Peripheral chemoreceptors

Stimuli through peripheral chemoreceptors (carotid and aortic bodies) are less dominant. They stimulate ventilation when PaO2 falls below normal. They also react to hypoperfusion (enhanced respiration in response to hypotension). A fall in (alveolar) PAO2 from 5.3 to 4.0 kPa (from 40 to 30 mmHg) increases ventilation by a factor of 1.5 to 1.7, whereas ventilation increases fourfold when blood pH decreases to 7.0 or tenfold when blood PaCO2 increases by 50 per cent. This is because the powerful PaCO2 and pH control mechanisms counteract the hypoxic stimulus ('braking' effect by compensatory mechanisms).

The stimulating effect of hypoxemia becomes much more significant when the compensatory effects of blood PaCO2 and pH are ineffective, for example when the CO2/pH regulation mechanisms become adapted to chronic hypoxemia (e.g. in COPD). However, in the case of combined respiratory pump and oxygenation failure the two mechanisms become additive. Ventilatory response to hypoxia is enhanced by exercise; however, it is virtually abolished during anesthesia or marked analgesia and sedation. During severe hypoxia the respiratory center is depressed.

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