Understanding AcidBase

1
Understanding Acid-Base
2
Lecture Objectives

• Identify Acids and Bases
• Calculate Henderson-Hasselbach
• Identify major electrolyte disorders
• Regulation of acid-base
• Interpretation of blood gases

3
Acids

• Ionize in solution
• Produce H ions
• The more H ions produced the stronger the acid
• Acids are proton (H) donors

4
Bases

• Ionize in solution
• Produce OH ions
• The more OH ions produced the stronger the base
• Bases are proton (H) acceptors

5
Buffers

• Limit extreme changes in the H and the OH
  concentration

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Intracellular Buffers

• proteins
• polypeptides

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Extracellular Buffers

• hemoglobin
• plasma proteins
• bicarbonate

8
pH is the negative logarithm of the hydrogen ion
concentration

• Or the power of the H ion

9
Conversion of H to pH

• Nanomoles
• pH of 7.4 40 nmol H / L
• each change in nanomoles by 2 shifts log units
  by 0.3
• i.e 40 nmol to 80 nmol 7.4 to 7.1 pH
• 100 nmol to 50 nmol 7.0 to 7.3 pH

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Normal pH values

• Arterial blood 7.35 - 7.45
• Venous blood 7.32 - 7.42

11
Determining pH

• Henderson - Hasselbach equation
• pH 7.4 HCO3- 24 mmol
• pK 6.1 PaCO2 40 torr
• H2CO3 0.03mmol
• pH pK log HCO3-
• H2CO3 x PaCO2
• 7.4 6.1 log 24
• 0.03 x 40
• 7.4 6.1 log 24
• 1.2

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Simplified Henderson - Hasselbach equation

• (H) 24 x PaCO2
• HCO3
• Shows relationship between 3 major factors
• H
• CO2
• HCO3

13
Conversion factors for simplified equation

• For large changes
• H 40 mmol/L _at_ pH 7.4
• Increase in pH of 0.10 changes H to 0.8 x
  starting H concentration
• Decrease in pH of 0.1 increases H by a factor of
  1.25

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Conversion factors for simplified equation

• For small changes
• Decrease pH 0.01 increase H by 1mmol / L
• Increase pH 0.01 decrease H by 1mmol / L

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Physiological Response to increase in H acid load

• Buffer
• Compensation
• Correction

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Adjusting pH

• Buffers
• Respiratory mechanisms
• Renal mechanisms

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Major Buffers

• Bicarbonate 50 of buffers
• Hemoglobin 35 of buffers
• Plasma proteins 6 of buffers

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Bicarbonate Carbonic acid

• Normal ratio of
• bicarbonate carbonic acid 20 1
• Addition of H drives the equation to the right
  with increased CO2

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Renal Buffer System

• Excretes hydrogen ions
• Produces and reabsorbs bicarbonate

20
Renal Tubule Cells

• H2O CO2 form H2CO3
• H2CO3 ionizes to H and HCO3
• H excreted in exchange for Na
• NaHCO3 reabsorbed and H combines with NH3 to
  form NH4 which is excreted

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Renal Effects

• Hydrogen is excreted
• Bicarbonate is formed and reabsorbed

22
Protein Buffers

• Hemoglobin is the most important
• Represents only 6 of total buffers

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The Anion Gap

• The anion gap is the difference between the
  measured anions and cations.
• Anion gap Na - (Cl HCO3)
• Normal 12 2
• gt14 consistent with met acidosis
• gt30 severe organic acidosis

24
Four acid-base Abnormalities

• Metabolic alkalosis
• Metabolic acidosis
• Respiratory alkalosis
• Respiratory acidosis

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Metabolic Alkalosis

• pH gt 7.45
• Mortality 65 when pH gt 7.65
• Most commonly iatrogenic
• Elevated CO2 on ABG or lytes
• (normal 24 / 25)

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Causes of Metabolic Alkalosis

• NG suction
• Loop Diuretic administration
• Renal hypoperfusion
• Adrenal hyperplasia
• Bicarbonate administration

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Effects of Metabolic Alkalosis

• Decreased serum potassium
• Decreased serum ionized calcium
• Dysrhythmias
• Hypoventilation / hypoxemia
• Increased bronchial tone / atelectasis
• Left shift of the Oxygen curve

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Metabolic Acidosis

• With normal anion gap (bicarb lost)
• Diarrhea
• biliary drainage
• renal tubular acidosis

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Metabolic Acidosis

• With increased anion gap
• Lactic acid
• Ketoacidosis
• Inadequate renal excretion of H
• Ingestion of toxic quantities of acids

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Physiologic Effects of Acidosis

• Reduced cardiac contractility
• Increased PVR
• Reduced SVR
• Impaired response to catecholamines
• Compensatory hyperventilation

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Anesthetic Implications

• Drug action may be altered
• Correct fluid deficit
• Rule out
• lactic acidosis
• ketoacidosis
• Uremia
• Toxic ingestion

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Treatment of Acidosis

• Correct the problem
• Maintain ventilation
• Consider giving bicarbonate
• calculate the deficit
• give 1/2 the calculated dose
• repeat ABGs in 5-10 min.

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Bicarbonate Deficit

• Determine extracellular fluid volume
• Multiply ECF volume x meq/l deficit to determine
  the total deficit
• Give half the calculated dose

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Example

• 65 kg female Height 70 inches
• Actual bicarbonate 14
• 20 of body weight is ECF
• 65 x 0.2 13 kg of ECF
• 13 l x 10 meq/L 130 meq deficit
• Give half 65 meq

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Example

• 80 KG male Height 71 inches
• Actual bicarb 16 (deficit 8)
• 80 x 0.2 16
• 16 x 8 128
• Give half 64 meq

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Respiratory Alkalosis

• pH gt 7.45
• Hypocarbia (PaCO2 lt 35)

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Causes of Respiratory Alkalosis

• Increased alveolar ventilation
• mechanical hyperventilation
• pain / anxiety
• hypoxemia
• labor
• sepsis
• anemia

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Physiologic Effects

• All effects of metabolic alkalosis apply to
  respiratory alkalosis
• Reduced cerebral blood flow
• doubling AV halves CBF
• will return to normal after 8-24 hours

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Respiratory Acidosis

• pH lt 7.35
• hypercarbia (PaCO2 gt45)
• May be decreased AV, increased CO2 production or
  both

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Causes of Respiratory Acidosis

• Respiratory depression
• drugs
• cerebral injury
• Increased work of breathing
• airway obstruction
• neuromuscular dysfunction

41
Causes of Respiratory Acidosis

• COPD
• Pulmonary embolism
• Fever / Sepsis
• high glucose IV feedings

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Anesthetic Implications

• Acute vs Chronic?
• If acute
• Ventilate to maintain normal pH
• correction causes alkalosis
• If Chronic
• maintain physiologic pH
• note bicarbonate is rarely given

43
Setting the Ventilator
44
Carbon Dioxide is inversely proportional to
alveolar ventilation
45
ABG Interpretation

• Is the pH life threatening? If so, take
  immediate action
• Is the pH above or below 7.40?
• Is the PaCO2 above or below 40?

46
ABG Interpretation
47
Base Excess / Deficit

• Number is calculated, not measured
• Golden rule is applied
• PaCO2 is corrected to 40
• Is the new bicarbonate 40?
• Difference from 24 is the Base Excess
• 
  Base Deficit

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Golden Rules for ABGs

• pH increases 0.10 for every 10mm/hg decrease in
  PaCO2
• HCO3 decreases 2meq/l for every 10mm/hg decrease
  in PaCO2

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Golden Rules for ABGs

• Over time, pH will nearly normalize if hypocarbia
  is sustained
• Bicarbonate will decrease 5-6 meq/L for each
  chronic 10mm/hg decrease in PaCO2