Shallow Water Blackout
& Secondary Drowning
Amy Peden
National Manager – Research and Policy,
Royal Life Saving Society – Australia and
Associate Professor Richard Franklin,
James Cook University and Royal Life
Saving Society – Australia
Drowning in Australia
• 292 people on average drowned in
Australia each year for the past 10 years
• The 2014 Royal Life Saving National
Drowning Database found
that 266 people drowned
in 2013/14
• 81% of all drowning victims
were male
Drowning in Australia 2013/14
• Largest number of drowning deaths
occur in rivers, creeks and streams
• Swimming pools overtook beaches as
the second leading location for drowning
Drowning in Australia 2013/14
• There are a diverse range of activities
being undertaken prior to drowning in
Australia
Drowning Defined
• “Drowning is the process of
experiencing respiratory impairment
from submersion/immersion in liquid.”
• Drowning outcomes should be classified
as: death, morbidity, and no morbidity.
• There was also consensus that the
terms wet, dry, active, passive, silent,
and secondary drowning should no
longer be used.
The Physiology of Drowning
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Mouth, nose submerged
Aspiration
Breath hold
Cold shock
Diving response
Autonomic conflict
Laryngospasm
Inhalation of Water
Ingestion
Cardiac Arrest
Shallow Water Blackout – How
does it happen?
• Shallow Water Blackout (SWB) occurs
because the normal, protective
‘breakpoints’ have not been triggered
before unconsciousness (due to
cerebral hypoxia).
• There are two chemical sensors in the
body which detect the levels of oxygen
and carbon dioxide (Co2) & protect us
from a lack of oxygen.
Shallow Water Blackout – How
does it happen?
• The CO2 sensor is the most sensitive.
• Hyperventilation i.e. ‘blowing off’ too
much Co2 before submerging can
interfere with the sensor which is not
triggered early enough to prevent
blackout.
The “Break Point”
Also Known As
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Shallow Water Blackout
Hypoxic Blackout
Underwater blackout
The Long dive
Long breath holding
Surface Blackout
Deep water blackout
Freediver blackout
Latent hypoxia
Why?
• The activity of breath-holding is common
and often an essential part of some
underwater sports, such as underwater
hockey, synchronised swimming and
free-diving (the activity of seeing how
deep/ far a person can swim on one
breath).
• It also occurs recreationally while trying
to swim as far as possible underwater
on a single breath.
Shallow Water Blackout
• There are challenges around quantifying
the role of SWB in broader drowning
fatalities
• Often the activity prior to drowning is
unwitnessed so hard to know if the
deceased was participating in breathholding activities
• No autopsies to rule out other potential
causes such as cardiac arrhythmias and
seizure disorders (Long QT Syndrome)
Differential Diagnoses
• Voluntary endurance underwater swimming with
pre-submersion hyperventilation
• Hypoxic seizures (convulsions) can result from
hypoxia following pre-dive hyperventilation.
• Hypoxic triggering of cardiac dysrhythmia
consequent upon pre-existing genetic or acquired
electrical conduction abnormalities of the heart.
• Cardiac dysrhythmias secondary to the normal
diving response, conduction defects sensitised by
pre-existing cardiac conditions.
• Association with the normal physiological “cold
shock” response.
Prevention Strategies
• A number of prevention strategies proposed.
These include:
– Increasing awareness of the dangers of
hyperventilation
– Use of lifeguards & facility staff to discourage
the practice at venues
– Ensuring the practice of hyperventilation is
not part of any sport; and
– Where breath-holding is undertaken,
appropriate medical and rescue personnel,
devices and procedures must be in place.
Known deaths in Australia
• Drowning deaths associated with Shallow
Water Blackout in Australia commonly fall
into one of three categories:
– Free-divers and spear fishermen: most
commonly in the ocean
– Those training to complete in activities
such as underwater hockey or
free-diving: most commonly in public
pools
– Those recreating in home swimming
pools and holding breath
Known deaths in Australia
• Between 1 July 2002 and 30 June 2012,
there were 16 deaths known to be related
to SWB.
• 8 deaths involved males between the ages
of 16 and 35 swimming laps underwater, in
swimming pools (5 public and 3 private).
• There were a further 8 deaths among 21 to
40 year old males whilst free-diving for the
purposes of spear fishing in the ocean
SWB – Prevention Strategies
• Public awareness &
public education
through fact sheets
• Royal Life Saving
Fact Sheet on Shallow
Water Blackout
SWB Prevention Strategies
• Diver specific
education
• Undertaken by Divers
Alert Asia Pacific
Policies
Case Studies & Media Attention
• Recent media has highlighted the
dangers related to activities involving
breath-holding and pre-immersion
hyperventilation:
• What follows are three real life cases
from recent years
Case Study 1
Case Study 2
Case Study - 3
An 18 year old male has drowned in a public
pool due to shallow water blackout.
The male was swimming laps of the pool
whilst holding his breath for the purposes of
training for underwater hockey.
The deceased had completed between 4 and
6 laps of the 50m pool underwater using
flippers and a face mask. The deceased had
swum two of the laps completely underwater
and decided to try swimming another on a
single breath without flippers.
Long QT Syndrome
• What is it?
Long QT Syndrome
• Triggers
• It is important that a genetic autopsy
following unexpected drowning in young
people including SWB is undertaken. It
is best practice, if a coroner identifies a
good swimmer who drowns, for the first
degree relatives (parent, child or full
siblings) to also be tested.
Secondary Drowning – A variety
of terms
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Secondary drowning
Dry drowning
Delayed drowning
Passive drowning
• Terms such as these should be
discouraged
• Please use the term drowning and add
appropriate outcome (e.g. fatal, non-fatal,
morbidity, no morbidity)
Secondary drowning
• Secondary Drowning occurs when
someone breaths in small amounts of
water.
• This triggers the muscles in the airway to
spasm and makes breathing difficult. Fluid
builds up in the lungs and causes trouble
breathing.
• What are the risks?
• Secondary or dry drowning is not common,
and is estimated to account for 1-2% of
drowning
Secondary drowning
• Prevention
– Supervision is key, especially for
children and inexperienced swimmers
– Restricting Access to water –
especially for children
– Resuscitation
Acknowledgements
• Professor John Pearn
• National Medical Advisor, Royal Life
Saving Society - Australia
Any questions?