Aetiology and risk factors for CVD

Question 1

What are observational studies?

Answer 1

• no intervention
• purely observational
• e.g.
  
  • descriptive: case series, case reports, ecological, cross-sectional
  • analytical: case-control, cohort

Question 2

What are interventional studies?

Answer 2

• change people’s circumstances with an intervention
• e.g. clinical trials, which are analytical (look at cause and effect)

Question 3

What is prevalence?

Answer 3

• the number of existing cases of an outcome of interest (e.g. risk factor or disease) in a defined population at one point or period in time
• expressed as a proportion or a percentage
• examples:
  
  • % of current smokers
  • % of 65yo Australian males with CHD
    
    • among these, % who smoked
  • % of current inpatients at RMH with heart failure

Question 4

What are the two main measures of disease burden?

Answer 4

prevalence and incidence

Question 5

What is incidence?

Answer 5

• the number of new cases of an outcome of interest arising from a defined population during a time interval
• expressed as a rate; the denominator includes a time component
  
  • ​must constantly be updated
• drawn only from longitudinal studies
• examples:
  
  • number of non-smokers who start smoking in 2012
  • number of 65yo males who develop CHD in 2012
  • number of pt admitted to RMH with HF out of all admissions in one week

Question 6

What is risk?

Answer 6

• probability of disease occurring in a disease-free population during a specified time period
• Risk = n/P
  
  • n = new cases in a defined period
  • P = population-at-risk
• examples:
  
  • in 1995, 3 cases of lung cancer developed out of 1000 men
    
    • risk = 3/1000 per year
    • assuming all men were followed up for a full year (often not the case)

Question 7

What is rate?

Answer 7

• probability of disease occurring in a disease free population during the sum of individual follow-up periods
• Rate = n/total person-time of follow-up
  
  • n = new cases in a defined period
  • total person-time = sum of individual follow-up periods of all indivdiuals
• beneficial because recruitment is often staggered over time, and this gives a more accurate measure
• example:
  
  • 3 cases of lung cancer developed out of 1000 person-years follow-up
  • rate = 3/1000 person-years
    
    • denominator is essentially the time the population at risk spent being at risk

Question 8

What is hazard?

Answer 8

• special type of rate
• continuously updated as a longitudinal study progresses
• rate applies to an exact point in time: an instantaneous rate
• derived from longitudinal studies, especially clinical trials with close follow-up
• as soon as someone is affected by what they are at-risk for, they are removed from the denominator

Question 9

Cause-and-effect is generally described by

Answer 9

relative and attributable risks

Question 10

What is the absolute risk (absolute rate)?

Answer 10

• isolated measurement of risk/rate
• no indication of association with exposure (ie no causes)
• examples:
  
  • 5 strokes/10000 men per year
  • 10 MIs/1000 person-years

Question 11

What do relative risk and attributable risk provide?

Answer 11

• provide an indication of association
• describe cause-and-effect relationship between exposure and outcome
• both rely on comparison of two absolute risk/rate measurements:
  
  1. risk/rate among exposed (Re)
  2. risk/rate among unexposed (Ru)

Question 12

What is relative risk (aka risk ratio, rate ratio)?

Answer 12

• indicates the relative magnitude of change in risk/rate of outcome associated with exposure
• RR = Re/Ru
  
  • Re = 10/100p-yr
  • Ru = 5/100p-yr
  • RR = 2.0
• if RR > 1, it implies that exposure causes likelihood of the outcome compared to non-exposure

Question 13

What is attributable risk (aka risk difference, rate difference)?

Answer 13

• indicates the absolute magnitude of change in risk/rate outcome associated with exposure; i.e. the absolute magnitude of likelihood of outcome due to exposure
• AR = Re - Ru
  
  • Re = 10/100p-yr
  • Ru = 5/100p-yr
  • AR = 5/100p-yr

Question 14

What is attributable risk percent?

Answer 14

• proportion of incident disease among exposed people that is due to exposure
  
  • e.g. percent of the disease among people which is attributable to the exposure
• AR% = [(Re-Ru)/Re] x 100

Question 15

What is the population attributable risk?

Answer 15

• indicates the additional or excess risk/rate of the outcome in the population due to the exposure
• PAR = Rt - Ru
  
  • Rt = risk/rate in whole population (weighted average of exposed and unexposed)

Question 16

What is the population attributable risk percent (aka preventable fraction)?

Answer 16

• proportion of incident disease among the whole population that is due to exposure
• PAR% = [(Rt - Ru)/Rt] x 100
• e.g. 38% of the incident disease among the whole population is due to the exposure (and 62% is due to something else)
  
  • implies that removing the risk factor from this population would actually prevent 38% of the disease in that population

Question 17

What are the Bradford Hill Criteria for causality?

Answer 17

• temporal relationship (exposure preceeds outcome)
• strength of association (greater the relative increase, the more likely to be causal)
• dose-response relationship (increase exposure increases outcome)
• consistency (consistent results in repeated studies)
• plausibility (has to make biological sense)
• specificity (specific relationship between cause and effect)
• coherence (similar to consistency)