PPS Epidemiology of Hypertension Flashcards
Epidemiology of Hypertension
PPS
What factors can systematically affect BP measurements?
Subject:
anxiety -> high BP
Observer:
technique -> high/low
personality -> high/low
Instrument:
characteristics -> high/low
Cuff size:
small cuff -> high/low
large cuff -> high/low
Environment:
noisy, cold -> high
No. of readings:
more readings -> lower (esp if BP high initially)
An initial high blood pressure is often followed by lower readings- why?
Accomodation’
1 -Accommodation – sympathetic response to initial blood pressure readings falls, blood pressure falls
2 -Statistical phenomenon `regression to mean’ – when the initial BP measure is high, repeat measures tend to be lower
Why is high blood pressure globally important?
Looking at global burden of disease causes, the top item on that is actually high blood pressure.
It’s the most important single cause of ill health and death in a global context.
Why is high blood pressure a problem?
Higher blood pressure levels cause markedly higher risks of cardiovascular disease (CVD), especially coronary heart disease (CHD) and stroke
High blood pressure levels high enough to increase CVD risk are very common in the general population
What does High BP increase the risks of?
Coronary (ischaemic) heart disease Stroke (all types) Ischaemic stroke Haemorrhagic stroke Subarachnoid haemorrhage Heart failure Hypertensive heart disease Sudden death Renal (kidney) failure All-cause mortality
What is a cohort (longitudinal) study?
An observational study, just observing associations not attmepting to change BP (eg Framingham study)
Take BP measurement (ideally based on repeated measurements providing ‘usual blood pressure’)
-> follow up after 5-10 years
Risk of developing cardiovascular disease or dying from it
What is the Framingham study?
The study began in 1948 by recruiting an Original Cohort of 5,209 men and women between the ages of 30 and 62 from the town of Framingham, Massachusetts, who had not yet developed overt symptoms of cardiovascular disease or suffered a heart attack or stroke. Since that time the study has added an Offspring Cohort in 1971 and more
Monitoring of the Framingham Study population has led to the identification of major CVD risk factors, as well as valuable information on the effects of these factors such as blood pressure, blood triglyceride and cholesterol levels, age, gender, and psychosocial issues. Risk factors for other physiological conditions such as dementia have been and continue to be investigated.
What are the two ways of describing the effect of high BP on risk?
Ratio of risks (high/normal, eg 6/2)
Difference of risks (high minus normal, 6-2)
How do we measure overall strength of association between BP and CVD?
USE OF RELATIVE RISK
Relative risk provides a good summary of the influence which high blood pressure has on cardiovascular risk across the population
Described as ratio:
= Risk of disease outcome in people with a cause/Risk of disease outcome in people without a cause
= Cardiovascular risk in people with high BP/Cardiovascular risk in people without high BP
= Cardiovascular risk in people with a higher BP/Cardiovascular risk in people with a lower BP
How strong are the relative risks of high blood pressure (70-79 years)?
Systolic: risk of stroke and CHD with high reading is around double, but ~0.5 with lower
Diastolic: risk of stroke and CHD with high reading is around double, but ~0.5 with lower
These proportional risks apply across all BP levels above SBP 115 mmHg, DBP 75 mmHg
RRs are slightly higher in younger people, slightly lower in olderpeople
How do measure strength of association between BP and CVD?
USE OF RELATIVE RISK
= Cardiovascular risk in people witha higher BP/Cardiovascular risk in people with a lower BP
Relative risk: Interpretation
= 1.0 Exposure has no effect on risk
= 2.0 Exposure associated with doubling of risk (twice as likely’)
= 0.5 Exposure associated with halving of risk (half as likely’) (`protective’)
In practice, relative risk estimates always have associated error, which needs to be considered in interpretation
How does complications of hypertension affect risk?
Complications of high blood pressure:
left ventricular hypertrophy
proteinuria or renal impairment
hypertensive retinopathy
….then the relative risks of CVD in people with high blood pressure are even higher….Emphasizes the need to control blood pressure before this happens
Which is more strongly related to risk, systolic or diastolic pressure?
Both are important, systolic slightly more so
In older people, high’ systolic BP can occur with normal’ diastolic pressure (isolated systolic hypertension), is associated with increased CV risk
So blood pressure is strongly related to CV risk, and making it more problematic is that high BP is widespread
How prevalent is high BP in England?
41% male, 33% female overall have high BP (1988)
Even average BP level in population associated with increased CHD risk (Average SBP is 150)
What is the problem with the ‘old’ view of high BP and normal BP?
Used to think by looking at distribution of people with normal BP we could deperate it out from people with high blood pressure who would be at the top end of the distribution, but this is incorrect.
Blood pressure is a single distribution. Impossible to pick out a seperation, though it may be skewed to the right
BLOOD PRESSURE IS CONTINUOUSLY DISTRIBUTED IN THE POPULATION: THERE IS NO CLEAR SEPARATION BETWEEN NORMAL’ AND HIGH’
THERE IS NO OBVIOUS CUT POINT IN THE ASSOCIATION BETWEEN BLOOD PRESSURE AND CVD RISK
How do we define high BP?
DEFINING HIGH BLOOD PRESSURE ON THE BASIS OF THE BP LEVEL ABOVE WHICH LOWERING BP IS BENEFICIAL
This is the definition generally used
Why do some people within a population have higher BP than others?
A small proportion of individuals with high blood pressure have a specific medical cause (secondary hypertension)<1% in general population, <5% in medical clinics
Most have no specific medical cause – this is referred to as primary’ or essential’ hypertension – we still do not entirely understand the cause
What are medical causes of secondary hypertension?
Coarctation of aorta Renal and renal vascular disease Adrenal disease: -cortex: Primary hyperaldosteronism, Cushing’s syndrome -medulla: phaeochromocytoma
Pregnancy
Drugs especially Oral Contraceptive Pill, Hormone Replacement Therapy
What are key factors contributing to higher BP in general population?
Biggest to lowest impact
High body fatness (high BMI) High alcohol intake High salt intake Low potassium intake Low fibre/high fat diet Physical inactivity Stress (uncertain how much)
Why does BP rise with age in adult life in Western populations but not in all populations?
In populations where BP does NOT rise with age:
rural communities in less developed settings
hunter gatherer, subsistence diet (low fat, low salt, less alcohol)
low BMI
high physical activity
low stress
Western population very different lifestyle
Do the low blood pressure levels have an environmental or genetic explanation?
Studies of migration and BP
Most robust studies have been in people moving from low blood pressure (origin) to higher blood pressure (adoption) populations
Generally show that blood pressure patterns change (increase) to match those of the adopted population
Change generally occurs within 6 months of migration
Strong evidence for mainly ADULT ENVIRONMENTAL influence on population BP
Risk factors for high BP
A risk factor is something which is associated with a high risk of a condition, but is not necessarily a cause of it. For high BP important risk factors are:
Age
Ethnicity
Family history
BMI
Alcohol intake
(gender not a strong risk factor for high BP)
What are the benefits for lowering BP?
Substantial evidence from randomized trials of blood pressure reduction and their effect on CVD risk
Sustained reduction in blood pressure over about 5 years (particularly using BP lowering medications) effectively reverses the risks of previously higher blood pressure on CVD risk shown in cohort studies
The more blood pressure level is lowered, the greater the reduction in cardiovascular risk
No obvious target for BP reduction above SBP of 120 mmHg, DBP 70 mHg
