TOF response %of receptors occupied
0 —————–100
1 ——————95
2—————— 90
3 ——————85
4 —————-<75
Category Archives: Exams EDAIC FRCA
DEFINITION OF SEDATION
The American Society of Anaesthesiologists uses the following definitions for levels of sedation:
# Minimal sedation (formerly known as anxiolysis) is a drug-induced state during which patients respond normally to verbal commands. Although cognitive function and coordination may be impaired, respiratory and cardiovascular stability is unimpaired.
# Moderate sedation (formerly known as conscious sedation) is a drug-induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation. It is important to remember that reflex response to a painful stimulus is not a purposeful response. No interventions are required to maintain a patent airway, and spontaneous ventilation is adequate. Cardiovascular stability is usually maintained.
# Deep sedation/analgesia is a drug-induced depression of consciousness during which patients cannot be easily roused but respond purposefully following repeated or painful stimulus. Patients may require assistance in maintaining a patent airway and spontaneous ventilation may be inadequate. Cardiovascular stability is usually maintained. In the UK deep sedation is considered to be a part of the spectrum of general anaesthesia.
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 circuits create 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 circuits allow current to flow, as the alternating direction of current flow prohibits a significant build up of charge on one of the plates.
- Reactance is 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
LASERS
- 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.
WHEATSTONE BRIDGE & STRAIN GAUGES
- 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 or vapour
- 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 – EEG – EMG 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
ELECTRICAL EQUIPMENT SAFETY
- Class 1 equipment is fully earthed.
- Class 2 equipment is double insulated.
- Class 3 is low voltage (24V).
PRESSURE & IT’S 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
The Lay Medical Man 