Arterial Blood Gases

1
Arterial Blood Gases

• Acid-Base Balance

2
Components

• Arterial partial pressure of oxygen (PaO2)
• Measures the amount of oxygen dissolved in the
  plasma
• Value is directly dependent upon the alveolar
  oxygen pressure (PAO2)
• Arterial pressure of carbon dioxide (PaCO2)
• Measures the amount of CO2 dissolved in the
  plasma
• pH
• The negative logarithm of the hydrogen ion
  concentration

3
Components, cont

• Bicarbonate (HCO3-)
• Calculated from the CO2 and pH using the
  Henderson-Hasselbach equation
• Allows assessment of the metabolic component of
  acid-base balance
• Total CO2
• Reflects
• Base excess (or deficit)
• A measure of the amount of acid or alkali that
  must be added to a sample under standard
  conditions to return the pH to 7.4
• Calculated from the pH and PaCO2

4
Interpreting ABGs

• Assess pH
• normal, alkalotic, acidotic
• Assess the respiratory component
• PaCO2 gt45 respiratory acidosis
• PaCO2 lt35 respiratory alkalosis
• Assess the metabolic component
• HCO3 gt 28 metabolic alkalosis
• HCO3 lt 22 metabolic acidosis
• Determine if there is metabolic or respiratory
  compensation occurring

5
Anion Gap

• The difference between the main positive and
  negative ions
• (Na K) (Cl HCO3)
• Normal is 10-18
• Increased anion gap indicates an accumulation of
  organic acids (ketoacids, lactic acid, other
  acids)

6
Acid-Base Balance

• Intracellular enzymes function best when the pH
  is 7.25 7.45
• Most metabolic processes produce acids
• Acid production increases with dx
• Lactic acid from ischemia/anaerobic metabolism
• Ketoacids from diabetes
• Methanol from alcohol ingestion
• Renal/respiratory/liver failure decrease acid
  removal from the body
• Loss of acid occurs with vomiting/NG suctioning
• Loss of bicarb occurs with diarrhea

7
Disorders of Acid-Base Balance

• Metabolic acidosis
• With a normal anion gap
• Renal bicarb loss
• Loss of bicarb from the gut
• Decreased renal hydrogen ion secretion
• Increased anion gap (acid accumulation)
• Lactic acidosis
• Type A sepsis, cardiac arrest, hypotension,
  methanol
• Type B insulin deficiency, decreased hepatic
  metabolism
• Ketoacidosis insulin deficiency, starvation
• Exogenous acids salicylates

8
Disorders, cont

• Metabolic alkalosis
• Hydrogen ion loss vomiting, renal loss,
  diuretics, hypokalemia, low Cl states
• Bicarb gain sodium bicarb administration,
  citrate administration
• Respiratory acidosis
• Airway obstruction, pneumonia, ARDS, PE
• Respiratory muscle weakness
• Trauma
• Respiratory depression

9
Disorders, cont

• Respiratory alkalosis
• High levels of anxiety or pain
• Altitude
• Excessive mechanical ventilation
• Respiratory stimulants/salicylate overdose
• Pulmonary embolism, asthma, edema

10
Control of acid-base balance

• The body prevents pH changes by regulating 2
  pathways for eliminating acid
• Respiratory
• Renal
• 100 times more acid equivalents are expired each
  day in the form of CO2 than are excreted by the
  kidney
• Buffers bind or release hydrogen ion according to
  the pH to limit the change in pH that occurs when
  acid is added to the blood

11
Main body buffers

• Bicarbonate
• CO2 combines with water to form carbonic acid
  which dissociates into bicarb and hydrogen ion
• CO2 H2O H2CO3 HCO3- H
• The normal ratio of bicarb to CO2 is 201as long
  as this ratio remains 201, the pH will be 7.4
• Hemoglobin
• Especially deoxygenated Hb

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Relationship between pH, PaCO2, HCO3

• When CO2 changes persist, pH is slowly corrected
  by renal compensation (retention or elimination
  of bicarb)
• Metabolic changes can be corrected by respiratory
  compensation but metabolic alkalosis is not
  compensated as it would require a drop in
  ventilation
• Mixed conditions (acidosis or alkalosis) can occur