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\begin{document}

\title{Case Report: The Physiology of a Preventable Near Death in a Hot Tub}



\author[1]{Bruce Becker}%
\affil[1]{University of Washington School of Medicine}%


\vspace{-1em}



  
  \date{May 03, 2024}


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\selectlanguage{english}
\begin{abstract}
This is a case report of a very preventable near-death event involving a
6-year-old female who had a syncopal event due to hyperthermia in an
overheated hot tub and drowned. She was resuscitated but due to the
combination of anoxia and hyperthermia has been left with permanent
brain damage.%
\end{abstract}%



\sloppy


\textbf{The Physiology of a Preventable Tragedy: Near Death in a Hot
Tub}

\textbf{Author:}

Bruce E. Becker, MD, MS, FACSM

Clinical Professor

University of Washington School of Medicine

\textbf{Corresponding Author:}

Bruce E. Becker, MD, MS, FACSM

beckerb2@uw.edu

\textbf{Keywords:}

Hyperthermia

Anoxia

Traumatic brain injury

Pediatric

Aquatic immersion

Hot Tubs

\textbf{Key Clinical Message:}

Hyperthermia in children is a well-recognized risk due to enclosed
vehicles.Exposure to an overheated hot tub poses a real risk in children
due to unique pediatric physiology. This physiology should be better
understood by medical professionals

\textbf{Abstract:}

Hot tubs are a common source of relaxation and recreation. While
recommendations exist for tub temperatures and guidelines regarding
their use by children and adults with potential risk factors, these are
commonly ignored. Environmental factors such as solar gain may
dramatically alter tub temperatures well beyond those set by the
regulating thermostat, as tubs have no cooling mechanisms. This article
describes a situation involving a hot tub with tragic consequences,
referencing the underlying physiology and describing means of prevention
of such adverse consequences.

\textbf{The Case Study:}

Mid-afternoon on a very hot day in August in the southern United States,
a mother, her two children and a friend entered a hotel water park,
anticipating a relaxing afternoon in the water playground. This was not
intended as an area for adults except as observers and guardians for
their children. It was filled with several slides, and many spray
venues, both overhead and ground level. In a corner of the area was a
circular hot tub, a carry-over from the previous facility which was more
adult-centric with a full pool.

The temperature was 100\selectlanguage{ngerman}° on this cloudless day following a lengthy
period of extended heat wave. The environment looked very appealing. The
mother and her children sat on recliners in the vicinity of the hot tub
which was empty at the time and still. The mother assumed because the
jets were not on that the tub had been turned off. The younger children
asked if they could go into the hot tub. The mother consented, and the
two youngest entered the tub using the stairs. The youngest child exited
to play in the spray features, and the 6-year-old, a skilled swimmer,
remained in the tub seated on the bench edge inside the tub with water
nearly reaching her neck. The mother was observing all of the children
across the area while her oldest child departed to change into his swim
suit. As he returned from the changing room, he saw that his sister was
face down in the tub and called to his mother. The mother and brother
immediately pulled the unconscious girl from the tub onto the concrete
pool deck, calling for help. A nurse visiting in the pool area was
alerted to the situation, ran to the child and assessed her, finding no
pulse or respiration. She asked for a towel to be placed under her and
immediately initiated CPR with another medical professional pool guest
assisting. 911 was called. After several cycles of CPR, the young girl
began breathing and her pulse returned, She vomited frothy material, but
she remained unconscious. EMTs arrived and assessed the situation,
supplying oxygen via facemask, and noting labored respiration, with
lungs, sounding wet. They recorded that her skin was very hot to touch
and that the young girl was unconscious with a Glasgow score of 8. She
was then evacuated on a to the closest emergency room after their
arrival. In route she had a seizure, without any prior seizure history.
Her temperature on ED arrival was 104.7° F. She remained very febrile
for a period of days with very low consciousness scores. Medical imaging
demonstrated significant brain injury. After a series of transfers
between hospitals over a period of several weeks she was discharged to
home with very apparent severe brain damage. Years later, she remains
very impaired.

\textbf{The Physical Environment:}

The hot tub thermostat had been set at 102°F, although on the day prior
and subsequently during the early morning, the County inspector had
measured the actual tub temperature at 104°F. This event happened in
mid-late afternoon, allowing solar gain to heat the hot tub well beyond
104°F. A systems engineer using finite element analysis estimated the
tub temperature to be between 106°-109° F. on the day of the event.
Using the same methodology, the temperature of the surrounding concrete
was estimated at 139°F. As a consequence of the solar gain on both tub
and concrete, this was a particularly dangerous environment for the risk
of hyperthermia.

\textbf{The Physiology:}

\emph{Hyperthermia}

Hyperthermia in children is quite common, potentially occurring
post-exercise, and environmentally, such as when inside enclosed
cars.\textsuperscript{1} The number of hot tub thermal injuries is
unknown. That said, at a temperature of 34°C, the only effective means
of body cooling is through the evaporation of perspiration, and within a
hot water environment there is no evaporative loss.\textsuperscript{2,
3} Since there is no effective biologic means of temperature reduction,
hot water immersion poses a substantial potential physiologic risk for
both children and adults. Children are more susceptible to hyperthermia
for several important reasons. Children's skin is thinner than adults,
and their surface area to body mass is much
higher.\textsuperscript{4}During heating, children also have elevated
skin temperatures when compared to adults, indicating higher levels of
peripheral vasodilatation.\textsuperscript{4} This allows faster heat
transport (but also faster heat return as in this case.) Children also
have a lower total blood volume per body mass than adults, presenting
decreased cooling opportunity and also a potential for decreased central
and brain blood flow when blood is diverted from central circulation to
the periphery.\textsuperscript{4} The combination of these two
vulnerabilities means that children raise their core temperatures much
faster than adults and thus face a faster and higher risk of
hyperthermia, especially when the peripheral circulatory increase only
serves to return heated blood into the circulation.\textsuperscript{5}

\emph{Cardiac Output}

Simple aquatic immersion produces a long-recognized increase in cardiac
output.\textsuperscript{6-10} This has not been studied in children
during immersion. In land-based exercise in warm environments children
do show increase in cardiac output but to a lesser degree than in
adults.\textsuperscript{4} In a warm or hot environment, the first
response of the body is to increase peripheral circulation, especially
to skin surfaces to facilitate sweating.\textsuperscript{3} The
magnitude of this response is substantial, increasing peripheral
circulation 15 cm\textsuperscript{3}/min for each 0.01°C rise in body
temperature, with potentially a 25-fold increase in cutaneous blood
flow.\textsuperscript{3, 11} Children have higher peripheral blood flow
than adults in comparable high temperatures.\textsuperscript{4} While
core temperature rises far more slowly than skin temperature, it is
highly likely that the initial neurologic signaling from the skin is
what triggers this response because this happens far quicker to protect
thermal homeostasis. Work done in our laboratory demonstrated nearly a
1200\% increase in peripheral circulation within the first 4 minutes of
warm water (39°C) immersion in young subjects, nearly double that seen
in an older cohort.\textsuperscript{12, 13} This increase in peripheral
blood flow is facilitated by the increase in cardiac output, but the
redistribution of blood reduces the blood flow to central structures and
the brain.\textsuperscript{3} The speed with which this occurred was
dramatic, with immediate increase in blood flow within seconds. In a
warm water environment, the heated blood returns uncooled and even
heated to the brain, signaling a demand for yet more peripheral blood
flow, creating a vicious cycle until maximal peripheral flow is
achieved.\textsuperscript{3}

\emph{Brain Metabolism and Blood Flow}

Despite the brain occupying only 2-3\% of total body mass, the brain
consumes around 20\% of total body oxygen
consumption.\textsuperscript{11} The brain has a very high overall
metabolic rate, a single neuron consuming many hundred-fold the power
consumption of an average body cell.\textsuperscript{14} This high level
of metabolism causes the brain to normally operate at temperatures
0.3-0.8°C above core temperature, and this figure can rise substantially
during increased demand periods, and particularly during
hyperthermia.\textsuperscript{14} This temperature increase is normally
managed by cerebral blood flow (CBF) bringing in cooler blood, so that
there is a differential between arterial inflow and warmed venous
outflow. In addition to providing oxygen and other needs to the brain,
it also transports heat away from it.\textsuperscript{15} Recent
publications on young adults have documented an increase in the cerebral
metabolic rate of oxygen consumption (CMRO\textsubscript{2)} of 10-20\%
during hyperthermia.\textsuperscript{15, 16}

Simple immersion and immersed exercise increases brain blood flow by
significant amounts, although human studies to date have not yet
included children.\textsuperscript{17-19} The studies demonstrating this
used thermoneutral water temperatures rather than warm water. The normal
increase in CBF from thermoneutral immersion is overwhelmed in the face
of hyperthermia. The normal physiologic response to hyperthermia uses
two mechanisms of cooling: hyperventilation and increasing cerebral
blood flow. Hyperventilation typically produces a reduction in arterial
CO\textsubscript{2} pressure (PaCO\textsubscript{2}), which causes
cerebral vasoconstriction.\textsuperscript{11} This is one factor in
reducing CBF. A 2020 study of passive thyperthermia corrected for this
respiratory alkalosis by using an end-tidal forcing apparatus to correct
PaCO2 to normothermic levels and found essential the same effects on
cerebral metabolism elevation and cerebral blood flow
reduction.\textsuperscript{16} As a consequence of a reduction in
cerebral blood flow with a simultaneous increase in cerebral metabolism
there is very real potential for cellular injury to brain cells. When
brain temperatures reach 40°to 41°C, particular cell lines are damaged
and when reaching 42°C, irreversible damage occurs.\textsuperscript{3,
20} The duration of hyperthermia is particularly relevant in promoting
cellular survival, so aggressive rapid cooling measures are
essential.\textsuperscript{21}

\emph{Syncope}

Disorders of cerebral function and syncope can occur with decreases of
CBF below 60\% of normal. When CBF drops to only 40\% of normal
unconsciousness occurs in seconds.\textsuperscript{3} Syncope results
during such a loss of CBF, and is exacerbated by the increased
O\textsubscript{2} demands of a hypermetabolic
brain.\textsuperscript{11}

\emph{Drowning}

Normally respiration does not cease with syncope, and on land when the
person collapses, cerebral blood flow is increased, resulting in return
to consciousness. When syncope occurs in an aquatic environment, the
body attempts to breathe, but instead of taking in air, water enters the
airway, triggering laryngospasm and consequent inability to breathe.
This initiates the drowning process.\textsuperscript{22} With increasing
hypoxia, the laryngospasm abates and further aspiration of water may
occur. This often (30-50\%) results in post-drowning pneumonia in
survivors.\textsuperscript{22} Such was the case in the case report
above. When drowning occurs in cold water environments, the body and
brain may cool sufficiently to prevent massive cerebral damage, but in
heated water, the increase in brain metabolism combined with hypoxia may
be catastrophic.\textsuperscript{22}

\emph{Hyperthermia following Cerebral Injury}

In the case report following immersion in an excessively heated body of
water, the initial

hyperthermia was a passive consequence of that environment. It was
followed by a prolonged period of resuscitation while laying on 139°C
concrete for at least 20 minutes, resulting in burns to her back and
extremities. As soon as her heart resumed function following CPR, the
superheated blood in her trunk and extremities was returned to her
central circulation and brain. Once she was hospitalized, aggressive
attempts to cool were made. Over the following week or more, her
temperature fluctuated dramatically. Traumatic brain injury is
associated with aseptic hyperthermia in 30-70\% of pediatric cases, and
is closely associated with the magnitude of brain injury and subsequent
recovery.\textsuperscript{20, 23} As a result, current best practice is
to provide cooling to normothermic or lower levels during the period
following resuscitation of cardiac arrest or hospital care following
TBI.\textsuperscript{23} The longer the brain is allowed to remain in a
hypermetabolic state, the poorer the ultimate
recovery.\textsuperscript{23}

\emph{Brain Injury}

The primary temperature sensitive elements of neural cells are
mitochondrial and plasma membranes.\textsuperscript{20} The protein
structures within these structures become disrupted and may undergo
irreversible abnormalities at temperatures even above
40°C.\textsuperscript{20} This is potentiated by
hypoxia.\textsuperscript{20} Children with their rapidly developing
brain cells may be at greater risk than adults, and while some cell
death occurs quickly, other cells may undergo a prolonged period of
apoptosis.\textsuperscript{20} Purkinje cells seem to be the most at
risk, showing cellular changes to the greatest
extent.\textsuperscript{20} As a consequence, the cerebellum may undergo
the greatest damage with loss of motor control.\textsuperscript{20} This
has been the case in the subject described above, with on-going deficits
in gait function, bimanual motor control, and coordination.

\textbf{Discussion}

The frequency of the events described above is unknown; there is no
national registry of pediatric drownings which includes
causation.\textsuperscript{24} Drowning is the leading cause of injury
and death in the USA between the ages of 1-4 and the
3\textsuperscript{rd} leading cause in the ages from 5-19 years of
age.\textsuperscript{24} Supervision remains the most essential means of
protection, but the Model Aquatic Health Code developed by the CDC is
both based on science and standards for safety of operation. The code
states maximum temperature ``shall not exceed
104°F.''\textsuperscript{25} The policy statement from the American
Academy of Pediatrics referenced fails to mention hyperthermia risk from
hot tub use.\textsuperscript{24} In the case scenario, these standards
were not met: the hot tub temperature was substantially in excess of
this code, and in actuality signage within the pool area failed to
describe risks of overheating. There were no depth markers thus leaving
visitors little forewarning of such risk. While the age of the case
described was over the age of 5, below which hot tub use is warned
against by the CDC, she was close enough to have raised concerns. The
presence of a hot tub with heater set to 104°F did not include any means
of cooling should that temperature be exceeded, and it is unclear as to
whether there was any means of alerting operators if this temperature
was exceeded. There were no thermometers in the hot tub. While the
mother did not accompany the child into the tub, she had believed that
the tub was off, as the jets were not working, and her child was a
skilled swimmer.

Hyperthermic syncope can occur suddenly in both children and adults,
with potentially catastrophic results. It is poorly recognized as a risk
by medical professionals, aquatic facility supervisors, lifeguards, and
the general public. This case demonstrated the risks, the unfortunate
outcome and the underlying physiology behind those risks.

\textbf{Prevention}

Far broader awareness of the risks of hyperthermia is needed by medical
professionals, the lay public and aquatic facility owners. This event
could have been prevented by a mother alerted to the risks presented by
an overheated hot tub, made aware by signage and by available
thermometers showing actual tub temperature. The aquatic attendant
should have been aware of both the tub temperature and the risks thus
presented, and equipped to manage adverse events. Facility management
should have made adaptations to the hot tub to avoid such risks through
appropriately covering the area to reduce solar gain, and setting the
temperature no higher than 100°F, as described in the Model Aquatic
Health Code. Facility management should also have had better oversight
of the tub vicinity than a single attendant over the entire large area.

\textbf{Funding Credits and Disclosure of Potential and Real Conflicts
of Interest}

No external funding was used in preparation of this manuscript. I have
no real or potential conflicts of interest.

\textbf{References:}

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\textbf{23.} Natale JE, Joseph JG, Helfaer MA, Shaffner DH. Early
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\textbf{25.} CDC. Model Aquatic Health Code. 2018;
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\textbf{Data Availability:}

Data sharing not applicable-no new data generated

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