Acid Base

Acid-Base Balance

Concept Overview

  • Definition of Acid-Base Balance: Refers to the process of regulating the pH, bicarbonate concentration, and partial pressure of carbon dioxide in body fluids.

  • Definitions:

    • Acid: A substance that releases hydrogen ions (H+).

    • Base: A substance that takes up hydrogen ions (H+).

  • Changes in pH:

    • Decrease in pH indicates acidosis.

    • Increase in pH indicates alkalosis.

  • Three Main Processes to Maintain Acid-Base Balance:

    • Acid Production: Generation of acid through metabolism.

    • Acid Buffering: Body fluids resist large changes in pH when acids or bases are added or removed.

    • Acid Excretion: Removal of acid from the body.

Arterial Blood Gas Components

  • Reference Ranges:

    • pH: 7.35 to 7.45 (Very closely regulated in the body).

    • PaCO2: 35 to 45 mmHg.

    • HCO3-: 22 to 26 mEq/L.

    • PaO2: 80 to 100 mmHg.

    • SaO2: > 90%.

  • Clinical Manifestations of Imbalances:

    • Alkalosis: Can lead to muscular irritability, cramps, tetany, spasms, confusion, and numbness/tingling in the face/hands/feet.

    • Acidosis: Symptoms include weakness, hyporeflexia, increased heart rate, headache, and confusion.

Testing for Arterial Blood Gas Values

  • Nursing Priority: Perform the Allen’s test to assess circulation before taking a sample.

  • Procedure:

    • Gather required tests and ice prior to collection.

    • Apply pressure after the stick, place the blood sample on ice quickly, and send to the lab.

Control of Acid-Base Balance by Hydrogen Ions

  • PaCO2 (Carbon Dioxide): Represents the amount of carbon dioxide dissolved in arterial blood.

    • Elevated PaCO2: Indicates potential acidosis due to CO2 accumulation.

    • Below normal PaCO2: Suggests alkalosis due to excessive CO2 excretion.

  • HCO3 (Bicarbonate): Indicates the kidneys' ability to excrete metabolic acid.

    • Below normal HCO3: Suggests acidosis (too much metabolic acid).

    • Elevated HCO3: Suggests alkalosis (too little metabolic acid).

  • PaO2 (Partial Pressure of Oxygen): Normal range 80-100 mmHg.

  • SaO2 (Oxygen Saturation): Indicates the percentage of hemoglobin saturated with oxygen, normal level >90%.

Classification of Acid-Base Disorders

Respiratory Factors

  • Respiratory Acidosis: Caused by impaired respiratory function leading to decreased exchange of O2 and CO2.

    • Causes: Hypoventilation, respiratory depression, inadequate chest expansion, airway obstruction, reduced alveolar-capillary diffusion.

    • Indicators: pH < 7.35 & PaCO2 > 45 mmHg.

Metabolic Factors

  • Metabolic Acidosis: Results from the body producing too much acid or kidneys failing to excrete acid.

    • Causes: Diabetic ketoacidosis (DKA), lactic acidosis, prolonged diarrhea (loss of bicarbonate), renal failure, pancreatitis, salicylate intoxication.

    • Indicators: pH drops < 7.35 & HCO3 < 22 mEq/L.

Clinical Maifestations of Acidosis

  • Cardiovascular (CV) Signs:

    • Delayed electrical conduction.

    • Hypotension and thready peripheral pulses.

  • CNS Symptoms: Depressed activity.

  • Neuromuscular Signs: Hyporeflexia, skeletal muscle weakness, flaccid paralysis.

  • Respiratory Signs: Kussmaul respirations, variable respiration rates.

  • Integumentary Signs: Warm, flushed, dry skin, with pale to cyanotic and dry skin due to poor perfusion.

Monitoring Electrocardiogram Changes in Hyperkalemia

  • Significance: Recognize changes on EKG associated with hyperkalemia, since excess potassium can affect cardiac conduction.

Metabolic Alkalosis

  • Definition: The amount of bicarbonate in the body exceeds the normal ratio, due to an increase of bases or a decrease of acids.

  • Causes: Gastric suction (prolonged), ingestion of bicarbonate antacids, prolonged vomiting, excessive use of potassium-losing diuretics, Cushing's syndrome, and whole blood transfusion.

  • Indicators: pH > 7.45 & HCO3 > 26 mEq/L.

Respiratory Alkalosis

  • Definition: Removal of too much CO2, as in cases of hyperventilation.

  • Causes: Hyperventilation, hysteria/anxiety, fever, excessive mechanical ventilation, salicylate overdose, shock, and hypoxemia.

  • Indicators: pH > 7.45 & PaCO2 < 35 mmHg.

Manifestations of Alkalosis

  • CNS Symptoms: Increased activity, anxiety, irritability, tetany, seizures, positive Chvostek’s and Trousseau’s signs, paresthesias.

  • Neuromuscular Symptoms: Hyperreflexia, muscle cramping, twitching, skeletal muscle weakness.

  • CV Symptoms: Increased heart rate, normal or low blood pressure, increased risk of digitalis toxicity.

  • Respiratory Symptoms: Increased ventilation rate and depth, with potential decrease in effort due to muscle weakness.

Compensation Mechanisms

  • Definition: Compensation occurs when the body alters values to return pH within normal limits.

  • Respiratory Compensation: Rapid response of seconds to minutes after pH change.

  • Renal Compensation: Occurs when the imbalance persists for several hours to days.

  • Clinical Consideration: In compensated blood gas analysis, the pH will be within normal range, but the PaCO2 and HCO3- levels may not be.

Rules of Interpretation

  1. **Evaluate blood gases systematically: **

    • Assess pH to categorize as normal, acidotic, or alkalotic.

    • Evaluate PaCO2 for the same classification.

    • Assess HCO3- similarly.

  2. Identify Relationships:

    • In acidotic scenarios, examine if either PaCO2 or HCO3- is also acidotic.

    • In alkalotic scenarios, identify which other measurement is also alkalotic.

ABG (Arterial Blood Gas) Interpretation

  • Process: After determining if each value is normal, acidic, or alkalotic, analyze the interrelationships to ascertain the underlying acid-base disturbance pattern.