diver's mask

Diving Physiology

  1. Physiologic stresses
    1. The severity of the stress will depend on the depth attained, the length of the dive, whether the breath is held, or breathing apparatus is used etc.
    2. Major physiologic stresses:
      1. Elevated ambient pressure
      2. Decreased effect of gravity
      3. Altered respiration
      4. Hypothermia
      5. Sensory impairment

  2. Effects of immersion up to the neck
    1. Respiration
      1. Pressure outside chest wall is now positive, averaging about 10 cmH2O ("negative pressure breathing"). FRC decreases. Intrathoracic pressure less negative at end-expiration.
        1. FRC decreases about 50%.
        2. ERV decreases about 70%.
        3. VC - slight decrease.
        4. IRV increases.
        5. Slight decrease in RV, probably due to increased pulmonary
        6. blood volume.

      2. Result: about a 60% increase in the work of breathing.

    2. Cardiovascular
      1. Increased venous return due to elevated abdominal pressure and decreased pooling in peripheral veins. (Decreased ambient temperature will also lead to venoconstriction).
      2. Increased venous return leads to increased central blood volume (approximately 500 ml). Right atrial pressure increases from about -2 to +16 mmHg.
      3. Cardiac output and stroke volume increase about 30%.

    3. A/ - probably better matched
    4. Renal
      1. Elevated intrathoracic blood volume "immersion diuresis". Urine flow increase 4-5 times but osmolal clearance increases very slightly.
      2. Consistent with ADH suppression or release of atrial natriuretic hormone.

  3. Breath-hold diving
    1. The Diving Reflex: Greatly decreased heart rate and increased systemic vascular resistance. Sensors unknown in man. (Same effect as chemoreceptor stimulation without lung inflation reflex).
    2. Gas exchange during a 10 meter, one minute breath-hold dive.
      1. Usually hyperventilate first
      2. Compression during descent, expansion during ascent.
      3. Before After
        Surface: PAO2 = 120 mmHg PAO2 = 41 mmHg
        PACO2 = 29 mmHg PACO2 = 42 mmHg
        PAN2 = 567 mmHg PAN2 = 631 mmHg

        10 meters depth
        PAO2 = 149 mmHg
        PACO2 = 42 mmHg
        PAN2 = 1143 mmHg

      4. Therefore, the transfer of O2 from alveolus to blood is undisturbed until ascent. However, the normal transfer of CO2 from blood to alveolus is reversed during descent and results in a significant retention of CO2 in the blood.

  4. Physiologic problems encountered in diving with underwater breathing apparatus.
    1. Breathing gases at ambient pressure
      1. Increased work of breathing - not as great a problem since breathing gases at ambient pressure. However increased gas density leads to increased airways resistance work of breathing. Helium is 1/7 as dense as N2, so at great depths breathe He-O2 mixture.
      2. Respiratory sensitivity to CO2 is decreased at depth because of low breathing rates, high PO2 and increased gas density.

    2. Other hazards at depth
      1. Barotrauma
        1. Descent ("squeeze"):
          ears
          sinus
          dental
          pulmonary - congestion, edema, hemorrhage
        2. Ascent: ear
          sinus
          dental
          pulmonary - "Burst Lung" - pneumothorax, air embolism
          > G.I. - eructation, flatus, abdominal discomfort

      2. Decompression Illness : Arterial Gas Embolism and Decompression Sickness (caisson disease, the "bends")
        1. During diving the high ambient pressure PN2. The high PN2 helps dissolve this normally poorly soluble gas in the body tissues, especially fat, which has a relatively high N2 solubility. Therefore, tissues become supersaturated with N2.
        2. During rapid ascent, the high ambient pressure falls rapidly, and bubbles of N2 form in the blood and body tissues, especially the joints and more dangerously, the CNS.
        3. Treatment: Recompression chamber
        4. Prevention: Decompression tables

        5. He instead of N2 (He is about ½ as soluble as N2)

        6. Can also get if travel in an airplane to soon after a dive.

      3. Nitrogen narcosis - high PN2 directly affects CNS - at 100 ft: euphoria; loss of memory, irrational behavior, etc. At greater depths - numbness of limbs, disorientation, motor impairment loss of consciousness. Mechanism of N2 narcosis is unknown.
      4. Oxygen toxicity - inhalation of 100% O2 at 760 mmHg or lower [O2] at high pressure can cause both alveolar and CNS damage (convulsions at greater than 2.5 Atm.) Mechanism of O2 toxicity is not known.
      5. High Pressure Nervous Syndrome - at very great depths divers experience decreased manual dexterity and tremors. Can be prevented by adding very small amounts of N2 to the inspired gas mixture.

Copyright 2000 M. G. LEVITZKY

Last updated Tuesday, November 25, 2003 2:48 PM
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