By J. F. Nunn
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Additional resources for Applied Respiratory Physiology. With Special Reference to Anaesthesia
If P c o 2 is raised above about 80 mm Hg, the linear relationship between P c o 2 and ventilation is lost. As P c o 2 is raised a point of maximal ventilatory 32 CHEMICAL CONTROL OF BREATHING stimulation is reached (probably within the range 1 0 0 - 2 0 0 mm Hg), and thereafter the ventilatory stimulation is reduced until, at very high P c o 2 , the ventilation is actually depressed below control value and finally apnoea results, at least in the dog and almost certainly in man as well. It does not appear to be possible to arrest breathing in the cat by this means in spite of elevation of P c o 2 to more than 500 mm Hg (Hornbein, personal communication; Raymond and Standaert, 1967).
In the control of breathing. F. of elevated P c o 2 and reduced pH. Leusen's work touched off a long series of studies aimed at localizing central chemoreceptors. F. could influence the respiratory neurones within the substance of the medulla in the short time required for development of the ventilatory response to inhaled carbon dioxide. These studies were carried out mainly in the University of Göttingen (Loeschcke and Keopchen) and the University of California, San Francisco (Mitchell and Severinghaus), although various combinations of many authors appeared in numerous publications which included joint work between members of the two universities.
It has been explained above that, at least in animals, rhythmic breathing may be governed by a negative feedback loop from the pulmonary stretch receptors via the vagus and the expiratory neurones of the medulla (Figure 4). It has also been shown in cats that the common inhalational anaesthetics, particularly trichloroethylene, sensitize the pulmonary stretch receptors and so would be expected to cause an enhancement of the inflation reflex (Whitteridge and Bülbring, 1944). This is in contrast to Head's conclusion (1889) that ether and chloroform paralysed vagal endings.