Introduction
HyperbarIC oxygen therapy
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- Bu səhifədəki naviqasiya:
- Risk considerations Hyperbaric chamber
- Venturi suction system (support)
- Venturi / Bernoulli principle
- Dynamic performance analysis
- Adiabatic gas law
- Cryotherapy
- Cryo spray
- Problems
HyperbarIC oxygen therapy
Figure 28 Hyperbar chambers Hyperbaric oxygen therapy is performed with the patient in a closed chamber and by slowly supplying pure oxygen washing out the nitrogen of the air in the chamber and in the patient. Slowly the oxygen pressure is increased up to e.g. 2.5 bar absolute pressure. At the end of treatment the pressure is slowly taken down to atmospheric pressure and the pure oxygen replaced by air. The chamber may be a complete room where patients can walk around (like the models used for divers). In hospital smaller units (Fig.28) are used for each patient and several patients can be treated simultaneously in one department. Oxygen itself does not burn or explode, but it increases the burning rate of combustible materials. A paper strip burns 30% faster at 25% than 20% oxygen. NASA allows 25.9% in its space shuttles. Oxygen rich environment is defined as >25% or 27.5 kPa partial pressure by IEC (IEC 2005). 100% oxygen implies explosive combustibility. In hospitals oxygen rich environment is used in infant incubators (chapter 13.7). It is well known from such use that too much oxygen is dangerous for the infant, in particular the eyes. The toxic property of oxygen is used in hyperbar therapy e.g. for enhanced healing of selected problem wounds.
Venturi suction system (support)Suction may be very important during surgery and intensive care. The operating field must be kept free from blood and liquids hindering visual control. During intensive care it is critical to keep the airways free from mucus and other obstacles. The suction is a vital part for respiratory systems. 
Fig 29 shows the main components of a Venturi suction system, it is gas (air) driven. Other models may use an electric pump to create the vacuum. The operator holds the suction tube handle in the airways or as a sterile instrument in the wound. The aspired debris is assembled in the bottle. The bottle is kept at low pressure via a second tube connected to the Venturi. For the operator it is important that suction is available when needed, and that the degree of vacuum and flow can be chosen. The suction must not be too strong so that tissue is destroyed, but strong enough to ensure safe removal of debris. The tubing must be reinforced so that it does not collapse at high vacuum. It must be sterile for many of the procedures. The suction device can be characterised by static and transient parameters. The static parameters are max. vacuum (min. absolute pressure) and flow capacity as a function of drive gas pressure. These parameters are determined by the Venturi construction and the driving gas flow rate. The dynamic parameter is the transient flow for instance in the starting phase if the operator has closed the suction inlet for vacuum build up. Flow and volume is then also dependent on the bottle volume which again is dependent on the degree of bottle filling.
A
s Fig.29 shows, a high pressure gas supply is coupled to the Venturi, where the gas must pass the cone. The increased kinetic energy during molecule acceleration is taken from the gas pressure. The local pressure is reduced (Bernoulli text box) and picked up by the perpendicular tube there. Notice that the Venturi outlet contains both the driving gas and the aspirated gas from the bottle. Equation 8 Bernoulli Ps + ½ v2 + gh = constant Ps = static (v=0) pressure [Pa]. = density [kg/m3]. v = velocity [m/s]. g = acceleration due to gravity [m/s2]. h = height difference [m]. Validity range: Laminar flow, any geometry, valid at any point along a line of flow, gases or liquids, no frictional (viscous) losses. Actually the Bernoulli equation is about the conservation of energy along a flow line, but it is usually given as here in terms not of energy, but pressure. 
F
are synonyms.
The static vacuum pressure may be too high for certain procedures; the sudden start from closed to open suction head may be too violent for the tissue concerned. A false air leakage device between the pump outlet and the room air can be inserted so that max. vacuum is reduced, without influencing the suction capacity at lower pressures. Dynamic performance analysisThe suction system is a simple medical device which lends itself well for the use of simple models to understand its function better. A usual practise is to use electronic equivalent circuit models, Fig.31, as knowledge of electric network theory is widespread. 
Figure 31 Equivalent electrical circuit for a dynamic suction system O T = temperature [o Kelvin] V = volume [m3] K = constant dependent on gas, e.g. 0,4 for O2 Validity range Closed volume thermally isolated (adiabatic condition) Ideal gas (far from condensation)
Electric formulas such as Ohm’s law or the relaxation time constant =RC can now be applied. The formulas are electric, but the quantities mechanical. From Fig.31 it can for instance be seen that the time constant for attaining vacumm in the bottle is smaller with closed handle ( = Rp||Rs C) than with open handle ( =Rp C). Example: Find the tube-bottle time constant . The tube is 1m long with inner diameter 20 mm and coupled to a closed bottle of volume 10L. R is found from Poiseuille (viscosity 10-3) to be 0.3 106 [Pas/m3]. C is V/RT equal to 4 10-6 [mol/Pa]. =RC is equal to 1.2 seconds.
Gas dependent, not electricity dependent Obstacles in the Venturi exit filter reversing vacuum suction pressure to positive blowing pressure. Using a tube of a diameter so that all debris may pass. Room air pollution, central vacuum installation ? Sterility, disinfections CryotherapyCryo- is a word of Greek origin meaning cold and is the antonym to thermo- (thermostat – cryostat). The cryo technique is used in general surgery and by ophthalmologists and dermatologists to destroy tissue. It is often an ablation method, meaning that the dead (necrotic) tissue is not cut out and taken away, but left in-situ. But it may also be a technique where frozen tissue can be taken out as an alternative to scalpel surgery. 
O Figure 32 Cryo principle according to a Joule-Thomson capillary model. nce the cells are destroyed, components of the immune system - primarily the white blood cells - clear out the dead tissue. A killing mechanism is ice formation only outside a cell that causes the cell to shrink as it gives up water by osmosis to replace the water that has turned into ice. As the area thaws, water rushes into the shrunken cell and causes it to burst. For this reason, cry therapy often consists of a series of steps in which the tissue is repeatedly frozen and thawed. At lower temperatures intracellular ice formation is important, below approximately -40°C intracellular ice crystals begin to form that destroys the cells completely. Tumour cells also die when their blood supply is choked off by ice formation within small tumour vessels.
Figure 33. Principle components of cryo equipment according to Joule-Thomson The cryo source is a gas under pressure at room temperature. The effect is based upon the temperature drop in an expanding gas, the Joule-Thomson effect. An expanding gas performs work, and the energy is taken from the internal heat energy of the gas, and the gas temperature drops. The basic model is described by the adiabatic gas equation 9 (see text box). In the original Joule-Thomson experiment gas was flowing through an insulated pipe with an obstacle in the middle, in the form of a porous disk or a silk handkerchief. The temperature and pressure were measured on each side of the disc. The usual practical construction is to replace the disk by a capillary, Fig.32. The high pressure room temperature gas is brought into the capillary so that one end is at e.g. 67 bar and the outlet at 1 bar. The pressure drop along the capillary is due to the viscous losses as illustrated by the law of Poiseuille. These losses generate heat reducing the cryo effect. Capillary length is determined to give the correct resistance and therefore a suitable flow rate. Due to the gradual fall in pressure the gas expands, and because the same number of molecules (mol) must pass a cross-sectional area per second, the gas velocity increases gradually. Due to the gas expansion the temperature drops if the adiabatic cryo effect is larger than the heat effect from the viscous forces. Coming out of the capillary the gas is at its minimum temperature, and is sent right to a wall constituting the internal part of the active cryo probe surface to be brought in contact with the tissue. The basic components of a cryo instrument is shown on Fig.33. The freezing cycle is started by pressing the footswitch. In approx. 5 seconds a freezing temperature of approx. -80°C (-176° F) is reached. After release of the footswitch the freezing cycle stops and the defrost cycle starts automatically as the pressure in the probe increases and the gas is compressed and heat is liberated. In this way the cryoprobe is defrosted within 5 seconds without use of electric heating. The tip is equipped with a cryometer (cf. thermometer). The same equipment may handle either carbon dioxide or nitrous dioxide without any modification. Special more performing systems use both argon and helium gas. It is important that the supply gas is very dry, since water will freeze and tighten the capillary nozzle. The gas passes the pressure regulator and then through a flexible but highly armoured tube out to the capillary. In the defrost cycle the high pressure will propagate also to the outlet tube, which accordingly also must be able to withstand maximum pressure. Argon, CO2 and nitrogen are not polluting, while N2O should be taken care of by a scavenging system.
Risk considerations capillary cryo
Cryo sprayThe phase diagram for CO2 on Fig.34 shows that at room temperature the pressure in a filled10 CO2-bottle is about 67 bar, and a part of the substance is in the liquid phase. If the bottle is in the vertical position and the gas (vapour) is abruptly let out in the room the pressure suddenly drops to 1 bar. Because of the gas expansion the temperature quickly drops, for a short time down to -78 oC or lower. The CO2 gas is transformed to the solid phase (“dry ice”) without passing the liquid phase. Because of the non-adiabatic conditions the snow soon reverts to the gas phase. The liquid phase is an impossible state for CO2 if the pressure is below 5.1 bar. The point in the phase diagram where all three phases meet is called the triple point (5.1 bar, -56 oC). A cryo source may also be cold, a liquid gas kept in a thermos bottle. Liquid nitrogen (LN2) may be kept in a thermos bottle at -196 oC and 1 bar, or at higher pressures and temperatures. Liquid nitrogen applied with a spray or probe (temperature of -196º C) is much colder than liquid nitrogen applied with a swab (-20º C), than cryogens that come in disposable spray cans (-55º C and -70º C), and than nitrous oxide (-75º C). Table 2 shows the boiling point of some usual gases. In its most simple form for patients a cryo liquid or vapour is applied directly on the tissue, e.g. the skin (cryospray). In dermatology or surgery liquid nitrogen (LN2) is often used because of its powerful cryogenic effect. Cryosurgical freeze times vary according to lesion type, size, depth, and location. The handling of liquid nitrogen is of course cumbersome, and therefore Joule-Thomson based cryo equipment is much used. Problems
1 Jean-Louis Poiseuille (1799-1869), French physicist 2 Avogadro (1776-1856), italian autodidact chemist and physicist 3 Boyle (1627-1691) Irish physicist; Mariotte (≈1620-1684) French physicist; Gay-Lussac (1778-1850) French physicist. 4 Pierre Simon Laplace (1749-1827), French astronomer, mathematician and physicist 5 Respirator and ventilator are usually considered to be synonyms, perhaps with a certain tradition that respirators are simpler devices, e.g. not electrically driven. 6 Used for patients with poliomyelitis in the large epidemic outbreak in the 1950ies. 7 tracer means a substance not harmful to the patient and the concentration of which can be conveniently determined. 8 Giovanni Baptista Venturi, 1746-1822, Italian physicist, studied hydraulics, sound and colours. 9 Daniel Bernoulli, 1700-1782, part of an important Swiss scientist family studying astronomy, mathematics, medicine, hydrodynamics. 10 CO2 in liquid form is incompressible. Therefore a CO2 bottle completely filled with liquid is dangerous. The supplier is not cheating when a “filled” bottle has a vapour pocket at delivery to allow for only a moderate pressure rise at rising ambient temperatures. Chapter 9 Yüklə 1,16 Mb. Dostları ilə paylaş:
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he Venturi principle is interesting and somewhat contra intuitive: Is it possible to create vacuum pressure from high pressure? The clue is to make the gas molecules pass a nozzle (cone) where they are accelerated. Venturi8 developed this practical method, Bernoulli9 had already explained it from his discoveries in kinetic gas theory and mathematical modelling, see Eq.8.
