.

  • Requires an anaerobic sample of arterial blood.

  • Classification: Invasive test carrying inherent risks.

• Key Parameters:

  • pH: No units. Normal range $7.35 - 7.45$.

  • $PaCO_2$ (Partial pressure of arterial carbon dioxide):

    • Units: $mm\,Hg$

    • Normal Range: $35 - 45\,mm\,Hg$

    • Mixed Venous Reference: $40 - 46\,mm\,Hg$

  • $PaO_2$ (Partial pressure of arterial oxygen):

    • Units: $mm\,Hg$

    • Normal Range: $80 - 100\,mm\,Hg$

    • Mixed Venous Reference: $40\,mm\,Hg$

  • Bicarbonate ($HCO_3^-$):

    • Units: $mEq/L$

    • Normal Range: $22 - 26\,mEq/L$

  • Base Excess (BE):

    • Units: $mEq/L$

    • Normal Range: $\pm 2\,mEq/L$

Oxygenation Assessment Formulas

• Alveolar Air Equation ($PAO_2$):

  • $PAO_2 = (Pb - PH_2O) \times FIO_2 - (PaCO_2 \times 1.25)$

• A-a Gradient:

  • Difference between alveolar and arterial oxygen tension.

  • Normal Value: In healthy persons, usually less than $20\,mm\,Hg$.

  • Concept: If ventilation matches perfusion, the blood leaving the capillaries should have an oxygen tension very close to the alveoli.

• Total Oxygen Content of Arterial Blood ($CaO_2$):

  • $CaO_2 = (Hb \times 1.34 \times SaO_2) + (PaO_2 \times 0.003)$

• Oxygen Delivery ($DO_2$):

  • $DO_2 = CaO_2 \times CO \times 10$

Acid-Base Disturbances and Classifications

• Respiratory Disturbances:

  1. Acute ventilatory failure (respiratory acidosis).

  2. Acute ventilatory failure with partial renal compensation.

  3. Chronic ventilatory failure with complete renal compensation.

  4. Acute alveolar hyperventilation (respiratory alkalosis).

  5. Acute alveolar hyperventilation with partial renal compensation.

  6. Chronic alveolar hyperventilation with complete renal compensation.

• Metabolic Disturbances:

  1. Metabolic acidosis.

  2. Metabolic acidosis with partial respiratory compensation.

  3. Metabolic acidosis with complete respiratory compensation.

  4. Combined metabolic and respiratory acidosis.

  5. Metabolic alkalosis.

  6. Metabolic alkalosis with partial respiratory compensation.

  7. Metabolic alkalosis with complete respiratory compensation.

  8. Combined metabolic and respiratory alkalosis.

Specific Clinical Examples

• Acute Ventilatory Failure (Respiratory Acidosis):

  • COPD exacerbation.

  • Drug overdose.

  • General anesthesia complications.

  • Head trauma.

  • Neurologic disorders.

• Acute Alveolar Hyperventilation (Respiratory Alkalosis):

  • Hypoxia.

  • Pain, anxiety, and fever.

  • Brain inflammation.

  • Stimulant drugs.

• Metabolic Acidosis:

  • Lactic acidosis.

  • Ketoacidosis.

  • Salicylate intoxication (aspirin).

  • Renal failure.

  • Uncontrolled diarrhea.

• Metabolic Alkalosis:

  • Hypokalemia and Hypochloremia.

  • Gastric suction or vomiting (loss of acid).

  • Excessive administration of corticosteroids.

  • Excessive administration of sodium bicarbonate.

  • Diuretic therapy.

  • Hypovolemia.

Anion Gap

• Purpose: Determines if metabolic acidosis is caused by fixed acid accumulation or the excessive loss of bicarbonate ions.

• Law of Electroneutrality:

  • Cations: Sodium ($Na^+$).

  • Anions: Chloride ($Cl^-$) and Bicarbonate ($HCO_3^-$).

• Interpretation: Distinguishes between normal versus high anion gap causes.

Specimen Collection Procedures

• Common Collection Sites:

  • Primary: Radial artery (most common).

  • Secondary: Brachial artery.

  • Alternative sites: Femoral, Dorsalis pedis, Posterior tibial, Umbilical.

  • Special case: Warmed Capillary Sample.

• Modified Allen’s Test:

  1. Instruct the patient to make a tight fist.

  2. Apply pressure to both the radial and ulnar arteries.

  3. Instruct the patient to open the fist; a blanched palm/fingers should be observed.

  4. Release pressure off the ulnar artery only.

  5. Visualize the flushing of the palm within $15$ seconds.

  6. If flushing occurs, it is a positive test (indicating collateral circulation). Proceed with collection.

• Clinical Considerations & Precautions:

  • Do not puncture through a lesion.

  • Do not puncture distal to a surgical shunt.

  • Peripheral Vascular Disease (PVD) and infections are contraindications at specific sites.

  • Query patients regarding the use of anticoagulant drugs.

  • Ensure patient comfort and practitioner comfort.

  • Always use standard precautions.

  • After collection and air expulsion, gently roll the syringe to mix the sample.