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Factors affecting PaCO2

Dr Swapnil Pawar March 16, 2023 346 5


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    Factors affecting PaCO2
    Dr Swapnil Pawar

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 ≈ (VCO2/VA)
PaC02 = (Vco2 x 0.863) / VA
= (Vco2 x 0.863) / (VE – VD)
Vco2: CO2 production by the body
VA: Alveolar ventilation
VE: Minute ventilation
VD: 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:

  • CO2 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

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