CLINICAL UNDERSTANDING OF ACIDBASE DISORDERS

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CLINICAL UNDERSTANDING OF ACID-BASE DISORDERS

• Gregory L. Burke PA-C

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pH Scale

• pH - based on H, pH and H inversely
  related
• ?H ?pH more acidic
• ?H ?pH more alkaline
• Normal 7.4 (7.35-7.45)

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Acids

• Substance containing 1 or more H ions
  (protons)that can be liberated into solution.
• A.K.A. a proton donor
• Two types of acids are formed by metabolic
  processes
• Volatile acids liquid ? gas. CO2 eliminated by
  lungs.
• CO2 H2O ?H2CO3 ? H HCO3-
• Nonvolatile or fixed acids cannot be converted
  to a gas and subsequently must be converted or
  eliminated by the kidneys
• Examples SO4, PO4, lactic acid, ketoacids
• The non-volatile portion is trivial when compared
  to the volatile CO2.

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Bases

• Substance that can capture or combine with
  hydrogen ions to form a solution.
• A.K.A. a proton acceptor
• Example HCO3- (bicarbonate)

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Buffers

• Chemical substance that minimizes the pH change
  in a solution caused by the addition of either an
  acid or base.
• There are four main buffer systems in the body
• Bicarbonate buffer system. (the MAIN one) 64
• NaHCO3 ? H2CO3
• Hemoglobin buffer system. 29
• HbO2- ? HHb
• Protein buffer system. 6
• Pr- ? HPr
• Phosphate buffer system. 1
• Na2HPO4 ? NaHPO4

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Regulation of Extracellular Fluid (ECF) pH

• Acids and bases continually enter the body via
  breakdown of ingested substances, normal body
  metabolism, IVFs, etc.
• Compensation must occur to keep the pH normal.
• Three regulatory mechanisms exist
• Buffer systems immediate (HCO3-)
• Respiratory control CO2 elimination or
  retention.
• Rapid (minutes)
• Renal regulation Bicarbonate level regulation.
• Kidneys can excrete H and/or retain/reabsorb
  HCO3- as needed.
• Slow (hours to days).

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HENDERSON-HASSELBACH EQUATION

• pH pKa log base (HCO3-)
• acid (H2CO3)
• pH HCO3- 20 (kidneys)
• PaCO2 1 (lungs)

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Semi-Logarithmic Relationship between pH and H
H
pH
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Acid/Base Terminology

• Acidemia /Acidosis
• Blood pH lt 7.4 (7.35)
• Caused by
• Loss of base
• Increase of acid
• Lowers the 201 ratio
• Less base/More acid

• Alkalemia /Alkalosis
• Blood pH gt 7.4 (7.45)
• Caused by
• Increase of base
• Loss of acid
• Raises the 201 ratio
• More base/Less acid

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pH
7.4
7.6
7.2
acidemia
alkalemia
HCO3-
CO2
Respiratory Component (acid)
Metabolic Component (base)
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Primary Acid-Base Imbalances

• Primary acid base imbalances occur when a single
  physiologic mechanism is not functioning properly
  or is overwhelmed.
• Primary respiratory acidosis
• Primary respiratory alkalosis
• Primary metabolic acidosis
• Primary metabolic alkalosis

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Primary Acid-Base Imbalances Respiratory Acidosis
or Alkalosis

• The normal process of CO2 excretion depends on
  three factors
• Ventilation depends on both the nervous system
  (chemoreceptors and respiratory center) and the
  mechanical aspects of respiration.
• Diffusion the CO2 has to cross the semipermeable
  alveolar membrane into the airspace and exit the
  body.
• Perfusion the blood supply must be delivered to
  the lungs for the CO2 to interface with the
  alveolar membrane.

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Primary Acid-Base Imbalances Respiratory Acidosis
or Alkalosis

• Respiratory imbalances are those in which the
  primary disturbance is in the concentration of
  CO2.
• Think lungs, heart or CNS!
• Respiratory acidosis an increase in the PaCO2
• CO2 retention, hypercapnia, alveolar
  hypoventilation
• Respiratory alkalosis a decrease in the PaCO2
• excess CO2 elimination, hyperventilation,
  hypocapnea

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Primary Acid-Base Imbalances Metabolic Acidosis
or Alkalosis

• Metabolic imbalances are those in which the
  primary disturbance is in the concentration of
  bicarbonate.
• Think kidney!
• Metabolic acidosis a 1o decrease in the HCO3-
• Metabolic alkalosis a 1o increase in the HCO3-

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Common causes of Respiratory Acidosis

• Inhibition of the Respiratory Center
• Drugs opiate, sedatives, anesthetic overdose
• Oxygen therapy in chronic hypercapnia
• Cardiac arrest
• Sleep apnea
• Chest Wall and Respiratory Muscle Disorders
• Neuromuscular disease myasthenia gravis,
  Guillian-Barre syndrome, polio, ALS
• Chest cage deformity kyphoscoliosis
• Extreme obesity
• Chest wall injury such as fractured ribs

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Common causes of Respiratory Acidosis

• Disorders of Gas Exchange
• COPD
• End -stage diffuse intrinsic pulmonary disease
• Severe pneumonia or asthma
• Acute pulmonary edema
• Hemo/pneumothorax
• Acute Upper Airway Obstruction
• Aspiration of foreign body or vomitus
• Laryngospasm or laryngeal edema
• Severe bronchospasm

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Common causes of Respiratory Alkalosis

• Central Stimulation of Respiration
• Psychogenic hyperventilation caused by emotional
  stress
• Hypermetabolic states fever, thyrotoxicosis
• CNS disorders
• Head trauma or CVA
• Brain tumors
• Hypoxia
• Pulmonary edema
• Congestive heart failure
• Pulmonary fibrosis
• High altitude residence
• Excessive Mechanical Ventilation
• Uncertain Gram-negative sepsis, Hepatic
  cirrhosis
• Exercise

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Primary Acid-Base Imbalances Metabolic Acidosis

• Caused by a primary decrease in in plasma HCO3-
• Generated by either a gain of acid or loss of
  HCO3-
• Usually accompanied by a K depletion which must
  be corrected along with the acidosis
• Typically classified according to whether or not
  there is an increased anion gap (AG)

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Anion Gap (AG)

• AG is a measure of the relative abundance of
  unmeasured anions.
• Used to evaluate patients with metabolic
  acidosis.
• High AG metabolic acidosis is due to the
  accumulation of H plus an unmeasured anion in
  the ECF.
• Most likely caused by organic acid accumulation
  or renal failure with impaired H excretion.
• Normal AG metabolic acidosis is caused by the
  loss of HCO3- which is counterbalanced by the
  gain of Cl- (measured cation) to maintain
  electrical neutrality.
• Most likely caused by HCO3- wasting from diarrhea
  or urinary losses in early renal failure.

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Determinants of the Anion GapAG UA - UC
Na-(Cl- HCO3-)

• Unmeasured Anions
• Proteins (15 mEq/L)
• Organic Acids (5 mEq/L)
• Phosphates (2 mEq/L)
• Sulfates (1mEq/L)
• UA 23 mEq/L

• Unmeasured Cations
• Calcium (5 mEq/L)
• Potassium (4.5 mEq/L)
• Magnesium (1.5 mEq/L)
• UC 11 mEq/L

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Common Causes of Metabolic Acidosis Increased
Anion Gap(ThinkMUDPILES)

• Methanol intoxication
• Uremic acidosis (advanced renal failure)
• Diabetic ketoacidosis
• Paraldehyde intoxication
• Iron overdose
• L-lactic acidosis
• Ethylene glycol intoxication
• Salicylate intoxication
• D-lactic acidosis
• Alcoholic ketoacidosis
• Denotes most common

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Common Causes of Metabolic Acidosis Normal anion
gap

• Mild to moderate renal failure
• Gastrointestinal loss of HCO3- (acute diarrhea)
• Type I (distal) renal tubular acidosis
• Type II (proximal) renal tubular acidosis
• Dilutional acidosis
• Treatment of diabetic renal tubular acidosis
• Ketones lost in urine
• Denotes most common

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Common causes of Metabolic Alkalosis

• Net loss of H from the ECF
• G.I. Loss
• Vomiting or nasogastric suctioning
• Chloride losing diarrhea chronic
  diarrhea/laxative abuse
• Renal loss
• Loop or thiazide type diuretics esp. in CHF and
  cirrhosis
• Mineralocorticoid excess
• Hyperaldosteronism
• Cushings syndrome

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Common causes of Metabolic Alkalosis

• Retention of HCO3-
• Excess administration of NaHCO3
• Milk-alkali syndrome antacids, milk, NaHCO3
• Massive (gt8 units) blood transfusion (citrate)
• Posthypercapnia metabolic alkalosis (after
  correction of chronic respiratory acidosis)

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Mixed acid-base disordersTwo or more simple
acid-base disorders coexist

• Metabolic acidosis Respiratory Acidosis
• pH usually very low
• Pa CO2 too high
• HCO3- too low
• Metabolic Alkalosis Respiratory Alkalosis
• pH usually very high
• Pa CO2 too low
• HCO3- too high

• Metabolic Acidosis Respiratory Alkalosis
• pH may be near normal
• Pa CO2 too low
• HCO3- too low
• Metabolic Alkalosis Respiratory Acidosis
• pH may be near normal
• Pa CO2 too high
• HCO3- too high

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