3 - Hypobaric and Hyperbaric Physiology (Clements)

Question 1

Objectives: Provide a summary of hyperbaric physiology

Answer 1

• High Pressure Environments - Most common injury is Barotrauma in diving
  
  • ​Hyperbaric Chambers (wound healing)
  • Wet pressure (SCUBA)
  • Construction workers (caissons)
• P1V1=P2V2: Gas will compress or expand to account for changes in pressure; Every 10 m BSL, ATM doubles
• Scuba Summary:
  
  • ​Compression on Descent
  • Expansion on Ascent

Question 2

Objectives: Provide a summary of hypobaric physiology

Answer 2

• Low pressure, usually High Altitude
• Main issue with hypoxia and it’s affect on ventilation
• Range of disease, from minor to severe
• CO₂ Acclimation exists, however hypoxia does NOT adapt

Question 3

Objective: Explain the following conditions:

Acute Mountain Sickness (AMS)

High Altitude Cerebral Edema (HACE)

High Altitude Pulmonary Edema (HAPE)

Answer 3

• Acute Mountain Sickness (AMS) - “Hangover”
  
  • Headache, nausea, weakness, insomnia
  • Associated with fluid retention, treated with diuretic
• High Altitude Cerebral Edema (HACE) - Neurologic medical emergency
  
  • Ataxia / inability to Heel-to-Toe Walk = Cardinal sign
  • Swelling in brain can cause ischemia/herniation
• High Altitude Pulmonary Edema (HAPE) - Pulmonary medical emergency
  
  • Rapid Respiration at Rest = Cardinal Sign
  • Highest mortality
  • Must return to lower altitude

Question 4

Explain the pathophysiology of the following:

Mask Squeeze

Ear Drum Rupture

Middle Ear Squeeze

Answer 4

• Hyperbaric Conditions w/SCUBA diving–Compression on Descent
• Mask Squeeze:
  
  • Negative pressure on mask can cause eye vessel / conjunctive hemorrhage
• Ear Drum Rupture:
  
  • If canal is blocked (ear plugs, etc) pressure can’t equalize and can rupture tympanic membrane
• Middle Ear Squeeze:
  
  • Failure to equalize pressure as you descend, causing Eustachian tube to collapse
  • Risk Factor: Taking a decongestant too early before diving

Question 5

Explain the pathophysiology of gas expasion on ascent when SCUBA diving?

Answer 5

• Failure to exhale on ascent is most common problem
  
  • Panic situations, hold breath
  • Volume of air in lungs rapidly expands
• Must continuously exhale when going to the surface
• Treatment:
  
  • Recompression / Life Support

Question 6

Explain the physiology of Oxygen and Nitrogen toxicity

Answer 6

• Oxygen Toxicity: > 760 torr of O₂ for extended time
  
  • ​Generates peroxide and free radical interactions within cells
  • Normal air O₂ is amplified by increased pressures
  • Solution: Mix Helium in SCUBA tanks for long, deep dives
• Nitrogen Toxicity: Acts as anesthetic, “Nitrogen Narcosis”
  
  • Poor decision making, drunk-like behavior
  • Decompression Sickness (“Bends” , “Caisson’s Disease”)
    
    • ​Nitrogen bubbles in the vasculature
    • Must ascend slowly, using dive tables to allow Nitrogen to equilibriate as you exhale

Question 7

Objective: How is Low P_AO₂ detected and adjusted?

Answer 7

• Peripheral Chemoreceptors detect hypoxia (< 60 mmHg)
• Result is increase in ventilation rate (hyperventilation)​
  
  • Decreases P_ACO₂ (may go below normal)
  • Increases P_AO₂
  • May result in acute respiratory alkalosis (low CO₂, increases pH)
  • Does NOT change F_IO₂
• ***NOT the result of CO₂ or H⁺

Question 8

Explain the process for acclimation to altitude?

Answer 8

• Hyperventilation
• Increased hematocrit / blood volume: Promotes O₂ carrying capacity of blood
  
  • Downside: Increases viscosity, and load on heart
• Increased angiogenesis: Reduces diffusion distance, and reduces vascular resistance
• Plasma Volume decreases due to hyperventilation and reduced water intake