2021B11 Explain the physiological effects of acute hypercapnia, caused by hypoventilation.

 

Hypercapnoea: PaCO2 >40mmHg

 

CVS

Heart:

·      ↑Catecholamines

o   Hypertension, tachycardia

o   ±Arrhythmias, ischaemia

·      Acidosis -> negative inotropy

o   Predominates at pH <7.2

Vasculature:

·      Systemic vasodilation (↓SVR)

·      Pulmonary vasoconstriction (↑PVR)

Resp

Hypoxaemia:

·      PAO2 = PiO2 – PaCO2 / R

·      N.B. supplemental O2 very effective

Compensation:

·      Bohr effect

o   OHDC right shift -> ↑tissue oxygenation

·      Stimulation of central and peripheral chemoreceptors -> ↑VA -> ↓PaCO2

CNS

Vasodilation:

·      ↑CBF, ↑CBV, ±↑ICP

·      Luxury perfusion: e.g. post-stroke, post-ROSC

Narcosis:

·      Sedation 60-80mmHg, anaesthesia 80-100mmHg

·      ↓MAC, ↓propofol Cp50

Acid-base

Effects:

·      Respiratory acidosis

o   ↑10mmHg CO2 -> pH ↓0.08 if acute (0.03 if chronic)

o   ↑10mmHg CO2 -> HCO3- ↑1mM acute (4mM chronic)

Kinetics:

·      Transport: in RBC as ~60% HCO3-, 30% carbamino, 10% dissolved

·      Distribution: diffusion into cells, conversion to H+ + HCO3-

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·      Buffering: 99% ICF (imidazole, phosphate), 1% ECF

·      Compensation: ↑fixed acid excretion in urine -> ↑HCO3-

 

 

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