Water covers over 70% of the earth, has varying depths and temperatures and contains much of the earth's resources. Head-out water immersion (HOWI) or submersion at various depths (diving) in water of thermoneutral (TN) temperature elicits profound cardiorespiratory, endocrine, and renal responses. The translocation of blood into the thorax and elevation of plasma volume by autotransfusion of fluid from cells to the vascular compartment lead to increased cardiac stroke volume and output and there is a hyperperfusion of some tissues. Pulmonary artery and capillary hydrostatic pressures increase causing a decline in vital capacity with the potential for pulmonary edema. Atrial stretch and increased arterial pressure cause reflex autonomic responses which result in endocrine changes that return plasma volume and arterial pressure to preimmersion levels. Plasma volume is regulated via a reflex diuresis and natriuresis. Hydrostatic pressure also leads to elastic loading of the chest, increasing work of breathing, energy cost, and thus blood flow to respiratory muscles. Decreases in water temperature in HOWI do not affect the cardiac output compared to TN; however, they influence heart rate and the distribution of muscle and fat blood flow. The reduced muscle blood flow results in a reduced maximal oxygen consumption. The properties of water determine the mechanical load and the physiological responses during exercise in water (e.g. swimming and water based activities). Increased hydrostatic pressure caused by submersion does not affect stroke volume; however, progressive bradycardia decreases cardiac output. During submersion, compressed gas must be breathed which introduces the potential for oxygen toxicity, narcosis due to nitrogen, and tissue and vascular gas bubbles during decompression and after may cause pain in joints and the nervous system.

Human Physiology in an Aquatic Environment.

Evaluation of thoracic mechanics in the ventilated patient part 1: Primary measurements

The cause of the initial hyperventilation, which occurs on exposure to O2 at high pressure (o.h.p.), has been investigated by measuring tidal volume (VT), frequency of breathing (f) and hence ventilation (VE) in thirty-six barbiturate-anaesthetized rats, with or without their glossopharyngeal (IX) nerves sectioned, during 30-60 min of exposure to o.h.p. at 4, 6 or 8 atm absolute. In intact rats the rates of rise of VT, f and VE with time during exposure to o.h.p. were smallest at 4 and greatest at 8 atm absolute. In IX-sectioned rats the rates of rise of VT at 4, 6 and 8 atm absolute and of f at 4 atm absolute were similar to those of intact rats. At 6 atm absolute and even more so at 8 atm absolute, however, f decreased. Hence the slope of VE in IX-sectioned compared with intact rats was similar at 4 atm absolute but smaller at 6 and 8 atm absolute. In fact at 8 atm absolute VE remained constant in IX-sectioned rats. Since the slope of VE versus time in intact rats was steeper the greater the pressure and since the removal of carotid bodies in IX-sectioned rats reduced the VE slope at 6 and 8 atm absolute, the stimulus to the hyperventilation induced by o.h.p. cannot be an accumulation of CO2 in the brain resulting from the lack of O2 desaturation of haemoglobin. This theory would predict that VE should be identical at all pressures above 3.5 atm absolute. The findings in the IX-sectioned rats indicate a major contribution of the carotid bodies to the f increase in o.h.p. They may be stimulated by a histotoxic hypoxia induced by early O2 poisoning. Since the VT increase on exposure to o.h.p. was both large and fairly similar in intact and IX-sectioned rats, it is suggested that a large part of the VT increase was caused by stimulation of the central chemoreceptors by lactic acidosis induced by an o.h.p.-induced histotoxic hypoxia of the brain.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1986.sp015946

Ventilation in intact and glossopharyngeal nerve sectioned anaesthetized rats exposed to oxygen at high pressure.

Some aspects of the linearity and the frequency response of flow measurement with Fleisch type pneumotachometers are considered. The transition from a uniform velocity distribution at the entrance of the capillary tubes of the transducer to a fully developed flow profile is discussed as one of the phenomena influencing the linearity of the set-up. Besides, an expression is derived for the frequency response of the Fleisch pneumotachometer. The frequency responses as derived from measurements show good agreement with theory. In the range of the lower frequencies the frequency response of the flow measuring set-up is proved to be largely determined by the frequency response of the pressure transducer with the connecting tubes. A simple way is indicated to measure the latter. This, in combination with the calculated frequency response of the flow transducer, makes it fairly casy to determine the frequency response characteristics of a flow measuring set-up containing a Fleisch type pneumotachometer.

Linearity and frequency response of Fleisch type pneumotachometers.

Giannella-Neto, A., C. Bellido, R. B. Barbosa, and M. F. Vidal Melo. Design and calibration of unicapillary pneumotachographs. J. Appl. Physiol.84(1): 335–343, 1998.—This study presents a method fo...

Design and calibration of unicapillary pneumotachographs

Measurement of ventilatory flow during pulmonary function maneuvers and in severe exercise by the pneumotachograph flow transducer in dense atmospheres is handicapped by the inverse relationship between the linear range ofsuch transducers and gas density. This limitation in the usefulness of the pneumotachograph can be overcome by mounting such flow transducers in parallel and taking into account the reduction in peak ventilatory flow transducers in parallel and taking into account the reduction in peak ventilatory flow which occurs with increasing density. The design of such assemblies, their static performance characteristics, and a drift subtractor, constructedto stabilize the output of the pneumotachograph pressure transducer, are described.

Pneumotachograph for flow and volume measurement in normal and dense atmospheres

Human tolerance to He, Ne, and N2 at respiratory gas densities equivalent to He-O2 breathing at depths to 1200, 2000, 3000, 4000, and 5000 feet of sea water (predictive studies III).

Four men, exposed to the pressure equivalent of 1,200 ft of seawater (37 ATA) for 6 days in a CO2-free, normoxic helium-oxygen environment (Predictive Studies III-1971, Aviation Space Environ. Med. 4: 843-855, 1977), had no evident respiratory distress at work, at rest, or asleep. Ventilatory responses of two men to CO2 were measured in 20-min acute exposures to mixtures of oxygen (173 Torr) with nitrogen, crude neon, and helium over a gas density range of 0.4-22 g/l and a pressure range of 1-37 ATA during stepwise compression of 37 ATA. Analysis of delta V/delta PACO2 as functions of pressure, of density, and of density-to-viscosity ratios shows that increased gas density, but not nitrogen narcosis, is associated with gross diminution of the ventilatory response to CO2. A small ventilatory response to CO2 is predicted for liquid breathing when viscosity is included as a parameter in the analysis. Other findings associated with increased gas densities and progressive elevation of inspired CO2 concentration are disruption of the normal patterns of tidal volume and frequency of breathing and reduction in the range of linear respiratory response to CO2.

Human respiratory control at high ambient pressures and inspired gas densities

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Pneumotachograph for flow and volume measurement in normal and dense atmospheres

Human tolerance to He, Ne, and N2 at respiratory gas densities equivalent to He-O2 breathing at depths to 1200, 2000, 3000, 4000, and 5000 feet of sea water (predictive studies III).

Human respiratory control at high ambient pressures and inspired gas densities