Acid Base Balance

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Acid Base Balance

• Cheryl McConnell RN MSN

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Definitions

• Acid
• releases hydrogen ions when dissolved in water
• Base
• binds hydrogen when dissolved in water
• Buffer
• capable of binding or releasing hydrogen ions
  from body fluids

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Balance

• Lungs control carbon dioxide Respiratory
• Kidneys control bicarbonate Metabolic

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Arterial Blood Gas

• Measures
• pH Percent of H Concentration
• pCO2 Pressure of Carbon Dioxide
• pO2 Pressure of Oxygen
• Sao2 Oxygen Saturation
• BE Base Excess
• HCO3 Bicarbonate

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Why ABGs?

• Measure respiratory function
• Make changes in treatment
• Ventilator settings
• Monitor acid base balance
• For example DKA

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pH

• How acidic or base a substance is
• Normal ABG pH 7.35 7.45
• ? 7.35 is acidic (acidosis)
• ? 7.45 is base (alkalosis)
• 7.40 considered exactly normal when determining
  compensation

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pCO2

• Measurement of carbon dioxide
• Controlled by the lungs
• Has an inverse ration with pH
• If pCO2 ? then pH ?
• Normal ABG pCO2 35 45 mm Hg
• ? 35 hyperventilation or base (alkalosis)
• ? 45 hypoventilation or acidic (acidosis)

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pO2

• Measures oxygen
• carried by red blood cells
• dissolved in plasma
• Normal ABG p02 80 100 mm Hg
• ? 80 is hypoventilation (hypoxia)

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HCO3

• Measurement of amount of bicarbonate in blood
• Is the bodys buffer system
• Controlled by the kidneys (metabolic)
• Normal ABG HCO3 22 26 mEq/L
• ??? 22 acidic (acidosis)
• ? 26 base (alkalosis)

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Base Excess

• Indication of how much extra base is available to
  the body
• Normal ABG -3 to 3
• ? -3 acidic (acidosis)
• ? 3 base (alkalosis)

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Review of Normal ABGs

• pH 7.35 7.45
• pCO2 35 45
• pO2 80 100
• HCO3 22 26
• BE -3 to 3
• O2 Sat ? 95

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Metabolic Acidosis

• Excess acid or reduced bicarbonate production
• Causes
• Starvation/fasting
• DKA
• Renal failure
• Diarrhea
• Shock

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Signs and Symptoms for Metabolic Acidosis

• Anorexia, N/V
• Fatigue
• Weakness, ? DTRs
• Warm, flushed skin and mucous membranes
• Tachycardia
• Kussmauls respirations (hyperventilation)
• ? LOC ? confusion ? stupor ? coma
• ?? K, possible ? Mg

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Treatment for Metabolic Acidosis

• Hydration
• Correct the cause
• Insulin
• Antidiarrheal medications
• Dialysis
• Sodium bicarbonate not routinely given

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Metabolic Alkalosis

• Excess base production
• Causes
• Vomiting/NG suctioning
• Medications
• TPN
• Blood transfusions

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Signs and Symptoms of Metabolic Alkalosis

• Confusion, ? LOC
• Seizures
• Hypotension
• Tingling numbness
• Shallow slowed breathing to respiratory failure
• Hallmark sign
• Increased bicarbonate level with rising pCO2

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Treatment for Metabolic Alkalosis

• Treat underlying cause
• Antiemetic medications
• Monitor electrolytes and replace as indicated
• KCL (? K Cl)
• Potassium sparing diuretic
• Seizure precautions

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Respiratory Acidosis

• Lungs fail to eliminate CO2
• Causes
• Respiratory Depression
• Chest or head trauma
• Neurological disorders
• Inadequate chest expansion
• Airway obstruction
• Alveolar disorders
• COPD

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Signs and Symptoms of Acute Respiratory Acidosis

• H/A, irritability,
• Confusion, ? LOC
• Weakness, ? DTRs
• Tachycardia
• Ineffective respiratory efforts
• ? K in acute respiratory acidosis

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Chronic Respiratory Acidosis

• Weakness
• Dull H/A
• Sleep disturbances
• Impaired memory
• Personality changes
• Hypokalemia

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Treatment for Respiratory Acidosis

• Maintain patent airway with enhanced gas exchange
• Meds
• Oxygen
• Pulmonary toilet
• Ventilatory support
• Frequent ABGs, O2 sats, etc.
• Assess LOC frequently
• Diet Low carbohydrate, high fat

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Respiratory Alkalosis

• Lungs eliminate too much CO2
• Causes
• Anxiety
• Hypoxemia
• Metabolic triggers
• Mechanical ventilation
• CNS stimulation

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Signs and Symptoms of Respiratory Alkalosis

• Rapid, deep respirations
• Anxiety, lightheadedness
• Tetany
• Chvosteks sign, Trousseaus sign
• Paresthesia
• Tachycardia
• Pale to cyanotic
• ?? K, ?Ca

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Treatment for Respiratory Alkalosis

• Watch for respiratory failure
• Decrease respiratory rate
• Focused breathing
• Decrease breaths per minute on ventilator
• Oxygen therapy
• Antianxiety medications

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Interpreting ABGs

• 1. Identify whether pH, pCO2, and HCO3 are
  abnormal
• pH 7.30
• pCO2 55
• HCO3 26
• Match like disorders
• pH and pCO2 are both
• So the patient has an acid problem
• In other words, the two matching values tell
  what the problem is Acidosis or Alkalosis

• Acid
• Acid
• Normal
• Acid

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Interpretation Continued

• Determine if abnormality is due to the kidneys
  (metabolic ) or the lungs (respiratory)
• pH 7.30
• pCO2 55
• HCO3 26
• Match the like abnormalities
• Respiratory (lung problem) and Acidosis
• Respiratory Acidosis

• Acid
• Acid LUNGS
• Normal Kidneys

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Try this one

• pH 7.50
• pCO2 21
• pO2 84
• HCO3 24

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Name That Gas

• pH 7.31
• pCO2 39
• PO2 90
• HCO3 17

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What Do You Think?

• pH 7.38
• pCO2 44
• pO2 96
• HCO3 25

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One More

• pH 7.22
• pCO2 60
• pO2 60
• HCO3 25

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How the Body Compensates

• Body attempts to maintain normal pH by
• Lungs
• Very sensitive and can compensate quickly by
  changing respiratory rate
• But tires easily so cant continue long term
• ?? pH (acid) respiratory rate?? to blow off
  CO2 (acid)

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More Compensation

• Kidneys
• More powerful but slower (hours to days)
• Changes the amount of hydrogen excreted and
  bicarbonate retained
• ? pH (acid) kidneys excrete more H and retain
  more bicarbonate so HCO3 ?

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Compensation

• The goal is to try to maintain normal pH
• If problem is with lungs then kidneys try to
  compensate
• If problem is with kidneys then lungs try to
  compensate

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How Do You Know If Compensation Is Occurring?

• If the compensation occurs fully, the pH returns
  to normal
• If the compensating organ cant work hard enough
  to compensate, the pH will still be abnormal

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Rules of Compensation

• pH will now have one value (7.40) instead of the
  range of 7.35 to 7.40
• Use the same rules as with regular interpretation
• Identify if each value is acid or alkaline
• Match like abnormalities to interpret ABG

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• Look at value that doesnt match
• If abnormal, the organ is trying to compensate
• Re-look at pH using the range of 7.35 to 7.45
• If in normal range - complete compensation has
  occurred
• If out of normal range partial compensation has
  occurred
• If the unmatched value is normal no
  compensation has occurred

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An Example of Compensation

• pH 7.38
• pCO2 56
• pO2 70
• HCO3 35
• What is happening?

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Try This One

• pH 7.42
• pCO2 50
• pO2 90
• HCO3 39

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What Do You Think?

• pH 7.29
• pCO2 67
• pO2 56
• HCO3 37

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What About This One?

• pH 7.49
• pCO2 51
• pO2 91
• HCO3 39