Def'n: the ratio of the amount of a substance present in one phase as compared with than in another, the two phases being of equal volume, the temperature must be specified and the phases in equilibrium.
the rate of diffusion of a substance across a unit area is proportional to the concentration gradient for that substance further, the diffusion of gas across a membrane, or into or out of a liquid, is proportional to the gases solubility in the liquid.
CO2 being more soluble than O2 diffuses far more rapidly across the alveolar membrane and into the RBC.
N2O being far more soluble than N2 may diffuse into and expand closed cavities during induction of anaesthesia.
Any process in which the rate of change of a quantity is proportional to the quantity is an exponential function. An example is the emptying of a bath: the rate of change of the volume of the bath (the plug-hole flow) is proportional to the volume remaining in the bath (which determines the pressure at the plug-hole):
Integrating with respect to time gives the exponential function:
Solids : which do not contain many unbound electrons and solutions with few ions are poor conductors and are known as insulators.
Semi-conductors contain electrons which are loosely bound and may conduct a current if electrons are given enough energy to become unbound.
This effect is seen in thermistors and photodetectors used in monitoring equipment. It is also the basis for transistors and silicon-based integrated circuits which are universally present in electronic equipment.
Potential difference is the difference in electrical energy between two points.
Its unit is the Volt (1 Joule/Coulomb) and it generates a electromotive force which drives a current of charged particles.
Resistance is a measure of the electromotive force required to drive a current through a material.
Its unit is the Ohm (1 Volt/Ampere). Thermistors display a change inresistance over a range of temperature and so with calibration the current flow for a specified voltage can be measured and used to determine temperature.
Impedance is the resistance of a component or circuit with a specified characteristic current flowing.
Resistance of many components (capacitors and inductors) varies with frequency of alternating current.
In a surgical diathermy device, a capacitor is part of the circuit, providing low impedance at the high frequency typically used (1 MHz), but high impedance to low frequency currents likely to cause arrhythmias (50 Hz).
Above a critical speed, laminar flow changes to turbulent flow. For a smooth cylindrical tube, the transition occurs when Reynolds number is approximately 2000. For rough or bent tubes, the transition occurs at lower numbers. Reynolds number (RN) is defined in terms of speed ( ν ), density ( ρ ) and viscosity ( η ) of the fluid and diameter (d) of the tube:
The cuff should cover at least 2/3rds of the upper arm.
The width of the cuff ’ s bladder should be 40% of the mid-circumference of the limb.
The middle of the cuff should overlay the brachial artery. The device has a fast rate of inflation and a slow cuff deflation. This avoids venous congestion and allows time to detect arterial pulsation.
Invasive arterial pressure monitoring provides beat-to-beat information about the perfusion of patient ’ s vital organs. Equipment
An indwelling Teflon arterial cannula (20/22G) is used. The cannula may be sited in the radial, ulnar, brachial, posterior tibial, femoral or dorsalis pedis artery. The radial artery is the preferred site. The cannula has parallel walls to minimise turbulent flow.
Allen ’ s Test Before cannulating the radial artery, Allen ’ s test should be carried out: (Inadequate collateral flow exists in 3% of hospitalised patients) Causes of Inadequate Ulnar Circulation [i] Normal anatomic variant [ii] Hypothenar Hammer Syndrome
Specific Complications Thromboembolism/vasospasm/thrombosis resulting in: [i] Compromise of circulation leading to blanching and possible necrosis or gangrene of tissues and extremities. [ii] Emboli. [iii] Damage to peripheral nerves.
Lead II is best for detecting arrhythmias. CM5 detects 89% of S-T segment changes due to left ventricular ischaemia. (Right arm electrode on manubrium, left arm electrode on V5 and indifferent lead on left shoulder).
CB5 useful in thoracic anaesthesia. Right arm electrode over the centre of the right scapula and left arm electrode over V5.
Interference currents are induced in the metal screen and not in the monitoring leads. The screening layer may often be covered by a second layer of insulation. Shivering can produce artefacts. Thus aim to place electrodes over bony prominences.
The differential amplifier measures the difference between the potential from two different sources.
Hence if there is interference common to the input terminals (e.g. mains frequency) it can be eliminated since it is only the difference between the terminals that is amplified by the differential amplifier.
This is known as common mode rejection.
The ratio of the output signal amplitude to the input signal amplitude is known as the gain of the amplifier.
In the following situations the pulse oximeter readings may not be accurate:
A reduction in peripheral pulsatile blood flow produced by peripheral vasoconstriction (hypovolaemia, severe hypotension, cold, cardiac failure, some cardiac arrhythmias) or peripheral vascular disease.
These result in an inadequate signal for analysis.
The excretion of CO2 is the final common pathway of metabolism. As such it provides a useful global indication that all is well.
V entilation must be sufficient to carry oxygen into the lungs oxygen is being transported to the mitochondria (cardiovascular function) aerobic metabolism is consuming this oxygen and producing carbon dioxide CO2 is being transported to the lungs (cardiovascular function) CO2 in the expired air gives an indication of adequate ventilation
oxygen is paramagnetic and is therefore attracted into a magnetic field this is due to the unpaired outer shell electrons of the oxygen molecule most other gases, such as N2, are weakly diamagnetic and are repelled from a magnetic field
CaO2 was originally measured volumetrically by the method of Van Slyke and Neill oxygen saturation is defined as the CaO2 / the oxygen capacity, expressed as a percentage this includes contributions from Hb-O2 and dissolved O2.
the normal PaCO2 = 40 mmHg (5.3 kPa) measurements are based on pH, due to the dissociation of carbonic acid the PCO2 is therefore related to the [H+] the Severinghaus CO2 Electrode provides a direct measure of PCO2 from the change is pH.
a. a closed cylinder of pH sensitive glass in the centre
b. 2 electrodes, one inside, the other outside the cylinder
c. a surrounding solution of sodium bicarbonate
d. a thin film of bicarbonate impregnated nylon mesh covering the end of the cylinder
e. a thin, CO2 permeable membrane covering the end of the electrode at the end of the electrode CO2 diffuses from the blood sample through the membrane into the nylon mesh and by the formation of carbonic acid lowers the pH of the bicarbonate solution this change in pH alters the V across the glass
b. alcohol thermometers cheaper than mercury useful for very low temperatures, mercury solid at -39°C unsuitable for high temperatures as alcohol boils at 78.5°C expansion also tends to be less linear than mercury
c. Thermocouple based on the Seebeck effect at the junction of two dissimilar metals a small voltage is produced,
the magnitude of which is determined by the temperature metals such as copper and constantan (Cu+Ni) requires a constant reference temperature at the second junction of the electrical circuit may be made exceeding small and introduced almost anywhere
Conduction is not an important means of heat loss in humans as gases are poor conductors.
Radiation is predominantly in the infrared spectrum and is determined by the temperature difference between the body and surrounding objects
the amount of heat loss by evaporation may be increased up to 10 fold by sweating all of these mechanisms depend upon the surface area of skin exposed to the environment thus, if this area is reduced heat loss is minimized
the heat required to convert 1 kg of a substance from one phase to another at a given temperature= latent heat of vapourization
therefore, the lower the temperature the greater the latent heat required as temperature rises, the latent heat falls until ultimately it reaches zero at a point which corresponds with the critical temperature
NB: fully saturated air at 20°C contains ~ 17 mg/l 37°C contains ~ 44 mg/l although relative humidity is expressed in terms of mass, as mass is directly proportional to the number of moles present, then by the ideal gas equation it becomes evident that,
Relative humidity = actual vapour pressure /saturated vapour pressure
the temperature of the wet bulb is reduced due to evaporation
the lower the humidity the greater the evaporative cooling and the greater the temperature difference tables relating T to % humidity air must be flowing over the wet bulb to prevent a local rise in the humidity
the stationary phase of the column is frequently a material such as fine silica-alumina coated with polyethylene glycol or silicone oil
through this column a flow of carrier gas is passed, such as argon or helium sample gases are then entered into the stream, and the speed with which they pass through the column is determined by their differential solubility between the two phases as solubility is temperature dependent,
the apparatus is maintained at a constant temperature this system is often termed gas liquid chromatography
CVP is an index of the circulating blood volume and preload to the right ventricle
the pulsatile characteristics of the CVP are a function of the uninterrupted return of venous blood to the right atrium, right atrial size and compliance, intrathoracic pressure, and the mechanical properties of the tricuspid valve and right ventricle .