Dead space and shunt

• Ventilation perfusion mismatch


  When the proper balance is lost between ventilated alveoli and good blood flow through the lungs
• Ventilation/perfusion ratio (V/Q)


  The ratio of ventilation to perfusion
• V/Q mismatch is common and often effects our patient's ventilation and oxygenation
• Types of V/Q mismatch

  • Dead space
  • Shunt
• Dead space


  The volume of air that is inhaled but does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused

  
• Types of dead space

  • Physiologic dead space
  • Mechanical dead space

  
• Physiologic dead space


  Anatomic dead space + alveolar dead space

  
• Anatomic dead space


  The volume of gas within the conducting zone (as opposed to the transitional and respiratory zones) and includes the trachea, bronchus, bronchioles, and terminal bronchioles

  
• Alveolar dead space


  The volume of gas within unperfused alveoli (and thus not participating in gas exchange)

  
• The ratio of physiologic dead space to tidal volume is usually about 1/3
• Ventilated patient


  The depth and frequency of breathing is determined by chemoreceptors and the brainstem, as modified by a number of subjective sensations. When mechanically ventilated using a mandatory mode, the patient breathes at a rate and tidal volume that is dictated by the machine.
• Taking deep breaths more slowly (e.g. ten 500 ml breaths per minute) is more effective than taking shallow breaths quickly,(e.g. twenty 250 ml breaths per minute) Although the amount of gas per minute is the same (5 L/min) , as a large proportion of the shallow breaths is dead space, and does not allow oxygen to get into the blood
• Factors that increase dead space

  • General anesthesia
  • Anesthesia apparatus/circuit
  • Artificial airway
  • Neck extension and jaw protrusion
  • Positive pressure ventilation
  • Upright posture as opposed to supine
  • Pulmonary embolus, PA thrombosis, hemorrhage, hypotension, surgical manipulation of pulmonary artery tree
  • Emphysema
  • Age
  • Anticholinergic drugs
• Shunt


  Alveoli that are perfused, but not ventilated. Blood flowing past poorly ventilated alveoli doesn't pick up additional oxygen, lowering the total oxygen content of the arterial blood.

  
• In a healthy lung, the V/Q ratio is approximately 0.8
• Common causes of shunt

  • Pneumonia and pulmonary edema
  • Tissue trauma
  • Atelectasis
  • Mucous plugging
  • Pulmonary arteriovenous fistulas