IMPEDENCE AND RESISTANCE

• Impedance and resistance are terms used to describe the opposition to electrical current flow.
• Resistance is used to describe opposition to flow in DC circuits, whereas impedance is used for AC circuits.
• Unlike direct currents, alternating currents exhibit reactance (capacitive and inductive) because they have frequency and are phase associated.
• In AC circuits, impedance (Z) consists of a real part (ohmic resistance) and an imaginary part (reactance of an inductor or capacitor). It is measured in ohms, and is the total opposition to current flow
• Capacitors placed in DC circuitscreate an increasing resistance to current flow. This is because the charge at the negative plate of the capacitor accumulates, until maximum capacitance is reached and current flow ceases.
• Capacitors in AC circuitsallow current to flow, as the alternating direction of current flow prohibits a significant build up of charge on one of the plates.
• Reactanceis the resistance to AC that a capacitor or inductor exhibits and is inversely proportional to frequency.
• This principle is used in filters to screen out DC currents and low frequency AC.
• Inductors are coils of conducting wire wound around a ferrous or air core.
• An increasing current flowing through an inductor generates a magnetic field around it. This magnetic field in turn creates an electromagnetic force, which opposes the current flow, known as back-emf.This effect is known as inductance, and its SI unit is the henry (H).
• In a circuit where the rate of current change is 1 A/s, an inductance of one henry would generate one volt across the inductor.
• Henry (H) = Voltage (V) × Time (s) / Amperes (A)
• When the power is switched off, collapse of the magnetic field induces flow of electrons in the inductor and circuit, prolonging the flow of current for a short period.
• Inductors placed in DC circuits will initially encounter transient resistance while the magnetic field is established. Once a steady state is reached, the reactance is negligible.
• Conversely, inductors in AC circuits encounter increasing reactance proportional to the frequency. This is because the creation and subsequent reversal of magnetic field development produces a constant back-emf resisting current flow. Therefore, high-frequency AC will cause a high reactance in the inductor.
• Inductors are used to filter out high-frequency alternating currents, or to smooth out the effect of power surges in monitoring equipment
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LASERS IN MEDICINE

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.

A laser comprises a laser tube constructed from an active lasing medium that can be a gas, solid or a liquid, with a mirror at each end of the tube

Lasers produce an intense parallel beam of coherent monochromatic light (one specific wavelength of light) by the stimulated emission of photons from excited atoms

Injecting energy from an external source (pumping) causes the lasing medium to become excited. Gas lasers are excited using an electric current applied to either end of the laser tube, while solid state and liquid lasers are excited using a high intensity light source. The mirrors cause photons to bounce back and forth within the laser medium, triggering further emission of photons by the process of stimulated emission. One mirror is partially reflective which allows some photons to escape in the form of the laser beam. The beam is then focused as required

Electrons of atoms within a lasing medium normally reside in a stable low-energy level known as the ground state. Pumping excites electrons, raising them to higher energy levels. Because higher energy states are unstable in comparison to the ground state, there is a tendency for electrons to release excess energy and return to lower energy levels. This process is known as decay.

As an electron decays from the metastable to ground state, it emits a photon of energy. If this photon strikes an excited electron in the metastable state, it incites it to emit another photon, which will have the same wavelength, waveform and direction as the incident photon. They are said to be in phase and coherent.

Red and near-infrared lasers have the deepest penetration.

Carbon dioxide lasers emit infrared light and have limited penetration, but are precise and can be used for cutting and vaporising.

Argon lasers predominantly produce blue–green light at 488 and 514 nm, and are commonly used in ophthalmology.

Neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers have the deepest penetration and can cut and coagulate. They are used to resect gastrointestinal and bronchial tumours, and gynaecological lesions.

LASERs are classified 1–4, 1 being least dangerous.

Most medical lasers are class 3B and class 4. They pose a high risk to staff and patients.

Lasers can ignite flammable material such as endotracheal tubes and surgical drapes. They can also cause airway and body cavity fires in the presence of high concentrations of flammable gases.

The risk of airway fires can be reduced by using the lowest inspired oxygen concentration possible that achieves suitable oxygen saturations. In addition, using laser-safe endotracheal tubes with the cuffs filled with saline and dye helps to further reduce the risk of fires. The water in the cuff acts as a heat sink to reduce the likelihood of perforating and igniting the cuff with the laser. The dye provides a visual indication in the event of cuff perforation.

The Wheatstone bridge

• The Wheatstone bridge is an electrical circuit that uses an arrangement of four resistors to measure an unknown electrical resistance.
• The typical Wheatstone bridge contains a power source, a galvanometer (G), two resistors of known resistance (R1, R2), a variable resistor (R4) and an unknown resistance, which is the one to be measured (R3). The connection across CD containing the galvanometer is known as the bridge.
• This circuit is sensitive to changes in the ratio of resistances across pairs of resistors.
• When the voltages at C and D are equal, the ratios of resistances equal each other (R1/R2 = R3/R4), no current will flow through the galvanometer and the bridge is balanced.
• By altering the resistance of the variable resistor R4 until the ratio of resistance across the limb ADB equals that of ACB, the bridge can be balanced, and no current flows across CD. By knowing the resistance required at R4 to
  balance the bridge, R3 can be calculated by using the equation R1/R2 = R3/R4
• A strain gauge is either a foil arrangement or a conductive metallic strip. In the arterial transducer, strain gauges are mounted on a diaphragm.
• The arterial pulsation is transmitted via a continuous column of fluid to the diaphragm, which causes it to stretch. The attached strain gauge will also stretch and its resistance increases. Conversely, when the diaphragm relaxes the resistance in the strain gauge falls. R3 is the strain gauge attached to the diaphragm, and the variable resistor R4 has been adjusted to match the resistance of R3 in the resting position, so that the bridge is balanced. Movement of the diaphragm would alter the resistance of R3, which unbalances the bridge and results a potential difference across CD. The resulting potential difference is quite small, so it is common to use a differential amplifier in place of the galvanometer to increase the sensitivity of the circuit in detecting the signal.

METHODS TO MEASURE THE CONCENTRATION OF A GAS

• The Rayleigh refractometer utilises the refractive index of a gas to calculate its concentration.
• Thermal conductivity is used in katharometers. In these devices, the cooling of a wire causes a change in resistance proportional to gas concentration.
• Solubility is employed in devices such as rubber strips when an increase in length accompanies gas absorption.
• Light emission features in the Raman light scattering measurement device.
• Both infrared and ultraviolet absorption are used in gas concentration measurement.

ECG EMG EEG ; THE BIOLOGICAL SIGNALS

• The movement of ions across cell membranes during the depolarisation and repolarisation of myocytes and neurones generates electric potentials.
• Silver metal electrodes covered with a layer of silver chloride gel within an adhesive sponge pad can be used to measure these potentials at the skin.
• Ion movement near the electrode–skin interface induces movement of chloride ions within the gel layer. The ion concentration gradient promotes electron production at the electrode.
• A lead wire and voltmeter attached to the electrode allows measurement of the potential relative to a reference point. The reference point is usually a second skin electrode.
• Signals are then amplified, processed and displayed.
• Skeletal and cardiac muscles have higher amplitudes than cerebral neurones.This is because the amplitude of biological potentials is proportional to the number of simultaneously depolarising cells.
• The frequency of potentials is related to the fluctuating ion activity across cell membranes.
• Skeletal myocytes which undergo tetany have high frequencies of
  up to 1 kHz.
• Conversely, cardiac myocytes have lower frequencies due to their refractory periods 

HUMIDITY AND ANESTHESIA

• Absolute humidity is the mass of water vapour present in a given volume of gas at
  defined temperature and pressure (expressed as g of H2O/m3 of gas).
• Relative humidity is the mass of water vapour present in a given volume of gas,
  expressed as a percentage of the mass of water vapour required to saturate the same volume of gas at identical temperature and pressure.
• The amount of water vapour required to saturate a known volume of gas increases with temperature, i.e. a gas saturated at 20°C contains less water than the same volume, saturated at 37°C
• Relative humidity (RH), can be calculated from the ratio of the mass of water vapour present (mP), to the mass required for satruation (mS) as
  RH = mP/mS
• If droplets are present, supersaturation has occurred and relative humidity exceeds 100%.
• From the gas laws, mass of a gas in a mixture is proportional to the partial pressure it exerts, thus: RH = water vapour pressure/ saturated water vapour pressure
• Instruments used to measure humidity are called hygrometers. Examples include:
  Regnault’s hygrometer
  Hair hygrometer
  Wet and dry bulb thermometers
  Humidity transducers

PRESSURE AND ITS MEASUREMENT BY MANOMETER

💎Pressure = Force/Area
💎SI unit is Pascal
💎1Pa = 1N/m-2
💎It is the simplest method of pressure measurement
💎lt does not need calibration
💎So it can be used to calibrate other devices
💎The pressure is balanced against a column of liquid of known density - usually water for low pressures and mercury for higher pressures
💎The pressure is equal to the depth multiplied by the liquid density multiplied by the acceleration due to gravity; hence the commonly used units cm of H2O and mm of Hg
💎Mercury is 13 times more dense than water
💎The vertical height gives the pressure value

NEBULIZERS

• Aerosols are small particles of liquids or solids suspended in a carrying gas
• Medical aerosols can be produced by a nebulizer
• The therapeutic efficacy of the aerosol is dependent on the liquid or solid’s ability to remain in suspension and the depth reached by the aerosol on inhalation, and is dependent on its stability. These are both determined by the particle size.
• For liquid medication to enter the alveolithe droplets must be smaller than the diameter of the terminal bronchioles and fall within the size range of 0.005 µm to 50 µm in diameter.
• For droplet sizes below 5 µm, gravity exerts a negligible effect.
• Particles or droplets in the range 5 to 10 µm tend to deposit in the upper airways,with material below 5 µm penetrating further into the lungs.
• Below 3 µm, the droplets enter the alveoli and become therapeutically beneficial.
• Droplets below 1 µm are ideal; but if significantly smaller than this, the particles will be exhaled without having a therapeutic effect.
• The temperature for an aerosol generated by a nebulizer must not exceed 37°C and the process must not alter the structure of the medication being carried.
• This is the essential difference between vaporizers that generate a vapour and nebulizers that produce liquid droplets.
• Jet or gas driven nebulizer (atomizers)
• A high flow of gas is driven over a capillary tube that is immersed into the fluid to be nebulized. The high pressure air driven through the small orifice, generates negative pressure as a result of the Venturi effect. These nebulizers are simple and low cost, but small variations in gas flow rate can result in inconsistent delivery of aerosol to the patient.
• Ultrasound driven nebulizer
• The ultrasound nebulizer incorporates a ceramic piezoelectric transducer that changes electrical energy into mechanical energy (pressure oscillations). The transducer sits at the bottom of the chamber and vibrates at a frequency of 1.5 MHz. The vibrations are transmitted through the water. The diaphragm is in contact with the solution to be nebulized and violently shakes the solution into particles. At low frequencies, larger particles are produced, but at higher frequencies, a fine mist is generated
• Ultrasonic nebulizers tend to produce a more consistent particle size than jet nebulizers and, as a result, produce a much greater deposition into the lungs.
• But long-term use of ultrasonic nebulization might inadvertently affect surface tension stability in the alveoli

Latent Heat and its applications in anesthesia practice

• Heat capacity: The heat energy required to raise the temperature of a given object by one degree. (J.K−1 or J.°C−1)

• Specific heat capacity: The heat energy required to raise the temperature of one kilogram of a substance by one degree. (J.kg−1.K−1 or J.kg−1.°C−1)

• But not all heat energy results in a temperature change. 

• Latent heat: This is the heat energy that is required for a material to undergo a change of phase. (J) The heat is not utilised for raising the temperature, but for changing the phase.

• If heat is applied to matter, temperature increases until the melting or boiling point is reached. At these points the addition of further heat energy is used to change the state of matter from solid to liquid and from liquid to gas. This does not cause a change in temperature. The energy required at these points is referred to as latent heat of fusion andlatent heat of vaporisation, respectively.

• Specific latent heat is the heat required to convert one kilogram of a substance from one phase to another at a given temperature.

• As temperature increases, the amount of additional energy required to overcome the intermolecular forces of attraction falls until the critical temperatureof a substance is reached. At this point the specific latent heat is zero, as no further energy is required to complete the change in state of the substance.
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• Variable bypass vaporisers function by passing a small amount of fresh gas through the vaporising chamber, which is fully saturated with anaesthetic vapour. This removes vapour from the chamber. Further vaporisation from the anaesthetic liquid must occur to replace the vapour removed, which requires energy from the latent heat of vaporisation. This cools the remaining liquid, reducing the saturated vapour pressure and thus the concentration of anaesthetic vapour delivered, resulting in an unreliable device.

• Temperature compensation features help to overcome this problem; a copper heat sink placed around the vaporising chamber is one such example. Copper has a high heat capacity and donates energy required for latent heat of vaporisation, maintaining a stable temperature and reliable delivery of anaesthetic agent.

• Evaporation of sweat is another example. It requires the latent heat of vaporisation, which is provided by the skin’s surface, exerting a cooling effect upon the body.

• Evaporation from open body cavities can be a cause of significant heat loss from patients while under anaesthesia.

• These principles are also applicable to blood transfusion. Blood is stored at 5°C and has a specific heat capacity of 3.5 kJ·kg−1·K−1. If cold blood were transfused into a patient without pre-warming, the heat energy required to warm the blood to body temperature would need to be supplied by the patient, which would have a significant cooling effect.

APPLICATIONS

• Variable bypass vaporisers function by passing a small amount of fresh gas through the vaporising chamber, which is fully saturated with anaesthetic vapour. This removes vapour from the chamber. Further vaporisation from the anaesthetic liquid must occur to replace the vapour removed, which requires energy from the latent heat of vaporisation. This cools the remaining liquid, reducing the saturated vapour pressure and thus the concentration of anaesthetic vapour delivered, resulting in an unreliable device.

• Temperature compensation features help to overcome this problem; a copper heat sink placed around the vaporising chamber is one such example. Copper has a high heat capacity and donates energy required for latent heat of vaporisation, maintaining a stable temperature and reliable delivery of anaesthetic agent.

• Evaporation of sweat is another example. It requires the latent heat of vaporisation, which is provided by the skin’s surface, exerting a cooling effect upon the body.

• Evaporation from open body cavities can be a cause of significant heat loss from patients while under anaesthesia.

• These principles are also applicable to blood transfusion. Blood is stored at 5°C and has a specific heat capacity of 3.5 kJ·kg−1·K−1. If cold blood were transfused into a patient without pre-warming, the heat energy required to warm the blood to body temperature would need to be supplied by the patient, which would have a significant cooling effect.

THERMISTORS AND THEIR USE IN ANESTHESIA

🔻A #thermistor is a temperature-sensitive resistor whose resistance changes with temperature.

🔻Most temperature-sensitive resistors are constructed from a semiconductor material (carefully chosen metal oxides) and the resistance increases with a fall in temperature (they have a negative temperature coefficient)

🔻So they are known as negative thermal conductivity (NTC) thermistors.

🔻A Wheatstone bridge circuit is used to measure the resistance accurately.

🔻The main disadvantage of thermistors is the non-linear resistance versus temperature characteristic, although this can be compensated for using an appropriate calibration equation programmed into an electronic measurement system.

🔻Thermistors remain highly popular due to their cost, miniature size and convenience.

🔻Thermistor probes are commonly placed in the nasopharynx, oesophagus, rectum or bladder (integrated with a urinary catheter).

🔻They have excellent accuracy and their small mass means that there is a quick response to variations in temperature.

🔻True or False? 'A thermistor comprises a junction of dissimilar metals'

🔻Answer: False. Dissimilar junctional metals are thermocouples

♈️#PhysicsForAnesthesiologist : Beer-Lambert Law

☢️The #pulseoximeter works based on Beer-Lambert law, which relates the attenuation of light to the properties of the material through which the light is travelling. 

☢️It helps us in the calculation of the absorbance of a solution.

☢️According to the law, the absorbance of a solution depends on:

🖍The concentration of that solution, i.e. the more molecules of a light-absorbing compound there are in the sample, the more light will be absorbed. 

🖍The path-length of light travelling through the solution, i.e. the longer the length of the sample container, the more light will be absorbed because the light will come into contact with more molecules.

🖍A = εlc where 

🔻A is absorbance of light

🔻ε is the molar extinction coefficient(l mol–1 cm–1). It compensates for variance in concentration and the path-length, to allow comparison between solutions. 

🔻l is the length of solution that the light passes through. 

🔻c is the concentration of the compound in solution, expressed in mol L–1

☢️In the pulse oximeter, the concentration and molar extinction coefficient are constant. The only variable becomes the path length, which alters as arterial blood expands the vessels in a pulsatile fashion.

#Anesthesia, #PhysicsAndMedicine , #MedicalExams