Brian Locke

Breath Hold Diving

Immersion: body surrounded by water; exposed to external hydrostatic pressure.

  • increases inspiratory effort (static load), reduces lung capacity (static load), shifts blood from lower body into chest (abd organs, for instance, are less dense than H2O - induces diuresis as sensed as fluid overload. Preload increased), and induces autonomic reflexes.

In terms of lung volumes, immersion to the neck = -28 cmH2O of pressure due to the water column on the chest.

Maximal inspiratory pressure that the chest/diaphragm can generate is 100-150 cmH2O, which corresponds to the longest tube you could theoretically breath through, even if it was large enough that flow resistance was negligible. -- you also get additive transmural pressures across the heart chambers, due to pressure of increased preload from hydrostatic pressure on body)

Normally, spleen holds/sequesters 10% of blood volume. Squeezed out in response to loud noise, sympathetic release, diving. (Diving mammals have huge spleens - scuba tanks for storing oxygen). Splenectomy lowers breath-hold time.

Asphyxia = triad of cellular hypoxia, hypercapnia, and acidosis caused by inadequate gas exchange with the environment. Apnea = halted gas exchange with environment.

Diving response - parasympathetic drop in HR, maintained MAP (thus increased SVR in non-critical organs). Most marked in aquatic mammals, but still present in terrestrial animals. Can lead to lactate production in those tissues. Activated by face submersion alone.

Lung mechanics: Boyles law: P1V1 = P2V2. At 1m of sea water (msw) is 10 atm, roughly. Thus, 90m down is 10 atm of pressure.

Thus, volume will be much below residual volume (max exp) because pressure on thorax will be much larger. RV will generally be reach at about 30 msw

Diaphragm (and abd sucking in), blood vessels fill the volume - and significant negative pleural pressures. Meaning, the positive pleural pressures from shallow depth pass 0 (lung at FRC) at the 'pressure reversal depth' - somewhere 20-40m depending on lung and chest wall compliance.

Why no Scuba for prior PTX? Below the pressure reversal depth, will get very negative pressures.

Additionally, more and more of the lung will get atelectatic - at depths below pressure reversal depth, and below RV depth... "the squeeze" (negative pressure injury - generally hemoptysis from bronchial or alveolar rupture, or pulmonary edema) becomes possible.

Why black out on the way back up? (Transit time of blood from lungs to brain approaches 8 s, so symptoms tend to occur a few seconds after surfacing and exhaling.)

  • the decreased ambient/barometric pressure means that a PaO2 concentration drops, causing a dip in PaO2
  • Remaining air in the lungs increases intrathoracic pressure (atelectasis resolves) and leads to decreased preload.
  • PaO2 < 30 leads to LOC (to 20 with training - increase cerebral blood flow to adapt)

Hypoxia should not be a factor throughout most of a dive because increased ambient pressure creates hyperbaric conditions at depth that raise oxygen partial pressures despite the diminishing oxygen fraction in alveoli

Breath holds:

In normals, CO2 limits - though trained panelists can run in to oxygenation limitation. Initially, PaCO2 raises rapidly to equilibrate with mixed-venous CO2 (in 20-30s), then linear rise after until the threshold is reached.

  • The normal rise in PaCO2 is 3 to 4 mmHg min−1.

Farhi and Rahn asked, “to what extent does man act as a CO2 absorber” (110).

The highest reported human PaCO2 was measured in a patient who was inadequately ventilated for several hours during general anesthesia, and remained deeply comatose after surgery (311).

Threshold/breakpoint PaCO2?

  • on room air, typically near 50 mmHg (diaphragmatic contractions occur at PaCO2 ~46-49)
  • on supplemental oxygen, can tolerate much longer (highly trained apneass can increase their conventional breakpoint to critical hypoxia, which is 20-30 mmHg O2)
  • can be extended by rebreathing gas (the sensation of air hunger is partially mediated by lack of movement in / out)

Note: O2 is consumed at around 0.25 L/min, CO2 generated at 0.2 L/min, thus 50 ml/min of volume decrease occurs.