Resp – Capnography

Master answer

Mixed venous blood

• Mixture of all venous blood from all tissue capillary beds.
• Sampled from pulmonary artery
• In reality, omits venous blood from physiological shunt

PCO₂

• Small portion of CO₂ carriage in blood – approx. 5%
  • The rest being HCO₃ (approx. 90%), Carbamino compounds (approx. 5%), and carbonic acid (negligable)
• Constitutes 10% of A-V difference in CO₂ carriage
  • The rest being HCO₃ (approx. 60%) and Carbamino compounds (approx. 30%)
• Obeys Henry’s Law
  • Mass of dissolved gas is proportional to its partial pressure
  • Solubility coefficient of CO₂ ~0.54
• Obeys Dalton’s Law
  • Sum of all partial pressures equals the environmental atmospheric pressure

P_MVCO₂

• Sum of P_aCO₂ and dissolved portion of CO₂ produced
• P_aCO₂
  • In ideal system equal to P_ACO₂ – 10% shunt only increases CO₂ by 0.7mmHg
  • P_ACO₂ = (CO₂ delivery to lung)/(Alveolar ventilation)
  • Alveolar ventilation = (V_t – V_d) x RR
    • V_t = Tidal volume – Approx. 7ml/kg
    • V_d = Dead space volume
      • Anatomical deadspace = first 16 divisions of airway –  approx 2ml/kg
      • Alveolar deadspace – Volume of non-perfused lung
        • Increases with
          • West zone 1
          • PE
          • Decreased lung perfusion
          • Decreased cardiac output
          • Positive pressure ventilation
          • Upright posture
    • RR – Regulated by medullary respiratory centre
      • Chemoreceptor
        • Peripheral
          • Sensitive to decreased pO₂, decreased pH, decreased blood flow and increased pCO₂
          • Increases respiratory rate
        • Central
          • Sensitive to CO₂ via conversion to [H⁺]
          • Increases respiratory rate
      • Baroreceptor
        • Decreased stretch of aortic and carotid baroreceptors increases respiratory rate
      • Pulmonary receptors
        • J fibres
          • Stimulation causes apnoea, bronchoconstriction, bradycardia and hypotention
        • Stretch receptors
          • Inflation reflex – Inhibits inspiration on lung inflation
          • Deflation reflex – Inhibits expiration on lung deflation
• CO₂ production
  • Occurs predominantly in mitochondria
    • From metabolism of glucose
      • Glucose + 6O₂ → 6CO₂ + 6H₂O
    • From
      • BMR – 40kcal/hr/m²
      • Increased muscle activity
      • Post-prandial metabolism
      • Thyroid hormones increase metabolism
      • Catecholamines increase metabolism
      • Disease states
        • Fever
        • Malignant hyperthermia
        • Tourniquet
      • Relative perfusion of tissues
        • Blood from highly metabolic tissues contributes relatively greater amounts of CO₂ per weight
        • Blood from less metabolic tissues contributes relatively less CO₂ per weight

Sakurai 2016

Answer by Sakurai