Predicting probability of cardiac arrest

Predicting probability of cardiac arrest

Predicting probability of cardiac arrest

Roll a six-sided dice three times.  What are the odds you’ll throw a six at least once?

This exercise is a staple for teaching statistics and probability, and there is more than one way to calculate the answer (which according to this, is 91/216).

Mathematically, determining probability is simply a question of calculation.  But the answer isn’t always satisfactory.

Take the Monty Hall Problem. Statistician, Steve Selvin named this conundrum after Monty Hall, the affable host of American TV game show, Let’s Make a Deal (pictured).

In the Monty Hall problem, a player has a choice of three doors.  Behind one door is a new car; behind each of the other two, is a goat.

After choosing door number one, the host opens door number three, revealing a goat.  He then gives the player a chance to rethink their choice, and pick door number two instead.  What should they do?

A mathematical determination of probability shows that, as a result of knowing more about what is behind the unchosen doors, the player should change their choice.

Selvin showed that the player now has a 2/3 chance of winning the car by amending their choice, and selecting door number two; if they stay with door one, their chances of winning remain at 1/3.

When this problem appeared in Parade magazine in 1990, more than 1,000 PhDs wrote in to dispute the conclusion.  You may share their opinion; even when the mathematical probability is clearly defined, the answer seems absurd.

We’re only human

The answer to the Monty Hall Problem is a paradox; something that seems ridiculous but may otherwise be true.  It challenges our instincts, using mathematical calculations of probability to test our assumptions.

As humans, we constantly experience the conflict between our assumptions of probability and the mathematical determination of certain outcomes.

We decide the likelihood of risks and rewards, based on what we consider to be the contributing factors to those outcomes.

As the Monty Hall Problem demonstrates however, our understanding of those factors is imperfect.

That scenario deals with rewards.  What happens when the problem we’re facing is a question of risk?

Consider Elvis Presley, whose tragic end was the result of a confluence of factors contributing to his eventual heart failure.  Any one of those factors – such as type II diabetes, obesity, poor diet and drug addiction – increase the probability of a heart attack or sudden cardiac arrest.

As University of Notre Dame Professor of Cardiology, David Playford told us recently, the chances of surviving your first heart attack is 50 per cent.  You’d get better odds of survival from a game of Russian Roulette.

The likelihood of surviving a sudden cardiac arrest is much, much lower.

Globally, the chances of surviving an out-of-hospital cardiac arrest is less than one per cent, and there are between seven and nine million sudden cardiac deaths every year.

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Calculating the probability of cardiac arrest

Being able to accurately determine probability requires recognition of the factors that influence possible outcomes.

This is precisely what the authors of this study were trying to achieve with their score for predicting risk of death from cardiovascular disease (CVD).

Their assessment of more than 47,000 patients led to the creation of a CVD risk score calculator, developed from 11 physical, medical and behavioural factors that contribute to risk.

Simpler versions, such as this calculator, have since been introduced, adding to the body of work dedicated to creating a reliable algorithm for predicting cardiovascular risk.

It is interesting to note however, that for all the science applied to creation of these models, they still adopt the parlance of games of chance.

Reducing our exposure to behavioural and environmental risks can reduce the probability of cardiovascular disease – but it cannot eliminate risk entirely.  It still comes down to playing the odds.

Professor Playford stressed this point: for all our knowledge, we still cannot conclusively determine if someone is at risk of sudden cardiac arrest.

His point was that the best way to increase chances of surviving a catastrophic coronary event, is to be prepared.

The best way to be prepared, is to firstly understand if you, or someone close to you, is at risk.

Second, it is being prepared to respond effectively in the event that something goes wrong.

That includes having access to the tools you need to save a life – such as knowing how to contact emergency services, being ready to provide CPR (cardiopulmonary resuscitation), and having access to an AED (automated external defibrillator).


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