INCIDENCE AND PREVALENCE
1
818 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 his questionary had spontaneously spoken of the impor- tance of religion in relation to health. But Dr. Dukes is convinced " that the little working bit of a man’s religious faith, not his outward professions or his theological creed but his inward secret convictions-these must ultimately influence his health to some extent, and might, if given more 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 well established in the treatment of many conditions ; and owing to advances in technique a continuous supply of this gas at concentrations of 60% or more can be assured. This improved efficiency in administration should give rise to further thought on the indications and dangers of the agent. Hitherto there has sometimes been a tendency to regard oxygen as a harmless physio- logical substance which " if it does no good, at least will do-no harm." It is increasingly clear, however, that oxygen therapy must be applied with circumspection. In chronic pulmonary disease, and notably emphysema, there is commonly a considerable accumulation of carbon dioxide in the arterial blood. If oxygen is then given the pulmonary ventilation diminishes owing to the cessation of anoxic impulses from the carotid body ; further retention of carbon dioxide results from the hypoventilation, and toxic levels may be reached. This train of events has been noted in the course of normal oxygen therapy.1 Bouterline-Young and Whitten- berger suggest that where anoxaemia is accompanied by hypercapnia effective pulmonary ventilation must be maintained by artificial respiration while oxygen is being administered. Storstein 3 submitted three groups of patients to cardiac catheterisation and measured their respiratory and circulatory responses to the inhalation of 97% oxygen for one hour. His first group comprised healthy people rendered acutely anoxic by the inhalation of low- oxygen mixtures. His second group was composed of patients with chronic pulmonary disease ; this group was divided into those with evidence of cardiac failure (cor pulmonale) and those without past or present signs of heart-disease. The last group consisted of patients with various chronic heart lesions. The respiratory responses tended to confirm the observations of other workers ; and as the experimental conditions were not ideal for respiratory stability Storstein concentrated on the circulatory findings. By measuring the pulmonary arterial pressure he found that pure-oxygen breathing reversed the pulmonary hypertension induced in healthy people by administering low-oxygen mixtures. Where, however, anoxia had long been present, as in his chronic lung-disease group, the lowering of pulmonary arterial pressure was far less striking. He suggests that with chronic anoxia early and efficient oxygen therapy might prevent the pulmonary vascular changes from becoming irreversible. Calculation of the cardiac output and work of the right ventricle in his -three groups showed that not only was the work of the right ventricle greatest in the cor-pulmonale group but that it was actually increased by the administration of 97% oxygen. This undesirable effect was due to an increase in cardiac output without compensatory reduction in pulmonary arterial pressure. Chronic anoxaemia is a persistent stimulus to high cardiac output even in the presence of cardiac failure ; and when irreversible pulmonary hypertension is present long- continued administration of oxygen will place an increased strain on the already over-burdened right ventricle. The administration of oxygen in high concentrations to the premature newborn infant has become a routine in many nurseries. That this measure may not be wholly beneficial has been demonstrated by Jefferson,4 who has shown that the incidence of retrolental fibroplasia in premature infants is closely related to the fashion of oxygen therapy. Incidence was greatest among infants who had been suddenly removed from an atmosphere containing 60% oxygen to room air. When removal from the oxygen atmosphere was made more gradual, the incidence of retrolental fibroplasia was lower. Equally striking is her success in treating babies with early signs of the disease by replacing them in high concen. trations of oxygen for a long time before gradually reducing 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 with mortality, problems of terminology were few. Death is an unequivocal fact, and it comes only once to each person. The epidemiology of acute diseases is also fairly straightforward, in that the affected can usually be clearly distinguished from the unaffected, and the outcome -recovery or death-is apparent within days or weeks. But the epidemiology of more chronic disorders may be complicated by a gradual onset, so that a person who regards himself as in normal health may be shown to be in the early stages of serious disease. Furthermore, the patient may remain affected for a long time; and this complicates surveys and the terminology in which their results are expressed á One of the chief trouble-makers is " incidence." Some years ago Hogben 6 objected to the practice among medical writers of using this word when "frequency" " was meant. A record of the number of cases in age and sex groups is a " frequency " distribution. To justify the use of " incidence " it is essential to know the size of the population from which the cases are drawn, so that the result may be expressed "per 1000 " or " per cent." But even then " incidence " may be used in any one of three different senses : (a) for the number of cases per 1000 examined at a given time ; (b) for the number of cases per 1000 examined, developing during an unspecified time in those known to have been normal originally ; (c) for the number of cases developing during a known period in those known to have been normal at 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 number of cases per 1000 examined at a given time- American workers apply the term " prevalence " ; and this has much to commend it. To most people " incidence " conveys the idea of the onset of some new condition ; but a mass-radiography survey, for example, includes cases that have existed unknown for many years, and the term " prevalence " seems etymologically more justifiable than " incidence." Stocks 5 recognises this, and defines " monthly prevalence-rate" as the " number of illnesses present in the population at any time during the month, regardless of when they began, per stated number of population." If " prevalence" " were to be generally adopted for the results of surveys applied once only to populations of known size in which the time of onset cannot be known, and " attack- rate " were used when both the population and the duration of risk are known, then " incidence " with its ambiguities could largely lapse, or be restricted to the second meaning—i.e., new occurrences in a known population during an unspecified time.
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Transcript of INCIDENCE AND PREVALENCE
818
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.
