acid base

Acid-Base Balance Overview

  • Course: NUR 113

Acid-Base Balance in Relation to Cellular Regulation

  • The body constantly produces acids through normal metabolism.

  • To maintain health, the body must balance acids and bases.

  • Optimal cellular function occurs when this balance is preserved.

  • Minor fluctuations in pH can disrupt metabolism, impair organ function, and lead to significant clinical issues.

Understanding Acid-Base Chemistry

  • Body fluids contain various acids and fewer bases.

    • Main Acids:

    • Carbonic acid (H₂CO₃), formed when CO₂ combines with water.

    • Main Base:

    • Bicarbonate (HCO₃).

  • Plasma pH indicates the concentration of hydrogen ions (H⁺) in the blood:

    • More H⁺ results in higher acidity.

    • Fewer H⁺ leads to increased alkalinity.

  • Homeostatic mechanisms (lungs, kidneys, buffers) regulate pH between 7.35 and 7.45.

Mechanisms of Acid-Base Balance

Acid-Base Balance Definition

  • Balance is achieved when the rate of acid/base production equals the rate of excretion.

  • Regulatory systems exist to maintain this balance through buffer systems that stabilize blood H⁺ levels.

Definitions of Acids and Bases

  • Acid:

    • A substance that contributes hydrogen ions (H⁺) to a solution.

    • Higher H⁺ concentration = more acidic environment.

    • Acids often arise as by-products of metabolism, such as carbonic acid from CO₂ and water.

    • To increase acidity (lower pH), add H⁺.

  • Base:

    • A substance that absorbs or binds hydrogen ions (H⁺) from solution.

    • Lower H⁺ concentration = more alkaline environment.

    • The primary base in the body is bicarbonate (HCO₃), which neutralizes acids.

    • To increase alkalinity (raise pH), remove or neutralize H⁺.

Acid-Base Disturbances

pH Ranges

  • Acidic:

    • pH below 7.35 suggests either excess acid or insufficient base.

  • Alkalotic:

    • pH above 7.45 indicates either excess base or insufficient acid.

    • Acidity and alkalinity are determined by hydrogen ion concentration.

Bicarbonate to Carbonic Acid Ratio

  • The normal bicarbonate (HCO₃) to carbonic acid (H₂CO₃) ratio is 20:1.

  • Maintaining this ratio keeps pH within the normal limits of 7.35 to 7.45.

  • pH Range and Effects:

    • 6.8: Death

    • 7.35: Normal

    • 7.45: Normal

    • 8.0: Death

    • 1 part H₂CO₃ to 20 parts HCO₃.

Understanding the pH Scale

  • The pH scale measures H⁺ concentration.

    • Greater H⁺ = lower pH (more acidic).

    • Lesser H⁺ = higher pH (more basic).

  • pH scale ranges from 1 to 14, with 7 as neutral.

Review Questions

  • What is the normal pH range for arterial blood?

  • What are the most abundant acid and base in the body?

  • What happens to pH levels when there is an excess of acid or base?

  • Define the terms for excess acid and excess base.

Body’s Buffer Systems

  1. Chemical Buffers:

    • React within seconds to imbalances.

  2. Respiratory System:

    • Acts within minutes to address imbalances.

  3. Renal System:

    • The strongest and slowest, operates over hours to days.

    • Chemical reaction:
      CO2+H2O⇌H2CO3⇌H++HCO3−CO₂ + H₂O \rightleftharpoons H₂CO₃ \rightleftharpoons H⁺ + HCO₃⁻

Chemical Buffers

  • Bicarbonate-Carbonic Acid Buffer System:

    • Neutralizes disturbances using bicarbonate and carbonic acid.

  • Phosphate Buffer System:

    • Functions within cells and kidneys.

  • Protein Buffers:

    • Hemoglobin and plasma proteins can bind or release H⁺.

  • Ion Shifts:

    • H⁺ can shift into cells, exchanging with K⁺.

    • Acidosis: Increased incidences of K⁺, Mg²⁺, and Ca²⁺.

    • Alkalosis: Decreased incidences of these ions.

Respiratory Buffer

  • Acts as the second line of defense against pH changes.

  • Adjusts respiration rate in response to CO₂ levels in the blood.

  • Chemoreceptors in the medulla are activated when CO₂ increases (causing pH to fall), leading to increased respiration to lower CO₂ and restore normal pH.

  • Conversely, if pH increases, respirations decrease to retain CO₂, correcting the imbalance.

Lungs and Acid-Base Balance

  • Control Mechanisms:

    • Blow off CO₂ (hyperventilation elevates pH).

    • Retain CO₂ (hypoventilation decreases pH).

  • Normal CO₂ levels range from 35 to 45 mmHg.

Renal Buffer

  • Kidneys regulate bicarbonate (HCO₃⁻), reflecting the metabolic component of acid-base balance.

  • The renal system provides a robust defense against pH fluctuations over hours to days.

  • Kidneys can reabsorb acids and bases or excrete them in urine, resulting in acidic or alkaline urine.

  • Normal bicarbonate levels are between 22-26 mEq/L.

Normal Values for Arterial Blood

Parameter

Arterial Blood

Mixed Venous Blood

pH

7.35–7.45

7.32–7.42

PCO₂

35–45 mm Hg

38–52 mm Hg

PO₂

>80 mm Hg

24–48 mm Hg

HCO₃⁻

22–26 mEq/L

19–25 mEq/L

Base Excess/Deficit

±2 mEq/L

±5 mEq/L

Oxygen saturation (SaO₂%)

>94%

65–75%

Compensation Mechanisms

  • Aim to minimize pH changes:

    • Return pH to normal or near normal via the unaffected component responding.

    • The respiratory system compensates for metabolic imbalances and vice versa.

Compensation Examples

Acidosis (pH low)

  • Metabolic Acidosis:

    • Increase respiration to expel CO₂.

  • Respiratory Acidosis:

    • Kidneys reabsorb HCO₃⁻ and excrete H⁺.

Alkalosis (pH high)

  • Metabolic Alkalosis:

    • Decrease respiration to retain CO₂.

  • Respiratory Alkalosis:

    • Kidneys excrete more HCO₃⁻ and retain more H⁺.

Practice Questions

  1. A nurse views lab results for a patient with a respiratory illness. Which lab indicates lung contribution to acid-base balance?

    • Options: WBC, RBC, CO₂, BNP.

  2. For a patient with trauma-induced respiratory acidosis, why is compensation absent?

    • A. Kidney trauma

    • B. Time for renal compensation

    • C. Fluid excess

    • D. Hypoxemia.

Comparing Acidosis and Alkalosis

Acidosis

  • Gain H⁺ or Loss HCO₃

    • pH ↓

    • HCO₃ ↓

    • H⁺ ↑

  • Respiratory compensation:

    • Hyperventilation leads to ↑ PCO₂.

  • Renal compensation:

    • Conserve HCO₃⁻, excrete H⁺.

Alkalosis

  • Loss H⁺ or Gain HCO₃

    • pH ↑

    • HCO₃ ↑

    • H⁺ ↓

  • Respiratory compensation:

    • Hypoventilation leads to ↓ PCO₂.

  • Renal compensation:

    • Excrete HCO₃⁻, conserve H⁺.

Compensation Overview

  • Respiratory Compensation:

    • Corrects metabolic imbalances.

    • Increases respirations for metabolic acidosis and decreases for metabolic alkalosis.

  • Metabolic Compensation:

    • Corrects respiratory imbalances.

    • Kidneys adjust HCO₃⁻ and H⁺ according to the imbalance.

Understanding Arterial Blood Gases (ABGs)

Analyzing ABGs

  • Assesses: pH, O₂, CO₂, HCO₃, O₂ saturation.

  • Procedure for collection:

    • Arterial puncture with a heparinized syringe, transport to the lab promptly.

    • Hold pressure at the site post-collection.

Modified Allen's Test

  • Method to assess collateral circulation prior to sampling the radial artery.

    • Patient clenches fist while both radial and ulnar arteries are occluded.

    • Upon opening the hand, the color change indicates perfusion.

Normal Ranges for ABGs

Parameter

Normal Range

pH

7.35-7.45

PaO₂

80-100

PaCO₂

35-45 mm Hg

HCO₃⁻

22-26 mEq/L

Reading ABGs

  • pH and CO₂ move inversely.

    • As acid (CO₂) rises, pH falls.

  • pH and HCO₃ move together.

    • Increase in base (HCO₃) results in an increase in pH.

Respiratory Acid Base Imbalances

Respiratory Acidosis

  • Cause: Inadequate ventilation leading to CO₂ retention (hypercapnia).

  • Acute Causes:

    • Airway obstruction, cardiac/respiratory arrest, neuromuscular disorders, mechanical ventilation issues.

  • Chronic Causes:

    • COPD, sleep apnea, obesity.

Signs and Symptoms

  • Headache, decreased level of consciousness, hypoventilation, hyperkalemia, cardiac dysrhythmias, severe cases lead to hypotension.

Diagnostic Findings

  • ABGs:

    • pH decreased below 7.35, PCO₂ increased above 45 mm Hg.

    • Kidneys start to compensate by retaining bicarbonate and excreting H⁺ after 2-3 days.

Medical Management Goals

  • Improve ventilation:

    • Use supplemental O₂ or mechanical ventilation.

    • Administer bronchodilators, antibiotics, and medications for hyperkalemia.

    • Provide pulmonary hygiene, hydration, and avoid sedatives.

Respiratory Alkalosis

  • Cause: Hyperventilation resulting in excessive CO₂ expulsion.

  • ABGs:

    • pH > 7.45, PCO₂ < 35 mm Hg.

  • Associated electrolytes: decreased K⁺ and Ca²⁺.

Signs and Symptoms

  • Rapid respiratory rate, light-headedness, difficulty concentrating, numbness and tingling in extremities, tachycardia, potential for carpopedal spasms.

Diagnostic Findings

  • ABGs: pH above 7.45, PCO₂ below 35 mm Hg.

    • Kidneys attempt to compensate but take longer by excreting bicarbonate.

Medical Management Steps

  • Monitor vital signs and ABGs, teach controlled respiration, reduce stimuli, use re-breather masks, provide sedation if necessary, and adjust ventilator settings.

Metabolic Acid Base Disorders

Metabolic Acidosis

  • Cause: Loss of bicarbonate, accumulation of metabolic acids, or both.

  • ABGs:

    • pH < 7.35, HCO₃ < 22 mEq/L.

Causes

  • Loss of bicarbonate due to diarrhea, renal impairment, ketoacidosis, drug toxicity.

Signs and Symptoms

  • Kussmaul breathing, gastrointestinal distress, malaise, hypotension, potential for cardiac arrhythmias, altered consciousness leading to coma or death.

Diagnostic Findings

  • High potassium levels due to H⁺ shifts, leading to hyperkalemia.

    • Normal anion gap is 8-12 mEq/L.

Medical Management Goals

  • Address underlying causes: use insulin for diabetic conditions, dialyze for renal failure, administer antidiarrheals, IV HCO₃, manage fluids, and intubation if necessary.

Metabolic Alkalosis

  • Cause: Accumulation of bicarbonate or loss of H⁺.

  • ABGs:

    • pH > 7.45, HCO₃ > 26 mEq/L.

Causes

  • Loss of K⁺ through diuretic use, excessive vomiting, and certain medications.

Signs and Symptoms

  • Neurological disturbances such as tingling, tachycardia, respiratory depression, and alterations in consciousness.

Diagnostic Findings

  • EKG changes, decreased potassium and calcium levels.

Management Steps

  • Remove the cause, using antiemetics, electrolyte management, NG irrigation, IV fluids, oxygen supplementation, and monitoring labs and vital signs.

Summary of Acid-Base Disorders

Disorder

Primary Cause

Compensation Mechanism

Effect on ABGs

Metabolic Acidosis

Excess nonvolatile acids; bicarbonate deficiency

Rate and depth of respiration increase, eliminating CO₂

↓ pH, ↓ HCO₃, ↓ PaCO₂

Metabolic Alkalosis

Bicarbonate excess

Rate and depth of respiration decreases; CO₂ is retained

↑ pH, ↑ HCO₃, ↑ PaCO₂

Respiratory Acidosis

Retained CO₂ and excess carbonic acid

Kidneys conserve bicarbonate to restore carbonic acid: bicarbonate ratio

↓ pH, ↑ PCO₂, ↑ HCO₃

Respiratory Alkalosis

Loss of CO₂ and deficient carbonic acid

Kidneys excrete bicarbonate and conserve H⁺ to restore carbonic acid: bicarbonate ratio

↑ pH, ↓ PCO₂, ↓ HCO₃

Reading and Interpreting ABGs

Steps to Analyze ABGs

  1. Determine acidosis or alkalosis using pH.

    • <7.35: acidosis, 7.35-7.45: normal, >7.45: alkalosis.

  2. Assess pCO₂ to identify respiratory effects.

    • <35: alkalosis, 35-45: normal, >45: acidosis.

  3. Assume metabolic cause if respiratory is ruled out.

  4. Use HCO₃ levels to confirm metabolic effects.

    • <22 indicates metabolic acidosis, >26 indicates metabolic alkalosis.

ABG Interpretation Mnemonic

  • ROME:

    • Respiratory = Opposite (pH and pCO₂).

    • Metabolic = Equal (pH and HCO₃).

Case Studies and Application

  • Utilizing the foundational knowledge of acid-base balance, students are prompted to engage with various patient scenarios to interpret ABGs, identify potential acid-base imbalances, and formulate nursing interventions.