Factors affecting PaCO2

Written by – Dr Madhuri Anupindi

Describe the physiological factors that affect PaCO₂

PaCO₂: partial pressure of CO2 in the arterial blood (normally 35 – 45mmHg)
PaCO₂ ≈ (V_CO2/V_A)
PaC0₂ = (V_co2 x 0.863) / V_A
= (V_co2 x 0.863) / (V_E – V_D)
V_co2: CO₂ production by the body
V_A: Alveolar ventilation
V_E: Minute ventilation
V_D: Dead space ventilation
0.863 converts the units to mmHg

Therefore factors affecting PaCO₂ are those related to CO2 production and those related to CO2 elimination. Generally, PaC02 is tightly controlled so increased production will cause an increase in ventilation.

Factors related to production:

• CO_{2 is} produced through mitochondrial aerobic metabolism via the Krebs cycle  influenced by metabolic rate and the metabolic substrate used
• ↑Metabolic rate  ↑CO₂ production :
  • ↑Temperature: malignant hyperthermia, sepsis, heat stroke
  • Exercise
  • Pregnancy
  • Hyperthyroidism
  • ↑Muscle mass
  • Male
  • Neonate
• Metabolic substrate:
  • Respiratory quotient = ratio of CO₂ produced: O₂ consumed
    • RQ of carbohydrates = 1
    • RQ of protein = 0.8
    • RQ of fats = 0.7
  • ↑Proportion of energy derived from fat will decrease CO2 production for every oxygen molecule consumed

Factors related to elimination: (alveolar ventilation)

Control of alveolar ventilation:

• Chemoreceptors:
  • Peripheral chemoreceptors – aortic and carotid bodies
    • ↓Pa0₂  ↑RR and tidal volume  ↑minute ventilation
    • Detect ↑PaCO₂ (synergistic response if also↓Pa0₂) _, ↑H⁺, ↓pH ↑minute ventilation
  • Central chemoreceptors – medulla
    • Detect ↓pH of CSF (CO₂ diffuses across blood-brain barrier from arterial blood into CSF  reacts with H₂O  H₂CO₃  ↓pH  ↑minute ventilation
  • Other causes of ↑minute ventilation:
    • Exercise: via activation of stretch receptors  stimulate respiratory centre  ↑RR
    • Pregnancy  ↑progesterone  ↓CO₂ response threshold of the respiratory centre  ↑tidal volume
    • Pain/anxiety/fear  autonomic inputs  respiratory motor neurones  ↑RR
    • ↓MAP  high-pressure baroreceptors stimulated  ↑RR

Dead space: ↑Dead space  ↓Alveolar ventilation

Mainly pathological, not physiological, factors which affect this as anatomical dead space is reasonably constant