What is Pressure? Atmospheric Pressure€¦ · pressure. The physical units for pressure is force...

American Osteopathic College of Occupational and Preventive MedicineOMED 2012, San Diego, Tuesday, October 9, 2012

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Physiological Altitude Threats:

Hypoxia, Trapped Gas, Decompression, etc

Lou Gilleran, MD, MPH, MSHSCAPT, MC (FS), USN

Outline

• Physiology of Decompressive Stress

• Respiratory Physiology

• Hypoxia and necessary protection

• Barotrauma

• Aviation DCS

• AGE

What is Pressure?

• From a microscopic pointof view, gas pressure iscaused by the collisionsof gas molecules on asurface. Each individualcollision provides a tinypush on the surface thatit contacts. The sum totalof all of these tiny forcesdetermines the airpressure. The physicalunits for pressure is forceper area.

Atmospheric Pressure

• Since the air (a gas) is afluid, the pressure forceacts in all directions, notjust downward. Thepressure force pushingdownward due to theweight of the air is thesame as the pressureforce acting sideways andeven upward.

14.7lbs.

One square inchcolumn of air fromSea level to space

Scale lbs Barometer Barometer

760mm Hg

29.92inches Hg

Measurement of Pressure Composition Of The Atmosphere

• Nitrogen 78% 597.0 mm Hg

• Oxygen 21% 159.0 mm Hg

• Other gasses & H2O 1% 4.0 mm Hg

760.0 mm Hg

@ Sea Level


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Atmospheric“Shells”

TroposphereSea level -10 km (7 miles above sea level)

Thermosphere80 km to space

Mesosphere50 km - 80 km (56 miles above sea level)

Stratosphere10 km - 50 km (35 miles above sea level)

Human Operational Parameters

• Man is designed for terrestrial existence up to 10K’

– Troposphere defines this physiologic zone

– Troposphere is where weather, clouds, winds have greatestaffect

• Adiabatic (Lapse) Rate defines temperature reductionof 3.5°F/1,000 ft (6.4°C/km) of altitude increase

18,000 ftSea Level

Life at Altitude

Though theatmosphericpercentages ofgases stay the samewith increasedaltitude the “partialpressure” of gasesdecreases at higheraltitudes

O2 and Altitude

• Lack of O2 is a relatively ineffective stimulus andbegins to occur at about pO2 of 60 mm Hg.– Corresponds to about 10,000 ft. on room air and 39,000 ft.

on 100% O2.

• 1st sign is usually increased tidal volume (depth ofrespiration) “air hunger”

• Elevated pCO2 is the primary stimulus for increasedrespiratory rate

Physiological Responses at Altitude

1) At high altitudes there's less O2, therefore less dissolved O2 inthe blood.

2) The response is stimulation of peripheral chemoreceptorswhich send signals to respiratory drives.

3) Hyperventilation leads to Respiratory Alkalosis.

4) Respiratory alkalosis shift the Hb dissociation curve to the leftso that Hb can pick up O2 easier.

5) The kidney responds to alkalosis by generating H ions and thiswill correct the pH back to normal.

Pre-existing Cardiopulmonary Disease

• FAA allows Cabin Altitude up to 8k’ wherePAO2 is ~65 and O2 Sat 90% in healthy.

• Therapeutic O2 required if needed at baselinealtitude.

– Recommended if unable to climb 1 flight of stairsor walk 150’ w/o rest or dyspnia and

• CHF NYHA Class III-IV or baseline PaO2 <70mm Hg

• Angina CCS Class III-IV

• COPD PaO2 <70

• Medications in carry-on!


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At 10,000 feet,alveolar pO2 is about60 mm Hg. This is onthe steep portion oftheoxygen/hemoglobindesaturation curve.Any additional climbin altitude withoutsupplemental oxygencan result in theinsidious onset ofhypoxia for theaircrewman.

Oxygen-Hemoglobin Dissociation Curve

Physiologic Atmospheric Grouping

Physiologic Deficient Zone10, 000 – 50, 000 ft

_____________________________ 10,000 ft

Physiological ZoneSea Level – 10,000 ft

Healthy individual fit without the aid ofspecial protective equipment

Aircraft Pressurization

Purpose and Types

• To ensure safety and comfort for the crew andpassengers

• Pressurization schedules

Commercial aircraft cabins are required to be pressurized such that the cabin pressure is not

to exceed an altitude of 8,000ft.

System Types

• Conventional

– used currently in all pressurized aircraft

– increase cabin pressure via on-board compressor

• Sealed Cabin

– used in space vehicles

– very high altitudes (80,000 ft)

Pressurization Systems• Isobaric systems maintain a constant cabin altitude

from a preselected altitude (generally from 2000-8000 ft) up to the service ceiling of the aircraft.

• Isobaric-differential systems maintain a constantaltitude until the pressure differential (usually 5psi) isreached and then the differential is maintained up tothe service ceiling of the aircraft.

• Sealed Cabin systems are used only in spacecraft andcarry their own supply of gases to createenvironment.


Advantages

• Reduces possibility of:

– Hypoxia

– Decompression sickness

– Trapped gas problems

• Comfort / mobility / fatigue

• Cabin temperature control

• Communication


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Disadvantages

• Increased operating cost

• Added weight

• Reduced aircraft performance

• Risk of accidental pressure loss


Jet EngineA/CUnit Cabin Outlet Valve

Ambient Air

Bleed AirOverboard

Vent

What Happens When Pressure Fails?• Decompression

– Explosive Decompression (less than 1 sec)

– Rapid Decompression (between 1-10 sec)

– Slow Decompression (greater than 10 sec)The NAVY averages 90 events per year or 2.5 occurrences /

100,000 flight hours

• Factors Affecting Severity– Volume of cabin

– Size of Opening

– Pressure Differential/ Ratio

– Flight Altitude

Physiological Effects of Decompression

• Rapid Decompression– Arterial Gas Embolism (AGE)

– Decompression Sickness(DCS)

– Hypoxia

– Trapped Gas Expansion (TGE)• G.I. Tract• Ears• Sinus• Lungs

• Slow– DCS

– Hypoxia (Insidious)

Arterial Gas Embolism

• Leakage of air from the lungs to thepulmonary circulation.

• Due to pressure – expansion oflungs.

• Pulmonary circulation goes right tothe brain.

• Coronary artery embolization canlead to myocardial infarction ordysrhythmia.

• Cerebral artery emboli can causestroke or seizures

Operational Problems

• Noise: Hissing to Explosion

• Fog: May be mistaken for smoke; decreasedvisual environment

• Flying debris, dust and dirt effect vision

• Temp changes:• Sea Level 70 F

• 25K 20 F

• 35K - 67 F

• 50K - 67 F


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Altitude Threat Protection• Cabin Pressurization System

0 to 8,000ftAircraft ALT = Cabin ALT

35,000ftCabin at 14,500ft

24,500 to 50,000ftCabin ALT = Differential

8,000 to 24,500ftCabin ALT = 8,000ft

(Isobaric)

POSITIVE PRESSURE BREATHING (PPB)

Positive Pressure Breathing (PPB) is the deliveryof a gas to the respiratory tract at a pressure

greater than the ambient.

TYPES of PPB

Intermittent PPB

Pressure applied only during inspiration phase.

Used in SCUBA

Continuous PPB

Pressure applied throughout the breathing cycle

Primary type used in Military aviation

REQUIREMENT for PPB

Based on the level of hypoxia considered operationallyacceptable.

The Navy uses a minimal alveolar pO2 of 60 mm Hg

Reached at 39,000 ft, breathing 100% O2.

PPB is an emergency condition in operational aircraft.

Altitude Threat Protection

• Supplemental Oxygen System-Regulators

Increased PPB

Oxygen (O2)

RESPIRATORY EFFECTS of PPB

1. Distension of lungs and chest

2. Increased Pulmonary Ventilation (+50%)

3. Intrapleural Pressure

4. Breathing Effort

5. Venous Pooling/Distension

6. Reduction of Effective Blood Volume

7. Reduction in Cardiac Output (-30%)


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Applications of PPB

The physiological ceiling is raised from 39,000 ft to45,000 ft.

In a sudden decompression (up to 50,000 ft) PPB canbe used to maintain consciousness in order to effect

an emergency descent.Applications in increased G-tolerance

Disadvantages of PPB

6,000 ft increase is fairly smallPotential physiological problems

Reverses the normal breathing pattern - training required tocompensate

FatiguingCommunications become very difficult

Can induce hyperventilation

Hypoxia

Definition

• State of oxygen deficiency in cells andtissues sufficient to cause impairmentof function.

• Hypoxemia refers specifically to adeficiency of O2 in the blood and willlikely result in hypoxia

Histotoxic(Poisoning)

Stagnant(Pooling)

Hypemic(Blood)O2

O2

O2O2

Hypoxic(Altitude)

Types Of Hypoxia

Hypoxic Hypoxia

Causes:

• Ascent to altitude

• Malfunctioning equipment

• Loss of cabin pressurization

• Improper O2 equipment usage

Diminished O2 available to lungs

Carbon Monoxide

Hemorrhage

Anemia

Sulfa Drugs

Hypemic Hypoxia

Causes:

Reduced O2 carrying capacity of blood


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Carbon Monoxide

Wear and deterioration of airframe seals and opening of seamsincreases susceptibility. This will allow exhaust fumes and toxicgases to infiltrate crew compartment.

Flight personnel effects

– Carboxyhemoglobin (COHb) levels of 15-25% produceheadache and nausea. With prolonged exposure, muscularweakness, dizziness, and confusion occur.

– 25%, electrocardiographic changes, stupor, and eventualunconsciousness will occur.

Stagnant HypoxiaPooling or reduced flow of blood,as seen in heart failure and coldenvironments

Can be caused by inactivity,restriction of movements

Or G-Forces (G-LOC)

Effect of G ForcesEffect of G Forces

Normal Grey/black Out GLOC

Gs

Histotoxic HypoxiaInability of cells to take up or utilize oxygen fromthe bloodstream, despite physiologically normaldelivery of oxygen to such cells and tissues.

• Alcohol

• Medication (Narcotics, etc)

• Cyanide

• CO also Hypemic Hypoxia cause

Factors Influencing Onset Rate

• Cabin altitude

• Rate of ascent

• Duration of exposure

• Individual tolerance

• Physical fitness and activity

• Self imposed and environmental stress

Signs and SymptomsObjective Symptoms

•Increased respiration rate and depth•Cyanosis (may not be evident in anemia)•Pallor•Mental confusion•Poor judgment•Muscle twitching•Cold clammy skin


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Signs and SymptomsSubjective Symptoms

•Air Hunger

•Apprehension

• Fatigue

•Nausea

•Headache

•Dizziness

•Hot and cold flashes

• Euphoria

• Belligerence

• Blurred vision

• Numbness

• Tingling

Stages of Hypoxia

• Indifferent Stage

• Compensatory Stage

• Disturbance Stage

• Critical Stage

Indifferent Stage

• Altitudes:

– Air: 0 - 10,000 feet

– 100% O2: 34,000 - 39,000 feet

• Symptoms: decrease in night vision @ 4000 feet

• acuity

• color perception

Compensatory Stage

• Altitudes:

Air: 10,000 - 15,000 feet

100% O2: 39,000 - 42,000 feet

• Symptoms: impaired efficiency, drowsiness,poor judgment and decreased coordination

Disturbance Stage

• Altitudes:

Air: 15,000 - 20,000 feet

100% O2: 42,000 - 44,800 feet

Critical Stage

• Altitudes

Air: 20,000 feet and above100% O2: 44,800 feet and above

• Signs: loss of consciousness,convulsions and death


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Time of Useful Consciousness (TUC)

Effective Performance Time (EPT)

The period of time from the loss of oxygen supply or

exposure to an oxygen poor environment to the time

when deliberate function is lost

FL 430 & above 9-12 seconds

FL 400 15 - 20 seconds

FL 350 30 - 60 seconds

FL 300 1 - 2 minutes

FL 280 2 1/2 - 3 minutes




Expected Performance Times

put back on oxygen

Time off Oxygen

1 minute

2 minutes

3 minutes

4 minutes

5 minutes

6 minutes

Warning

Effect of Rapid Decompression on TUC/EPT

Time of useful consciousness (TUC) from

hypoxia can be reduced by as much as

30 to 50 percent following a rapid

decompression

Treatment of Hypoxia

• Maximum oxygen under pressure• Connections check – check security• Breathe at a rate and depth slightly less

than normal until symptoms disappear.• Descent below 10,000 feet and land as

soon as conditions permit.


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Oxygen Paradox

• Temporary worsening ofhypoxia symptoms afteradministration of 100%O2

• Caused by physiologicalreaction to a suddenflood of oxygen richblood

• Very rare occurrence

• Remain on 100% O2

Hyperventilation• Definition: An abnormal increase in the rate

and or depth of breathing.

• Causes:• Emotional Stress

• Improper Pressure Breathing

• Altitude hypoxia

• Primary Physiological Results

– Hypocapnia (loss of carbon dioxide [CO2])

– Alkalosis (shift in pH balance)

HYPERVENTILATION - EFFECTS

• Respiratory System

– CO2 + H2O H2CO3 H+ + HCO3-

– Direction of flow is determined by pCO2

– Hyperventilation causes a decrease in pCO2

(hypocapnia), therefore a decrease in H+

– The result is alkalosis


• Central Nervous System– Brain protects itself against chemical imbalance of

blood by restricting blood flow.

– The combined effects of the vasoconstriction anddilation and the Bohr effect cause a stagnanthypoxia to the brain.

– This stagnant hypoxia can result in LOC

– The alkalosis leads to neuromuscular irritabilitywhich can lead to spasms and tetany


• Cardiovascular System

– The hypocapnia/alkalosis causes

• peripheral vasodilation

• cerebral vasoconstriction

– The result is a restriction in blood flow to thebrain

– The Bohr Effect

• Shift of the oxyhemoglobin curve to the left

• restricts oxygen off-loading in the brain

• Dizziness• Numbness• Tightening of muscles• Coolness• Muscle tremors• Tingling• Faintness• Vision impairment• Tetany• Slight nausea

Hyperventilation Symptoms

Onset of symptoms slow and gradual; muscleactivity spastic especially in upper extremities; skinis pale or clammy.


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Treatment of Hyperventilation

• Maximum oxygen under pressure• Connections check – check security• Breathe at a rate and depth slightly less

than normal until symptoms disappear.• Descent below 10,000 feet and land as

soon as conditions permit.

Hyperventilation vs. Hypoxia

Wayne –April,1958 Aviation Med: 307-15

Hypoxia

Dizziness –18.0 %

Visual Disturb –17.6%

Lightheaded – 9.9%

Tingling – 6.0%

Hot or Cold Flash –3%

BOTTOM LINE: Impossible to determine clinically w/o blood gas.

GENERAL RULE: If below 10K’ = Hyperventilation

If above 10K’ = Hypoxia

Hyperventilation

– Dizziness – 27%

– Visual Disturb – 4.2%

– Lightheaded – 26%

– Tingling 15.7%

– Hot or Cold Flash –3%

Trapped Gas Expansion (TGE)Barotrauma

• Causes

• Symptoms

• Prevention

• Treatment

Boyle’s Law

• At constant temperature for a fixed mass, theabsolute pressure and the volume of a gas areinversely proportional

Middle Ear

Cause of block — Blocked or constrictedeustachian tube or upper respiratory infection(URI); usually occurs on descent

Treatment (Ascent)Yawning, Chew & Swallow

Treatment (Descent)Valsalva

Avoid valsalva on ascent andStay ahead of pressure

changes

Post Flight Ear Block

TREATMENT = Frequent Valsalva


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Maxillary

Treatment is to stop ascent or descent then follow protocolPrevention is better option!

Frontal

Ethmoids

Sphenoids

The Sinuses

If your sinuses become blocked by a cold, pressure can’t be equalized withaltitude change and pain will result

Warning:

• Onset rate of SinusSqueeze (Block) mayoccur faster than MiddleEar problems.

• Leaves little time foraircrew decision making

• Sinus pain may beincapacitating if painsevere enough.

Teeth Barodentalgia

• Tooth pain normally occurs on ascent

• Causes: Caries or failing and/or illfittingdental repair/appliance; loose filling;abscess; swollen maxillary sinus(impacted wisdom tooth).

• Treatment

–Descend

–Dental treatment

Gastrointestinal Tract

• Occurs on ascent when trapped gasexpands

• Treatment :

– Start as soon as symptoms appear

– Relieve gas by belching or passingflatus

– Position change / Massage affectedarea

– If problems continue, descend

Vasovagal Syncope:

• Brief loss of consciousness caused by a sudden drop in yourheart rate and blood pressure, which reduces blood flow to yourbrain.

• Vagus nerve is overstimulated by parsympathetic nerves fromabdominal organs and causes the body's blood vessels to dilateand the heart to slow down. This anti-adrenaline effectdecreases the ability of the heart to pump blood upward to thebrain against gravity.

Prevention


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Recommendations

• Watch Diet: Avoid foods that can cause excessgas formation such as cauliflower, broccoli,onions, cabbage and pumpkin.

• Avoid carbonated beverages or drinkingrapidly before flight.

• Avoid chewing gum on ascent.Cola Cola

Intro to Decompression SicknessGases In Solution – Henry’s Law

At a constant temperature, the amount of a given gasdissolved in a given type and volume of liquid is directly

proportional to the partial pressure of that gas in equilibriumwith that liquid.

55.1 in Hg

1.2 mm Hg CO2

34 degrees C

29.92 in Hg

(1 atm = 101.3 kPa = 14.7 psi = 760 mm Hg = 29.92 in Hg)

0.3 mm Hg CO2

34 degrees C

Decompression Sickness

• Decreased pressureforms releases nitrogen.

• Under normalcircumstances, the bodyis able to “off-gas” theexcess nitrogen.

• Rapid ascent or highaltitudes can result inbubbles.

• Bubbles block bloodflow.

• Venous Gas Embolism(VGE) occurs commonlyand is cleared by lung -rarely results in DCS

DCS is uncommon below 18,000 feet

Decompression Sickness (DCS)

• Joint Pain - Bends

• CNS disorders - Staggers

• Lung Involvement - Chokes

• Skin symptoms – Creeps

Decompression Sickness Factors

• Rate of ascent

• Altitude

• Time

• Body Composition

• Age

• Activity

• Dehydration

• Previous injury

Symptoms

• Local Joint Pain 89%

• Leg 30%

• Arm 70%

• Dizziness (The Staggers) 5.3%

• Paralysis 2.3%

• Shortness of Breath 1.6%

• Extreme Fatigue and Pain 1.3%

• Collapse with unconsciousness 0.5%


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DCS Incidence While Breathing 100%Oxygen

Delayed Decompression Sickness

80% of delayed DCS cases occur within 1 hour ofexposure.

Occasionally occur within 24 hours of exposure.

24 HOURRestriction BetweenSCUBA and Flying

Haldane Equation

• Dissolved gas followed byevolved gas.

• Body could tolerate up to a2:1 ratio.

Sea level - 760 mm Hg

18,000 ft - 380 mm Hg

• 760:380 = 2:1

30 FSW – 1520 mm Hg

Sea level – 760 mm Hg

• 1520:760 = 2:1

30 FSW to 8,000 ft ?1520:545 = 2.8 : 1

Protection/Prevention

• Cabin pressurization (primary method)

• Denitrogenation (Preoxygenate 30 min+)

• Exercise during preoxygenationenhances denitrogenation process.

Training

Hypoxia Symptoms are individualized and fairly consistent over time makinghypoxia training a worthy endeavor for aircrew enabling early

recognition and corrective action.


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Last scary thing

•Ebullism: Formation of gas bubbles in bodily fluids due toreduced pressure.

•A system of liquid & gas at equilibrium will result invaporization of liquid to gas. Water pressure is 47mmHg andthe PB at 60k’ is 47mmHg. Voila bubbles and their negativeramifications

•This is known as Armstrong’s line.

•Pressure suits as worn in high altitude surveillance aircraftand extravehicular space travel are required to prevent thispotentially lethal ocurrence

What is Pressure? Atmospheric Pressure€¦ · pressure. The physical units for pressure is force...

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Transcript of What is Pressure? Atmospheric Pressure€¦ · pressure. The physical units for pressure is force...