Blood gasses

• Venturi system:

  • Use the Venturi effect to deliver a precise amout of oxygen
  • As the oxygen moves through the narrow part of the mask, the speed of oxygen increases and this pulls in air
  • By controlling the size of the opening, the mask can mix oxygen with room air
  • Useful for patients who need controlled oxygen therapy - COPD
• Base excess:

  • Amount of strong base which would be needed to be added or subtracted from a substance in order to return the pH to normal
  • The amount of H+ ions that would be required to return the pH of blood to 7.35 if the pCO2 were normal
  • Value outside the normal range (-2 to +2) suggests a metabolic cause for the acidosis or alkalosis
  • E.g. if bicarb 18 (22-30), the base excess would be -4, suggests metabolic acidosis
• Nasal cannula adds 4% to normal atmospheric FiO2 per 1L

  E.g. 2L from nasal cannula = 21% + 8% = 29% FiO2
• Alveolar-arterial gradient:

  • Comparison of the partial pressure of O2 in the alveoli and arterial blood
  • Helps narrow causes of hypoxemia
  • Calculated as the alveolar partial pressure of oxygen (PAO2) minus the arterial pressure of oxygen (PaO2)
  • Elevated gradient indicates that the the pressure of oxygen is higher in the alveoli than in arterial blood - V/Q mismatch
• Causes of elevated A-a gradient:

  • Dead space ventilation - pneumonia, asthma, COPD, PE
  • Left to right shunt - pulmonary oedema, ARDS, pneumonia
  • Alveolar hypoventilation - pulmonary fibrosis, ILD
• Standard face mask:

  • FiO2 30-50%
  • Flow rates 6-10 L/min
  • Imprecise delivery of oxygen
• Nonrebreather mask:

  • FiO2 80-100%
  • Flow rate 10-15 L/min
• pH buffering:

  • Maintained through buffering and excretion of acids
  • Buffers are substances that prevent changes in the pH by binding or releasing hydrogen ions
  • Buffers: sodium bicarbonate-carbonic acid, phosphate, protein
  • Hydrogen ions are excreted via the kidney and carbon dioxide excreted via the lungs
• Causes of acidosis:

  • Strenuous physical exercise
  • Obesity
  • Starvation
  • Raised lactate
  • Depressed respiratory drive
  • Pulmonary oedema
  • Cardiac arrest
  • Renal failure
  • Addison's disease
  • DKA
• Causes of alkalosis:

  • Hyperventilation
  • Anaemia
  • Shock
  • Aspirin OD
  • Hypokalaemia
  • Vomiting/diarrhoea
  • Burns (third space fluid loss)
  • Diuretics
• Increased CO2:

  • Pulmonary oedema
  • Obstructive lung disease
  • Depressed respiratory drive e.g. neuromuscular disease
• Decreased CO2:

  • Hyperventilation
  • Hypoxia
  • Anxiety
  • Pregnancy
  • PE
• V/Q mismatch is usually the cause of type 1 respiratory failure:

  • Pneumonia
  • PE
  • Pneumothorax
  • Acute asthma
  • ARDS
• FiO2 = fraction of inspired oxygen
• Kussmaul breathing is seen in DKA or other causes of metabolic acidosis where the body is trying to compensate and blow off CO2