- Physiologic stresses
- 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.
- Major physiologic stresses:
- Elevated ambient pressure
- Decreased effect of gravity
- Altered respiration
- Sensory impairment
- Effects of immersion up to the neck
- Pressure outside chest wall is now positive, averaging about 10 cmH2O ("negative pressure breathing"). FRC decreases. Intrathoracic pressure less negative at end-expiration.
- FRC decreases about 50%.
- ERV decreases about 70%.
- VC - slight decrease.
- IRV increases.
- Slight decrease in RV, probably due to increased pulmonary blood volume.
- Result: about a 60% increase in the work of breathing.
- Increased venous return due to elevated abdominal pressure and decreased pooling in peripheral veins. (Decreased ambient temperature will also lead to venoconstriction).
- Increased venous return leads to increased central blood volume (approximately 500 ml). Right atrial pressure increases from about -2 to +16 mmHg.
- Cardiac output and stroke volume increase about 30%.
- A/ - probably better matched
- Elevated intrathoracic blood volume "immersion diuresis". Urine flow increase 4-5 times but osmolal clearance increases very slightly.
- Consistent with ADH suppression or release of atrial natriuretic hormone.
- Breath-hold diving
- 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).
- Gas exchange during a 10 meter, one minute breath-hold dive.
- Usually hyperventilate first
- Compression during descent, expansion during ascent.
- 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.
- Physiologic problems encountered in diving with underwater breathing apparatus.
- Breathing gases at ambient pressure
- 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.
- Respiratory sensitivity to CO2 is decreased at depth because of low breathing rates, high PO2 and increased gas density.
- Other hazards at depth
- Descent ("squeeze"):
pulmonary - congestion, edema, hemorrhage
pulmonary - "Burst Lung" - pneumothorax, air embolism
> G.I. - eructation, flatus, abdominal discomfort
- Decompression Illness : Arterial Gas Embolism and Decompression Sickness (caisson disease, the "bends")
- 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.
- 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.
- Treatment: Recompression chamber
- Prevention: Decompression tables
- Can also get if travel in an airplane to soon after a dive.
- 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.
- 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.
- 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.
|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|
He instead of N2 (He is about ½ as soluble as N2)
Copyright 2000 M. G. LEVITZKY
Last updated Tuesday, November 25, 2003 2:48 PM
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