Atrial Blood Gas Analysis

• Acid


  A substance which donates hydrogen ions when in a solution
• Base


  A substance which accepts hydrogen when in a solution
• pH


  Term used to express the acidity or alkalinity
• Buffers


  Dissolved substances that stabilise pH by either providing or removing hydrogen from a solution, they may be:

  Weak acid – which donates hydrogen

  Weak bases – which accepts hydrogen
• Buffer systems


  A buffer system consists of a weak acid and the -ve ion is released when it in the solution

  The most important one is the carbonic acid-bicarbonate buffer system in ECF

  Other buffer systems:

  Protein buffer systems

  The Phosphate buffer system
• Mechanisms of regulation


  Buffers act quickly to temporarily bind with hydrogen to remove it from solution but not from the body

  The respiratory system responds rapidly to eliminate carbonic acid in the form of carbon dioxide and water

  The metabolic system (kidneys) responds much more slowly, taking hours or days to alter acid and bicarbonate levels in the blood
• Elimination of CO2


  Eliminating CO2 by increasing ventilation provides a rapid, efficient and responsive mechanism for maintaining pH within normal limits

  Large quantities of hydrogen ions are eliminated in this way
• Elimination of metabolic acid

  Non-carbonic or 'metabolic' acid must be eliminated by the kidney using urinary buffers – primarily phosphate (HPO42-) and ammonia (NH3)

  A little free H+ is also eliminated in the urine

  It is a slower, less responsive process
• Arterial blood gas analysis

  ABGs give an indication of:

  Gaseous exchange (PaO2)

  Ventilation (PaCO2)

  Acid-base status (pH, PaCO2 and HCO3-)
• PaO2


  The partial pressure of oxygen in the arterial blood and represents the 1.5% of oxygen dissolved in the blood plasma

  Gives an indication of oxygenation

  PaO2 should be > 10 kPa breathing air

  PaO2 below 10 kPa indicates hypoxaemia

  A PaO2 below 8 kPa defines respiratory failure
• pH


  Reflects the acid-base balance

  Responds to respiratory and metabolic changes

  Normal pH range is between 7.35 – 7.45

  If the H+ concentration is high the pH will be low < 7.35 (acidic)

  If the H+ concentration is low the pH will be high >7.45 (alkalotic)
• Base Excess


  The base excess (or deficit) is calculated by measuring the amount of strong acid or base that needs to be added to a sample of blood under the precise conditions (37°C, PaCO2 5.3 kPa) to give a pH of 7.4

  The BE increases in metabolic alkalosis and decreases (or becomes more negative) in metabolic acidosis
• PaCO2 and Standard Bicarbonate HCO3-

  PaCO2 is the partial pressure of carbon dioxide in arterial blood and reflects alveolar ventilation

  HCO3- is the standard bicarbonate and reflects the concentration of bicarbonate in plasma under standard conditions: 37°C, PaCO2 5.3 kPa
• Primary acid-base disorders


  • Respiratory acidosis

  Respiratory alkalosis

  Metabolic acidosis

  Metabolic alkalosis
• Respiratory acidosis


  Characterised by an increased PaCO2 and a decreased pH

  Caused by alveolar hypoventilation
• Respiratory alkalosis


  Characterised by a decreased PaCO2 and an increased pH

  Caused by alveolar hyperventilation
• Metabolic acidosis


  Characterised by a decreased bicarbonate concentration and a decreased pH

  Caused by the accumulation of metabolic acids or a loss of bicarbonate
• Metabolic alkalosis


  Characterised by an increased bicarbonate and an increased pH

  Caused by excess bicarbonate or loss of acid
• Compensation


  Respiratory disturbances are compensated by the kidney

  Metabolic disturbances are compensated by the respiratory system
• Degrees 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
• Partially compensated respiratory acidosis

  If the pH is not within normal limits and both carbon dioxide and bicarbonate are out of range, any compensation is PARTIAL
• Fully compensated respiratory acidosis

  If the pH is within normal limits and both carbon dioxide and bicarbonate are out of range, any compensation is FULL