American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 Physiological Altitude Threats: Outline Hypoxia, Trapped Gas, Decompression, etc • Physiology of Decompressive Stress • Respiratory Physiology • Hypoxia and necessary protection • Barotrauma • Aviation DCS • AGE Lou Gilleran, MD, MPH, MSHS CAPT, MC (FS), USN What is Pressure? Atmospheric Pressure • From a microscopic point • Since the air (a gas) is a of view, gas pressure is fluid, the pressure force caused by the collisions of gas molecules on a acts in all directions, not surface. Each individual just downward. The collision provides a tiny pressure force pushing push on the surface that downward due to the it contacts. The sum total of all of these tiny forces weight of the air is the determines the air same as the pressure pressure. The physical force acting sideways and units for pressure is force even upward. per area. Measurement of Pressure Composition Of The Atmosphere • Nitrogen 78% 597.0 mm Hg One square inch column of air from • Oxygen 21% 159.0 mm Hg Sea level to space • Other gasses & H2O 1% 4.0 mm Hg 760.0 mm Hg @ Sea Level 760 29.92 mm Hg inches Hg 14.7 lbs. Scale lbs Barometer Barometer N-1 American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 Atmospheric Human Operational Parameters “Shells” • Man is designed for terrestrial existence up to 10K’ Thermosphere – Troposphere defines this physiologic zone 80 km to space – Troposphere is where weather, clouds, winds have greatest affect Mesosphere • Adiabatic (Lapse) Rate defines temperature reduction 50 km - 80 km (56 miles above sea level) of 3.5°F/1,000 ft (6.4°C/km) of altitude increase Stratosphere 10 km - 50 km (35 miles above sea level) Troposphere Sea level -10 km (7 miles above sea level) Life at Altitude O2 and Altitude • Lack of O2 is a relatively ineffective stimulus and begins to occur at about pO2 of 60 mm Hg. Though the – Corresponds to about 10,000 ft. on room air and 39,000 ft. atmospheric on 100% O2. percentages of gases stay the same • 1st sign is usually increased tidal volume (depth of with increased respiration) “air hunger” altitude the “partial pressure” of gases • Elevated pCO is the primary stimulus for increased decreases at higher 2 respiratory rate altitudes Sea Level 18,000 ft Physiological Responses at Altitude Pre-existing Cardiopulmonary Disease 1) At high altitudes there's less O2, therefore less dissolved O2 in the blood. • FAA allows Cabin Altitude up to 8k’ where PAO2 is ~65 and O2 Sat 90% in healthy. 2) The response is stimulation of peripheral chemoreceptors which send signals to respiratory drives. • Therapeutic O2 required if needed at baseline altitude. 3) Hyperventilation leads to Respiratory Alkalosis. – Recommended if unable to climb 1 flight of stairs or walk 150’ w/o rest or dyspnia and 4) Respiratory alkalosis shift the Hb dissociation curve to the left • CHF NYHA Class III-IV or baseline PaO2 <70mm Hg so that Hb can pick up O2 easier. • Angina CCS Class III-IV 5) The kidney responds to alkalosis by generating H ions and this • COPD PaO2 <70 will correct the pH back to normal. • Medications in carry-on! N-2 American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 At 10,000 feet, alveolar pO2 is about Physiologic Atmospheric Grouping 60 mm Hg. This is on the steep portion of the Physiologic Deficient Zone oxygen/hemoglobin 10, 000 – 50, 000 ft desaturation curve. Any additional climb in altitude without _____________________________ 10,000 ft supplemental oxygen can result in the insidious onset of Physiological Zone Sea Level – 10,000 ft hypoxia for the aircrewman. Healthy individual fit without the aid of special protective equipment Oxygen-Hemoglobin Dissociation Curve Aircraft Pressurization System Types Purpose and Types • Conventional – used currently in all pressurized aircraft • To ensure safety and comfort for the crew and – increase cabin pressure via on-board compressor passengers • Pressurization schedules • Sealed Cabin Commercial aircraft cabins are required to be pressurized such that the cabin pressure is not – used in space vehicles to exceed an altitude of 8,000ft. – very high altitudes (80,000 ft) Pressurization Systems Aircraft Pressurization • Isobaric systems maintain a constant cabin altitude Advantages from a preselected altitude (generally from 2000- 8000 ft) up to the service ceiling of the aircraft. • Reduces possibility of: • Isobaric-differential systems maintain a constant – Hypoxia altitude until the pressure differential (usually 5psi) is – Decompression sickness reached and then the differential is maintained up to – Trapped gas problems the service ceiling of the aircraft. • Comfort / mobility / fatigue • Sealed Cabin systems are used only in spacecraft and • Cabin temperature control carry their own supply of gases to create • Communication environment. N-3 American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 Aircraft Pressurization Aircraft Pressurization Ambient Air Disadvantages • Increased operating cost A/C Jet Engine Outlet Valve Unit Cabin • Added weight • Reduced aircraft performance Overboard Bleed Air Vent • Risk of accidental pressure loss What Happens When Pressure Fails? Physiological Effects of Decompression • Decompression • Rapid Decompression – Explosive Decompression (less than 1 sec) – Arterial Gas Embolism (AGE) – Rapid Decompression (between 1-10 sec) – Decompression Sickness(DCS) – Slow Decompression (greater than 10 sec) – Hypoxia The NAVY averages 90 events per year or 2.5 occurrences / – Trapped Gas Expansion (TGE) 100,000 flight hours • G.I. Tract • Ears • Factors Affecting Severity • Sinus – Volume of cabin • Lungs – Size of Opening • Slow – Pressure Differential/ Ratio – DCS – Flight Altitude – Hypoxia (Insidious) Arterial Gas Embolism Operational Problems • Leakage of air from the lungs to the • Noise: Hissing to Explosion pulmonary circulation. • Due to pressure – expansion of • Fog: May be mistaken for smoke; decreased lungs. visual environment • Pulmonary circulation goes right to • Flying debris, dust and dirt effect vision the brain. • Coronary artery embolization can • Temp changes: lead to myocardial infarction or • Sea Level 70 F dysrhythmia. • 25K 20 F • Cerebral artery emboli can cause • 35K - 67 F stroke or seizures • 50K - 67 F N-4 American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 Altitude Threat Protection POSITIVE PRESSURE BREATHING (PPB) • Cabin Pressurization System 0 to 8,000ft 8,000 to 24,500ft 24,500 to 50,000ft Aircraft ALT = Cabin ALT Cabin ALT = 8,000ft Cabin ALT = Differential Positive Pressure Breathing (PPB) is the delivery (Isobaric) 35,000ft Cabin at 14,500ft of a gas to the respiratory tract at a pressure greater than the ambient. Intermittent PPB Based on the level of hypoxia considered operationally Pressure applied only during inspiration phase. acceptable. Used in SCUBA The Navy uses a minimal alveolar pO of 60 mm Hg TYPES of PPB REQUIREMENT for PPB2 Reached at 39,000 ft, breathing 100% O2. Continuous PPB Pressure applied throughout the breathing cycle PPB is an emergency condition in operational aircraft. Primary type used in Military aviation Altitude Threat Protection RESPIRATORY EFFECTS of PPB • Supplemental Oxygen System-Regulators 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 Increased PPB 7. Reduction in Cardiac Output (-30%) Oxygen (O2) N-5 American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 Applications of PPB Disadvantages of PPB The physiological ceiling is raised from 39,000 ft to 6,000 ft increase is fairly small 45,000 ft. Potential physiological problems In a sudden decompression (up to 50,000 ft) PPB can Reverses the normal breathing pattern - training required to be used to maintain consciousness in order to effect compensate an emergency descent. Fatiguing Communications become very difficult Applications in increased G-tolerance Can induce hyperventilation Hypoxia Types Of Hypoxia Definition • State of oxygen deficiency in cells and Hypoxic tissues sufficient to cause impairment (Altitude) Histotoxic (Poisoning) of function. O2 O2 Hypemic • Hypoxemia refers specifically to a O2 (Blood) O2 deficiency of O2 in the blood and will Stagnant likely result in hypoxia (Pooling) Hypoxic Hypoxia Hypemic Hypoxia Diminished O2 available to lungs Reduced O2 carrying capacity of blood Causes: Causes: • Ascent to altitude • Malfunctioning equipment Carbon Monoxide • Loss of cabin pressurization Anemia • Improper O2 equipment usage Sulfa Drugs Hemorrhage N-6 American Osteopathic College of Occupational and Preventive Medicine OMED 2012, San Diego, Tuesday, October 9, 2012 Carbon Monoxide Stagnant Hypoxia Pooling or reduced flow of blood, as seen in heart failure and cold Wear and deterioration of airframe seals and opening of seams environments increases susceptibility. This will allow exhaust fumes and toxic gases to infiltrate crew compartment. Can be caused by inactivity, restriction of movements Flight personnel effects – Carboxyhemoglobin (COHb) levels of 15-25% produce Or G-Forces (G-LOC) headache and nausea. With prolonged exposure, muscular