High Altitude Physiology

• High altitude reduces the partial pressure gradient, decreasing oxygen diffusion

Oxygen and Carbon Dioxide Transport

• Hyperventilation due to hypoxia leads to decreased arterial partial pressure of carbon dioxide (pCO2)
• This shift results in the hemoglobin dissociation curve shifting to the left, initially hindering oxygen dissociation from hemoglobin
• Over time, increased 2,3-BPG from red blood cells shifts the curve to the right
• Anaerobic metabolism during hypoxia can cause lactic acid buildup, potentially leading to metabolic acidosis

Pulmonary Edema

• Cerebral edema (brain swelling) is more likely with cerebral hyperfusion
• Excessive blood flow to the brain increases pressure and fluid leakage
• Hypocapnia (low CO2 levels) causes strong cerebral vasoconstriction
• Vasoconstriction raises blood vessel pressure and edema
• Hypoxia acts as a strong vasodilator and increases blood flow through vessels
• Increased pressure from blood flow leads to edema

Acetazolamide

• Acetazolamide inhibits carbonic anhydrase to minimize bicarbonate in the body
• It reduces bicarbonate reabsorption and production
• It acts as a diuretic
• Can prevent hypoxic pulmonary vasoconstriction from developing

Diving Stressors

• Increased ambient pressure is a physiological stressor while diving
• Effects of gravity during diving are decreased
• Respiration is altered during diving
• Hypothermia can occur when diving, depending on water temperature
• Sensory impairment can occur when diving, depending on depth

Factors Determining Diving Stress

• Depth of dive is a factor
• Length of time is a factor
• Whether a breath hold or breathing apparatus is used

Immersion Effects

• Increased ambient pressure on the thorax, abdomen, and limbs, making it harder to inflate the lungs
• Changes in size of thoracic cavity affects pulmonary blood flow
• Immersion diuresis is the urge to urinate
• Atrial stretch increases venous return
• This stimulates ANP which lowers blood volume
• Reduced outside pressure allows less blood to be pulled into lower extremities
• Stroke volume and cardiac output increase by 30%
• Increased pulmonary blood flow leads to increased mean pulmonary artery pressure
• Recruitment and distension improves ventilation-perfusion matching

Breath-Hold Diving

• Every 33 feet in sea water adds 1 atmosphere (atm) of pressure
• Ambient pressure increases by 1 atm (760 mmHg) for every 33 feet of sea water
• The human body is nearly incompressible, but gases are compressible
• Gas volume in the lungs is inversely proportional to depth
• Lung volume is halved at 33 feet (2 atm)
• Gas compression increases density, making exhalation harder

Diving Reflex

• The diving reflex causes bradycardia, apnea, and coldwater reaction
• It also decreases heart rate in cold water

Self-Contained Underwater Breathing Apparatus (SCUBA)

• SCUBA introduces ambient pressure
• SCUBA gear has a tank of compressed gas delivered via a regulator
• Expelled gas is released as bubbles

Gas Density and Pressure

• Increased gas density and partial pressure is related to diving
• Airway resistance and work of breathing increase with depth
• Helium, being 1/7 as dense as nitrogen, can replace nitrogen to reduce gas density

Decompression Illness

• "The Bends" or Decompression illness involves bubbles forming in blood and joints
• Recompression raises pressure in a chamber, forcing bubbles back into solution
• Arterial gas embolism and immune attack are caused by bubbles
• Pressure change from 30,000 ft to sea level can cause remaining compressed gases to form bubbles
• As a diver descends, increased pressure increases gas solubility
• As nitrogen dissolves in the blood, rapid ascent causes bubble formation
• To avoid decompression illness, divers should ascend slowly to allow controlled gas release
• Divers should not fly too soon after diving to avoid decompression illness from the pressure difference between sea level and altitude