Transcript Anesthesia at the Extremes of Altitude and Environment

Anesthesia at the Extremes of
Altitude and Environment
Major Eric Weissend, M.D.
Department of Anesthesiology
Wilford Hall Medical Center
Lackland AFB, Texas
Environmental Challenges in the
Practice of Anesthesiology
• Air Force anesthesiology providers are now
going far and wide in support of combat and
humanitarian operations. The majority of
postings are to minimally developed areas
where patients and providers are subject to
environmental extremes.
Environmental Challenges in the
Practice of Anesthesiology
• Deployment presents numerous personal
and professional challenges. Caring for
trauma and surgical disease in the deployed
environment can be physically and
intellectually challenging. The extremes of
heat, cold, and altitude further complicate
the care of our patients.
Environmental Challenges in the
Practice of Anesthesiology
• Military operations in Afghanistan and Iraq
serve to illustrate theses types of
environments.
Heat
• Iraq: Daytime temperatures in the summer
regularly reaching well over 100 degrees F.
Cold
• Afghanistan, Operation Anaconda: Soldiers
fought for extended periods in temperatures
well below freezing in the mountainous
Shah-I-Khot region.
Altitude
• Afghanistan, Operation Anaconda (again):
Combat Ops took place between 8,000 and
12,000 feet above sea level.
• Bagram Air Base located at 5,000 feet
above sea level.
Environmental Challenges in the
Practice of Anesthesiology
• To provide safe and effective anesthesia
services to our patients we must understand
the effects that extremes of heat, cold, and
altitude have on our patients, ourselves, and
our equipment.
How Does Excessive Ambient
Heat Effect Anesthetic Practice?
• Effects on volatile anesthetics
• Heat injuries
Inhaled Anesthesia and Heat
• Temperature effects vaporizer output minimally in
normal ranges of temperatures.
• All vaporizers in use in the EMEDS and MFST
systems are temperature compensated.
• In a consistently climate controlled environment
altered output should not be an issue.
Narkomed M
Inhaled Anesthesia and Heat
• Narkomed M: Currently the Air Force standard
anesthesia machine for field anesthesia operations.
This machine is equipped with the Draeger Vapor
2000 anesthetic vaporizer.
• Draeger Vapor 2000 vaporizer is temperature
compensated with an operating range of 10 to 40
degrees C (50-104 degrees F)
Ohmeda Portable Anesthesia Circuit
(PAC) with Draw-over Vaporizer
Inhaled Anesthesia and Heat
• The Ohmeda Portable Anesthesia Circuit
(PAC) Draw-Over Vaporizer System
(primarily still in use with MFST).
• Operating temperature for the PAC
vaporizer is 18 to 35 degrees C (65-95
degrees F).
Inhaled Anesthesia and Heat
• Use above this ambient temperature range
may lead to “potentially hazardous
excessive concentrations of anesthetic
agent.”
Inhaled Anesthesia and Heat
• “Under no circumstances must the
temperature of the anesthetic agent reach
boiling point, as the output concentration
will then become impossible to control.”
• The boiling points for isoflurane, halothane,
and sevoflurane are 48.5, 50.2, and 58.5
degrees C (119, 122.4, 137 degrees F)
respectively at 760 mm Hg.
Inhaled Anesthesia and Heat
Inhaled Anesthesia and Heat
• Is it conceivable that in Iraq, in July, the
HV/AC system may fail intraoperatively?
If using volatile anesthetics at
high ambient temperatures
• Ensure you are operating in a consistent
climate controlled environment.
AND/OR
• Use only with end tidal anesthetic gas
monitoring (RGM or other) to minimize the
risk of volatile anesthetic overdose.
Inhaled Anesthesia and Heat
• There currently is no means of monitoring
inspiratory or expiratory anesthetic gas in
any Air Force deployable anesthetic system.
Other Anesthetic Options at High
Ambient Temperatures
• Total Intravenous Anesthesia
• Regional Anesthesia
Neither method is known to be effected
by high ambient temperatures.
Heat Injuries
• Heat illness is the inability of normal regulatory
mechanisms to cope with a heat stress
• Minor injuries include muscle cramps, edema,
rash, syncope, and tetany
• Major injuries are heat exhaustion and heat stroke
• All heat injuries are manifestations of
dehydration
Heat Injuries
• Patients who are injured in and evacuated
from areas with high ambient temperatures
may suffer heat injuries in addition to their
traumatic wounds.
• Medical personnel suffering heat injuries
may have difficulty or even be unable to
care for their patients.
Heat Injuries
• Any condition that increases heat gain or
decreases heat loss may result in a major
heat illness.
• Hot environments and physical exertion
increase the heat load.
• Strenuous exertion can increase endogeonous heat
production ten to twenty-fold.
• High temperatures and high humidity
inhibit heat loss.
Heat Injuries
Photo by Wally Santana/The
Associated Press
Heat Injuries
• Peripheral vasodilation and sweating are
the primary mechanisms of heat loss
• Evaporation of sweat from the skin is the
most important mechanism of heat
dissipation.
• As humidity increases, the efficiency of
sweating decreases.
Heat Injuries
Heat Exhaustion
• Caused by dehydration with inadequate
fluid and electrolyte replacement.
• Usually in nonacclimatized persons who
have been working in the heat for several
days.
Heat Injuries
Heat Exhaustion
• Symptoms
Weakness, fatigue, frontal headache, impaired
judgement, vertigo, nausea and vomiting, muscle
cramps
Orthostatic dizziness and syncope
Sweating persists, often profuse
Core temperature less than 40 C
No signs of severe CNS damage
Heat Injuries
Heat Exhaustion
• Volume depletion is the primary problem
• Treatment
Rest in cool environment
Fluid resuscitation
Heat Injuries
Heat Stroke
• A catastrophic life threatening medical
emergency
• The failure of normal homeostatic cooling
mechanisms
• Leads to extremely high temperatures
(>40.5C), multisystem tissue damage and
organ dysfunction.
Heat Injuries
Heat Stroke
• Symptoms
• Profound CNS dysfunction is the Hallmark
Delerium and coma are common
Any neurologic manifestation is possible
• Dry hot skin, though sweating can persist
• Cardiovascularly hyperdynamic
• Hepatic dysfunction with massive rise in
transaminases
• Coagulopathy
• Renal damage with acute renal failure in up to 30%
of cases.
Heat Injuries
Heat Stroke
• Treatment
• Core Temperature Cooling
Evaporative cooling with fans and skin wetting
Ice-water immersion
Ice packs, cooling blankets, cool body
cavity lavages
• Supportive Therapy
Airway management (aspiration and seizures
are common)
Resuscitation and invasive monitoring
Anesthesia At Altitude
• As altitude increases atmospheric pressure
decreases.
• Decreased atmospheric pressure has
profound effects on inhaled anesthetics and
human physiology.
• Safe and effective anesthesia care requires
an understanding of all of these effects.
Anesthesia At Altitude
• The composition of the atmosphere is fixed
and is independent of altitude. Oxygen is
always ~21% of the ambient atmosphere
pressure.
• As atmospheric pressure decreases with
elevation however, the partial pressure of
oxygen (PO2) declines.
Anesthesia At Altitude
• Recall the alveolar gas equation:
PAO2=FiO2(PB-PH2O)-PaCO2/RQ
At 5000ft elevation, PB is 632 mmHg, PaO2
is 81 mmHg with SaO2 95%.
At 10,000ft elevation, PB is 522 mmHg,
PAO2 is 59 mmHg, SaO2 84%.
Oxygen-Hemoglobin Dissociation Curve. Approximate
oxygen saturations are marked for several altitudes
Sutton JR, et al: J Appl Physiol
64:1309, 1998
Anesthesia At Altitude
• “In addition, it is important to maintain a higher
concentration of oxygen both during and after
administration of the anesthetic to support
adequate oxygenation. It is suggested that 30%
oxygen be the minimum at 5000 ft and that 40%
oxygen be the minimum at 10,000 feet, for both
intraoperative anesthetic management and
postoperative recovery.”
Anesthesia At Altitude
• Recommendations for anesthesia at altitude: “The
major risk of anesthesia at high altitude is that
anesthetized patients can become hypoxic despite
the fact that adequate oxygen concentrations are
being administered.”
Anesthesia At Altitude
• Nitrous Oxide
• Essentially irrelevant. Unlikely to be available in
the deployed environment. Efficacy of N2O is
decreased by 50% at 5000 ft and essentially
insignificant at 10,000 ft.
Anesthesia At Altitude
• Volatile anesthetic agents
• “The saturated vapor pressure of a volatile
anesthetic agent depends only on temperature and is
practically independent of total environmental
pressure”
Anesthesia At Altitude
• Given the relative scarcity of gaseous (or liquid)
oxygen in the deployed environment it may be
reasonable to conduct as much anesthesia under
regional techniques.
• At altitude, maximizing the use of regional
anesthesia not only decreases use of scarce
resources, but may improve patient safety
postoperatively. Minimizing opioid use decreases
the risk of postoperative respiratory depression.
Anesthesia At Altitude
• If general anesthesia is required oxygen
requirements may be minimized using TIVA
techniques.
Altitude Illness
• Military personnel deployed rapidly to high
altitude regions are all at risk for altitude related
illnesses.
• High altitude begins at 1500m (~5000ft) above sea
level.
• Very high altitude begins at 3500m (~11,500 ft)
• Extreme altitude begins at 5500m (~18,000 ft)
Altitude Illness
• Physiologic adjustment to altitude requires
time and patience.
• Sudden exposure to very high and extreme
altitude (above 11,500 ft) can be fatal.
• Unconciousness can occur within minutes
and death may follow without supplemental
oxygen.
Physiologic Response to Altitude
• Lower PB leads to lower PAO2, decreased
SAO2 and PaO2 and elevated Alveolararterial oxygen gradients.
• Hypoxic Ventilatory Response to low PaO2
leads to hyperventilation.
• Hyperventilation leads to decreased PaCO2.
Physiologic Response to Altitude
• As hyperventilation is the primary means of
adaptation to ascent, the ability to tolerate
hypoxic environments depends largely on
sufficient pulmonary reserve.
Physiologic Response to Altitude
• 2,3-DPG levels rise due to hypoxic stress,
shifting O2-Hgb dissociation curve back
toward the right. This facilitates O2
unloading into tissues.
• Erythropoiesis
• Increased cardiac output secondary to
Hypoxia
Altitude Illness
• High Altitude Illness can take several forms
that often overlap and share common
pathophysiology.
• Acute Mountain Sickness (AMS)
• High Altitude Pulmonary Edema (HAPE)
• High Altitude Cerebral Edema (HACE)
Acute Mountain Sickness
• All visitors to higher altitudes are
susceptible to AMS.
• Overexertion, poor hydration, and young
age may contribute. Physical fitness and
gender don’t seem to effect incidence.
Acute Mountain Sickness
• Symptoms:
• Early symptoms (12-24 hours): headache refractory
to standard analgesics, nausea, anorexia, lassitude,
sleep disturbances.
• Can progress to shortness of breath, intense snoring,
vomiting, hallucinations, and impaired cognitive
function,
• Advanced symptoms: severe dyspnea, cyanosis,
decreased SaO2, ataxia.
Acute Mountain Sickness
• Definitive treatment is descent.
• Often descent of 500 to 1000m leads to
complete resolution of symptoms.
• Rest, hydration, analgesics, oxygen all can
help.
• Acetazolamide 250 mg q 8-12 hours
improves symptoms and SaO2 (especially
during sleep)
Acute Mountain Sickness
• Prevention
• Ascend slowly, not always possible in military ops
• Daily altitude gain of no more than 300m above
3000m.
• After ascending 1000m spend two consecutive
nights.
• Rest on arrival at altitude, avoiding overexertion,
adequate hydration
• Acetazolemide 250mg q8 hours beginning at least
24 hours before ascent and continued for 2 to 3 days
after reaching highest altitude.
High Altitude Pulmonary Edema
(HAPE)
• A malignant form of AMS with similar early
symptoms. Life threatening.
• May occur in any healthy individual after rapid
ascent above 2500 m (8200 ft)
• Dyspnea, tachypnea, chest pain, rales,
tachycardia,dry cough, followed by the production
of pink frothy sputum
• Respiratory failure and death can quickly ensue.
High Altitude Pulmonary Edema
(HAPE)
• CXR shows patchy infiltrates, which spare
lung bases and costophrenic angles.
• Elevated pulmonary artery pressure
secondary to hypoxia.
• ECG shows right heart strain
• LV function is normal
High Altitude Pulmonary Edema
(HAPE)
• Treatment
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Rapid descent to lower altitude
Supplemental O2
Morphine ?
PEEP
If descent is not possible, consider Gamow bag
High Altitude Cerebral Edema
(HACE)
• Another severe form of AMS, also be life
threatening.
• Thought to be due to increased cerebral
blood flow and alterations in blood-brain
barrier permeability (due to severe
hypoxemia)
• Early symptoms similar to AMS.
High Altitude Cerebral Edema
(HACE)
• Early symptoms
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Headache
Anorexia
Nausea
Emesis
Photophobia
Fatigue
Irritability
Decreased socialization
• Late symptoms
• Ataxia (appendicular to
truncal)
• Irrationality
• Hallucinations
• Visual disturbances
• Focal neurological
deficits
• Abnormal reflexes
High Altitude Cerebral Edema
(HACE)
• Patients may have concurrent HAPE
symptoms
• Death may be imminent when symptoms of
HACE become severe
High Altitude Cerebral Edema
(HACE)
• Lumbar puncture may show markedly
elevated CSF pressure.
• CT suggestive of brain edema
High Altitude Cerebral Edema
(HACE)
• Treatment
• Immediate, rapid descent
• Dexamethasone 10 mg IV or IM, then 6 mg q 6 hrs.
• Supplemental O2, may be helpful if pulmonary
symptoms are present
• Diuretics may reduce brain edema, but may worsen
an already dehydrated state
The Gamow Bag
The Gamow Bag
• Portable, lightweight, fabric hyperbaric
chamber.
• Can generate 103 mm Hg of pressure above
ambient pressure.
• Simulates a descent of 4000 to 9000 ft at
moderate altitudes.
Cold Injuries
• Frostbite
• Hypothermia
• Defined as core temperature below 35 degrees C
Cold Injuries
Cold Injuries
Frostbite
• Peripheral vasoconstriction limits radiant
heat loss in cold ambient temperatures
• Occurs when tissue temperature decreases
to less than 0 degrees C
• Ice crystal formation leads to cellular architectural
damage
• Microvascular stasis and thrombosis
• Extent of injury is determined by duration and
extent of cold contact with the skin
Cold Injuries
Frostbite
• Distal extremities, nose, ear, and penis are
most at risk
• Numbness is most common presenting
symptom
Cold Injuries
Frostbite
• Treatment
• Rapid rewarming by immersion bath in 37-40
degree C water. Reheating with static heat is much
more injurious to tissue.
• If patient requires surgical care and anesthesia allow
for passive rewarming to minimize risks of
worsening injury.
Cold Injuries
Hypothermia
• Mild: Core body temperature 32-35 C
• Excitation stage, to retain and generate heat (shivering,
increased heart rate, cardiac output, and blood pressure)
• Moderate: 30-32 degrees C
• Slowing stage, to decrease oxygen utilization and CO2
production (shivering ceases, HR. CO, BP all decrease)
• Severe: Below 30 degrees C
• ECG changes and dysrhythmias (Osborn J waves T-wave
inversions, PR, QRS, and QT prolongation, sinus bradycardia
to atrial fibrillation with slow ventricular response to
ventricular fibrillation to asystole)
Cold Injuries
Hypothermia
Osborn J wave/ From Marx: Rosen's
Emergency Medicine
Cold Injuries
Hypothermia
• Other Manifestations
• Pulmonary
Tachypnea, bronchorrhea, diminished cough and gag
reflex (increased aspiration risk)
• CNS
Confusion, lethargy, incoordination, decreased
consciousness, coma
• Leftward shift of Oxyhemoglobin dissociation curve
Impairs release of O2 to tissues
Cold Injuries
Hypothermia
• Other manifestations
• Renal
Decreased renal concentrating abilities leads to “cold
diuresis” and severe dehydration
• Heme
Hemoconcentration, disseminated intravascular
coagulation (decreased enzymatic function at lower
core body temperatures)
• GI
Pancreatitis, decreased hepatic function (impaired drug
metabolism)
Cold Injuries
Hypothermia
• Treatment
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Handle Gently
Oxygen (warmed, humidified)
IV fluids (warmed)
Monitor: core temperature, oxygen saturation,
cardiac rhythm
Dysrhythmias may be refractory to treatment until
patient is rewarmed
Cold Injuries
Hypothermia
• Treatment
• Rewarming
Passive: allow patients to rewarm passively and slowly
Active: rewarm with external (water immersion, radiant
heat, forced warm air heating blankets) and core
techniques (heated IV fluids, body cavity lavage,
cardiopulmonary bypass pump
Resuscitation with Lactated Ringers should be avoided
as the cold liver inefficiently metabolizes lactate
• Neither passive nor active rewarming has been
shown to be superior, however…
Cold Injuries
Hypothermia
• Indications for rapid rewarming
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Cardiovascular instability
Moderate or severe hypothermia (<32.2 C)
Inadequate rate or failure to rewarm
Endocrine insufficiency
Traumatic or toxilogic peripheral vasodilation
Secondary hypothermia impairing thermoregulation.
Cold Injuries
Hypothermia
• As anesthesiology providers we are most
likely to become involved with hypothermia
caring for patients suffering from traumatic
injury in addition to hypothermia.
• Surgical requirements will likely force
active treatment of hypothermia while
undergoing surgical stabilization.
Cold Injuries
• No known effect of cold ambient
temperature to the delivery of any form of
anesthesia.
References
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