High pressure and diving

Effect of pressure on alveolar P co ₂ and P o ₂

Pressure has complicated and important effects on P co ₂ and P o ₂ . The alveolar concentration of CO ₂ equals its rate of production divided by the alveolar ventilation (page 107). However, both gas volumes must be measured under the same conditions of temperature and pressure. Alveolar CO ₂ concentration at 10 ATA will be about one-tenth of sea-level values, that is, 0.56% compared with 5.3% at sea level. When these concentrations are multiplied by pressure to give P co ₂ , values are similar at sea level and 10 atm. Thus as a rough approximation, alveolar CO ₂ concentration decreases inversely to the environmental pressure, but the P co ₂ remains near its sea-level value.

Effects on the P o ₂ are slightly more complicated. The difference between the inspired and alveolar oxygen concentrations equals the ratio of oxygen uptake to inspired alveolar ventilation. This fraction, like the alveolar CO ₂ concentration, decreases inversely with the increased pressure. However, the corresponding partial pressure will remain close to the sea-level value, as does the alveolar P co ₂ . Therefore the difference between the inspired and alveolar P o ₂ will remain roughly constant, and the alveolar P o ₂ , to a first approximation, increases by the same amount as the inspired P o ₂ ( Fig. 17.1 ). However, these considerations only take into account the direct effect of pressure on gas partial pressures. There are other, more subtle, effects on respiratory mechanics and gas exchange which must now be considered.

Effect on mechanics of breathing

Two main factors must be considered. First, there is the increased density of gases at pressure, although this can be reduced by changing the composition of the inspired gas. The second factor is the pressure of water on the body, which alters the gravitational effects to which the respiratory system is normally exposed.

Gas density is increased in direct proportion to pressure. Thus air at 10 atm has 10 times the density of air at sea level, which increases the resistance to turbulent gas flow (page 28). As a result airway resistance is increased, both at rest and, to a greater extent, during exercise, and there are limits to the maximal breathing capacity (MBC) that can be achieved. In fact, it is usual to breathe a helium/oxygen mixture at pressures in excess of about 6 atm because of nitrogen narcosis (see later). Helium has only one-seventh the density of air, and so is easier to breathe. Furthermore, lower inspired oxygen concentrations are both permissible and indeed desirable as the pressure increases ( Table 17.1 ). Therefore, at 15 atm it would be reasonable to breathe a mixture of 98% helium and 2% oxygen. This would more than double the MBC that the diver could attain while breathing air at that pressure. Hydrogen has even lower density than helium and has been used in gas mixtures for dives to more than 500 m deep.

The effect of immersion is additional to any change in the density of the respired gases. In open-tube snorkel breathing, the alveolar gas is close to normal atmospheric pressure, but the trunk is exposed to a pressure depending on the depth of the subject, which is limited by the length of the snorkel tube. This is equivalent to a standing subatmospheric pressure applied to the mouth, and it is difficult to inhale against a ‘negative’ pressure loading of more than 5 kPa (50 cmH ₂ O). This corresponds to a mean depth of immersion of only 50 cm, and it is therefore virtually impossible to use a snorkel tube at a depth of 1 m. However, the normal length of a snorkel tube assures that the swimmer is barely more than awash, so these problems should not arise.

Negative pressure loading is prevented by supplying gas to the diver’s airway at a pressure close to the hydrostatic pressure surrounding the diver. This may be achieved by providing an excess flow of gas with a pressure-relief valve controlled by the surrounding water pressure. Such an arrangement was used for the traditional helmeted diver supplied by an air pump on the surface. Free-swimming divers carrying their own compressed gas supply rely on inspiratory demand valves, which are also balanced by the surrounding water pressure.

These arrangements supply gas close to the hydrostatic pressure surrounding the trunk. However, the precise ‘static lung loading’ depends on the location of the pressure-controlling device in relation to the geometry of the chest. Minor differences result from the various postures that the diver may assume. Thus if he or she is ‘head up’ when using a valve at mouthpiece level, the pressure surrounding the trunk is higher than the airway pressure by a mean value of about 3 kPa (30 cmH ₂ O). If he or she is ‘head down’, airway pressure is greater than the pressure to which the trunk is exposed. The head-down position thus corresponds to positive pressure breathing, and the head-up position to negative pressure breathing. The latter causes a reduction of functional residual capacity of about 20% to 30%, but breathing is considered to be easier head up than head down.

Apart from these considerations, immersion has relatively little effect on respiratory function, and the additional respiratory work of moving extracorporeal water does not seem to add appreciably to the work of breathing.

Effect on gas exchange ^{, ,}

The best measure of the efficiency of oxygenation of the arterial blood is the alveolar/arterial P o ₂ gradient. Measurement of arterial blood gas partial pressures presents formidable technical difficulties at high pressures. However, studies at 2.8, 47 and 66 ATA have reported only small increases in alveolar/arterial P o ₂ gradient. Because it is customary to supply deep divers with an inspired oxygen partial pressure of at least 0.5 ATA, arterial hypoxaemia is unlikely to occur either from hypoventilation or from maldistribution of pulmonary ventilation and perfusion in healthy subjects.

The position for arterial P co ₂ is less clear. Hypercapnia is a well-recognized complication of diving, and divers may have a blunted P co ₂ /ventilation response of unknown cause. Hypercapnia in divers at rest is uncommon, but during exercise elevated end-tidal and arterial P co ₂ levels are described, reaching levels in the range of 6.2 to 8.3 kPa (47–62 mmHg) during exercise at 66 ATA. This is potentially hazardous, because 9 kPa is approaching the level at which there may be some clouding of consciousness, which is potentially dangerous at depth. High gas density at depth causing increased work of breathing is believed to be responsible for the inadequate ventilation during exercise.

Oxygen consumption

The relationship between power output and oxygen consumption at pressures up to 66 ATA, whether under water or dry, is not significantly different from the relationship at normal pressure shown in Figure 13.1 . Oxygen consumption is expressed under standard conditions of temperature and pressure, dry (STPD; see Appendix C ) and therefore represents an absolute quantity of oxygen. However, this volume, when expressed at the diver’s environmental pressure, is inversely related to the pressure. Thus an oxygen consumption of 1 L.min ⁻¹ (STPD) at a pressure of 10 atm would be only 100 mL.min ⁻¹ when expressed at the pressure to which the diver is exposed. Similar considerations apply to CO ₂ output.

The ventilatory requirement for a given oxygen consumption at increased pressure is also not greatly different from the normal relationship shown in Figure 13.5 , provided that the oxygen consumption is expressed at STPD, and minute volume is expressed at body temperature, saturated with water vapour and at the pressure to which the diver is exposed (see Appendix C ). Considerable confusion is possible as a result of the different methods of expressing gas volumes, and although the differences are trivial at sea level they become very important at high presures.