2005A12 Describe the physiological factors influencing the carbon dioxide tension in arterial blood.

 

List:

·         Intro

·         ↑↓VCO2

·         ↑↓VA

·         Control of VA

·         ↑↓K

 

Intro:

CO2 homeostasis

·   CO2 is produced by Krebs cycle in mitochondria during aerobic respiration

·   PaCO2 tightly controlled. 40 +/- 1-2mmHg usually

Equation

·   PACO2 = VCO2/VA x k

Where PACO2 ≈ PaCO2 if minimal dead space and at equilibrium

And K = 0.863, derived from ideal gas laws

·   *Note PACO2 ∝ production/elimination

·   *Note negative feedback loop: ↑PaCO2 -> ↑VA -> ↓PaCO2

·   *Note metabolic rate matched to VA via control of PaCO2

 

CO2 production:

↑MR -> ↑VCO2:

·   ↑temp (7% per 1°C) e.g. sepsis, malignant hyperthermia

·   Exercise

·   Youth: neonate 2x

·   Male sex

·   ↑size

·   Pregnancy (1.2x)

↓ MR > ↓VCO2:

·   ↓ temp (7% per 1°C)

·   Cachexia

Histiotoxicity

·   e.g. cyanide -> ↓VCO2 -> ↓PaCO2

Respiratory quotient

i.e. ratio VCO2:VO2

·   Carbohydrate: RQ 1.0

·   Fat: RQ 0.8

·   Protein: RQ 0.7

·   Alcohol 0.7

·   Ketones 0.7

 

Alveolar ventilation:

Equation:

VA = RR x (VT – VD)

↑RR or VT

·   CNS: pain, anxiety, pregnancy (progesterone)

·   Hypoxia (synergistic with ↑PaCO2 at peripheral chemoreceptors)

·   Metabolic acidosis: PaCO2 = 8 + 1.5 x HCO3- (also synergistic)

↓RR or VT

·   Drugs: general anaesthetics, opioids

·   CNS: e.g. stroke

·   Metabolic alkalosis PaCO2 = 40 + 0.7 x (HCO3- - 24)

·   Muscular: e.g. dystrophy

·   Skeletal: e.g. scoliosis

·   Pulmonary: e.g. severe COPD

↑VD

·   Apparatus: e.g. long tubing distal to Y piece

·   Anatomical: reasonably fixed

·   Alveolar: (i.e. West zone 1)

o ↑alveolar pressure: positive pressure ventilation, PEEP

o ↓pulmonary arterial pressure: e.g. haemorrhage

Perfusion limitation

·   CO2 30x more soluble than O2

·   Small partial pressure gradient from mixed venous blood to alveolar air (46 -> 40mmHg)

 

Control of alveolar ventilation:

Afferents

·   Peripheral chemoreceptors: sense PaCO2 directly

·   Central chemoreceptors: sense PaCO2 indirectly via pH in brain ECF (CO2 crosses blood-brain barrier)

Controller

·   Respiratory centre in medulla

Efferents

·   Inspiratory +/- expiratory muscles

Circuit

·   ↑PaCO2 -> ↑afferent stimulation -> ↑discharge from resp centre -> ↑RR, ↑VT -> ↓PaCO2

 

Factors affecting PaCO2:CaCO2

Temperature

·   ↓T -> ↑solubility -> ↓ PaCO2:CaCO2 -> i.e. ↓ K

Haldane effect

·   ↓ PaO2/SaO2 -> ↑Hb affinity for CO2 -> ↓PaCO2:CaCO2

·   70% of increment due to ↑carbamino formation

·   30% of increment due to ↑pKa imidazoles to 8.2 hence better buffer

 

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