psinyc once bubbled...
So that pause in the breathing cycle is normal both underwater and topside.
Yes indeed, psinyc.
I think this subject is covered in most basic diving lectures but I feel it may benefit readers if I reinforce these basic physiological concepts. Please refer to the attachment.
At the end of a maximal inspiration an individuals lungs contain a fixed amount of air. This is known as the total lung capacity (TLC) which is between about 5 and 7 litres in a normal healthy adult male. When the subject empties his lungs as forcefully as he can he expels his vital capacity (VC), which is the absolute maximum amount of air that can be moved in and out of the lungs between a maximum inspiration and expiration. The vital capacity is between about 4 or 5 litres. It is smaller than the total lung capacity because a small volume of air must remain in the lungs and airways as dead space. This dead space is known as the reserve volume (RV) and of course is the difference between total lung capacity and vital capacity. VC = TLC - RV.
At rest, because of the elasticity of the chest wall and diaphragm, the volume of gas held in the lungs is just under half of the total lung capacity. This normal resting volume is oddly known as the functional residual capacity (FRC). During normal breathing at rest the intercostal muscles actively raise the ribs (and the diaphragm contracts downwards) thus expanding the lungs against this elasticity by an amount necessary to ventilate the alveoli and reduce the pp CO2 by the added air. The amount thus inhaled is known as the tidal volume (TV). During normal quiet breathing the glottis is not involved and inspiration is active and expiration passive;- the immediate almost reflex consequence of relaxation of the muscles of inspiration. Thus during normal quite breathing at rest and during sleep - which is totally reflex - the respiratory pattern tends to be inspiration, expiration, pause, inspiration, expiration, pause and so on. (Listen to someone snoring!)
During any form of activity the amount of air needed to excrete the excess CO2 produced obviously increases proportionately. From the position of rest at the FRC a maximal expiration to empty the lungs as much as possible gives us the expiratory reserve volume ERV, while the inspiratory excursion - above that of the resting tidal volume - gives us the inspiratory reserve volume IRV. (See the attachment). It is not difficult to work out that IRV+ TV+ ERV = VC. During moderate exercise, of course, the lungs move somewhere between the resting tidal volume (TV) and the vital capacity (VC) and, in addition, as the respiratory rate increases the overall result is a considerable increase in ventilation.
At any lung volume apart from the functional residual capacity an effort has to be made to hold one's breath against both the elastic forces AND the respiratory drive. When the lungs are inflated the easiest way to do this is to close the glottis and the resulting increase of pressure within the lungs holds the lungs inflated against these elastic forces. Once the glottis relaxes the air is forcefully exhaled, often producing a grunting sound. This is what happens during childbirth and other forms of straining. The glottis is not usually involved when the subject holds his breath at volumes less than the functional residual capacity. The diaphragm and intercostal muscles are simply held at a fixed position in opposition to the elastic forces within them. No "grunting" is heard. In simple terms the glottis is only used to hold air in the inflated lungs at higher than ambient pressures
During scuba diving, the control of the lungs is under conscious control until the diver is comfortable and relaxed, this may be when the temptation to conserve air is often overwhelming in inexperienced divers anxious to conserve their air and make it last longer. In my experience the way to safely conserve air is to relax and not to try too hard to restrict respiration. I feel that many incidents of pulmonary barotrauma may result from inexperienced divers attempting to overcome the immediate urge to breath out at the end of breathing in, simply in order to conserve air, by a conscious closure of the glottis against the lungs natural elasticity.
It is obviously far, far safer, to breath normally and deeply. The surface air consumption (SAC) will soon drop to acceptable levels once the diver is comfortable in the water.
Thus from my perspective, if any form of breath holding is contemplated
(and it should not be!) it ought only to be contemplated at the neutral position, that is at the end of a passive expiration at the functional residual capacity (FRC). The glottis naturally remains open at this lung volume since there are no elastic forces to overcome, the only force to overcome being the respiratory drive mediated by the levels of CO2 in the venous blood. The natural rhythm being inspiration, expiration, pause.
Remember the respiratory drive is normally an active inspiration to dilute the carbon dioxide in the alveoli (and the veins) with a gas mix which contains no CO2.
At the end of a normal expiration, as Mouth Breather describes, the venous pp CO2 gradually increases until there is an overwhelming desire to take another breath.
:doctor:
