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science that body and spirit are not two but one," had,he believed, achieved true health. He was the more

surprised that only one of the doctors answering hisquestionary had spontaneously spoken of the impor-tance of religion in relation to health. But Dr. Dukes isconvinced " that the little working bit of a man’s religiousfaith, not his outward professions or his theological creedbut his inward secret convictions-these must ultimatelyinfluence his health to some extent, and might, if givenmore scope for expression, often improve it."

1. Weterings, P. A. A. Acta med. scand. 1948, 130, 232.2. Bouterline-Young, H. S., Whittenberger, J. L. J. clin Invest.

1951, 30, 838.3. Storstein, O. Acta med. scand. 1952, 143, suppl. 1.

OXYGEN THERAPY

THE use of high concentrations of oxygen is now wellestablished in the treatment of many conditions ; and

owing to advances in technique a continuous supply ofthis gas at concentrations of 60% or more can beassured. This improved efficiency in administrationshould give rise to further thought on the indicationsand dangers of the agent. Hitherto there has sometimesbeen a tendency to regard oxygen as a harmless physio-logical substance which " if it does no good, at least willdo-no harm." It is increasingly clear, however, thatoxygen therapy must be applied with circumspection.

In chronic pulmonary disease, and notably emphysema,there is commonly a considerable accumulation ofcarbon dioxide in the arterial blood. If oxygen is then

given the pulmonary ventilation diminishes owing to thecessation of anoxic impulses from the carotid body ;further retention of carbon dioxide results from the

hypoventilation, and toxic levels may be reached. Thistrain of events has been noted in the course of normal

oxygen therapy.1 Bouterline-Young and Whitten-berger suggest that where anoxaemia is accompaniedby hypercapnia effective pulmonary ventilation must bemaintained by artificial respiration while oxygen is beingadministered.

Storstein 3 submitted three groups of patients tocardiac catheterisation and measured their respiratoryand circulatory responses to the inhalation of 97%oxygen for one hour. His first group comprised healthypeople rendered acutely anoxic by the inhalation of low-oxygen mixtures. His second group was composed ofpatients with chronic pulmonary disease ; this groupwas divided into those with evidence of cardiac failure

(cor pulmonale) and those without past or present signsof heart-disease. The last group consisted of patientswith various chronic heart lesions. The respiratoryresponses tended to confirm the observations of otherworkers ; and as the experimental conditions were notideal for respiratory stability Storstein concentrated onthe circulatory findings. By measuring the pulmonaryarterial pressure he found that pure-oxygen breathingreversed the pulmonary hypertension induced in healthypeople by administering low-oxygen mixtures. Where,however, anoxia had long been present, as in his chroniclung-disease group, the lowering of pulmonary arterialpressure was far less striking. He suggests that withchronic anoxia early and efficient oxygen therapy mightprevent the pulmonary vascular changes from becomingirreversible. Calculation of the cardiac output and workof the right ventricle in his -three groups showed thatnot only was the work of the right ventricle greatest inthe cor-pulmonale group but that it was actually increasedby the administration of 97% oxygen. This undesirableeffect was due to an increase in cardiac output withoutcompensatory reduction in pulmonary arterial pressure.Chronic anoxaemia is a persistent stimulus to high cardiacoutput even in the presence of cardiac failure ; andwhen irreversible pulmonary hypertension is present long-continued administration of oxygen will place an increasedstrain on the already over-burdened right ventricle.

The administration of oxygen in high concentrationsto the premature newborn infant has become a routinein many nurseries. That this measure may not be whollybeneficial has been demonstrated by Jefferson,4 whohas shown that the incidence of retrolental fibroplasiain premature infants is closely related to the fashion ofoxygen therapy. Incidence was greatest among infantswho had been suddenly removed from an atmospherecontaining 60% oxygen to room air. When removalfrom the oxygen atmosphere was made more gradual, theincidence of retrolental fibroplasia was lower. Equallystriking is her success in treating babies with earlysigns of the disease by replacing them in high concen.trations of oxygen for a long time before graduallyreducing the concentration.

4. Jefferson, E. Arch. Dis. Childh. 1952, 27, 329.5. Stocks, P. Sickness in the Population of England and Wales

in 1944-47. H.M. Stationery Office, 1949.6. Hogben, L. Brit. med. J. 1945, i, 884.

INCIDENCE AND PREVALENCE

WHEN medical statistics were concerned mainly withmortality, problems of terminology were few. Death isan unequivocal fact, and it comes only once to eachperson. The epidemiology of acute diseases is also fairlystraightforward, in that the affected can usually beclearly distinguished from the unaffected, and the outcome-recovery or death-is apparent within days or weeks.But the epidemiology of more chronic disorders maybe complicated by a gradual onset, so that a personwho regards himself as in normal health may be shownto be in the early stages of serious disease. Furthermore,the patient may remain affected for a long time; andthis complicates surveys and the terminology in whichtheir results are expressed á

One of the chief trouble-makers is " incidence."Some years ago Hogben 6 objected to the practice amongmedical writers of using this word when "frequency" "was meant. A record of the number of cases in age andsex groups is a " frequency

" distribution. To justifythe use of " incidence " it is essential to know the sizeof the population from which the cases are drawn, sothat the result may be expressed "per 1000 " or " percent." But even then " incidence " may be used in anyone of three different senses : (a) for the number of casesper 1000 examined at a given time ; (b) for the numberof cases per 1000 examined, developing during an

unspecified time in those known to have been normaloriginally ; (c) for the number of cases developing duringa known period in those known to have been normalat the beginning of the observation period, expressed as,for example,

"

per 1000 per annum." -

The third of these fa more usually known as an" attack-rate." To the first-namely, the numberof cases per 1000 examined at a given time-American workers apply the term " prevalence " ; andthis has much to commend it. To most people" incidence " conveys the idea of the onset of some newcondition ; but a mass-radiography survey, for example,includes cases that have existed unknown for manyyears, and the term " prevalence " seems etymologicallymore justifiable than " incidence." Stocks 5 recognisesthis, and defines " monthly prevalence-rate" as the" number of illnesses present in the population at anytime during the month, regardless of when they began,per stated number of population." If " prevalence" "were to be generally adopted for the results of surveysapplied once only to populations of known size inwhich the time of onset cannot be known, and " attack-rate " were used when both the population and theduration of risk are known, then " incidence " with itsambiguities could largely lapse, or be restricted to thesecond meaning—i.e., new occurrences in a knownpopulation during an unspecified time.