Definitions

This appendix is a list of key definitions that are common to many topics.

A

• Absolute Humidity
  Mass of water vapour in a given volume of air. Measured in mg.L^-1.

• Absorption
  The rate at which a drug leaves its site of administration and the extent to which this occurs.

• Accuracy
  The ability of a measuring device to match the actual value of the quantity being measured.

• Acid
  A proton donor.

• Acidaemia
  Arterial blood pH < 7.35.

• Acidosis
  A process which leads to an excess of hydrogen ions, and may lead to acidaemia if there is inadequate compensation. Can be subdivided into:
  • Respiratory acidosis: PaCO₂ > 45
  • Metabolic acidosis: HCO₃^- < 22

• Activity
  The effective concentration of a substance in a reacting system.

• Acute Pain
  Defined as pain of:
  • Recent onset
  • Limited probable duration
  • Identifiable causal and temporal relationship to injury or disease

• Adiabatic
  A process that occurs without transfer of heat or matter. For example, gases heat up when compressed (greater than the energy used to compress them), and cool when allowed to expand (adiabatic cooling).

• Affinity
  Ability of a drug to bind to a receptor.

• Afterload
  Ventricular wall stress at the onset of systole.

• Aging
  Naturally occurring, physiological decline in the structure and functional reserve of all organ systems.

• Agonist
  Drug which produces a maximal response at receptor site.

• Alkalaemia
  Arterial blood pH > 7.45.

• Alkalosis
  A process which leads to a deficit of hydrogen ions, and may lead to alkalaemia if there is inadequate compensation. Can be subdivided into:
  • Respiratory alkalosis: PaCO₂ < 35
  • Metabolic alkalosis: HCO₃^- > 26

• Allodynia
  Pain caused by a previously non-painful stimulus.

• Allosteric Modulator
  Substance which binds a receptor distant to the ligand-binding site, and modifies (positively or negatively) the effect of the ligand. Has no activity in absence of a ligand.

• Anaesthesia
  Without sensation.

• Analogue Signal
  Where the output of the transducer varies with the input signal.

• Anion
  Negatively charged ion.

• Anode
  The electrode which conventional current flows into.

• Anrep effect
  Method of myocardial autoregulation in which an increase in afterload causes an increase in contractility.

• Antagonist
  Drug which produces no response at the receptor, but prevents other ligands binding.

• Autoregulation
  Ability of an organ to maintain homeostasis in the presence of dynamic physiological conditions.

• Azeotrope
  A mixture of two substances that cannot be separated by fractional distillation, as each component shares same boiling point. This is typically temperature dependent.

B

• Base
  Proton acceptor.

• Base Excess
  Amount of acid that must be added to a solution to lower its pH to 7.4, at 37°C and with a PaCO₂ of 40mmHg.

• Bathmotropy
  Degree of myocardial excitability. Used with either positive or negative bathmotropy.

• Bell-Magendie Law
  The principle that in the spinal cord the dorsal roots are sensory and the ventral roots are motor.

• Bias
  The systematic distortion of the estimated intervention effect away from the “truth”, caused by inadequacies in the design, conduct, or analysis of a trial.

• Black-body radiation
  Electromagnetic radiation given off by all bodies at greater than 0°K. Wavelength of radiation emitted depends on the temperature of the body.

• Bohr Effect
  An increase in [H⁺] or PaCO₂ decreases Hb affinity for O2.

• Boiling Point
  The temperature at which the vapour pressure of a liquid equals the environmental pressure surrounding the liquid.
  • Therefore boiling point decreases as environmental pressure falls, as there is less external pressure keeping molecules in their liquid state
  • Boiling differs from evaporation as molecules anywhere in the liquid may enter the gaseous phase, whilst evaporation occurs only at the surface

• Bowditch Effect
  Increase in contractility seen with an increase in HR. Also known as the Treppe effect.

• Boyle's Law
  Pressure of a gas is inversely proportional to volume.

• Buffer
  Solution containing a weak acid and its conjugate base and will resist a change in pH when a stronger acid of base is added.

C

• Calibration
  A process of checking a monitoring device for linearity of correlation between actual and measured values over a given measurement range.

• Capacitance
  Ability of a system to store electrical charge. Measured in Farads.

• Central Blood Volume
  Volume of blood in heart and lungs.

• Central Sensitisation
  Increased responsiveness of nociceptive neurons in the central nervous system (i.e., post-synaptic) to their normal or subthreshold afferent input.

• Chemotaxis
  Movement of cells along a gradient of increasing concentration of an attracting molecule.

• Chronic Pain
  Pain that:
  • Persists beyond the time of tissue healing
  • Frequently has no clearly identifiable cause

• Clearance
  Volume of plasma completely cleared of a substance per unit time.

• Coronary Blood Flow
  At rest is ~5% of CO, or 225 ml.min^-1, and may increase 3-4x during exercise.

• Colloid
  Substance evenly dispersed throughout another solution in which it is insoluble.

• Colligative Properties
  The properties of a solution that depend on the ratio of solute to solvent, and not on the type of molecules present. These include:
  • Vapour pressure
  • Boiling point
  • Freezing point
  • Osmotic pressure

• Compliance
  Distensibility of a system. Expressed as the change in volume for a given change in pressure.

• Concentration Effect
  Describes the disproportionately rapid rise in Fi/FA ratio of nitrous oxide, as its rapid diffusion across the alveolar membrane increases the concentration of alveolar gas, and also augments respiration by drawing in dead space gas.

• Context-Sensitive Half-Time
  Time taken for plasma drug concentration to fall to 50% of its starting value after cessation of a drug infusion aimed to maintain a constant plasma concentration. Varies with the context, or duration, of drug infusion.

• Contractility
  Factors affecting myocardial performance, independent of preload and afterload.

• Critical Length
  The length of axon which must be blocked in order to prevent action potential transmission. It is dependent on myelination and fibre diameter.

• Critical Point
  The point on a phase diagram where the liquid and gas phases of a substance have the same density, and are therefore indistinguishable.
  • This point is where a substance is at both its critical temperature and critical pressure

• Critical Pressure
  Pressure required to liquefy a vapour at its critical temperature.

• Critical Temperature
  Temperature above which a substance cannot be liquified, irrespective of how much pressure is applied.

• Critical Volume
  The volume occupied by a given amount of substance at its critical point.

D

• Dalton
  Unit of mass equal to 1/12^th of the mass of Carbon-12.

• Dalton's Law
  The partial pressure of a gas in a mixture is equal to the pressure that gas would exert if it occupied the volume alone.

• Dead Space
  Inspired gas not participating in gas exchange. Includes:
  • Apparatus dead space
    Gas in the ventilator or breathing circuit.
  • Anatomical dead space
    Gas in the conducting zone of the lung.
  • Alveolar dead space
    Alveolar gas not participating in gas exchange. Also known as West Zone 1.
  • Physiological dead space
    Sum of alveolar and anatomical dead space.

• Density
  Mass per unit of volume.

• Dependence
  When a characteristic withdrawal syndrome occurs when a drug is withdrawn, or an antagonist administered.

• Diffusion
  Passive movement of a substance down an activity gradient by Brownian motion.

• Diffusion Hypoxia
  Fall in alveolar PAO₂ due to dilution of alveolar gas by N₂O diffusing from blood to alveoli.

• Digestion
  Process of breaking down macromolecules into readily absorbed compounds.

• Doppler Effect
  Alteration in frequency of a signal due to a relative difference in velocity between the emitter and observer. Detected frequencies will be:
  • Higher if the emitter is moving toward the observer
  • Lower if the emitter is moving away from the observer

• Down regulation
  Decrease in receptor number due to chronic agonist exposure.

• Drift
  A fixed deviation from the true value at all points in the measured range.

• Drug
  Substance administered to cause a change in a physiological system.

• Duplicate Publication
  Where the same set of results are published in multiple journals. Academically unethical, and will cause a systematic bias in a meta-analyses as the same set of patients are included twice.

• Dyne
  Force required to accelerate 1g by 1cm.sec^-2.

E

• Efficacy
  Maximal effect produced by a drug. Analogous to intrinsic activity.

• Electrocardiogram
  Graphical recording of the vector sum of cardiac electrical activity, as measured by electrodes on the skin.

• Emulsion
  A fine dispersion of minute droplets of one liquid in another in which it is not soluble or miscible.

• Enzyme
  Biological catalyst.

• Eutectic
  A mixture of substances with the lowest possible melting point than any other mixture of the same substances (and lower than that of either substance).

• Excitability
  How rapidly an excitable cell depolarises. Given by the gradient of phase 0 of the action potential, and is dependent on the function of voltage-gated sodium channels.

• Exponential Function
  Mathematical function where the rate of change is proportional to the current value.

• External Validity
  How well findings from one setting can be applied to another.

F

• Fahraeus-Lindqvist effect
  Decrease in apparent viscosity that occurs when a suspension (e.g. blood) flows through a tube of smaller diameter.

• Fasting
  Metabolic state achieved after complete digestion and absorption of a meal prior to the onset of starvation.

• Fick Principle
  Blood flow to an organ equals the uptake of a tracer substance by that organ, divided by the arterio-venous concentration difference.

• Flow
  Quantity of fluid passing a point per unit time.

• Fourier Analysis
  Deconstruction of a complex waveform by separating it into its constituent sine waves. The slowest component is known as the fundamental frequency.

• Free radical
  Extremely reactive molecular constituent carrying an unpaired electron.

• Freezing point
  Temperature at which molecular movement begins.

• Functional Residual Capacity
  Volume of gas in the lungs at the end of a normal tidal expiration, when the recoil pressure of the lungs equals the expansile pressure of the chest wall.

G

• Galvanometer
  Device to measure electrical current, usually via deflection of a wire in a magnetic field.

• Gas
  Substance above its critical temperature.

• General anaesthesia
  Drug induced, controlled, and reversible production of unconsciousness.

• Gibbs-Donnan Effect
  Describes the tendency of diffusable ions to distribute themselves such that the ratios of the concentrations are equal when they are in the presence of non-diffusable ions.

• Grahams Law
  The speed of diffusion of a gas through a membrane is inversely proportional to the square root of the molecular weight.

H

• Haldane effect
  Deoxygenated blood forms carbamino compounds and buffers H⁺ better than oxygenated blood.

• Half-Life
  Time taken for drug concentration (typically in plasma) to fall by half.

• Heat
  Describes the transfer of thermal energy from one body to another body; measured in Joules.

• Henry's Law
  Amount of gas dissolved in a substance is directly proportional to the partial pressure of gas at the gas-liquid interface.

• Heterometric autoregulation
  Change in ventricular function based on myocardial fibre length. Also known as Starling's Law.

• Homeometric autoregulation
  Mechanisms which alter myocardial performance independent of fibre length.

• Hormone
  Chemical messenger secreted by a ductless gland and has action on a distant target cell.

• Hyperalgesia
  Greater than normal amount of pain from a noxious stimulus. May be:
  • Primary
    Occurring in the region of tissue damage, e.g. in an inflamed area around a wound.
  • Secondary
    Extending beyond the region of tissue damage.

• Hypoxaemia
  When PaO₂ is less than 60mmHg.

• Hypoxia
  The point at which inadequate oxygenation of tissues results in anaerobic metabolism.

• Hysteresis
  When the future state of a system depends not only on its current state, but on the states preceding it.

I

• Ideal Gas
  A gas which will obey the ideal gas law. An ideal gas must have:
  • Negligible intermolecular attraction
  • A small molecular volume compared to the space between the molecules

• Idiosyncrasy
  An effect of a drug affecting only a small number of patients, typically due to the action of a particular metabolite.

• Inductance
  Property of a conductor by which a change in current induces an electromotive force in the conductor and any nearby conductors.

• Inotrope
  Drug which alters myocardial contractility.

• Intrinsic Activity
  Maximal effect produced by a drug. Analogous to efficacy.

• Impedance
  Resistance to alternating current.

• Internal Validity
  Where a causal relationship between variables has been properly demonstrated, i.e. a lack of bias.

• Irritability
  How easily an excitable cell can be stimulated. Given by how close the resting membrane potential is to threshold potential.

• Isomer
  Compound with the same chemical formula, but different chemical structure or arrangement of atoms.

• Isotherm
  Line of constant temperature drawn on a pressure-volume graph for a gas, which describes the relationship between pressure, temperature, and volume for a particular gas.

J

• Joule
  Energy transfered to an object when it is acted on by 1N for 1m.

L

• Laminar Flow
  Flow occurring smoothly and without turbulence.

• Local Anaesthetic
  Drug which reversibly prevents the conduction of the nerve impulse in the region to which it is applied, without affecting consciousness.

M

• MAC
  The minimal alveolar concentration (measured in % of 1 atm) at steady state which prevents a movement response to a standard surgical stimulus (midline incision) in 50% of a population.

• Manometer
  Device which measures gas pressure.

• Mean Systemic Filling Pressure
  The pressure measured anywhere in the systemic circulation when all flow of blood is stopped.

• Mixed Venous Blood
  Blood from the IVC, SVC and coronary sinus, which has been mixed by the pumping action of the RV and is typically sampled from the pulmonary artery.

• Mole
  Amount of a substance which contains as many representative particles as there are atoms in 12g of carbon-12.

• Molality
  Number of moles of solute per kg of solvent.

• Molarity
  Number of moles of solute per L of solvent. Varies with:
  • Temperature
  • Solvent density
  • Solute volume

N

• Natural Frequency
  Frequency at which a system will oscillate at if disturbed and left alone.

• Nausea
  Unpleasant subjective sensation associated with urge to vomit.

• Neuropathic Pain
  Pain caused by a lesion or disease of the somatosensory nervous system.

• Nociception
  Neural process of encoding a noxious stimulus.

O

• Odds Ratio
  Estimate of risk, where the OR is the ratio of odds of an outcome in those treated vs. those not treated. OR = 1 suggests no effect, ≤1 suggests reduced risk >1 suggests increased risk.

• Ohm
  Resistance which will allow one ampere of current to flow per volt of potential difference.

• Opiate
  Naturally occurring substance with morphine-like properties.

• Oncotic Pressure
  Proportion of osmotic pressure due to colloid.

• Opioid
  Describes any substance with activity at opioid receptors, and which can be reversed by naloxone.

• Osmosis
  Movement of a solvent across a semipermeable membrane to an area of greater solute concentration.

• Osmotic Pressure
  Pressure that must be applied to a solution to prevent the movement of a solvent from entering a solution with higher osmolality.

• Oxygen Flux
  Volume of oxygen delivered to the tissues per minute.

P

• p50
  The partial pressure at which an oxygen-carrying protein is 50% saturated.

• Pain
  Unpleasant sensory and emotional experience associated with actual or potential tissue damage, or expressed in terms of such damage.

• Partition Coefficient
  Describe the relative affinity of an agent for two phases. It is defined as the ratio of the concentration of agent in each phase, when both phases are of equal volume and the partial pressures are in equilibrium at STP.

• Pasteur Point
  PO₂ at which oxidative phosphorylation ceases.

• PEEP
  Supra-atmospheric airway pressure at the end of expiration.

• pH
  The power of hydrogen. Describes the activity of hydrogen ions in a solution, and is expressed as $pH = -log_{10}[H^+]$.

• Preload
  Load imposed on a muscle before contraction, and measured as the average myocardial fibre length at the onset of systole. May be approximated clinically using EDV.

• Precision
  The ability of a measurement device to provide reproducible results upon repeated measurement.

• Pseudo-critical temperature
  Temperature at which a gas mixture will separate into its constituent components.

R

• Radiation
  Transfer of energy via electromagnetic radiation.

• Receptor
  Component of a cell which binds to a ligand and results in a change in function.

• Reduction
  Reaction which results in a gain of an electron.

• Reflex
  Unconscious, predictable response to a stimulus.

• Regurgitation
  Passive passage of gastric contents into the mouth.

• Relative Humidity
  Ratio of mass of water vapour in a given volume of air, to the mass required to saturate that volume at that temperature. Expressed as a percentage.

• Respiratory Exchange Ratio
  Ratio of CO₂ produced to O₂ consumed at any given point.

• Respiratory Quotient
  Ratio of CO₂ produced to O₂ consumed at steady-state.

• Reynolds Number
  Dimensionless index which predicts the likelihood of turbulent flow.

S

• Saturated Vapour
  Vapour which is in equilibrium with its own liquid state, i.e. there are as many molecules entering the vapour phase as there there are those condensing into the liquid phase.
  • A saturated vapour contains the least amount of energy possible without condensing

• Saturated Vapour Pressure
  Pressure exerted by a vapour which is in equilibrium with its liquid state. Increases with temperature, since as the kinetic energy (heat) content of molecules increase, more of them enter the vapour phase.

• Second Gas Effect
  Disproportionately rapid rise in FA/Fi ratio seen when an anaesthetic agent is co-administered with nitrous oxide.

• Seebeck effect
  The generation of a potential difference at the junction of two dissimilar metals, with its value dependent on the temperature of the junction.

• Shivering
  Involuntary, oscillatory, muscular activity that augments metabolic heat production.

• Shunt
  Blood entering the left side of the circulation without being oxygenated via passage through the lungs.

• Specific Gravity
  Density of a liquid, in mass per unit volume.

• Specific Heat Capacity
  Amount of heat energy required to raise the temperature of 1kg of a substance by 1°K without a change in state.

• Standard Base Excess
  The base excess calculated for an Hb of 5g.L^-1, and which gives a better representation of ECF pH.

• Surface Tension
  Describes the tendency of a fluid to minimise its surface area.

• Suspension
  Particles of any phase dispersed in a liquid.

• Synergism
  When two drugs interact to produce a greater effect than would be expected.

T

• Temperature
  Average kinetic energy of molecules within a material, measured in degrees.

• Thirst
  Conscious sensation of the physiological urge to drink.

• Time constant
  Time it would take for an exponential function to complete if the initial rate of change continued. A process is:
  • 63% complete at 1T
  • 86.5% complete at 2T
  • 95% complete at 3T

• Tonicity
  Effective osmolality of a solution. Given by the osmolality, minus the concentration of freely diffusable osmoles (in plasma, these are urea and glucose).

• Tonometer
  Device which measures pressure of liquid.

• Transducer
  Device which changes a signal from one energy form to another.

• Treppe Effect
  Increase in contractility with an increase in HR. Also known as the Bowditch effect.

• Turbulent Flow
  Irregular movement in radial, axial, and circumferential axes.

V

• Valsalva Manoeuvre
  Forced expiration against a closed glottis.

• Vapour
  Substance in a gaseous phase below its critical temperature.

• Vapour pressure
  Pressure experted by a vapour.

• Venous admixture
  Amount of mixed venous blood that must be added to pulmonary end-capillary blood to give the observed arterial oxygen content.

• Viscosity
  Describes the tendency of a fluid to resist flow.

• Volt
  Potential difference which dissipates 1W of energy per 1A of current.

• Volume of Distribution
  Apparent volume into which a drug is distributed to produce the identified plasma concentration.