Basic Physics for the Anaesthetist
BASIC DEFINITIONS
It is now customary in medical practice to employ the International System (Système Internationale; SI) of units. Common exceptions to the use of the SI system include measurement of arterial pressure and, to a lesser extent, gas pressure. The mercury column is used commonly to calibrate electronic arterial pressure measuring devices and so ‘mmHg’ is retained. Pressures in gas cylinders are also referred to frequently in terms of the ‘normal’ atmospheric pressure of 760 mmHg; this is equal to 1.013 bar (or approximately 1 bar). Low pressures are expressed usually in the SI unit of kilopascals (kPa) whilst higher pressures are referred to in bar (100 kPa = 1 bar). The basic and derived units of the SI system are shown in Table 14.1.
The fundamental quantities in physics are mass, length and time.
From these basic definitions, several units of measurement may be derived:
Density is defined as mass per unit volume:
Velocity is defined as the distance travelled per unit time:
Acceleration is defined as the rate of change of velocity:
Force is that which is required to give a mass acceleration:
Momentum is defined as mass multiplied by velocity:
Work is undertaken when a force moves an object:
Power (P) is the rate of doing work. The SI unit of power is the watt, which is equal to 1 J s− 1:
= joules per second = watt (W)
Pressure is defined as force per unit area:
FLUIDS
Behaviour of Gases
Combining these three gas laws:
where suffixes 1 and 2 represent two conditions different in P, V and T of the gas. Note that where a change of conditions occurs slowly enough for T1 = T2, conditions are said to be isothermal, and the combined gas law could be thought of as another form of Boyle’s law.
Clinical Application of the Gas Laws
Filling Ratio: The degree of filling of a nitrous oxide cylinder is expressed as the mass of nitrous oxide in the cylinder divided by the mass of water that the cylinder could hold. Normally, a cylinder of nitrous oxide is filled to a ratio of 0.67. This should not be confused with the volume of liquid nitrous oxide in a cylinder. A ‘full’ cylinder of nitrous oxide at room temperature is filled to the point at which approximately 90% of the interior of the cylinder is occupied by liquid, the remaining 10% being occupied by nitrous oxide vapour. Incomplete filling of a cylinder is necessary because thermally induced expansion of the liquid in a totally full cylinder may cause cylinder rupture. Because vapour pressure increases with temperature, it is necessary to have a lower filling ratio in tropical climates than in temperate climates.
Entonox: Entonox is the trade name for a compressed gas mixture containing 50% oxygen and 50% nitrous oxide. The mixture is compressed into cylinders containing gas at a pressure of 137 bar (2000 lb in–2) gauge pressure (see below). The nitrous oxide does not liquefy because the two gases in this mixture ‘dissolve’ in each other at high pressure. In other words, the presence of oxygen reduces the critical temperature of nitrous oxide. The critical temperature of the mixture is −7 °C, which is called the ‘pseudocritical temperature’. Cooling of a cylinder of Entonox to a temperature below −7 °C results in separation of liquid nitrous oxide. Use of such a cylinder results in oxygen-rich gas being released initially, followed by a hypoxic nitrous oxide-rich gas. Consequently, it is recommended that when an Entonox cylinder may have been exposed to low temperatures, it should be stored horizontally for a period of not less than 24 h at a temperature of 5 °C or above. In addition, the cylinder should be inverted several times before use.
Pressure Notation in Anaesthesia
Measurement of central venous pressure is sometimes referred to in cm H2O because it can be measured using a manometer filled with saline, but it is more commonly described in mmHg when measured using an electronic transducer system. Note that, although we colloquially speak of ‘cm H2O’ or ‘mmHg’, the actual expression for pressure measured by a column of fluid is P = ρ.g.H, where ρ is fluid density, g is acceleration due to gravity and H is the height of the column. Because mercury is 13.6 times more dense than water, a mercury manometer can measure a given pressure with a much shorter length of column of fluid. For example atmospheric pressure (PB) exerts a pressure sufficient to support a column of mercury of height 760 mm (Fig. 14.1).
FIGURE 14.1 The simple barometer described by Torricelli (not to scale). (A) Filled with mercury. (B) Filled with water.
1 atmospheric pressure = 760 mmHg
GAS REGULATORS
The Heidbrink valve is a common component of many anaesthesia breathing systems. In the Magill breathing system, the anaesthetist may vary the force in the spring(s), thereby controlling the pressure within the breathing system (Fig. 14.2). At equilibrium, the force exerted by the spring is equal to the force exerted by gas within the system:
force (F) = gas pressure (P) × disc area (A)
Pressure-Reducing Valves (Pressure Regulators)
Pressure regulators have two important functions in anaesthetic machines:
They reduce high pressures of compressed gases to manageable levels (acting as pressure-reducing valves).
They minimize fluctuations in the pressure within an anaesthetic machine, which would necessitate frequent manipulations of flowmeter controls.
The principle on which the simplest type of pressure-reducing valve operates is shown in Figure 14.3. High-pressure gas enters through the valve and forces the flexible diaphragm upwards, tending to close the valve and prevent further ingress of gas from the high-pressure source.
Pressure Demand Regulators
These are regulators in which gas flow occurs when an inspiratory effort is applied to the outlet port. The Entonox valve is a two-stage regulator and its mode of action is demonstrated in Figure 14.4. The first stage is identical to the reducing valve described above. The second-stage valve contains a diaphragm. Movement of this diaphragm tilts a rod, which controls the flow of gas from the first-stage valve. The second stage is adjusted so that gas flows only when pressure is below atmospheric.
Flow of Fluids
In this context, velocity gradient is equal to the difference between velocities of different fluid layers divided by the distance between layers (Fig. 14.5B). The units of the coefficient of viscosity are Pascal seconds (Pa s).
Laminar Flow
Laminar flow through a tube is illustrated in Figure 14.5A. In this situation, there is a smooth, orderly flow of fluid such that molecules travel with the greatest velocity in the axial stream, whilst the velocity of those in contact with the wall of the tube may be virtually zero. The linear velocity of axial flow is twice the average linear velocity of flow.
Turbulent Flow: Flow of Fluids Through Orifices
In turbulent flow, fluid no longer moves in orderly planes but swirls and eddies around in a haphazard manner as illustrated in Figure 14.6. Essentially, turbulent flow is less efficient in the transport of fluids because energy is wasted in the eddies, in friction and in noise (bruits). Although viscosity is the important physical variable in relation to the behaviour of fluids in laminar flow, turbulent flow is more markedly affected by changes in fluid density.
It may be seen from Figure 14.7 that the relationship between pressure and flow is linear within certain limits. However, as velocity increases, a point is reached (the critical point or critical velocity) at which the characteristics of flow change from laminar to turbulent. The critical point is dependent upon several factors, which were investigated by the physicist Osborne Reynolds. These factors are related by the formula used for calculation of Reynolds’ number:
FIGURE 14.7 The relationship between pressure and flow in a fluid is linear up to the critical point, above which flow becomes turbulent.
the square root of the pressure difference driving the flow
the square of the diameter of the vessel
Applications of Turbulence in Anaesthetic Practice:
In upper respiratory tract obstruction of any severity, flow is inevitably turbulent downstream of the obstruction; thus for the same respiratory effort (driving pressure), a lower tidal volume is achieved than when flow is laminar. The extent of turbulent flow may be reduced by reducing gas density; clinically, it is common practice to administer oxygen-enriched helium rather than oxygen alone (the density of oxygen is 1360 kg m− 3 and that of helium is 160 kg m− 3). This reduces the likelihood of turbulent flow and reduces the respiratory effort required by the patient.
In anaesthetic breathing systems, a sudden change in diameter of tubing or irregularity of the wall may be responsible for a change from laminar to turbulent flow. Thus, tracheal and other breathing tubes should possess smooth internal surfaces, gradual bends and no constrictions.
Resistance to breathing is much greater when a tracheal tube of small diameter is used (Fig. 14.8). Tubes should be of as large a diameter and as short as possible.
THE VENTURI, THE INJECTOR AND BERNOULLI
assuming that the predominant fluid flow is horizontal such that gravitational potential energy can be ignored.
In a venturi, in order that the fluid flow be continuous, its velocity must increase through its narrowed throat (v2 > v1). This is associated with an increase in kinetic energy and Bernoulli’s equation shows that where this occurs, there is an associated reduction in potential energy and therefore in pressure. Beyond the constriction, velocity decreases back to the initial value and the pressure rises again. The principle is illustrated in Figure 14.9