Heart Rate Response to Barorecptor Feedback

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Heart Rate Response to Barorecptor Feedback

• Experiment 5 Anatomy and Physiology

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Heart Rate Response to Baroreceptor Feedback
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Baroreceptors

• Special sensors in tissue in aortic arch and
  carotid arteries
• A sudden increase or decrease in stretch sends
  signals to medulla
• Medulla then sends signal through vagus nerve to
  heart to increase or decrease heart rate
• Called a feedback loop

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Disorder

• Orthostatic Hypotension

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Orthostatic Hypotension

• (OH) describes an extreme drop in blood pressure
  that may occur when a person stands up suddenly
  and the blood pools in the blood vessels of the
  legs. Because of this pooling, the amount of
  blood carried back to the heart by the veins is
  decreased. Subsequently, less blood is pumped out
  from the heart, resulting in a sudden drop in
  blood pressure.

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Orthostatic Hypotension

• By definition, the drop in blood pressure must be
  greater than 20 mm of mercury during contraction
  of the heart muscles (systole) and more than 10
  mm of mercury during expansion of the heart
  muscles (diastole).

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Orthostatic Hypotension

• Among children and teenagers, short-lived
  episodes of OH are normal and not uncommon.
• Episodes among the elderly are always to be taken
  seriously.

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Orthostatic Hypotension

• Normally, specialized cells in the body
  (baroreceptors) quickly respond to changes in
  blood pressure.
• These baroreceptors then activate the autonomic
  nervous system to increase, via reflex action,
  levels of catecholamines (e.g. epinephrine,
  norepinephrine) in the body.
• Increased catecholamine levels rapidly restore
  the blood pressure.
• A defect in this spontaneous response (reflex),
  prevents the heart rate and blood pressure from
  rising adequately and orthostatic hypotension
  results.
• Fainting and falling are the usual consequences.

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Orthostatic HypotensionCauses

• Hypovolemia (a decreased amount of blood in the
  body), resulting from the excessive use of
  diuretics, vasodilators, or other types of drugs,
  dehydration, or prolonged bed rest.
• Addison's disease
• Atherosclerosis (build-up of fatty deposits in
  the arteries)
• Diabetes
• Certain neurological disorders including
  Shy-Drager syndrome and other dysautonomias.
• Pregnancy

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Orthostatic HypotensionTreatments

• When orthostatic hypotension is caused by
  hypovolemia due to medications, the disorder may
  be reversed by adjusting the dosage or by
  discontinuing the medication.
• When the condition is caused by prolonged bed
  rest, improvement may occur by sitting up with
  increasing frequency each day.
• In some cases, physical counterpressure such as
  elastic hose or whole-body inflatable suits may
  be required.
• Dehydration is treated with salt and fluids.

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Career

• Hypertension Specialist

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Hypertension Specialist

• Physicians with expert skills and knowledge in
  the management of clinical hypertension and
  related disorders.
• These physicians can act as local and regional
  consultants for the more complex and difficult
  cases and also assist in advice regarding
  guidelines and process improvement.
• 1115 physicians practicing in the United States,
  Canada and other countries.

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Hypertension Specialist

• Successful treatment of hypertension is among the
  most effective means to prevent cardiovascular
  morbidity and mortality.
• Unfortunately, nearly 70 of all hypertensive
  persons in the United States do not achieve the
  goal of lt140/90 mmHg..
• Most hypertensive patients can be effectively
  managed by primary care providers internists,
  family practitioners and other providers (i.e.
  supervised nurse clinicians and physician
  assistants).
• A fraction of the hypertensive population
  requires specialized expertise for optimal care
  due to a variety of conditions.
• It has become apparent that the increasingly
  overburdened primary care practitioner, facing a
  broad variety of medical problems in daily
  practice, can be helped by experts functioning as
  a resource for advice.

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How can I become a Hypertension Specialist?

• Be a currently licensed physician in the United
  States or Canada
• Be certified by a relevant primary care board
  (American Board of Medical Specialties (ABMS) or
  equivalent)
• Be certified by a relevant subspecialty board
  (ABMS or equivalent) (e.g., cardiovascular
  disease, nephrology, endocrinology)
• Demonstrate appropriate experience and practice
  activity in the management of complex
  hypertension problems (e.g., letter from Chief of
  Service or other recognized senior physician).
• Pass the Qualifying Examination for Specialists
  in Clinical Hypertension

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Can you answer this question correctly on the
Hypertension specialist exam?

• A 42 year old man, seen for a check-up has a
  blood pressure of 135/85 mmHg, with a body mass
  index of 29, and no symptoms. He reports
  occasional exercise and consumption of 4-5 beers
  per week. The exam and usual tests are normal.
  Which of the following lifestyle interventions is
  the best strategy for management?
• Weight loss
• Stress management
• Decreased alcohol intake
• Dietary supplementation with potassium

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Congrats, you have become a hypertension
specialist!!!!

• The answer is A
• The body mass index of 29 is considered overweight

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Technology

• Astronauts can have orthostatic hypotension!!
• Here is an audioclip explaining the problems
  astronauts can have.

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Astronauts and Space

• Astronauts experience orthostatic hypotension
  because of the way human bodies respond to
  gravity, explains Richard Cohen of the
  Harvard-MIT Division of Health Sciences and
  Technology. (Cohen leads the Cardiovascular
  Alterations Team at the National Space Biomedical
  Research Institute, or "NSBRI.") On Earth gravity
  pulls blood toward the lower body. But in space
  -- either in free-fall or far from a source of
  gravity -- blood that normally pools in the legs
  collects in the upper body instead. That's why
  astronauts have puffy-looking faces and spindly
  "chicken legs."

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Astronauts and Space

• Astronauts don't feel orthostatic hypotension
  while they're traveling through space, but they
  do begin to feel it during re-entry (when
  g-forces mimic gravity) and after landing. Blood
  returns to the lower body and blood pressure to
  the head is suddenly reduced. Hence the
  dizziness. (The sensation can continue for a
  while after landing, too.)
• It's a classic case of "use it or lose it."
  Veins in human legs contain tiny muscles that
  contract when the veins fill with blood. Their
  function is to send blood uphill toward the heart
  and so maintain blood pressure. But in space
  there is no "uphill," so those tiny muscles in
  the veins are less-used -- a normal adaptation to
  weightlessness.

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Astronauts and Space

• For many years astronauts have tried to
  counteract orthostatic hypotension by drinking
  lots of salt water, which increases the volume of
  bodily fluids.(There is a general loss of body
  fluids during space missions.) Astronauts also
  wear "G-suits" -- rubberized full-body suits that
  can be inflated with air. This action squeezes
  the extremities and raises blood pressure.
• Such countermeasures are only partially
  effective. "Almost all returning astronauts
  experience changes in gait and balance, explains
  NASA Chief Medical Officer Rich Williams.
  Nevertheless, "most are able to walk around just
  fine. A small number experience orthostatic
  changes that render them quite dizzy."

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Astronauts and Space

• An anti-dizzy pill would be helpful, but until
  recently there was no such thing.
• Enter Midodrine Midodrine is the first drug
  approved by the United States Food and Drug
  Administration to treat orthostatic hypotension.
  It constricts blood vessels and so increases
  blood pressure. "By increasing blood pressure
  when patients need it, Midodrine can help people
  lead a more normal life," writes Low.
• Cohen thinks it might help astronauts, too. An
  anti-dizzy pill would be helpful, but until
  recently there was no such thing.

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Astronauts and Space

• An important advantage to Midodrine is that it
  can be administered just before re-entry or even
  after landing. The benefits are immediate.
  Astronauts wouldn't have to take it throughout
  their mission when it might interfere with their
  body's own (and welcome) adaptations to zero-g.
• "A flight study protocol has recently been
  approved to test the drug's effects on space
  shuttle astronauts and ISS crewmembers," says
  Cohen.
• Perhaps soon astronauts returning home from space
  will feel lightheaded -- but only due to elation.
  Orthostatic hypotension will have nothing to do
  with it.

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Experiment 5 Heart Rate Response to Barorecptor
Feedback

• In this experiment, you will observe heart rate
  response to squatting and to standing from a
  squatting position. In squatting, there is a
  rapid increase in venous return to the heart as
  veins in the leg muscles are compressed. This
  causes a sudden increase in stroke volume and
  pressure sensed by the baroreceptors. In standing
  from a squatting position, there is a sudden
  reduction in venous return to the heart because
  of pooling of blood in the legs. This results
  in a decrease in stroke volume and pressure.
• Have fun!!

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References

• http//liftoff.msfc.nasa.gov/news/2002/news-dizzy.
  asp
• http//www.dizziness-and-balance.com/disorders/med
  ical/orthostatic.html
• http//www.aafp.org/afp/20031215/2393.html
• http//www.ninds.nih.gov/disorders/orthostatic_hyp
  otension/orthostatic_hypotension.htm
• http//www.bchealthguide.org/kbase/nord/nord769.ht
  m
• http//www.ash-us.org/about/index.htm