ventilation

see also:

• hypercapnia
• assisted ventilation
  • non-invasive ventilation
  • non-invasive positive pressure ventilation (NIPPV)
  • noninvasive positive pressure ventilation (NPPV or NIPPV)
  • Intermittent Mandatory Ventilation (IMV)
  • Assist-Control Ventilation (ACV)
  • pathophysiology of mechanical ventilation

• respiratory ventilation is generally closely controlled to adjust breathing rate and depth to keep oxygen, carbon dioxide, and, blood and CSF pH stable
  • inadequate ventilation results in hypercapnia and respiratory acidosis
  • excessive ventilation (hyperventilation) results in low carbon dioxide levels, resp_alkalosis and electrolyte shifts causing low phosphate levels, hypokalaemia, etc

• this estimates an alveolar gas partial pressure of oxygen given a specified inspired oxygen and a blood carbon dioxide as a marker of ventilation
• this equation is often used to ascertain the A-a gradient which is the calculated PAO₂ - actual arterial PaO₂ (normal for a healthy adult is 5-10mmHg)
• where:
  • PAO₂: Alveolar partial pressure of oxygen (mmHg)
  • FiO₂: Fraction of inspired oxygen (typically 0.21 for room air)
  • P_atm: Atmospheric barometric pressure (typically 760 mmHg at sea level)
  • PH₂O: Water vapor pressure (typically 47 mmHg at body temperature)
  • PaCO₂: Arterial partial pressure of carbon dioxide
  • RQ: Respiratory quotient (ratio of CO₂ produced to O₂ consumed, typically 0.8)

PAO₂ = (FiO₂ x (P_atm - PH₂O) ) - (PaCO₂/RQ) which approximates to = 150 - (PaCO₂/0.8) for room air

A-a gradient = calculated PAO₂ from the alveolar gas equation - actual arterial PaO₂

• normal ranges vary with age and FiO2:
  • in room air, generally increases by 1mmHg for every 10 yrs of age
  • for a healthy adult is 5-10mmHg in room air (may be 14mmHg in the elderly)
  • on 100% inspired oxygen, for a healthy adult is about 31mmHg (may be 56mmHg in the elderly)

• hypoxia with normal A-a gradient suggests either:
  • raised PaCO₂:
    • hypoventilation with hypercapnia
  • normal or low PaCO₂
    • low inspired oxygen pressures
      • high altitude or hypoxic environment
• hypoxia with raised A-a gradient suggests either:
  • V/Q mismatch eg. pulmonary embolism (PE), asthma
  • right to left shunt eg. intracardiac, pulmonary
  • increased oxygen extraction
    • elevated metabolic activity
      • physical exercise
      • hyperthyroidism
    • decreased oxygen delivery
      • anaemia
      • low blood flow to tissues eg. the shocked patient
  • diffusion defect (less common)

• medulla oblongata:
  • contains the dorsal respiratory group (DRG) for inspiration and the ventral respiratory group (VRG) for rhythm and forced breathing.
• pons:
  • fine-tunes the transition between breathing in and out via the apneustic and pneumotaxic centers

• Central chemoreceptors:
  • located in the medulla; sense changes in brain fluid pH caused by carbon dioxide
• Peripheral chemoreceptors:
  • located in the carotid and aortic bodies; detect large drops in blood oxygen, high carbon dioxide, and acid levels
• Mechanoreceptors:
  • found in the lungs and joints; protect against over-inflation (Hering-Breuer reflex) and signal movement during exercise

• Respiratory Muscles:
  • signals travel down the spinal cord via the phrenic nerve to contract the diaphragm and via intercostal nerves to the intercostal muscles.
• Higher Brain Centers:
  • the cerebral cortex allows voluntary overrides for speaking, singing, or holding breath, while the hypothalamus and limbic system adjust breathing for emotions and pain.