Respiratory Therapy and Acid-Base Physiology Flashcards

Weak Acids and Weak Bases in the Carbonic Acid Hydration Reaction

• Definition of Acid/Base Strength in Dissociation:
  • Strong Acids and Bases: These dissociate completely in a mixture. Once they split, they have difficulty reforming or do not go back together well at all.
  • Weak Acids and Bases: These dissociate in a fluid mixture (like water) but not completely. They dissociate reversibly, meaning they can reform. This reversible nature allows the chemical equation to be pushed in either direction (left or right) depending on the needs of the system.
• Carbonic Acid Hydration Equation Components:
  • Weak Acid Example: Carbonic Acid ($H_2CO_3$) is the middle portion of the hydration equation. It is considered weak, unstable, and volatile.
  • Weak Base Example: Bicarbonate ($HCO_3^-$) is the highlighted weak base in the reaction.
  • Strong Acid Example: The Hydrogen ion ($H^+$) is considered a strong acid in this context.
• The Physiological Path of $CO_2$ Transport:
  • Metabolism in the tissues (e.g., leg muscles) creates $CO_2$ as a byproduct.
  • Under normal resting conditions, the body produces $200\,mL$ of $CO_2$ per minute.
  • $CO_2$ flows from the tissues into the systemic capillary bed and enters the venous system.
  • Upon entering the bloodstream, $CO_2$ hits water ($H_2O$), creating carbonic acid ($H_2CO_3$).
  • Because carbonic acid is volatile and unstable, it dissociates into bicarbonate ($HCO_3^-$) and a hydrogen ion ($H^+$). This is how $CO_2$ is carried back to the lungs.
  • Once it reaches the lungs, the reaction reverses, turning the components back into $CO_2$ and water, allowing the $CO_2$ to be exhaled.

The Three Systems of Acid-Base Regulation

• 1. Chemical Buffer System:
  • This is the first line of defense against acid-base imbalances.
  • It responds the fastest to changes.
  • It is subdivided into three main systems: the Carbonic Acid-Bicarbonate system, the Phosphate system, and the Protein system.
• 2. Respiratory System:
  • This responds slower than the chemical system but is more powerful.
  • It possesses twice the buffering power of all chemical buffering systems combined.
• 3. Renal (Kidney) System:
  • This is the slowest to respond but is essential for long-term regulation and the removal of fixed acids.

The Chemical Buffer Systems: Detailed Subdivisions

• Carbonic Acid-Bicarbonate Buffer System:
  • Operates primarily in the blood plasma, which is roughly $90\%\,H_2O$.
  • Highly flexible because it can flux in either direction to maintain pH balance.
  • Considered the most powerful chemical buffer because of its flexibility.
• Henderson-Hasselbalch Equation Concepts:
  • Used as a conceptual mathematical proof that a normal pH is $7.4$.
  • $pK$: A mathematical constant associated with a specific acid; for carbonic acid, this constant never changes.
  • Ratio of Base to Acid: Under normal conditions, the ratio of bicarbonate (base) to carbonic acid (acid) is $20:1$.
  • $\text{pH}$ Shifts based on Ratios:
    • If the ratio decreases (e.g., to $15:1$), the $\\text{pH}$ becomes more acidic (less than $7.4$).
    • If the ratio increases (e.g., to $30:1$), the $\\text{pH}$ becomes more alkaline (greater than $7.4$).
• Phosphate Buffer System:
  • Similar function to the bicarbonate system but only about $\frac{1}{6}$ as effective in the blood plasma.
  • It is highly effective in the intracellular fluid and the renal system (specifically within the glomerular filtrate, which eventually becomes urine).
• Protein Buffer System:
  • Proteins are found in the plasma and inside cells.
  • They provide $75\%$ of the buffering power of body fluids, largely due to intracellular proteins.
  • Amphoteric Molecules: Proteins (which are polymers of amino acids) are unique because they can function as either an acid or a base depending on the environment.
  • Hemoglobin Case Study: Hemoglobin is an amphoteric molecule. At the tissue level, when hemoglobin loses oxygen (becomes reduced), it carries a negative charge. This allows it to bind with free hydrogen ions from the $CO_2$ hydration reaction, effectively decreasing the acidity of the plasma.

The Respiratory System and Ventilation Dynamics

• Equilibrium Shifts in the $CO_2$ Hydration Reaction:
  • Right Shift: Caused by an increase in $CO_2$. This occurs during high metabolic demand or hypoventilation (where $CO_2$ builds up in the system).
  • Left Shift: Caused by a decrease in $CO_2$. This occurs during hyperventilation (where excessive $CO_2$ is blown off).
• Definitions of Ventilatory States:
  • Eucapnia: Normal levels of $CO_2$ in the blood ($PaCO_2$ of $40\,mmHg$). Under normal resting conditions, alveolar ventilation ($V_A$) is approximately $4\,L/min$, and $CO_2$ elimination equals production ($200\,mL/min$).
  • Hypoventilation: Defined by a decrease in minute ventilation (respiratory rate $\times$ tidal volume) accompanied by an elevated $PaCO_2$ (Hypercapnia). If alveolar ventilation is halved, $CO_2$ level doubles.
  • Hyperventilation: Defined by an increase in minute ventilation accompanied by a reduced $PaCO_2$ (Hypocapnia). If alveolar ventilation is doubled, $CO_2$ level is halved.
  • Note on Diagnosis: Hypo- and hyperventilation cannot be technically diagnosed without an arterial blood gas (ABG) value for $PaCO_2$, regardless of observed respiratory rate or effort.

The Renal (Kidney) System Functions

• Fixed Acid Excretion: The kidneys rid the body of non-volatile (fixed) acids that the lungs cannot excrete, including:
  • Phosphoric acid.
  • Lactic acid.
  • Ketones.
• Bicarbonate Regulation: The kidneys manage alkaline substances by either retaining or excreting bicarbonate ($HCO_3^-$) to maintain hydrogen ion balance in the extracellular fluid.

Arterial Blood Gas (ABG) Analysis and Normal Ranges

• ABG Basics:
  • The most basic test of lung function.
  • Requires an anaerobic sample (no air bubbles) of arterial blood.
  • Air Bubble Contamination: If atmospheric air (low $CO_2$) enters the syringe, internal $CO_2$ will diffuse out of the blood into the bubble, falsely lowering the measured $PaCO_2$ and altering pH.
• Normal ABG Values:
  • $\\text{pH}$: Normal range is $7.35$ to $7.45$. The technical physiologic limit for life is roughly $6.9$ to $7.8$.
  • $PaCO_2$ (Respiratory Component): $35$ to $45\,mmHg$ (or Torr).
  • $PaO_2$ (Oxygen Tension): $80$ to $100\,mmHg$ at rest. Mixed venous $PvO_2$ is approximately $40\,mmHg$.
  • $HCO_3^-$ (Metabolic/Renal Component): $22$ to $26\,mEq/L$ (reported as $24 \pm 2$).
  • Base Excess (BE): An evaluation of all bases in the system. Normal range is $-2$ to $+2\,mEq/L$.
• The A-a Gradient ($P(A-a)O_2$):
  • Represents the difference between alveolar oxygen and arterial oxygen.
  • Normal Gradient: In healthy people, this is usually less than $20\,mmHg$.
  • Anatomical Shunts: Normal shunting (bronchial circulation and thebesian veins) prevents the gradient from being zero.
  • Factors Decreasing $PaO_2$ (Widening the Gradient): Hypoventilation, diffusion defects (Fick's Law: thickness, surface area, pressure gradient), V/Q mismatching, and high altitude.

Oxygen Delivery and Content Formulas

• Arterial Oxygen Content ($CaO_2$):
  • Measures total oxygen in the blood (bound to hemoglobin + dissolved in plasma).
  • Formula: $CaO_2 = (Hb \times 1.34 \times SaO_2) + (PaO_2 \times 0.003)$.
• Oxygen Delivery ($DO_2$):
  • Depends on arterial oxygen content and cardiac output.
  • Formula: $DO_2 = CaO_2 \times Cardiac\,Output \times 10$.

The Anion Gap

• Purpose: Used during metabolic acidosis to determine if the cause is accumulation of fixed acids or excessive loss of bicarbonate.
• Law of Electroneutrality: The sum of cations must equal the sum of anions to maintain neutrality.
• Calculation:
  • Primary Cation: Sodium ($Na^+$).
  • Primary Anions: Chloride ($Cl^-$) and Bicarbonate ($HCO_3^-$).
  • Formula: $Anion\,Gap = [Na^+] - ([Cl^-] + [HCO_3^-])$.
• Interpretation:
  • Normal Range: $9$ to $14\,mEq/L$.
  • High Anion Gap (> 14): Indicates an accumulation of fixed acids. Examples: Lactic acidosis (anaerobic metabolism), Ketoacidosis (low insulin), salicylate (aspirin) intoxication, renal failure.
  • Normal Anion Gap: Indicates the acidosis is due to a direct loss of bicarbonate. Example: Chronic/uncontrolled diarrhea.

Metabolic Disturbances: Acidosis and Alkalosis

• Metabolic Acidosis Examples:
  • Lactic Acidosis: Result of anaerobic metabolism when oxygen needs are not met.
  • Ketoacidosis: Glucose cannot enter cells due to low insulin; ketones accumulate.
  • Diarrhea: Direct loss of base (bicarbonate).
• Metabolic Alkalosis Examples (Rare in clinical practice):
  • Hypokalemia or Hypochloremia.
  • Gastric Suctioning: Losing stomach acid via an NG tube (especially continuous vs. intermittent suction).
  • Excessive vomiting.
  • Steroid therapy (corticosteroids).
  • Diuretic therapy.
  • Bicarbonate overdose during medical emergencies (codes).

Clinical Specimen Collection and Procedures

• Common Arterial Sites:
  • Radial: Most common due to collateral circulation.
  • Brachial: Medial to the antecubital fossa.
  • Femoral: Large artery in the groin/inguinal area.
  • Other: Dorsalis pedis (top of foot), Posterior tibial (inner ankle), Umbilical (neonates only).
• Modified Allen's Test:
  • Performed before radial puncture to ensure the ulnar artery provides adequate collateral blood flow.
  • Procedure: Patient makes a tight fist; therapist obstructs both radial and ulnar arteries. Patient opens hand (it should be blanched). Release pressure from only the ulnar artery.
  • Result: If the palm flushes (becomes pink/re-perfuses) within $15\,seconds$, the test is positive, and the radial artery can be safely punctured.
• Clinical Considerations and Safety:
  • Contraindications: Puncturing through lesions, distal to surgical shunts (e.g., dialysis shunts), or in areas of infection/peripheral vascular disease (PVD).
  • Mastectomies: Avoid the arm on the side of a mastectomy due to lymphedema risks.
  • Anticoagulants: If the patient is on blood thinners, pressure must be held longer post-puncture to prevent hematomas.
  • Needle Gauge: Generally a small $23$ to $25$ gauge needle.
  • Heparinization: Syringes often contain a heparin pellet to prevent the sample from clotting; the syringe must be mixed gently after collection.