Why does hyperventilation in severe COPD cause hypercapnia?

Why does hyperventilation in severe COPD cause hypercapnia?

THERE ARE 2 mechanisms…

1…PATIENTS with COPD (severe cases of COPD with long history ),have ventilatory drive at severe hypoxemia,almost ANOXIC level becoz chronic hypoxia reduces neuronal firing RESPIRATORY drive which is a basic principle of neurology called HABITUATION OF NEURONS,In such PTs with ventilatory drive happening at severe hypoxemia level if HIGH flow O2 therapy is given,it further reduces ventilatory drive and hypoventilation and later hypercapnia.

2 V/Q mismatch,normally Hypoxia causes pulmonary vasoconstriction,which is a safety mechanism,as it diverts blood to BETTER Ventilated areas.

In COPD,as we know a lot of alveoli are expanded,with air trapping called increase in dead space,with reduced minute ventilation so perfusing these areas is wasteful,as a result hypoxia in COPDs is again a protective factor.SO in PTS with COPD ,a high O2 therapy will cancel this hypoxia induced VASOCONSTRICTION,so areas with poor minute ventilation will get more perfusion leading to diversion of blood from better Ventilated areas n later impairement of gas exchange n later hypercapnia.

SO qs is how to combat this effect?

Becoz withholding O2 therapy wud be unwise decision as O2 therapy is only definitive RX of COPD.

Studies have shown that COPd pts given a high flow O2 therapy with TARGET saturation of higher than 95% have higher mortality as compared to PTs with Target Saturation of low level about 88-90%.

So,it wud be wise to keep a low TARGET Saturation,while giving O2.

(I think we have discussed it before