Applied PhysicsCHAPTER 1

Physics is an important component of the curriculum of postgraduate students. A thorough understanding of the physical principles of various pieces of equipment is essential in order to use them safely. This chapter covers the major areas of physics as applicable in anesthesia practice. In each main area, the various details are further explained in alphabetical order, except for some principles that need to be explained together for a better understanding.

Anesthetists use compressed gases in their day-to-day practice. At room temperature, some agents exist mostly as liquids and some only as gases. A nonliquefied compressed gas is a gas that does not liquefy at ordinary ambient temperatures regardless of the pressure applied. Oxygen and air are examples of such gases. Oxygen, when supplied at very low temperatures is a cryogenic liquid. A liquefied compressed gas is one that becomes liquid to a large extent in containers at ambient temperature and at pressures from 25 psig to 1500 psig (172 to 10,340 kPa). Examples include nitrous oxide and carbon dioxide. Different principles that require to be understood for safe use of compressed gases are detailed below.

The behavior of gases depends on the constancy of pressure, temperature or volume. The relationship between these variables is described by ideal gas laws. For a change to occur in the state of a gas, heat energy is either added or taken away from the gas. If the state of a gas is altered without a change in heat energy, it is said to undergo adiabatic change. An adiabatic process describes the change in the state of gas which occurs without transfer of heat or matter between a system and its surroundings.

If compressed gas expands adiabatically, heat energy is taken from the surrounding and cooling occurs. This principle is used in the commercial manufacturing of oxygen and cryoprobe.

Joule-Kelvin or Joule-Thompson effect: The phenomenon was investigated in 1852 by the British physicists James Prescott Joule and William Thomson (Lord Kelvin). When a high-pressure gas is released rapidly through a valve or porous opening, the resulting expansion leads to decrease in temperature. This can result in cooling of the gas which eventually liquefies. This principle is known as Joule-Kelvin principle and can be used for production of liquid air. By using the fractional distillation of liquid air (due to the different boiling points of nitrogen and oxygen), oxygen can be produced commercially.

If a gas is rapidly compressed its temperature rises. When a gas cylinder connected to the anaesthetic machine is turned on too quickly the temperature rises in gauges and pipelines and in the presence of oil or grease may lead to a fire or explosion.

The atmospheric pressure is the pressure exerted by the atmosphere at sea level and is the result of the weight of atmospheric gases bearing down on the surface of the earth.

Pressure: Anesthetists make pressure measurements both in patients and in machines, and hence should have a good understanding of pressure and its units of measurement. Pressure is expressed as force per unit area (Fig. 1.1). The SI unit is Pascal; since this represents a tiny pressure, more commonly, kilopascal (kPa) is used. The other units commonly used are presented in Table 1.1. The relationships between gas volume, pressure and temperature of compressed gases is shown in Table 1.2.

Critical temperature is defined as the temperature above which gases cannot be liquefied by applying pressure. Critical pressure is the pressure at which a gas can be liquefied at its critical temperature (critical pressure of oxygen—49.7 atm). The critical temperature of nitrous oxide (N₂O) is 36.5°C, and hence N₂O exists as a liquefied gas in the cylinders at room temperature. The critical temperature of oxygen is – 118°C and hence at room temperature oxygen exists as compressed gas. Liquid oxygen (LOX) can only be obtained if the oxygen is cooled below its critical temperature and pressurized.

In order to maintain this temperature, special insulated and refrigerated containers are required for supply of liquid oxygen (Chapter 2).

Described by Poynting and also known as pseudocritical temperature. The Poynting effect refers to the phenomenon of physical properties of a mixture of gases, differing from the individual gases. The critical temperature of a gas including its boiling point is altered, when mixed with another gas.

Oxygen at room temperature exists in gaseous state in compressed gas cylinders because it has a very low critical temperature. Nitrous oxide exists as liquefied gas at room temperature, since its critical temperature is higher. However, when nitrous oxide is mixed with oxygen (Entonox), nitrous oxide remains in a gaseous state even at a pressure of 137 atmospheres. This effect of oxygen which lowers critical temperature of the mixture (pseudocritical temperature), is attributed to the Poynting effect. This principle allows us to use Entonox cylinder to provide obstetric analgesia, analgesia to transport patients with long bone fractures and in dental practice.

Force is that which changes or tend to change the state of rest or motion of an object. In SI system force is measured in newtons (N). A newton is that force which gives a mass of 1 Kg an acceleration of 1 meter per sec per sec (Kg.m.s^-2). Force, pressure and area are interrelated. Force is pressure acting on unit area (Fig. 1.1). If force is applied over smaller area (like a smaller syringe) the pressure is 3higher. Similarly, high pressure acting over small area can be balanced by reduced pressure acting over larger area. This concept is used in the construction of pressure regulators (Chapter 3).

Equal volumes of gases at the same temperature and pressure contain equal number of molecules. This number of particles is 6.022 × 10²³ and is known as Avogadro's number. One mole of any gas occupies 22.4 liters at STP and 24 liters at RTP (20°C at sea level). This principle is used to calculate:

The first perfect gas law. Boyle's law states that at a constant temperature, the volume of a given mass of gas varies inversely with the absolute pressure. Another way to put the Boyle's law is to say that with any increase in the pressure of a gas at a constant temperature, the volume of the gas decreases. Similarly, with any decrease in the pressure, while maintaining a constant temperature, the volume of the gas will increase. Furthermore, any increase or decrease in the volume of the gas at a particular temperature will decrease and increase the pressure of the gas respectively. Two important practical uses of Boyle's law are:

Second perfect gas law states that at a constant pressure, the volume of a given mass of gas varies directly with absolute temperature. This principle is used to calculate the contents of a cylinder containing liquefied gases (Chapter 2). As per this principle, the volume of a mole of gas at STP is 22.4 L and it becomes approximately 24 L when correction is made for RTP.

Third perfect gas law: At a constant volume, the absolute pressure of a given mass of gas varies directly with temperature. As temperature increases in the cylinders, the pressure increases resulting in the possibility of an explosion. Hence, gas cylinders should not be exposed to extremes of temperatures and stored in cool places and regulations place a limit to filling density of liquefied compressed gas.

The ideal gas law relates the four quantities: pressure, volume, moles and temperature.

The three (3) gas laws are combined to give the universal gas constant:

The Ideal Gas Law for a mixture of two gasses can be rewritten as:

Henry's law states that at a particular temperature, the amount of a given gas dissolving in a given liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid (The partial pressure of the gas in gaseous phase is also termed as the tension of the gas in liquid). According to this law, the volume of gas that dissolves in a liquid is equal to its solubility coefficient multiplied by its partial pressure.

Application: Used to calculate the amount of oxygen dissolved in plasma at a given partial pressure, where the solubility coefficient for O₂ is 0.003 mL/dL. So, at 100 mm Hg of oxygen tension, the amount of oxygen dissolved would be 0.3 mL.

A gas refers to a substance that has a single defined thermodynamic state at room temperature, whereas a vapor refers to a substance that is a mixture of two phases at room temperature, namely gaseous and liquid phase. A gaseous phase of a volatile liquid, below the critical temperature is known as vapor. A vapor can be liquefied using pressure, whereas gas cannot be liquefied unless it is cooled to critical temperature.

Evaporation occurs at the surface of the liquid when its temperature is below the boiling temperature at a given pressure. The SVP of a liquid is independent of the ambient pressure, but increases with increasing temperature. If we continue to heat a liquid, eventually its SVP equals the ambient pressure (usually 101 kPa or 760 mm Hg). The temperature at this point is called as its boiling point of any liquid. Boiling points of some anesthetic agents are listed in Table 1.3.

Energy is required when a molecule changes from liquid to gas. Latent heat of vaporization is the number of calories required to change 1 gram of liquid into vapor without changing temperature.

Thus, in the absence of an outside temperature source, volatile liquids will cool significantly and lead to decreasing vaporization. The latent heats of vaporization among the common volatile anesthetic gases are similar.

In a closed container, molecules from a volatile liquid escape the liquid phase into gaseous phase (vaporization), and vice versa. The pressure exerted by these gaseous molecules on the wall of the container is known as vapor pressure. This process of vaporization will proceed until an equilibrium between the gas and liquid phases is reached. At this point number of molecules entering gaseous phase equals the number entering liquid phase, and the pressure exerted by the vapor (usually expressed in mm Hg) is called the saturated vapor pressure (SVP). The relation between SVP and ATP will enable the manufacturers to determine the splitting ratio of plenum vaporizers (Chapter 3). SVP of some volatile anesthetic agents at 20°C is given in Table 1.3.

Fluid is a substance that has the ability to flow because its particles are not rigidly attached to one another, and 5includes both liquids and gases. The behavior of a fluid in flow is predominantly related to two intrinsic properties of the fluid: density and viscosity.

Density is the pressure exerted by a column of fluid and is defined as the mass of a substance occupying a unit volume. The units of density are kg/m₃. Density is dependent on temperature and as the temperature of fluid increases the density decreases.

Flow is the movement of a gas or a liquid through a tube or other system. Flow Q is defined as the volume of fluid passing a point in unit time:

Where, V = change in volume

t = the time interval over which the flow is measured.

Flow can be described as laminar, turbulent or a mixture of both.

Orderly movement of a fluid that complies with a model in which parallel layers have different velocities relative to each other. At relatively low flow velocities, the flow can be modeled on layers or cylinders of flow at differing rates, the fastest velocity occurring in the center of the tube and the slowest at the edge where there is friction between the wall of the tube and the fluid. This distinctive velocity profile is maintained as long as laminar flow exists, that is in the absence of eddies or turbulence.

Hagen-Poiseuille's law: Hagen (in 1839) and Poiseuille (in 1840) described the laws governing laminar flow through a tube. It applies to laminar flow. It states that flow through the tube is directly proportional to the pressure gradient and the 4^th power radius and inversely proportional to the length and viscosity of the gas.

Law follows the formula:

Where,

Fluid flow which is unpredictable with multiple eddy currents and is not parallel to the sides of the tube through which it is flowing. Turbulent flow is facilitated by corners, 6irregularities and sharp angles. Turbulent flow is affected by the density of the gas. The factors that affect turbulent flow and their interrelation are:

Where, ∆P = Pressure gradient

Where,      Re = Reynold's number

Bernoulli's principle is named after the Dutch-Swiss mathematician Daniel Bernoulli who discovered this principle over 300 years ago. In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

Bernoulli's equation:

Where,      P = Pressure

The Venturi effect is based on the Bernoulli's principle. The pressure drop induced by the increase in velocity of a fluid passing through a narrow orifice can be used to entrain air or a nebulizer solution.

The venturi oxygen mask is based on the Venturi principle in that relatively rapidly moving oxygen molecules pull along (entrainment) air molecules by two processes:

The Coanda effect is an established physical phenomenon of fluid flow. This effect was named after a Romanian aircraft designer Henri Coanda, who discovered the effect after an aircraft he designed went up in flames as a consequence of this effect. A jet stream adheres to the boundary wall and therefore produces a lower pressure along the opposite wall. Essentially any fluid coming into contact with a curved surface will cling to this surface and alter its direction of flow. This can be illustrated by running a thin stream of water from a tap, and bringing the curved surface of a spoon to touch it. The water follows the surface of the spoon (Fig. 1.7). It does so because the solid stationary surface of the spoon slows the layer in immediate contact. This has a drag effect on the other layers, in effect pulling them into the line of the curved surface. The Coanda effect is said to explain the maldistribution of air in the pulmonary tree after a constricted portion of bronchiole, as the flow will stream along one fork of the division, leading to unequal distribution of gas flow and V/Q mismatch.

Surface tension is the result of attraction between molecules across the surface of a liquid either lining a tube or a sphere. The SI unit for surface tension is newton per meter.

Diffusion is the process by which there is a movement of solute molecules due to their random thermal motion. It is a passive process, and net movement of the solute occurs along a concentration gradient (from high concentration to low concentration).

If two aqueous solutions with different concentrations of particles are separated from each other by a semipermeable membrane then water will move across the membrane from the solution with the lower concentration to the solution with the higher concentration. The movement of the water will depend on the difference in the concentration of the particles and the nature of permeability of the membrane. This movement of water is termed osmosis and the pressure which would need to be exerted to halt its movement is called the osmotic pressure. The osmotic pressure is determined by the total number of particles in solution, regardless of molecular nature. The total number of particles will thus depend on the degree of dissociation of solutes (Fig. 1.9).

Osmometry is a technique for measuring the concentration of particles in a solution, i.e. the osmolar concentration. Osmolar concentration can be expressed in two ways:

Osmolality is a thermodynamically more precise expression because solution concentrations expressed on a weight basis are temperature independent while those based on volume will vary with temperature in a manner dependent on the thermal expansion of the solution.

Although the terms tonicity and osmolality are often used interchangeably, there is a clear distinction. Osmolality is a physical property dependent on the total number of solute particles present in a solution, whereas tonicity is a physiological process dependent upon the selectively permeable characteristics of a membrane. For example, solutes such as urea and ethanol permeate cells freely and therefore will have no effect on tonicity but will increase the measured osmolality.

Colloid is a term used to describe solute particles with a molecular weight greater than 30,000. Colloid osmotic pressure, or oncotic pressure, describes an equilibrium pressure measurement when two solutions, one of which contains colloid, are separated by a semipermeable membrane. Interest in its measurement has come from studies in critical care medicine in the prediction of intercompartmental body water movements, in particular as a useful prognostic indicator of pulmonary edema, and of mortality in the critically ill.

Heat is a form of energy that passes between two samples owing to the difference in their temperatures. Heat energy tends to pass along a temperature gradient from high to low temperatures. Heating a substance gives its constituent molecules increased kinetic energy. This results in a rise in temperature, or changes its state. Loss of heat results in the reverse process.

Base SI unit for heat energy is joules (J).

In terms of other SI units:

Temperature is the property of matter which determines whether the heat energy will flow to or from another object of a different temperature. This is expressed according to a comparative scale. It is that property by which we may quantify the heat energy of a substance.

The temperature scale most commonly used tends to be the Celsius scale or the centigrade scale (named after 18th century Swedish astronomer Anders Celsius). It uses the boiling and freezing points of water (100°C and 0°C respectively), at atmospheric pressure.

Water can exist in three phases, as vapor, liquid and ice. These phases depend on temperature and pressure. The transition between water vapor and water is demarcated by the boiling point of water (P), which is 100°C at 1 atm, but which increases with increasing pressure.

The specific heat of a substance is the number of calories required to increase the temperature of 1 g of a substance by 1°C. The concept of specific heat is important to the design, operation, and construction of vaporizers as it is applicable in two ways:

Thermal conductivity is a measure of the speed with which heat flows through a substance. The higher the thermal conductivity, the better the substance conducts heat. Vaporizers are constructed of metals that have relatively high thermal conductivity, thus maintaining a uniform internal temperature.

Humidity is an important aspect of delivering gases to patients in theaters and intensive care. The importance of humidity for anesthetists lies in the comfort of the theater environment, safety with regard to static electricity, adverse effects of dry medical gas supplies, and heat loss from the body. Humidity is a measure of the amount of water vapor in a gas. It can be expressed in a number of ways.

Absolute humidity is defined as actual mass of water vapor present in a known volume of gas, and is expressed as mg/mL or g/m³.

11The absolute humidity of air in the upper airway of humans is about 34 g/m³ and it reaches a peak of 43 g/m³ as it reaches the alveoli.

Relative humidity is defined as the ratio of the mass of water vapor in a given volume of gas to the maximum amount of water vapor that the same gas can hold at the same temperature. Relative humidity is expressed as a percentage.

Hygrometers are instruments that measure humidity; they can measure absolute humidity, relative humidity and the dew point—which is the point at which vapor condenses. They range from simple mechanical hygrometers, through psychrometers (wet and dry bulb hygrometer), to more complex electrical and optical instruments.

Some instruments include:

Hair hygrometer: The length of hair increases with increasing humidity.

Wet and dry hygrometer: It involves utilization of two mercury thermometers, one in ambient temperature and one in contact with water through a wick. The difference in temperature reading reflects the rate of evaporation of water which depends on humidity.

Regnault's hygrometer: Air is blown through a silver tube containing ether. This technique is more accurate than the previous two and gives the relative humidity.

Mass spectrometer: Utilizing the principle of reduction in the ultraviolet light transmitted through the medium containing water vapor.

Humidity transducers: Humidity transducers are used in those places where accurate measurement of humidity is required. They can transform a physical quantity of air humidity into a standard signal which is transferred to a controller. They are normally used in laboratories, ventilation and air-conditioning systems and in any other production process where it is necessary to control air humidity. They have an accuracy of ± 2% over a range of 0 to 100% relative humidity. Humidity transducers generate alarms or turn off a ventilation system when the predefined maximum and minimum values are exceeded.

While nose breathing at rest, inspired gases become heated to 36°C and are about 80–90% saturated with water vapor by the time they reach the carina, largely due to heat transfer in the nose. Mouth breathing reduces this to 60–70% relative humidity. This humidification maintains mucosal integrity, ciliary activity, prevents the drying of secretions and helps in easy expulsion of respiratory secretions when coughing. Lack of humidification (e.g. ventilating a patient with dry gas through a tracheal or tracheostomy tube) can result in cracking of mucosa, drying of secretions, keratinization of the tracheobronchial tree, reduction in ciliary activity, atelectasis and infection. Tracheal temperature and humidity fall with increased ventilation, particularly when the inspired gases are cold and dry.

If totally dry gases were inspired and fully saturated gases exhaled, the total water loss from ventilation at rest would be about 300 mL/day in the average adult. Normally the efficiency of nose, and humidity of inspired air minimize this loss by approximately 150 mL/day. Bypassing the nose with an ETT and not humidifying gases causes maximal losses.

Humidification can be used with any breathing circuit and may be provided for air, oxygen and a mixture of anesthetic gases.

Heat and moisture exchanger filters contain materials such as ceramic fiber, paper, cellulose, fine steel or aluminum fibers in a hygroscopic medium such as calcium chloride or silica gel.

Direct interception: For particle more than 1 μm, it is physically prevented from passing through the pores.

Inertial impaction: Particles less than 0.5 μm are held by the filtering medium by Van der Waals electrostatic forces.

Diffusional interception: Particles less than 0.5 μm move freely and randomly (Brownian movement) and subsequently swell up and get filtered by the pores.

Electrostatic attraction: Charged particles are attracted by oppositely charged fibers.

Advantages of HME filters:

Disadvantages of HME filters:

A simple cold-water bath humidifier allows gas to flow through water and carries water vapor as it bubbles out. This type is less efficient as bubbles are large and the loss of heat from the latent heat of vaporization reduces humidity.

The main problems of hot water humidifier are:

Nebulizers produce water vapor in the form of microdroplets (1–20 μcm).

There are three types of nebulizers:

The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes—the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwave, infrared light, ultraviolet light, X-rays and gamma rays (Table 1.5). Infrared light is used in several clinical monitors (infrared analyzers, pulse oximetry, etc.).

This law represents a combination of two laws, which combine to form a mathematical means of expressing how light is absorbed by matter.

The Beer-Lambert law is relevant to pulse oximetry (Chapter 7), a monitoring technique that works on the basis of spectrophotometry. The pulse oximeter probe, usually placed on a digit, emits light at different wavelengths The blood absorbs a certain proportion of light, which is dependent on the relative concentrations of deoxyhemoglobin and oxyhemoglobin present.

A photodetector placed at a constant path length away on the opposite side of the probe (and thus digit) senses the amount of light that has been absorbed and processes this electronically to give oxygen saturation and pulse waveform, or plethysmograph.

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometers (nm), or 400 × 10⁻⁹ m, to 700 nanometers—between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). Often, infrared and ultraviolet are also called light. The wavelength (which is related to frequency and energy) of the light determines the perceived color. The edges of the visible light spectrum blend into the ultraviolet and infrared levels of radiation.

While there is some ambiguity to where the boundaries are for the various “forms” of electromagnetic radiation, the infrared band is typically taken to begin just beyond the red wavelengths of the optical band, around 0.74 micrometers, and extends up to 300 micrometers. When IR radiation passes through a sample, some is absorbed by the sample and some of it passes through (transmitted). The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. Like a fingerprint, no two unique molecular structures produce the same infrared spectrum.

Molecules that contain two or more different atom species absorb infrared radiation, because of the nature of the bond between the dissimilar atoms. This property can be used to analyze gases like carbon dioxide, nitrous oxide and all anesthetic vapors, but not oxygen or nitrogen. Asymmetric, polyatomic molecules absorb the light IR radiation at different wavelengths.

Ultraviolet (UV) radiation is defined as that portion of the electromagnetic spectrum between X-rays and visible light, i.e. between 40 nm and 400 nm. The UV spectrum is divided into vacuum UV (40–190 nm), far UV (190–220 nm), UVC (220–290 nm), UVB (290–320 nm) and UVA (320–400 nm). The sun is our primary natural source of UV radiation. Artificial sources include tanning booths, black lights, curing lamps, germicidal lamps, mercury vapor lamps, halogen lights, high-intensity discharge lamps, fluorescent and incandescent sources, and some types of lasers (excimer lasers, nitrogen lasers and third harmonic Nd:YAG lasers). Unique hazards apply to the different sources depending on the wavelength range of the emitted UV radiation.

Electricity is broad term for the physical phenomena associated with the “flow” of electrons within imbalanced electrodes, usually from a positive to a negative electrode. Electricity may also be described just as the existence of a “charge” between the subatomic particles—electrons and protons, without necessarily having a “flow”. In the context of anesthesia and intensive care, it is important to understand the concepts of both electrical “charge” as well as “flow” of the electrically charged particles.

The electromagnetic force existing within the electrons and protons (and the imbalance existing between them) gives rise to the “electrical charge” of a system. When this charge is transferred to another point or a system, there is a “flow” of electricity resulting in an electrical “current”.

Electrical charge is a physical property exhibited by the subatomic particles of any matter, causing a “force” created by their electromagnetic interaction. Thus, conventionally, the charge of an electron is –1 and that of a proton is +1. Like charges repel and opposite charges attract. When charge is transferred between points, there exists a difference of the “electrical potentials” between the points that facilitates the transfer. Thus, the “electrical potential” of a point in a conductor is the work done to move a positive charge from infinity to that point.

Electrical charge, flow of current, the difference in the electrical potential between conductors, resistance and the capacitance of the conductors are all inter-related giving rise to the different definitions of the units of measurements in electricity.

Ampere: The SI unit of measurement of current is “ampere”, which is one of the basic seven units of the “Système International”. One ampere (A) is the amount of transfer of the electric charges (current) passing a point of a conducting system per second. The other units of measurements in electricity are “derived units” and not basic units.

Capacitance in simple terms is the storage of “electrical charge” or energy in a conductor or a set of conductors. When electrical charge is transferred between two conductors, which were not charged earlier, these conductors can then become charged, one positively and another negatively. This results in the establishment of a difference of “electrical potential” between them. Capacitance is calculated as the ratio of the product of the charges of the conductors (numerator) and the potential difference (denominator).

Coulomb: One coulomb (C) of electrical charge is defined as the quantity of charge when a current of one ampere (see below) passes through a conductor within one second (A•s).

Conductance: Electrical conductance is measured in terms of siemens (S).

Farad: One farad (F) is defined as the capacity of system, when charged with one coulomb of electricity, has a potential difference of one volt.

Electrical impedance is a concept that extends the property of resistance to alternating currents.

Ohm: One ohm (Ω) is defined as the resistance between two points of a conductor with a potential difference of one volt, when one ampere of current flows between the points.

Ohm's law: Ohm's law states that the current flow is directly proportional to the potential difference (V) and inversely proportional to the resistance (R). Thus, the relationship between “current” (I), “potential difference” (V) and “resistance” (R) can be expressed as follows:

Volt: One volt (V) is the difference in the electrical potential energy between two points of a system, when unit charge is transferred from one point to another.

Watt: Watt is a SI unit for measurement of “power” which is the energy converted or transferred per unit time (Joule/second). In electricity, one watt (W) is the amount of electrical energy transferred when one ampere of current flows between points with a potential difference of one volt.

When a current (AC) progresses in a sinusoidal waveform as shown in Figure 1.17, the distance between two consecutive corresponding points in the wave (red arrows) denote the wavelength (λ). Amplitude is the distance of the highest point of the wave from a zero point (green arrow). Amplitude may also be measured as peak-to-peak (purple arrow), which is the distance between the highest and the lowest points of the wave.

Static electricity is the property of a system, caused by the discharge of the “electrical charges” of one system to another, when they are not in equilibrium. However, when a current flows from one system to another, there is no net gain or loss to any system.

The principle of Wheatstone bridge is used in many clinical measurements and monitoring like invasive pressure, cardiac output, temperature, etc.

Wheatstone bridges are typically applied in transducers that may include strain gauges also known as “piezoresistive sensors”, light-dependent resistors (photoresistive sensors), potentiometers or positional sensors and thermistors, which are resistors measuring temperature.

The ultrasound plays a major role in anesthesia, pain and critical care. The rapidly evolving technology of ultrasound in anesthesia, leads away from the typically “blind” interventions based on anatomical landmarks, done at the risk of their very many variations to the normal. Ultrasound guidance is becoming standard practice, and the future generations of anesthetists need to develop a thorough understanding of this technology and practical skills. It should be part of the core training of every anesthetist.

The discovery of piezoelectric effect and its utility in construction of high-frequency mechanical vibrating sources coupled with high-frequency electronic drives provided the basis of ultrasound.

Ultrasound transducers work on the principle of piezoelectricity. Within the transducer are arrays of piezoelectric crystals, which have the property of changing shape when an electrical voltage is applied. Application of a voltage enables electrical energy to be converted into sound energy.

Recognizing structures on ultrasound takes practice and a good knowledge of the anatomy is a big help. What follows is a brief description of some of the features that make up the image.

Presentation: In almost all applications, the top of the screen represents the probe and as you look further down the screen you are seeing progressively deeper tissues.

Depth: The depth to which you can see is normally shown on a scale running alongside the image. This can be adjusted using the depth adjustor, which will be a prominent control on any scanner. The maximum depth is dependent on the frequency, with lower frequencies penetrating much further, but at the cost of reduced resolution. Hence, lower frequency (2.5 MHz) used in transthoracic echo for visualization of deeper structures. High-frequency probes using frequencies of around 10 MHz, is used in transesophageal echocardiography (TEE).

These detect and amplify the backscattered energy and manipulate the reflected signals for display.

The Doppler principle is the phenomenon in which sound transmitted from a moving object is perceived by a stationary observer to be of a different frequency depending upon the velocity and direction of travel. Thus, changes in frequency (frequency shift) can be used to calculate velocity of movement of blood.

Procedures include:

Advantage of using USG guidance:

Transesophageal echocardiography is used in anesthesia for the following procedures:

Advantage of using ultrasound imaging for gaining vascular access: