This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
The most marked effects of carbonic acid poisoning (from breathing the gas) are exerted upon the nervous system. An amount of 3 per 1,000 in the atmosphere of a room will cause throbbing headache, with fulness and tightness across the temples, and giddiness: more of the gas may induce fainting, muscular weakness, somnolence, or insensibility, coma, or convulsion.
Brown-Sequard taught that carbonic acid was a muscular excitant, because of the uterine contractions observed after injecting into the vagina, the excitement induced by arterial injections of blood charged with carbonic acid, and the convulsions said to be caused by directing the injections toward the head. Cyon further taught that the cardiac arrest caused by this gas was due to excitement of the vagus, but more modern observations lead us to regard the gas rather as a sedative. I have already mentioned the local anaesthesia it can produce, and Leven always found in his experiments anaesthesia with slowing of respiration and circulation, and finally cardiac arrest - no convulsion. I should say with Rabuteau that in therapeutical doses it modifies sensibility, while in toxic quantity it abolishes at once the functions of nerve and muscle. On the organs of special sense, anaesthetic effects are preceded not only by prickling and warmth, but also by muscae volitantes, tinnitus, and other phenomena connected with congestion. Herpin found that the gas, when diluted with 80 to 90 per cent. air, produced gradual anaesthesia without suffocation or pain.