Acid base/ abg

Acid-Base Balance

  • Primary Goal: Maintain acid-base homeostasis.

    • Key Organs Involved:

    • Lungs (Respiratory): Control rapid changes in acid-base balance by altering CO2 excretion.

    • Kidneys (Metabolic): Adjust bicarbonate levels to maintain pH over longer periods.

Overview of Acid-Base Homeostasis

  • Recap:

    • Tissues produce CO2 as a waste product, which gets transported into the blood and exhaled out.

    • If blood CO2 levels rise excessively, the brain regulates breathing depth and rate to expel CO2, adjusting quickly to maintain acid-base balance.

Clinical Interpretation of Arterial Blood Gases

Terminology

  • Acute Ventilatory Failure:

    • Characterized by a sudden rise in arterial partial pressure of carbon dioxide (PaCO2) with a corresponding decrease in blood pH, without renal compensation.

  • Chronic Ventilatory Failure:

    • Chronic elevation of PaCO2, normal or near-normal pH due to renal compensation with elevated bicarbonate (HCO3-).

  • Acute Ventilatory Failure Superimposed on Chronic Ventilatory Failure:

    • A sudden rise in PaCO2 in a patient already suffering from chronic ventilatory failure.

Types of Hyperventilation and Variants

  • Acute Alveolar Hyperventilation:

    • Sudden fall in PaCO2 with an increase in pH, no metabolic compensation occurring.

  • Chronic Alveolar Hyperventilation:

    • Chronic decrease in PaCO2 with a compensated normal or near-normal pH.

  • Acute Alveolar Hyperventilation Superimposed on Chronic Ventilatory Failure:

    • Sudden decrease in PaCO2 in a patient with chronic ventilatory failure.

Mechanisms of Acid-Base Regulation

Role of the Lungs

  • Lungs excrete carbonic acid (H2CO3) by providing a route for CO2 expulsion.

  • The brain adjusts breathing rates to match the rate of CO2 production by tissues.

Role of the Kidneys

  • Kidneys produce bicarbonate (HCO3-), the primary buffer for fixed acids.

  • They regulate blood pH by excreting or retaining bicarbonate:

    • Acidic blood: Kidneys retain HCO3- to neutralize acidity.

    • Alkalotic blood: Kidneys can excrete excess HCO3-.

pH and H+ Ion Relationship

  • Definition: pH is the negative logarithm of hydrogen ion (H+) concentration.

  • Relationship:

    • An increase in pH denotes a decrease in H+ concentration.

    • A decrease in pH indicates an increase in H+ concentration.

pH Homeostasis

  • Blood pH regulation involves:

    • Kidneys

    • Lungs

    • Blood buffers

  • Blood buffers help maintain pH within narrow limits, preventing drastic changes.

Acids and Bases

Definitions

  • Acids: Substances that release (donate) H+ ions; higher acid concentrations lower pH.

  • Bases: Substances that can accept H+ ions; help in raising pH.

  • Kidneys: Regulate blood bases, particularly bicarbonate; controlled in the nephron.

Regulation of H+ Ions

  • Types of Acids:

    • Volatile: Can be eliminated through respiration (e.g., H2CO3).

    • Fixed (Non-volatile): Cannot be excreted by lungs, handled exclusively by kidneys (e.g., lactic acid, sulfuric, phosphoric acid).

Clinical Conditions - Diabetic Ketoacidosis

  • Cause: Untreated diabetes leads to excess acid production; increased H+ stimulates increased ventilation for CO2 removal.

  • Diabetic Ketoacidosis: A state where the body can't produce enough insulin, forcing it to metabolize fats which leads to a buildup of ketones (toxic acids) in the bloodstream.

Buffer Systems

Bicarbonate Buffer System

  • Characteristics:

    • “Open” system; H2CO3 can be exhaled through ventilation.

    • Responsible for 53% total buffering capacity:

    • Plasma: 35%

    • Erythrocyte: 18%

  • Mechanism:

    • Increased lactic acid combines with bicarbonate to produce carbonic acid, which dissociates into CO2 and H2O, exhaled unless ventilation fails.

Non-bicarbonate Buffer System

  • Characteristics:

    • “Closed” system; all components of acid-base reactions are retained.

    • 47% of total buffering capacity:

    • Hemoglobin (Hb): 35%; most crucial in buffer systems.

    • Organic phosphates: 3%

    • Inorganic phosphates: 2%

    • Plasma proteins: 7%

  • Function: Buffers H+ ions from any acid but reacts slower than the bicarbonate system.

Kidneys and Acid-Base Control

  • Governing Actions:

    • Excretion of Fixed Acids: Managed by the kidneys.

    • Bicarbonate Reabsorption: Primarily occurs in the proximal tubules; driven by:

    • Luminal HCO3- concentration

    • Luminal flow rate

    • Arterial PaCO2

    • Angiotensin II

    • An increase in any of these variables triggers enhanced bicarbonate reabsorption.

Acid-Base Disturbances

Normal Values

  • HCO3-: Approximately 22-26 mEq/L.

  • PaCO2: Maintained between 35-45 mmHg.

  • Normal pH: Approximately 7.40 (Henderson-Hasselbalch equation).

Clinical Terminology

  • Hypercapnia: PaCO2 > 45 mmHg indicates insufficient ventilation (hypoventilation).

  • Hypocapnia: PaCO2 < 35 mmHg indicates excessive ventilation (hyperventilation).

Interpreting Acid-Base Results

Step 1 – Label the pH

  • Normal pH range: 7.35 to 7.45.

    • pH > 7.45 = Alkalosis

    • pH < 7.35 = Acidosis

Step 2 – Label PCO2

  • PCO₂ < 35 = Alkalosis

  • PCO₂ > 45 = Acidosis

Step 3 – Label HCO3- (Metabolic)

  • HCO3- < 22 = Acidosis

  • HCO3- > 26 = Alkalosis

Labeling ABG Values

Example Case

  • pH: 7.30

  • PCO2: 55

  • HCO3-: 26

  • Diagnosis:

    • Acidosis (idiosyncratic).

    • Respiratory Acidosis due to elevated PCO2.

    • Metabolic status normal as HCO3- is within the normal range.

    • Compensation: Kidneys will attempt to reabsorb more HCO3-.

Respiratory Acidosis

  • Also known as:

    • Acute Ventilatory Failure, Hypercapnia, Hypoventilation.

  • Compensatory Mechanism: Renal reabsorption of HCO3-.

  • Causes:

    • Respiratory: Upper airway obstruction, severe airway obstruction, massive pulmonary edema, pneumonia.

    • Non-respiratory: Drug overdose, spinal cord trauma, neuromuscular disease, head trauma, thoracic trauma, obesity.

Chronic Respiratory Acidosis

  • Characterized by an elevated PaCO2 with a normal pH due to renal compensation (HCO3- often > 26 mEq/L).

  • Associated Conditions: COPD leading to hypoventilation because of decreased response to low oxygen and high CO2 levels, increased ventilation-perfusion mismatch, and decreased diaphragmatic function.

Ventilatory Failure

  • Defined as PA elevated due to hypoventilation, leading to respiratory acidosis.

  • Acute Failure: Sudden changes in gas exchange efficiency due to pathological conditions.

  • Chronic Failure: Long-standing conditions such as COPD and other chronic lung diseases leading to sustained elevated PaCO2.

Metabolic Alkalosis

  • Definition: Elevated pH with elevated HCO3- levels.

  • Compensatory Mechanism: Hypoventilation to retain CO2.

  • Common Causes:

    • Hypokalemia, NG suctioning, persistent vomiting, diuretic use, and excessive sodium bicarbonate administration.

Metabolic Acidosis

Definition

  • Decrease in bicarbonate concentration to < 22 mEq/L.

Mechanisms

  • Gain of fixed acids or loss of bicarbonate.

  • Anion Gap Calculation:

    • extAnionGap=[Na+]−([Cl−]+[HCO3−])ext{Anion Gap} = [Na^+] - ([Cl^-] + [HCO_3^-])

Normal Ranges

  • Normal anion gap: 7–16 mEq/L;

  • Increased anion gap indicates excess fixed acids (e.g., diabetic ketoacidosis, lactic acidosis).

Causes of Metabolic Acidosis

  • Increased Fixed Acids: Ketoacidosis, renal failure, lactic acidosis, acid ingestion (methanol).

  • Loss of Bicarbonate: Diarrhea, renal tubular acidosis, pancreatic fistula, etc.

Total Parenteral Nutrition and Acid-Base Balance

  • Definition: Provides intravenous hyperalimentation for malnourished patients.

  • Components: Carbohydrates, fat, protein, electrolytes, vitamins, minerals, trace elements.

  • Consideration: TPN solutions are often acidic, potentially affecting acid-base balance.

Respiratory Alkalosis

  • Characteristics: Acute alveolar hyperventilation results in decreased PaCO2 and increased blood pH.

  • Compensatory Mechanism: Kidneys increase HCO3- excretion.

  • Signs & Symptoms: Tachypnea, dizziness, muscle spasms, confusion.

  • Causes: Hypoxemia, anxiety, pain, respiratory distress.

Mixed Disturbance

  • Example Cases:

    • 7.12, 60 mmHg, 40 mmHg, 19 mEq/L: Indicates mixed respiratory and metabolic acidosis (e.g., cardiac arrest leading to tissue hypoxia and anaerobic metabolism).

    • 7.58, 32 mmHg, 48 mmHg, 30 mEq/L: Indicates mixed metabolic and respiratory alkalosis (e.g., anxiety or severe illness).

Determining Compensation

  • Normal pH: Fully compensated systems.

  • Partially Compensated: Opposite system attempting to balance with measures outside normal limits.

  • Uncompensated: The opposite system has not adjusted to aid balance.

Oxygenation and PO2 Levels

  • Definition of Hypoxemia: Low oxygen concentration in the blood.

  • Normal PO2 values:

    • 80 to 100 mmHg normal.

    • 60 – 79 mmHg = mild hypoxemia;

    • 40 – 59 mmHg = moderate hypoxemia;

    • < 40 mmHg = severe hypoxemia.

ABG Examples

  • Normal ABG Values:

    • pH: 7.39

    • PCO2: 44 mmHg

    • HCO3-: 25 mEq/L

    • PO2: 89 mmHg

    • Conclusion: This ABG is normal, indicating proper acid-base regulation.