Definitions Flashcards
(234 cards)
1
Q
Absolute zero
A
Temperature at which all molecular motion stops; 0K or -273.16 degrees C.
2
Q
Ampere
A
SI unit of current. 1A is the current which produces a force of 2 x 10 to power -7 N/m between two parallel wires, of infinite length, 1m apart in a vacuum. 1A = 1C/s.
3
Q
Boiling point
A
Temperature at which the vapour pressure of a liquid equals the surrounding ambient pressure.
4
Q
Calorie
A
Amount of energy required to increase the temperature of 1g water by 1 degree C. (1kCal = 4.16J)
5
Q
Candela
A
SI unit of luminous intensity. 1cd is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10 to power 12 Hz and that has a radiant intensity in that direction of 1/683 Watt per steradian.
6
Q
Coulomb
A
Unit of charge. 1C is the amount of charge passing a given point per second, when a current of 1A is flowing. 1C is the magnitude of charge possessed by 6.2 x 10 to power 18 electrons.
7
Q
Critical temperature
A
Temperature above which a gas cannot be liquified by pressure alone.
8
Q
Freezing point
A
Temperature at which the solid and liquid phases of a substance of specified composition are in equilibrium at a given pressure.
9
Q
Force
A
That which changes a body’s state of rest or motion. Force = mass x acceleration (Newton’s second law).
10
Q
Gas
A
Gaseous substance above its critical temperature.
11
Q
Heat capacity
A
The amount of energy required to raise the temperature of a body by 1 degree C. Total HC = SHC x mass of body.
12
Q
Hertz
A
Unit of frequency. 1Hz = 1 cycle per second.
13
Q
Joule
A
Unit of energy. 1J is the energy expended when the application of a 1N force moves 1m in the direction of that force. 1J = 1Nm.
14
Q
Kelvin
A
SI unit of temperature. 1K = 1/273.16 of the thermodynamic scale temperature of the triple point of water.
15
Q
Kilogram
A
SI unit of mass. 1kg is the mass of a cylindrical piece of platinum-iridium alloy kept in Sevres, France.
16
Q
Kinetic energy
A
The energy of motion of a body, equal to the energy that would be expended if that body were brought to rest.
17
Q
Latent heat
A
The heat energy required when a substance changes phase at a given temperature.
18
Q
Latent heat of fusion
A
The amount of heat required to convert a unit mass of solid at its melting point into a liquid, without an increase in temperature.
19
Q
Latent heat of vaporisation
A
The amount of heat required to convert a unit mass of a liquid at its boiling point into a vapour, without an increase in temperature.
20
Q
Mass
A
The amount of matter contained in a body. Unlike weight, it does not alter under differing conditions of gravity.
21
Q
Metre
A
SI unit of length. Originally the length of a platinum-iridium bar kept in France, but now defined according to the speed of light in a vacuum.
22
Q
Mole
A
SI unit of amount of substance. A quantity containing the same number of particles as there are atoms in 12g of carbon-12 (i.e. 6.022 x 10 to the power 23; Avogadro’s number).
23
Q
Newton
A
Unit of force. 1N is the force required to accelerate a mass of 1kg by 1m/s/s.
24
Q
Absolute humidity
A
Mass of water vapour in a given volume of air; g/m3.
Trachea - 34 g/m3
Alveoli - 44 g/m3
Room temp - 17 g/m3
25
Ohm
Unit of electrical resistance. 1 ohm is the resistance between two points on a conductor when a constant potential difference of 1V between them produces a current of 1A.
26
Pascal
Unit of pressure. 1Pa represents a force of 1N acting over 1m2.
27
pH
Negative logarithm to the base 10 of hydrogen ion concentration in mol/L.
pH = -log10[H+].
Normal [H+] is 35-45 nmol/L.
28
Potential energy
Energy of a body or system as a result of its position in an electric, magnetic or gravitational field. It is the potential to do work.
29
Power
Rate of energy expenditure, measured in Watts. 1W = 1J/s.
30
Pressure
Force per unit area. Unit is the Pascal. 1 bar = 1 atm/14.5psi/101kPa/760mmHg or torr/1020cmH2O.
31
Relative humidity
Ratio of the mass of water in a given volume of air in relation to the mass of water vapour it could hold if fully saturated at a given temperature. Expressed as %. Equal to VP/SVP.
32
Resistance
Property of a conductor to oppose the flow of current through it. Opposite of conductance.
33
Saturated vapour pressure
Pressure exerted by a vapour when in contact with and in equilibrium with its liquid phase within a closed system.
34
Second
SI unit of time. Defined according to the frequency of radiation emitted by caesium-133 in its ground state.
35
Specific heat capacity
Amount of energy required to raise the temperature of 1g of a substance by 1 degree C. SHC of water = 4.16J (i.e. 1 Cal). SHC of human body = 3.5kJ.
36
Specific latent heat of vaporisation
Energy required to change 1kg of liquid into vapour. SLHV of water at 37.5 degrees C = 2420kJ/kg. SLHV of water at 100 degrees C is 2260kJ/kg.
37
Triple point of water
Conditions in which water exists in equilibrium in all three phases (0.01 degrees C and 0.06 atm).
38
Vapour
Gaseous substance below its critical temperature.
39
Volt
Unit of electrical potential. 1V is the potential difference between two points when 1J of work is done to move 1C of charge across them. 1V will 'push' a current of 1A through a resistance of 1 ohm.
40
Watt
Unit of power. 1W = 1J/s.
41
Weight
Gravitational force acting on an object. It is the product of mass and gravitational acceleration (9.81m/s/s). A mass of 1kg therefore has a weight of 1N acting on it.
42
Accuracy
The proximity of output value to the true value. Expressed as a percentage range.
43
Sensitivity
Determines how small a change in input will result in a change in output. A higher sensitivity means a narrower range of operation.
44
Drift
Movement of the output value away from the true input value. Types include offset drift and gradient drift.
45
Gain
Degree of amplification of a measurement system (i.e. the output to input ratio).
46
Damping
The tendency of a system to resist oscillation. Results from the frictional forces within a system.
47
Response time
Time taken for the output to reach 90% of its final reading.
48
Rise time
Time taken for the output to rise from 10% to 90% of its final reading.
49
Hysteresis
The property of a system whereby the output alters depending on whether the input is rising or falling.
50
Calibration
A process in which the output of a measuring device is compared to a known standard, in order to determine the accuracy of the device. Three-point or higher calibration is required to assess linearity.
51
Precision
The degree to which repeated measurements under the same conditions show the same results. Related to reproducibility and repeatability.
52
Signal noise
Unwanted external information that is fed unintentionally into a transducer, resulting in the output being altered. The magnitude of noise is described by comparing the two amplitudes to give a signal:noise ratio. Overcome by filters
53
Resonance
The tendency of a system to oscillate at maximum amplitude at certain frequencies. Determined by mass and stiffness.
54
Ideal gas
A theoretical gas in which the molecules behave as individual particles that move in a random fashion independently of each other and of any inter-molecular forces. An ideal gas obeys the ideal gas laws.
55
Filling ratio
Weight of usual cylinder contents / weight of water cylinder could hold if full. For N2O, 0.75 in temperate regions and 0.67 in the tropics.
56
Pseudocritical temperature
The temperature at which a mixture of gases separates out into its individual constituents.
57
Flow
The quantity of a fluid passing a point in unit time.
58
Critical velocity
The velocity above which the flow of a fluid within a given tube is likely to change from laminar to turbulent (e.g. critical velocity in a 9mm ETT is 9L/min.
59
Avogadro's hypothesis
Equal volumes of gases at a given temperature and pressure contain the same numbers of molecules. One mole of gas at STP will occupy 22.4L and will contain 6.022 x 10 to the power 23 particles.
60
Dalton's law of partial pressures
The total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each of the individual gases within the mixture.
61
Poynting effect
The dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.
62
Henry's law
The amount of gas dissolved in a liquid is proportional to the partial pressure of gas in equilibrium with that liquid at a given temperature (the warmer the liquid, the less gas is dissolved in it - hence why boiling water bubbles).
63
Bernoulli effect
The drop in pressure that occurs at a constriction in a tube. The kinetic energy of the fluid increases, so the pressure drops in order for total energy to remain constant.
64
Boyle's law
At constant temperature, the absolute pressure of a given mass of gas is inversely proportional to the volume.
65
Gay-Lussac's law
At constant volume, the absolute pressure of a given mass of gas is directly proportional to its absolute temperature.
66
Avogadro's number
The same number of particles as there are atoms in 12g of carbon-12 (6.022 x 10 to the power of 23).
67
Charles' law
At constant pressure, the volume of a given mass of gas is directly proportional to the absolute temperature.
68
Preload
Initial length of cardiac muscle fibre prior to contraction. Represented by PCWP/LVEDV.
69
Afterload
Tension needing to be generated in cardiac muscle fibres before shortening will occur. The resistance to ventricular ejection. Represented by SVR.
70
Frank-Starling law
Force of contraction of a muscle fibre is proportional to its original length. Therefore stroke volume increases in line with increasing end diastolic volume, such that cardiac output matches venous return.
71
Seebeck effect
At the junction of two dissimilar metals, a voltage is produced in proportion to the temperature difference between them.
72
Basal metabolic rate
Minimal rate of energy expenditure per unit time by endothermic animals at rest. Conditions: physically and psychologically undisturbed, thermally neutral environment, post-absorptive state. Normal = 30 kCal/kg/day.
73
Raoult's law
Vapour pressure is directly proportional to the molecular concentration of a solute.
74
Temperature
Thermal state of a substance. Determines whether it receives or donates heat energy from or to its surroundings. Measured in K.
75
Blood:gas (or oil:gas) partition coefficient
Ratio of the amount of substance present in equal volume phases of blood (or oil) and gas in a closed system at equilibrium and STP.
B:G relates to speed of onset/offset.
O:G relates to potency.
76
Defibrillator
Device for delivering electrical energy to the heart in a controlled fashion. Aims to simultaneously depolarise a critical number of cells to allow the natural pacemaker to regain control and sinus rhythm to be restored. Types: mono/biphasic; external/internal. Consists of a capacitor, a rectifier, an inductor and a transformer (CRIT).
77
Colligative properties
Those which depend purely on the number of solute particles present, rather than other characteristics. E.g. SVP, osmotic pressure, freezing and boiling points.
78
Closing capacity
The lung volume at which small airway closure begins. CC = CV + RV.
79
Closing volume
The lung volume above RV at which small airway closure begins. Increased by age, supine, smoking, high BMI. Reduced by GA, infancy and pregnancy.
80
Vacuum
A space devoid of matter. Also used to describe an enclosed space from which matter has been partially removed, resulting in a negative pressure relative to atmospheric.
Vacuum = opposite of plenum, in which gas is contained at greater than atmospheric pressure.
81
Filtration
Process by which particles are removed from a fluid stream by a semi-permeable membrane. Divided into screen and depth filters.
82
Inertia
Tendency of a body to resist changes in speed or direction.
83
Decontamination
Process of rendering a piece of equipment ready for use by the removal of contaminants in quantities sufficient to prevent a harmful reaction. Divided into cleaning, disinfection and sterilisation.
84
Diathermy
A surgical tool which uses electric current to cause localised heating, permitting cutting of tissue and coagulation of blood. Divided into unipolar and bipolar.
85
Doppler effect
The apparent change in wave frequency that occurs when the source of a wave is in motion relative to the receiver. Types in medicine: continuous wave, pulsed wave, colour.
86
Electricity
A form of energy resulting from the existence of charged particles (such as electrons or protons), either statically as an accumulation of charge or dynamically as a current.
87
Capacitance
The ability of a object to store electrical charge.
88
Concentration effect and second gas effect
N2O is 30x more soluble in blood than N2 so diffuses into blood faster than N2 diffuses out. Result = reduced alveolar volume and rise in conc of remaining gases (conc effect). The greater conc gradient means faster diffusion of volatile into blood and therefore faster onset of GA (2nd gas effect).
89
Adiabatic process
A thermodynamic process during which no energy is transferred as heat outside the system. Energy is drawn from the kinetic energy of the molecules concerned. E.g. the cryoprobe cools due to rapid gas expansion.
90
Heat
A form of energy associated with the movement of atoms and molecules; it moves from one site to another depending on the temperature difference between them. Measured in J.
91
Meyer-Overton hypothesis
When sufficient amount of agent dissolves into a neuronal lipid membrane, anaesthesia is achieved (i.e. it is the no of molecules that matters, not the agent itself). If correct, the product of MAC and O:G would be a single constant. In reality, older agents have a product of 100 and newer ones 200, suggesting there may be different sites or mechanisms of action.
92
MAC
Minimum alveolar concentration, at equilibrium (after 15m), at sea level, in 100% O2, at which 50% of the population will fail to respond to a standard noxious stimulus. This is MAC50. Other MACs = MAC95, MAC-awake (eye opening in response to verbal command - 0.3-0.5 MAC), MAC-intubation, MAC-BAR (block autonomic reflexes - 1.7-2.0 MAC).
93
Coanda effect
The tendency of a fluid jet to be attracted to a nearby surface and therefore not to divide evenly between pathways (e.g. 'wall-hugging' jets of MR).
94
Venturi principle
Entrainment of a fluid into an area of low pressure caused by a constriction in a tube (Bernoulli effect). Used in Venturi masks, nebulisers, spray atomisers and vaporisers.
95
Farad
Unit of capacitance. 1F is the capacity to store 1C of charge for an applied potential difference of 1V.
96
Coronary steal and cerebral steal
Coronary: diversion of blood from poorly perfused areas of myocardium to those already adequately perfused. May be caused by vasodilator substances acting on small coronary arteries but not on larger epicardial vessels.
Cerebral: vasodilatation diverts blood away from damaged areas of brain. Inverse steal: inducing vasoconstriction of normal areas may divert blood towards damaged areas of brain (e.g. thiopentone and hypocapnoea).
97
Half life
Time taken for the plasma concentration of a drug to decrease by half (in first order kinetics).
98
Time constant
The time in which an exponential process would be completed if the rate of change were maintained at its initial value. At 3τ it is 95% complete.
99
Clearance
The volume of plasma from which a drug is completely cleared per unit time.
100
Volume of distribution
The apparent volume into which a drug disperses to produce the observed plasma concentration at time zero.
101
Dead space
Volume of inspired gas that does not participate in gas exchange. Physiological dead space = anatomical dead space (Fowler's method, about 2ml/kg) + alveolar dead space (usually zero).
102
Haldane effect
DeoxyHb has a higher affinity for CO2 than oxyHb, and vice versa. Double Haldane effect occurs in the placenta.
103
Rate constant
The constant of proportionality linking the rate of the change of the quantity to its value (i.e. k = rate/quantity). It is the reciprocal of the time constant (τ). Unit = reciprocal time (time -1)
104
Autoregulation
The ability to maintain constant blood flow in the face of variable perfusion pressure. Mechanisms include metabolic, myogenic, endothelial, autonomic and hormonal.
105
Contractility
The intrinsic ability of cardiac muscle fibres to do work with a given preload and afterload.
106
GFR
Volume of plasma filtered at the glomerulus per unit time. Normal = 125ml/min (180L/day). 10% lower in women.
107
Filtration fraction
Proportion of filterable fluid that is actually filtered. Normal = 20%.
108
Tonicity
Describes the relative solute concentrations of two solutions separated by a semi-permeable membrane. Usually taken to mean tonicity of an IV fluid vs. internal environment of an RBC, with reference to the cell membrane of the RBC. Only influenced by particles unable to cross the membrane (i.e. not urea or glucose).
109
Osmotic pressure and colloid osmotic pressure (oncotic pressure)
Osmotic pressure: that required to prevent solvent migration by osmosis. Pressure exerted by osmotically active particles.
Colloid osmotic pressure/oncotic pressure: the osmotic pressure exerted by plasma proteins. Measured with an oncometer.
110
Osmole
The amount of solute that exerts an osmotic pressure of 1 atm when placed in 22.4L of solution at 0 degrees C. e.g. NaCl = 2 osmoles; glucose = 1 as does not dissociate.
111
Osmosis
Movement of water molecules from a less to a more concentrated solution across a semi-permeable membrane.
112
Ageing
An irreversible process causing a gradual reduction over time in the reserve of all body systems.
113
Pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage or expressed in terms of such damage.
114
Gate control theory of pain
Postulates that pain transmission from primary to secondary afferents is 'gated' by interneurones in the substantia gelatinosa. Pain can therefore be modulated either pre- or post-synaptically by A-beta fibres or A-delta/descending pathways, respectively.
115
Stress response
Term encompassing the metabolic and hormonal changes following an insult such as trauma or surgery. A catabolic state results, the magnitude and duration of which is proportional to the severity of the insult.
116
Endothelium
The simple squamous epithelium which lines organs, blood vessels and body cavities. Consists of a single cell layer overlying a basement membrane. Types include continuous (e.g. BBB), discontinuous (sinusoids) and fenestrated (glomerulus).
117
Portal circulation
A circulation in which blood from the capillary bed of one organ structure drains into the capillary bed of another, though a larger vessel. E.g. hepatic portal, placenta, renal, hypothalamo-hypophyseal, ovarian, testicular. Advantages: speed of transport and no risk of dilution/destruction in systemic circulation.
118
Reflex
A neuronal pathway that produces a rapid, automatic and predictable response to a stimulus. Types include somatic and visceral. e.g. stretch (monosynaptic), inverse stretch, withdrawal (polysynaptic - A-delta and C afferents to a-alpha efferents).
Components: receptor, afferent nerve, central integrating system, efferent nerve, effector.
The ANS is also predicated on reflexes.
119
Countercurrent multiplier
A mechanism that expends energy to maintain a concentration gradient (e.g. in kidney - permits water reabsorption).
120
Ligand
A substance able to bind to a receptor and bring about a biological change within a cell.
121
Receptor
A protein with selective ligand-binding site(s).
122
Law of mass action
The rate of a reaction is directly proportional to the concentrations of the reactants (e.g. drug and receptor).
123
Drug interaction
When the action of one drug is altered by the concurrent administration of another. Types: physicochemical (sugg/roc), pharmacokinetic (LA+adren), pharmacodynamic (prop/remi - synergism).
124
Ka
Dissociation constant. Indicates how readily an acid in solution gives up its H+ to a base. pKa is -log to base 10 Ka.
125
Bioavailability
The fraction of the drug that reaches the systemic circulation.
126
First pass metabolism
Process by which drugs absorbed from GIT must pass through the liver via the hepatic portal circulation, and hence are partially metabolised before reaching the systemic circulation.
127
Genetic polymorphism
A term describing genotypic and subsequent phenotypic variability, often in reference to differences in drug handling between individuals.
128
Potency
A measure of drug activity expressed in terms of the amount required to produce an effect of a given magnitude.
129
Affinity
How avidly a drug binds to its receptor.
130
Efficacy/intrinsic activity
The ability of a drug to produce the maximal response once bound to its receptor. Ranges from -1 (full inverse agonist) through 0 (antagonist) to 1 (full agonist).
131
Tachyphylaxis
Acutely reduced response to a drug after repeated administration (e.g. ephedrine depleting NA stores).
132
Desensitisation and tolerance
Reduced response to a drug after repeated administration over time e.g. adrenaline. Mechanisms include structural receptor changes, receptor sequestration and receptor downregulation. Larger doses required to achieve same effect.
133
Context sensitive half-time
The time taken for the drug concentration to halve once an infusion at steady state is stopped.
'Context' means duration of infusion.
Balance between distribution and elimination clearances - low Vd and high elimination desirable to reduce CSHT.
134
Dependence
Need for repeated administration to avoid withdrawal.
135
Addiction
Behaviour resulting from dependence; includes craving and compulsive use despite harm.
136
Awareness
Recall of time under anaesthesia; divided into explicit and implicit memory.
137
Collision broadening
The presence of one gas broadening the absorption spectrum of another - e.g. CO, CO2 and N2O. Accounted for by calibrating instrument to same background gas mixture as that to be analysed.
138
Beer's and Lambert's laws
The absorption of radiation increases exponentially as the concentration (Beer) and thickness (Lambert) of the medium through which it is passing increases.
139
Power (statistics)
Ability of a statistical test to reveal a difference of a certain magnitude. Acceptable power is normally considered 80% (i.e. a beta error of 20%).
140
P value
Likelihood of an observed difference being due to chance alone.
141
Type 1 error
To wrongly accept the alternative hypothesis. Chance of this occurring = alpha. To reduce the risk of this, reduce the accepted p value.
142
Type 2 error
To wrongly accept the null hypothesis. Chance of it occurring = beta. To reduce the risk of this, increase the sample size.
143
Meta-analysis
A statistical technique that combines the results of several independent studies that address a similar research question.
144
Bias
A systematic error that produces an incorrect result.
145
Significance (statistical and clinical)
Statistical: a result unlikely to be due to chance.
Clinical: a result with importance when applied to clinical practice.
146
Macroshock
Passage of current from one part of the body to another.
147
Microshock
Passage of current directly to the myocardium.
148
Penaz principle
The force exerted on a body can be determined by measuring an opposing force that prevents physical disruption. Similar principle to null deflection.
149
Laser
Light Amplification by Stimulated Emission of Radiation. A device that produces a beam of light that is monochromatic (single wavelength), coherent (in phase) and collimated (parallel).
150
Supramaximal stimulus
One with sufficient current amplitude to cause 100% of motor neurones to be depolarised in a given area. Usually 20% above required current.
151
Faraday cage
Enclosure formed by conductive material which is impervious to electric fields and electromagnetic radiation.
152
Conductors, insulators and semi-conductors
Conductors - materials with free electrons that can be made to flow. Insulators - very few free electrons. Semi-conductors - crystalline materials that are insulators when pure but conduct when impure or when stimulated by e.g. Light, heat, voltage. E.g. Silicon. Used to make diodes.
153
Impedance
The opposition that a circuit presents to a current when a voltage is applied. Sum of the resistance (opposition to current) and reactance (opposition to change in current).
154
Sine wave
Smooth, repetitive oscillation. A graph of the function sine.
155
Amplitude
Peak deviation from zero
156
Frequency
Cycles per second
157
Phase
Specifies where in its cycle the oscillation is at t = 0. When not 0, the wave appears shifted.
158
Period
Length of one cycle of the wave (peak to peak or trough to trough).
159
Difficult airway
The clinical situation in which a conventionally trained anaesthetist experiences difficulty with facemask ventilation, supraglottic device ventilation, tracheal intubation or all three.
(ASA)
160
STP/NTP
Standard temperature and pressure. 0C (273.16K) and 1 atm.
Normal temperature and pressure. 20C and 1 atm.
161
Reactance
Opposition to change in current.
162
Amplifier
A device that increases signal amplitude.
163
Capacitor
A device which stores energy in an electric field.
Consists of two conductive plates and a dielectric.
164
Inductor
A device which stores energy in a magnetic field.
Consists of a conducting coil +/- a ferromagnetic core.
165
Transformer
A device which can adjust voltage and current.
Uses principle of induction. Consists of two magnetically coupled coils. Can be step up, step down or isolation transformers (latter is the basis of the floating circuit).
166
Earth
The ground considered as a medium for completing an electric circuit.
167
Diode
Device that allows current to pass in only one direction.
168
Battery
A power supply that uses chemical energy to make electricity. Can be primary/non-rechargeable or secondary/rechargeable depending on whether the electrochemical reaction is reversible or not. Batteries supply DC current (mains = AC).
169
Transducer
A device that converts one form of energy to another.
170
Circuit breaker
An automatic device for stopping the flow of current in an electric circuit as a safety measure. Protects against voltage and current overloads. Reusable.
171
Fuse
An abbreviation for fusible link. Protects against current overload. Consists of a metal wire which will melt when overheated, thereby breaking the circuit. Single use.
172
Rectifier
Device that converts AC to DC. Opposite of an inverter.
173
Inverter
Device that converts DC to AC. Opposite of a rectifier. e.g. ambulance vehicle battery needs inverter to be able to power medical equipment.
174
Valsalva manouvre
Forced expiration against a closed glottis.
REVERT trial - Valsalva + leg elevation cf. Valsalva alone --> resolution of SVT in 43% cf. 17%.
Opposite = Mueller's manouvre (attempted inspiration against closed glottis at end expiration) - used in diagnosis of OSA as can differentiate upper from lower airway collapse when performed with nasendoscopy.
175
Sepsis 3.0
Life-threatening organ dysfunction caused by a dysregulated host response to infection.
Organ dysfunction is an acute change in total SOFA score ≥2 points. Mortality 10%.
Septic shock is sepsis with persisting hypotension requiring vasopressors to maintain MAP ≥65mmHg and having a serum lactate level >2mmol/L, despite adequate volume resuscitation. Mortality >40%.
176
Hypotensive anaesthesia
The intentional lowering of systemic blood pressure (by >30% of resting values) during general anaesthesia in an attempt to minimise operative blood loss and improve operating conditions.
Requires arterial access. CI in IHD, DM, severe anaemia.
177
MET
Metabolic equivalent. A unit for describing the energy expenditure of physical activities. 1 MET is the resting rate of oxygen consumption (3.5 ml O2/kg).
Light intensity activity = 1-3 METs
Moderate 3-6 METs
Vigorous 6 or more METs
Patients for major surgery should be able to perform at least 4 METs (equivalent of climbing a flight of stairs).
Health benefits are gained by expending 500-1000 MET-minutes per week.
178
VO2 and VO2 max
Rate of oxygen consumption. VO2 max is the maximal rate of O2 consumption, which is effort-dependent. VO2 can increase by 50% after major surgery. Pre-surgical exercise training is being explored as a method of pre-optimisation.
179
Anaerobic threshold
VO2 at which anaerobic metabolism begins during exercise, as oxygen demand starts to outstrip supply. Marker of combined efficiency of heart, lungs and circulation. Independent of effort, therefore reliable and repeatable. Usually occurs halfway through a CPET.
AT of at least 11 ml/kg/min is required for major surgery.
180
Autonomic dysreflexia/hyperreflexia
An exaggerated sympathetic response to stimulation below the level of a spinal cord lesion.
181
Stridor
A harsh, vibratory sound produced due to partial airway obstruction, resulting in turbulent flow.
Stridor at rest implies a reduction in airway diameter of >50%.
Inspiratory (laryngeal): obstruction above glottis
Expiratory (tracheobronchial): obstruction below glottis
Biphasic: obstruction at glottic level, or critical obstruction at any level.
e.g. infection, FB, tracheomalacia/stenosis.
182
ECG
The recording and display of cardiac electrical activity. Components: electrodes, amplifier, filter, display.
183
TEG
Point of care coagulation test utilising the viscoelasticity of whole blood.
184
Splitting ratio
The adjustable proportion of gas that either enters or bypasses a vaporising chamber in a variable bypass vaporiser.
185
Difficult intubation
Clinical situation in which an experienced unassisted anaesthetist needs >10m or >3 attempts to intubate with conventional laryngoscopy.
186
Difficult FMV
Clinical situation in which an experienced unassisted anaesthetist has difficulty maintaining SpO2 >92%, or preventing/reversing signs of inadequate ventilation during positive pressure FMV under GA.
187
Moisture deficit
Humidity added by upper airways to inspired gas to enable optimal gas exchange.
188
Neuropraxia
Temporary loss of motor or sensory function due to impaired nerve conduction.
189
Malnutrition
A state in which a deficiency or excess of energy, protein and other nutrients causes measurable adverse effects. Includes under- and overnutrition.
190
ICU-acquired weakness
Clinically detectable weakness in patients in whom there is no plausible aetiology other than critical illness. It is generalised, symmetrical, flaccid and spares the cranial nerves. MRC mean score is <4 in all muscle groups on two occasions over 24h apart. Divisible into myopathy, polyneuropathy and neuromyopathy.
191
Shunt
Blood which passes through unventilated areas of lung (true shunt). Physiological shunt refers to the amount of venous admixture which rejoins the circulation without being oxygenated (bronchial veins and thebesian veins).
192
Osmolarity and osmolality
Osmolarity: osmoles per L of solution
Osmolality: osmoles per kg of solvent
Osmolality is more accurate as not affected by temperature (as volumes change but mass is constant).
193
Osmolar gap
Difference between measured and calculated osmolarity. Indicates presence of an osmotically active particle that is not normally present e.g. ethanol.
Normal = <10.
194
PPH and MOH
>500ml SVD or >1L LSCS
MOH = >1.5L, drop 4g Hb or requiring 4u PRC
195
Enhanced recovery / fast-track surgery
The outcome of applying a range of multimodal strategies that are designed to prepare and optimise patients before, during and after surgery, ensuring prompt recovery and discharge. Aims to minimise disruption and promote early return to normal function.
196
Watershed effect
When two streams of fluid flowing in opposite directions meet, resulting in blood flowing in neither direction and so a risk of ischaemia.
e.g. mid-thoracic spine has greatest risk of ischaemia.
197
Warm ischaemic time
Time between onset of inadequate perfusion and/or oxygenation, and onset of cold perfusion.
198
Superconductor
A conductor with minimal or no resistance when below a critical temperature. e.g. MRI gradient coils when cooled with liquid helium.
199
Death
Irreversible loss of circulatory and respiratory functions, or irreversible loss of all brain function.
Brain (stem) death: irreversible loss of all brain functions. Coma, apnoea and absence of brainstem reflexes.
200
Bronchopleural fistula
Abnormal communication between the pleural cavity and bronchial tree. Defined as a persistent air leak >24h after development of PTX, or failure to reinflate the lung despite presence of a chest drain for 24h.
201
Target controlled infusion
An infusion controlled in such a manner as to achieve a user defined drug concentration in a body compartment of interest.
Components: user interface, microprocessor and infusion device.
202
Allodynia
Pain caused by a stimulus that is not normally painful.
203
Hyperalgesia
An exaggerated response to a stimulus that is normally painful.
204
Validity
That a study measures what it is designed to.
205
Reliability
Repeatability/reproducibility. Related to precision (the degree to which repeated measurements under the same conditions show the same results).
206
Prodrug
A prodrug is a compound that has little or no activity on a desired pharmacological target, but is converted to an active, or more active, entity by an endogenous metabolic reaction. Prodrugs can improve pharmacokinetics, reduce toxicity, or facilitate delivery of the drug to specific tissues or cells.
207
Burst-suppression
EEG pattern consisting of bursts of high-amplitude activity interspersed with longer episodes of suppression (isoelectric trace).
When burst-suppression is induced by anaesthetic agents, CMRO2 reduces by up to 50%.
Indications for burst-suppression = refractory status epilepticus, refractory raised ICP (e.g. TBI) and intraoperative neuroprotection (although levels of evidence are low). No evidence of benefit post cardiac arrest.
208
Cor pulmonale
An alteration in the structure and function of the right ventricle caused by a primary disorder of the respiratory system.
209
Never event
Serious incidents that are entirely preventable if published guidance has been followed. They have the potential to cause serious harm and death.
210
Consciousness and awareness
Consciousness is the state of being aware of and responsive to one's surroundings. Awareness does not include the latter.
211
Disseminated intravascular coagulation
DIC is a disorder characterised by systemic activation of blood coagulation, resulting in generation and deposition of fibrin microclots in various organs and contributing to multiple organ dysfunction syndrome (MODS). Consumption and subsequent exhaustion of coagulation factors and platelets may then induce severe bleeding.
212
Pollution
The introduction into an area of a substance which is harmful to living creatures or the environment. Divisible into matter (solid/liquid/gas) and energy (heat/noise/radiation) pollution.
213
Pre-optimisation
A strategy of manipulating physiology preoperatively to achieve supranormal oxygen delivery (target DO2 600ml/min often quoted). Borne out of the observation that morbidity and mortality is higher in patients who accrue a postoperative 'oxygen debt' due to inability to increase tissue oxygen delivery.
Facets: organ oxygenation and perfusion, attenuation of stress response, reversing reversible problems.
214
Two-stage arthroplasty
Gold standard mx for infected joint prosthesis. 1st stage = removal of original prosthesis and cement, aggressive debridement and abx-loaded spacer inserted. Then 6/52 abx, plus abx-free period. 2nd stage = removal of spacer, repeat debridement, new prosthesis.
215
Extraction ratio
The proportion of a drug removed from blood by a single pass through the liver. Equal to 1 - bioavailability.
216
Bioequivalence
Said of two drugs that have the same rate and extent of absorption. May not translate into therapeutic equivalence.
217
Sympathomimetics
Drugs with similar actions to the SNS. Also known as adrenergics.
218
Enzyme
A biological protein that catalyses a reaction, but remains unchanged itself.
219
Synaptic plasticity
The strengthening or weakening of synapses over time. Cellular basis for learning and memory. Thought to be responsible for opioid tolerance and dependence.
220
Gibbs-Donnan effect
The phenomenon by which charged particles achieve electrostatic equilibrium across a semipermeable membrane.
221
Spinal shock and neurogenic shock
Spinal shock: initial phase of flaccidity, areflexia and loss of sphincter tone that occurs acutely after a spinal cord injury. Lasts hours-weeks. Not a true form of shock as it is neurological, not cardiovascular.
Neurogenic shock: hypotension, paradoxical bradycardia, vasodilatation. Due to SNS damage from lesions above T6. Above T4, cardiac sympathetic supply is also lost.
222
Anaphylaxis
A life-threatening systemic hypersensitivity reaction, sub-divided into ‘allergic’ and ‘non-allergic’ reactions.
223
Weber effect
Initially increased reporting of adverse events to new drugs.
224
Elastance
Reciprocal of compliance (i.e. change in pressure per unit change in volume).
225
Compliance
Change in volume per unit change in pressure. Static compliance is always greater than dynamic. Measured using oesophageal pressure as a proxy.
Lung and chest wall compliance are each 200ml/cmH2O. Therefore total compliance is 100ml/cmH2O.
226
Pacing threshold
The amount of energy needed to obtain electrical capture. Threshold is set at 2-3 times the current at which capture is lost.
227
Hypothermia
A body temperature below normal. Mild (34-36.5), moderate (27-34), profound (<27).
Inadvertent perioperative hypothermia: mild (down to 35, moderate 34-35, severe <34).
228
Principle of black box radiation
Every object with a temperature above absolute zero emits radiant energy of an intensity proportional to the fourth power of its temperature. Used in infrared temperature measurement.
229
Base deficit (or excess)
The amount of base or acid required to restore 1L of blood to normal pH at a PCO2 of 5.3kPa and a temperature of 37C.
230
Hypoxic pulmonary vasoconstriction
A protective physiological reflex that aims to divert blood flow away from hypoxic areas of the lungs to areas with better ventilation and oxygenation.
231
Neural inertia
The tendency of the CNS to resist transitions between arousal states.
232
Density
Mass per unit volume of a material substance.
233
Velocity
A vector expression of the displacement that an object or particle undergoes per unit time. Consists of both magnitude and direction.
234
Viscosity
A measure of the resistance of a fluid to gradual deformation by shear or tensile stress. Caused by friction between molecules of a fluid.
