Preoxygenation and O2 flux

• Preoxygenation

  Administration of oxygen to a patient prior to intubation to extend the safe apnoea time
• Preoxygenation


  • Primary mechanism is denitrogenation of the lungs
  • Maximal preoxygenation is achieved when the alveolar, arterial, tissue, and venous compartments are all filled with oxygen
• Safe apnoea time


  Duration of time following cessation of breathing/ventilation until critical arterial desaturation occurs (typically considered SaO2 88% to 90% in clinical settings)
• Denitrogenation


  Using oxygen to wash out the nitrogen contained in lungs after breathing room air, resulting in a larger alveolar oxygen reservoir
• Oxygen consumption during apnea is approximately 250-200 mL/min (~3 mL/kg/min) in healthy adults
• Goals of Preoxygenation


  • Denitrogenation of the lungs
  • Achieve as close to SaO2 100% as possible
  • Oxygenate the plasma
• Factors that decrease safe apnoea time

  • Critical illness
  • Inadequate preoxygenation
  • Obesity
  • Pregnancy
  • Shunt physiology
  • Airway occlusion
  • Increased oxygen consumption (e.g. high metabolic rate, Fasciculations from suxamethonium)
  • Anaemia or dyshaemoglobinaemia
• Preoxygenation procedure


  1. IF adequate respiratory drive, preoxygenate by At least 3 minutes of tidal ventilations, or 8 breaths with full inspiration/ expiration to achieve vital capacity in <60 seconds (requires patient cooperation)

  2. IF inadequate respiratory drive, preoxygenate by: Positive pressure ventilation (e.g. assisted breaths with BVM) at 15 +/- 2 L/min or more
• FIO2


  The fraction of inspired oxygen during mechanical ventilation
• The FIO2 should always be set at 100% until adequate arterial oxygenation is documented
• A short period with an FIO2 of 100% is not dangerous to the patient receiving mechanical ventilation and offers the clinician several advantages
• Oxygen flux


  Amount of O2 leaving the left ventricle per minute in the arterial blood
• Cardiac output


  Stroke volume x heart rate, where stroke volume depends on preload, contractility and afterload
• SaO2


  Arterial oxygen saturation
• PaO2


  Arterial partial pressure of oxygen