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INCIDENCE AND PREVALENCE
818 science that he
body and spirit
are
not two but
believed, achieved true health.
He of the doctors
was
one," had, the
more
answering his surprised that only one had of the imporquestionary spontaneously spoken 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." 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 physiological substance which " if it does no good, at least will It is increasingly clear, however, that do-no harm." must be applied with circumspection. oxygen therapy 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 Whittenbergersuggest that where anoxaemia is accompanied by hypercapnia effective pulmonary ventilation must be maintained by artificial respiration while oxygen is being administered. Storstein3 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 lowoxygen 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 The respiratory with various chronic heart lesions. 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 He suggests that with pressure was far less striking. 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 longcontinued administration of oxygen will place an increased strain on the already over-burdened right ventricle. 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.
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.
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&a cute; "
One of the chief trouble-makers is incidence." Some years ago Hogben6 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 timeAmerican workers apply the term " prevalence " ; and To most people this has much to commend it. 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." Stocks5 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 " attackrate " 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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4. Jefferson, E. Arch. Dis. Childh. 1952, 27, 329. Sickness in the Population of England and Wales 5. Stocks, P. in 1944-47. H.M. Stationery Office, 1949. 6. Hogben, L. Brit. med. J. 1945, i, 884.
body and spirit
are
not two but
believed, achieved true health.
He of the doctors
was
one," had, the
more
answering his surprised that only one had of the imporquestionary spontaneously spoken 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." 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 physiological substance which " if it does no good, at least will It is increasingly clear, however, that do-no harm." must be applied with circumspection. oxygen therapy 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 Whittenbergersuggest that where anoxaemia is accompanied by hypercapnia effective pulmonary ventilation must be maintained by artificial respiration while oxygen is being administered. Storstein3 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 lowoxygen 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 The respiratory with various chronic heart lesions. 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 He suggests that with pressure was far less striking. 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 longcontinued administration of oxygen will place an increased strain on the already over-burdened right ventricle. 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.
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.
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&a cute; "
One of the chief trouble-makers is incidence." Some years ago Hogben6 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 timeAmerican workers apply the term " prevalence " ; and To most people this has much to commend it. 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." Stocks5 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 " attackrate " 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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4. Jefferson, E. Arch. Dis. Childh. 1952, 27, 329. Sickness in the Population of England and Wales 5. Stocks, P. in 1944-47. H.M. Stationery Office, 1949. 6. Hogben, L. Brit. med. J. 1945, i, 884.