ABGs

• PaO2


  Partial pressure of oxygen in arterial blood, represents 1.5% of oxygen dissolved in the blood (gas exchange)
• PaCO2


  Partial pressure of carbon dioxide in arterial blood, reflects alveolar ventilation and is the respiratory component of acid-base balance
• HCO3-


  Standard bicarbonate, reflects the concentration of bicarbonate in plasma under standard conditions, is the metabolic component of acid-base balance
• As the PO2 falls

  The SO2 falls
• Acid


  Substance which donates hydrogen ions in a solution
• Base


  Substance which accepts hydrogen ions in a solution
• pH


  Acidity to alkalinity scale (6.8-7.7 is compatible with life)
• Buffers


  Dissolved substances which stabilise pH by either donating or accepting H+
• Base excess


  Amount of strong acid or base needed to give pH 7.4
• Buffer systems


  1. A weak acid donates a negative ion to make the pH more acidic

  2. Proteins regulate intra and extracellular fluid pH- including haemoglobin in RBC

  3. Phosphates regulate intracellular fluid and urine
• Carbonic acid-bicarbonate buffer


  Most important buffer in extracellular fluid
• Carbonic acid-bicarbonate buffer


  CO2 + H2O H ⟺ 2CO3 ⟺ H+ + HCO3-
• Buffers


  • Act quickly to temporarily bind with H+ to remove it from solution but not from the body
• Respiratory System

  The lungs respond rapidly to eliminate carbonic acid (H+) in the form of carbon dioxide and water. Carbon dioxide may also be retained to increase H+ concentration.
• Metabolic System


  The kidneys respond much more slowly, taking hours or days to alter acid and bicarbonate levels in the blood.
• CO2 is in equilibrium with carbonic acid. Eliminating CO2 by ventilation therefore maintains pH levels.
• 4 main acid-base disorders

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


  Low pH, PaCO2
• Respiratory acidosis


  • Alveolar hypoventilation- low tidal vol and RR
  • Causes- coma, drug overdose, lung disease, pneumothorax, chest injury, paralysis of respiratory muscles
• Respiratory alkalosis


  High pH, low PaCO2
• Respiratory alkalosis


  • Alveolar hyperventilation- large tidal vol and RR
  • Causes- anxiety, pain, acute asthma, pneumonia, pulmonary embolism, hyperventilation in panic attack etc.
• Metabolic acidosis


  Low pH, low HCO3-
• Metabolic acidosis


  • Low bicarbonate means less buffering
  • Causes- renal failure, tissue hypoxia, diabetes, ingested toxins or loss of bicarbonate (diarrhoea)
• Metabolic alkalosis

  High pH, low HCO3-
• Metabolic alkalosis


  • High bicarbonate means more buffering
  • Causes- loss of acid (vomiting, from kidney) or excess bicarbonate (antacids)
• Compensation

  1. Respiratory disorder= compensated by kidney

  2. Metabolic disorder= compensated by respiratory system
• Types of compensation

  • Uncompensated= abnormal pH and change in PaCO2 or bicarbonate
  • Partially compensated= abnormal pH and change in PaCO2 and bicarbonate
  • Fully compensated= normal pH and change in PaCO2 and bicarbonate
• Respiratory acidosis compensation

  If hypoventilation continues over days (3-5) the kidney excrete H+ and retain bicarbonate. That helps establish the degree o renal compensation.
• Metabolic acidosis compensation

  Ventilation increases to excrete H+
• Corrections are not usually complete so its okay for the pH to remain a little low
• Determining primary disorder

  If both PaCO2 and HCO3 are out of range, you can tell which is the primary disorder by looking if pH is alkali or acidic. The primary will be the same as the pH. So if it's acidic the primary is PaCO2 and if its alkali then it's the HCO3.
• • pH: 7.35-7.45
  • Partial pressure of oxygen (PaO2): 75 to 100 mmHg
  • Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg
  • Bicarbonate (HCO3): 22-26 mEq/L
  • Oxygen saturation (O2 Sat): 94-100%[4]