In 1843, John Stuart Mill wrote a book titled "A System of Logic: Ratiocination and Induction." This text was used to judge causal relationships through the following means: method of agreement, method of difference, joint method of agreement and difference, method residues, and method of concomitant variation (Dishman, Washburn, and Heath: 2004). During that period this was the basic tool used to determine causation between two variables. Not until 1965 when Sir Austin Bradford Hill (Hill, 1965) developed another causation criteria that is used till this very day. His proposed criteria included the following criterion, Consistency, Dose Response Relationship, Temporality, Coherence, Theoretical Plausibility, Experimental Evidence, Specificity, Strength of Association and Analogy.
They form the fundamental prerequisites and assessment criteria of the cause-effect relationship (Jenicek, 2004)
Hill’s criteria apply to stress and hypertension because of several arguments that suggests that stress cause or do not cause hypertension. Many argue that there is little doubt that both physical and mental stress can cause significant elevations of high blood pressure. But long term exposure to high blood pressure is hypertension. So can stress play a role in development of hypertension?
It will be useful to define each of the 9 criteria which are Analogy, Coherence, Theoretical plausibility, Dose response relationship, Experimental evidence, Consistency, Strength of association, Specificity and Temporality.
Table 1: Hill’s Criteria Definitions
illustration not visible in this excerpt
A search was carried out using CINAHL, MEDLINE, and Pub Med databases and their reference list of included study and other internet sources. Keywords like stress, hypertension, plausibility, dose-response relationship, temporality, coherence, analogy, and epidemiology was used as the search criteria. Combinations of the above mentioned words were also used during the search, this was to aid in specific areas of the search. Results from the search that were of no importance were discarded and more key words like blood pressure and mortality in combination with the above mentioned terms yielded the desired search results.
Each of the criteria proposed by Dr. Hill was assessed independently as an individual entity. The result of the literature review was separated by Analogy, Coherence, Theoretical plausibility, Dose response relationship, Experimental evidence, Consistency, Strength of association, Specificity and Temporality. Overall, studies show a satisfactory link between stress and hypertension.
Specificity of causes and analogy was not consistent with the findings of the study
The strength of the evidence for concluding that there is a cause and an effect association between two entities must be judged by Hill's Criteria (Dishman, 2004)
If Stress is thought to be a cause, then there must logically be an effect. When a statistical association between a risk factor (e.g., stress) and a disease outcome (e.g., hypertension) can be demonstrated, Hill's Criteria increases the probability that the association is causal (Dishman, Washburn , Heath: 2004). It would seem that this is an accurate assumption when the results of confounding variables and effect modification have been controlled.
Strength of association
Strength is defined as the size of the risk as measured by appropriate statistical tests (Gordis, 2004) and is based on measurement (Colley, Morgan, Haas: 1998). It essentially asks the question "is the disease rate many times greater among an exposed population?" (Jenicek, 2004) In other words, the larger the association, the more likely the exposure is causing the disease (Rothman, 2002).
A spike in blood pressure is a direct result of stress (mayo clinic). often times, people who have stressful situations have hypertension (Anderson, 2007)
John Hall suggested in Hurst’s textbook on heart disease that, “Chronic stress may lead to long-term increases in blood pressure (Hypertension). Epidemiologic studies show that air traffic controllers, lower socioeconomic groups and other groups who are believed to have stressful lives, have increased prevalence of hypertension (Kulkarni, 1998)
These findings suggest that there is strength in association between stress and hypertension in relation to a causal link.
Experimental evidence is defined as the condition can be altered (prevented or ameliorated) by an appropriate experimental regimen (Last, 2001). Rothman (2002) argued that, as a criterion, experimental evidence cannot be applied to all settings. However, the one area in which the experimental evidence criterion can be applied to is in the animal model.
In relation to this study, Animal studies support the point of view that stressful situations can lead to high blood pressure which will eventually lead to hypertension. For example, mice develop high blood pressure when crowded in small cages. (Green, Kreuter, 1999). Similar trends are described in dogs and non human primates (Jenicek, 2004) and it is very familiar that the physiologic make up of these animals are very similar to man. The study of overcrowding in mice suggests that people living in dense urban environments should endeavor to visit recreational parks and centers. These study provided experimental evidence in the causal link between stress and hypertension.
Dose Response Relationship
Dose-response is defined as an increasing level of exposure (in amount and/or time) increases the risk (Last, 2001). Dose-response can be approached by asking the question "does the association show a dose response effect i.e. does the more exposure a group of people has proportionately increases the frequency of disease experienced?" (Jenicek, 2004) This criterion is an extension of the strength criterion and is based on measurement (Jenicek, 2004). It can be explained as the measurement of the relationship amount of exposure in duration, intensity, quality and the size of the impact.