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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