Definitions Flashcards
1
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
2
Q
pH
A
Negative logarithm to the base 10 of hydrogen ion concentration in mol/L.
pH = -log10[H+].
Normal [H+] is 35-45 nmol/L.
3
Q
Pressure
A
Force per unit area. Unit is the Pascal (1Pa = 1N over 1 m2). 1 bar = 1 atm/14.5psi/101kPa/760mmHg or torr/1020cmH2O.
4
Q
Relative humidity
A
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.
5
Q
Accuracy
A
The proximity of output value to the true value. Expressed as a percentage range.
6
Q
Sensitivity
A
Determines how small a change in input will result in a change in output. A higher sensitivity means a narrower range of operation.
7
Q
Drift
A
Movement of the output value away from the true input value. Types include offset drift and gradient drift.
8
Q
Gain
A
Degree of amplification of a measurement system (i.e. the output to input ratio).
9
Q
Damping
A
The tendency of a system to resist oscillation. Results from the frictional forces within a system.
Optimal damping = balance between rapidity of response and excessive oscillation. Optimal damping coefficient = 0.64.
10
Q
Response time
A
Time taken for the output to reach 90% of its final reading.
11
Q
Rise time
A
Time taken for the output to rise from 10% to 90% of its final reading.
12
Q
Hysteresis
A
The property of a system whereby the output alters depending on whether the input is rising or falling.
13
Q
Calibration
A
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.
14
Q
Precision
A
The degree to which repeated measurements under the same conditions show the same results. Related to reproducibility and repeatability.
15
Q
Signal noise
A
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.
16
Q
Resonance
A
The tendency of a system to oscillate at maximum amplitude at certain frequencies. Determined by mass and stiffness.
17
Q
Flow
A
The quantity of a fluid passing a point in unit time.
18
Q
Critical velocity
A
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.
19
Q
Bernoulli principle and Venturi effect
A
Bernoulli: 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.
dP = 4V2
Venturi: entrainment of a fluid into an area of low pressure caused by a constriction in a tube.
20
Q
Preload
A
Initial length of cardiac muscle fibre prior to contraction. Represented by PCWP/LVEDV.
21
Q
Afterload
A
Tension needing to be generated in cardiac muscle fibres before shortening will occur. The resistance to ventricular ejection. Represented by SVR.
22
Q
Frank-Starling law
A
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.
23
Q
Basal metabolic rate
A
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.
24
Q
Defibrillator
A
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. Biphasic = polarity of the shock is reversed midway, meaning a lower energy is required, there is less myocardial stunning post shock, and the defib can be battery powered.
25
Closing capacity
The lung volume at which small airway closure begins. CC = CV + RV.
26
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.
27
Filtration
Process by which particles are removed from a fluid stream by a semi-permeable membrane. Divided into screen and depth filters.
28
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.
29
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.
30
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.
31
Capacitance
The ability of a object to store electrical charge. Measured in Farads.
32
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).
33
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).
34
Half life
Time taken for the plasma concentration of a drug to decrease by half (in first order kinetics).
35
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.
36
Clearance
The volume of plasma from which a drug is completely cleared per unit time.
37
Volume of distribution
The apparent volume into which a drug disperses to produce the observed plasma concentration at time zero.
38
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).
39
Haldane effect
DeoxyHb has a higher affinity for CO2 than oxyHb, and vice versa. Double Haldane effect occurs in the placenta.
40
Autoregulation
The ability to maintain constant blood flow in the face of variable perfusion pressure. Mechanisms include metabolic, myogenic, endothelial, autonomic and hormonal.
41
Contractility
The intrinsic ability of cardiac muscle fibres to do work with a given preload and afterload.
42
GFR
Volume of plasma filtered at the glomerulus per unit time. Normal = 125ml/min (180L/day). 10% lower in women.
43
Filtration fraction
Proportion of filterable fluid that is actually filtered (i.e. ratio of ultrafiltrate:blood flow). Normal = 20%.
(In RRT, set at 25% or below)
44
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).
45
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.
46
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.
47
Osmosis
Movement of water molecules from a less to a more concentrated solution across a semi-permeable membrane.
48
Ageing
An irreversible process causing a gradual reduction over time in the reserve of all body systems.
49
Pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage or expressed in terms of such damage.
50
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.
51
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).
52
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.
53
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.
54
Countercurrent multiplier
A mechanism that expends energy to maintain a concentration gradient (e.g. in kidney - permits water reabsorption).
55
Law of mass action
The rate of a reaction is directly proportional to the concentrations of the reactants (e.g. drug and receptor).
56
Drug interaction
When the action of one drug is altered by the concurrent administration of another. Types: physicochemical (sug/roc), pharmacokinetic (LA+adren), pharmacodynamic (prop/remi - synergism).
57
Bioavailability
The fraction of the drug that reaches the systemic circulation.
58
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.
59
Genetic polymorphism
A term describing genotypic and subsequent phenotypic variability, often in reference to differences in drug handling between individuals.
60
Tachyphylaxis
Acutely reduced response to a drug after repeated administration (e.g. ephedrine depleting NA stores).
61
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.
62
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.
63
Dependence
Need for repeated administration to avoid withdrawal.
64
Addiction
Behaviour resulting from dependence; includes craving and compulsive use despite harm.
65
Awareness
Recall of time under anaesthesia; divided into explicit and implicit memory.
66
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.
67
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.
68
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%).
69
P value
Likelihood of an observed difference being due to chance alone.
70
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.
71
Type 2 error
To wrongly accept the null hypothesis. Chance of it occurring = beta. To reduce the risk of this, increase the sample size.
72
Meta-analysis
A statistical technique that combines the results of several independent studies that address a similar research question.
73
Bias
A systematic error that produces an incorrect result.
74
Significance (statistical and clinical)
Statistical: a result unlikely to be due to chance.
Clinical: a result with importance when applied to clinical practice.
75
Macroshock
Passage of current from one part of the body to another.
76
Microshock
Passage of current directly to the myocardium.
77
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. Basis of non-invasive continuous BP device Finapres NOVA.
78
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.
79
Sine wave
Smooth, repetitive oscillation. A graph of the function sine.
80
Amplitude
Peak deviation from zero
81
Frequency
Cycles per second
82
Phase
Specifies where in its cycle the oscillation is at t = 0. When not 0, the wave appears shifted.
83
Period
Length of one cycle of the wave (peak to peak or trough to trough).
84
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)
85
Capacitor
A device which stores energy in an electric field.
Consists of two conductive plates with a dielectric (insulator) between.
Main component of a defib circuit.
86
Inductor
A device which stores energy in a magnetic field.
Consists of a conducting coil +/- a ferromagnetic core.
Found in a defib circuit where it prolongs the duration of current flow by providing opposition to current flow (i.e. inductance - unit = the Henry). This increases the probability of a successful defibrillation.
87
Transducer
A device that converts one form of energy to another, e.g. pressure transducer in arterial line (kinetic to electrical signal). Other types = strain gauge, piezoelectric transducer.
88
Rectifier
Device that converts AC to DC. Opposite of an inverter.
89
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.
90
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 during nasendoscopy.
91
Sepsis 3.0 and septic shock
Life-threatening organ dysfunction caused by a dysregulated host response to infection. Acute change in total SOFA score ≥2 points. Mortality 10%.
Septic shock is a subset of sepsis in which underlying circulatory and cellular/metabolic abnormalities are profound enough to substantially increase mortality. Manifested by ongoing hypotension despite adequate fluid resuscitation. Acute change in SOFA score at least 2 + lactate >2 + vasopressor requirement to maintain MAP>65. Mortality 40%.
92
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.
93
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/kg/min).
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.
94
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.
95
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.
96
Autonomic dysreflexia/hyperreflexia
An exaggerated sympathetic response to stimulation below the level of a spinal cord lesion.
97
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.
98
ECG
The recording and display of cardiac electrical activity. Components: electrodes, differential amplifiers, filter, display.
Common mode rejection = ability of the differential amplifiers to reject interference common to both inputs, which is important when the signal of interest is of such low voltage (ECGs = 1-2mV).
99
TEG
Point of care coagulation test utilising the viscoelasticity of whole blood.
100
Difficult intubation
Clinical situation in which an experienced unassisted anaesthetist needs >10m or >3 attempts to intubate with conventional laryngoscopy.
101
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.
102
Malnutrition
A state in which a deficiency or excess of energy, protein and other nutrients causes measurable adverse effects. Includes under- and overnutrition.
103
Critical illness/ICU-acquired weakness
Clinically detectable weakness in patients in whom there is no plausible aetiology other than critical illness itself. 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.
104
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).
105
Osmolarity and osmolality
Osmolality: osmoles per kg of solvent
Osmolarity: osmoles per L of solution
Osmolality is more accurate as not affected by temperature (as volumes change but mass is constant).
106
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.
107
PPH and MOH
>500ml SVD or >1L LSCS
MOH = >1.5L, drop 4g Hb or requiring 4u PRC
108
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.
109
Warm and cold ischaemia times
WIT: time between onset of inadequate perfusion and/or oxygenation, and onset of cold perfusion.
CIT: time from cold perfusion to implantation with perfusion.
110
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.
111
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.
112
Validity
That a study measures what it is designed to.
113
Reliability
Repeatability/reproducibility. Related to precision (the degree to which repeated measurements under the same conditions show the same results).
114
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.
115
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.
116
Cor pulmonale
An alteration in the structure and function of the right ventricle caused by a primary disorder of the respiratory system.
117
Never event
Serious incidents that are entirely preventable if published guidance has been followed. They have the potential to cause serious harm and death.
118
Consciousness
Consciousness = wakefulness + awareness together
- Coma = neither awake nor aware
- Vegetative state = awake but not aware (cortical damage, brainstem preserved); persistent = >4/52
- Minimally conscious state = minimal, fluctuating awareness (global neuronal damage)
- Locked-in syndrome (bilateral pontine lesions)
- Akinetic mutism (bilateral frontal lobe/3rd ventricle pathology)
- Catatonia (psychiatric)
119
Disseminated intravascular coagulation
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.
120
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.
121
Extraction ratio
The proportion of a drug removed from blood by a single pass through the liver. Equal to 1 - bioavailability.
122
Sympathomimetics
Drugs with similar actions to the SNS. Also known as adrenergics.
123
Gibbs-Donnan effect
The phenomenon by which charged particles achieve electrostatic equilibrium across a semipermeable membrane.
124
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. Priaprism may be present. Lasts hours-weeks. Not a true form of shock as it is a neurological, not cardiovascular, phenomenon.
Neurogenic shock: hypotension, paradoxical bradycardia, vasodilatation. Due to SNS damage from lesions above T6. Above T4, cardiac sympathetic supply is also lost.
125
Anaphylaxis and anaphylactoid reactions
Anaphylaxis is an immune mediated type I allergic reaction.
Anaphylactoid reactions are non-immune/non-IgE-mediated.
They produce an indistinguishable clinical picture.
126
Weber effect
Initially increased reporting of adverse events to new drugs.
127
Elastance
Reciprocal of compliance (i.e. change in pressure per unit change in volume).
128
Compliance
Change in volume per unit change in pressure.
129
Pacing threshold (output)
The amount of current in mA needed to obtain electrical capture. Threshold is set at double the current at which capture is lost.
(Sensitivity is measured in mV)
130
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).
131
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.
132
Peak and plateau pressure
Peak pressure = max pressure during respiratory cycle, taken to represent pressure applied to large airways.
Plateau pressure = airway pressure measured during an inspiratory hold, taken to represent pressure applied to the alveoli.
133
Control, cycle and trigger
Control = target that the ventilator is set to achieve.
Cycle = variable that terminates inspiration and allows expiration.
Trigger = variable that initiates inspiration.
134
Recruitment manoeuvre
A deliberate, transient increase in intrathoracic pressure applied with the aim of improving oxygenation and/or compliance.
Types: sigh breath, sustained inflation, extended sigh, incremental PEEP.
135
APRV
Airway pressure release ventilation. A continuous airway pressure (Phigh) with a time-cycled release (Plow). Similar to high level CPAP with short releases.
To initiate:
- Phigh set to Pplat (if had been on VCV) or Ppeak (PCV)
- Plow 0
- Thigh 4-6s
- Tlow 0.4-0.6s (set so that inspiratory cycle starts when expiratory flow at 75%, so total deflation does not occur)
136
NAVA
Neurally adjusted ventilatory assist. Novel form of ventilation for spontaneously breathing pts. Level of support (the 'gain') is proportional to magnitude of Edi signal in microV (electrical activity of diaphragm) detected by specialised NGT. Edi of a certain level (the sensitivity) is required to trigger. Pros: reliable trigger, physiological cycling off, support tailored on breath-by-breath basis. Needs backup mode in case signal lost.
137
Dynamic hyperinflation (gas trapping, stacking)
Seen in severe airflow limitation. Expiratory time is insufficient to allow complete expiration, leading to increased residual volume with each breath. Shifts lungs up compliance curve, reducing compliance; compensatory tachypnoea increases WOB; reduces venous return and cardiac output. Quantified by measuring iPEEP during an expiratory hold in a ventilated pt.
138
Post cardiac arrest syndrome
Syndrome caused by the systemic ischaemia and reperfusion associated with cardiac arrest. 4 facets:
* Brain injury (--> coma, seizures)
* Myocardial dysfunction (can recover in 48-72h)
* Systemic ischaemia/reperfusion response (apoptosis of cells)
* Persisting precipitating pathology (e.g. CAD)
139
Ventilator associated event (VAE)
Significant event or condition that results in a >20% increase in O2 requirement, or >3cm H2O increase in PEEP, after being at a stable baseline for at least 48h.
140
Weaning delay/failure
Failure: failing >3 SBTs and requiring >7d MV after the first SBT; or MV requirement for >3/52 in the absence of non-respiratory factors preventing weaning (>2/52 = delay).
Simple wean - 60%
Difficult - 30-40%
Prolonged - 6-15%
Long term wean - >6h/day for >21d
Causes:
- RS: unresolved/new pathology, effusion, airway pathology
- CVS: cardiac dysfunction (pre-existing/new/weaning-induced)
- Metabolic/renal: metabolic alkalosis, low phosphate, hypothyroidism, fluid overload
- CNS: depression, delirium, latent/new pathology, ICU acquired weakness
141
Pulse contour/pulse pressure analysis
Method of monitoring SV and CO on a beat-to-beat basis from the arterial pulse pressure waveform.
142
Pulse power analysis
Non-morphology based method (i.e. not a contour method). Assumes net power change is equal to difference between stroke volume and mass of blood lost to the periphery during a beat.
143
Fick principle
The rate of flow to an organ = clearance of substance / A-V difference in substance concentration
e.g. CO = VO2 / (CaO2-CvO2) or RPF = PAH clearance / A-V conc diff
144
Haemofiltration
Mode of RRT based on convection (CVVH). Movement of water occurs across a semi-permeable membrane due to hydrostatic pressure (ultrafiltration). This creates a convective current that 'drags' additional small and medium-sized solutes across.
145
Haemodialysis
Mode of RRT based on diffusion (CVVHD). A countercurrent flow of dialysate runs in the opposite direction to blood with a semi-permeable membrane between the two, which enhances diffusion by maintaining high concentration gradients.
146
Delirium
An acute change in mental status with a fluctuating course, characterised by inattention, altered consciousness or disordered thinking.
Hypo/hyperactive/mixed.
147
Refeeding syndrome
A clinical syndrome occurring in malnourished patients undergoing nutritional support. Pts not fed >5d are at risk, as are any with chronically reduced intake, reduced absorption or increased catabolism.
Sudden carb load causes insulin surge as body switches from catabolic to anabolic state. K/Mg/PO4 move intracellularly. Multisystem effects including seizures, arrhythmia, resp muscle weakness, risk of death.
148
Hospital Standardised Mortality Ratio (HSMR) and Summary Hospital Mortality Indicator (SHMI)
HSMR: the ratio of the observed to expected deaths, multiplied by 100, with expected deaths derived from statistical models that adjust for available case mix factors such as age and comorbidity. It is (controversially) used to indicate excess/avoidable deaths.
Problems:
- Does not include deaths post discharge
- Only includes deaths from the 50 most common diagnoses
- As 98% pts survive hospital (and only a fraction of the deaths are avoidable), it measures only a tiny fraction of a hospital's activity
- Easily manipulated by changes in medical coding (e.g. by coding as 'palliative')
SHMI: partly addresses the problems with HSMR as it includes deaths within 30d of discharge and includes all diagnoses. Both are limited by clinical coding and data collection variability.
SMRs can be used to assess performance of an ICU over time. It should not be used to compare units as casemix will vary.
149
Quality indicators
Tools that permit measurement of the medical care provided. They can provide warning signs and identify problems or improvements and departures from standard care.
Can relate to structure (e.g. WRs, staffing), process/operational (e.g. OOH discharges), outcomes (e.g. SMR), equipment, data collection etc.
ICU specific QIs: CRBSI, readmission rate, GPICS standards.
150
Pulsus paradoxus
A drop in SBP >10mmHg during inspiration. Classically seen in cardiac tamponade but can also be present in obstructive lung disease, PE, tension PTX, RV infarction and others.
151
Pulsus alternans
Alternating systolic arterial pressure. Associated with severe LV dysfunction.
152
Electrical alternans
Alternating amplitude of QRS complexes. Associated with massive pericardial effusion.
153
Antibiotic
A substance produced by a micro-organism, which has the capacity to kill or inhibit the growth of another.
154
Bacteraemia
Presence of viable live bacteria in blood.
155
Toxin
Enzyme-like protein with biological activity.
Endotoxins are inherent components of bacterial structure, found primarily in Gram-negative organisms.
Exotoxins are proteins secreted by bacteria.
156
Major haemorrhage
- Loss of >1 blood volume within 24h
- Loss of half total blood volume within 3h
- Bleeding rate >150ml/min
- Bleeding with physiological derangement such that SBP<90 and HR>110
157
Dissociative anaesthesia
Lack of response to external stimuli due to ‘disconnection’ of the thalamoneocortical system from the limbic system.
158
Major trauma
Injury affecting more than one body system. ISS>15.
159
Damage control surgery
Limited surgical interventions that serve to control haemorrhage and minimise contamination, until the patient has sufficient physiological reserve to undergo definitive interventions.
The strategy aims to bring the ‘lethal triad’ under control, so that the patient will be able to tolerate definitive surgery later.
DCS is one facet of damage control resuscitation (the other two are permissive hypotension and haemostatic resuscitation).
160
Permissive hypotension (trauma)
The intentional targeting of a subnormal BP to prevent disruption of clot.
161
Drowning
Primary respiratory impairment from submersion or immersion in liquid (airway below/above surface respectively).
162
FEV1, FVC, PEFR
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FEV1 = volume of air forcefully exhaled in 1 second
FVC = volume of air forcefully exhaled in 1 breath
PEFR = max rate of air flow one can generate on forced exhalation
```
163
Mean arterial pressure
The average arterial BP over the course of one cardiac cycle.
164
Care bundle
A small group of evidence-based interventions which, when performed together consistently, improve patient outcomes.
Key features: brief, binary, backed by level 1 evidence, more than the sum of its parts.
165
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Guideline
Recommendation
Standard
Protocol
Policy
```
Guideline: evidence-based statements that help practitioners to make decisions about care in specific clinical circumstances; informs but does not replace clinician autonomy
Recommendation: evidence-based suggestion, with a specific strength (GRADE rating)
Standard: a statement of desired outcome, often used as a measure of success in audit
Protocol: agreed framework for care in specific circumstances, not normally deviated from
Policy: formal written statement that is contractually binding
166
Fluid creep
Excess iatrogenic fluid loading in burns resuscitation, leading to oedema. Can predispose to respiratory failure, limb and abdominal compartment syndrome and ocular hypertension.
167
Burn shock
Combination of hypovolaemic and distributive shock. Characterised by severe intravascular depletion and increased capillary permeability. Occurs in burns >20-30% TBSA. Affects both burned and non-burned tissues. Reduced contractility, elevated SVR, max drop in CO in first few hours. Improved but not reversed by fluids.
168
Burn
A coagulative lesion of the surface layers of the body, of variable area and depth.
169
Stroke volume variation
Percentage change in stroke volume that occurs during a ventilator cycle. Normal = 5-10%, >12-15% implies pt would be fluid responsive.
170
Plasmapheresis/plasma exchange
Extracorporeal blood purification technique whereby certain plasma constituents are removed by either membrane filtration or centrifugation. Fluid is then replaced with HAS/FFP/cryo.
Can get acquired sux apnoea as plasma cholinesterase is plasmapheresed off.
171
Shock
Life-threatening acute circulatory failure associated with inadequate oxygen delivery to tissues.
Hypodynamic
- Hypovolaemic and cardiogenic
- Low CO
- High resistance state
Hyperdynamic
- Septic
- High CO
- Low resistance state
172
Sieving coefficient
In RRT, the ratio of solute concentration in ultrafiltrate to its concentration in the unfiltered plasma.
Coefficient of 1 = solute is freely filtered. Low = solute is poorly removed. However does not take Vd into account - if this is high, substance will be poorly removed despite a high sieving coefficient (e.g. that of digoxin is 0.96 but its Vd is 4-7L/kg).
173
Frailty
A syndrome of physiological decline in late life, beyond that expected by chronological age alone. Predisposes to the accumulation of deficits and adverse outcomes from acute stressors.
174
Capacity and competence
Capacity - medical assessment (time and decision specific)
Competence - legal term
175
Craniotomy vs craniectomy
Craniotomy - skull bone flap replaced at end of procedure
Craniectomy - bone flap not replaced to allow brain swelling; may be replaced later
176
LocSSIPs / NatSSIPs
Local/National Safety Standards for Invasive Procedures. Their development was recommended by a 2015 Patient Safety Alert with the aim of reducing peri-procedural harm/never events. They take the form of checklists based on the WHO sign in/time out/sign out procedure.
177
Risk
The probability that a specific adverse event will occur. Understood as the combination of likelihood and consequence of a hazard being realised.
A clinical risk is the chance of an adverse outcome resulting from an aspect of patient care.
178
Root cause analysis
A tool to identify how and why an incident occurred. A factor is considered a root cause if its removal from the sequence of events would have prevented the incident. May be physical, organisational, human factors or others.
Processes: the five whys, fishbone analysis (cause and effect brainstorm).
179
Pharmacokinetics and pharmacodynamics
Pharmacokinetics: how the body handles the drug (ADME)
Pharmacodynamics: what the drug does to the body (by organ system)
180
Lusitropy
Rate of diastolic relaxation.
181
Orthodeoxia
Hypoxaemia exacerbated by the erect position. Can occur in hepatopulmonary syndrome with chronic liver disease.
182
Massive haemoptysis
Blood loss within the airways at a rate that poses an immediate threat to life. Death is by asphyxiation rather than exsanguination.
183
Preload responsiveness
The ability of the heart to significantly increase stroke volume in response to volume expansion. Assessed by static and dynamic parameters.
184
Base excess
Actual base excess: the amount of acid that must be added to return pH in vitro to pH 7.4 under the standard conditions of 37 ° C and PaCO2 of 5.33 kPa; it is measured in mEq.l– 1.
Standard base excess: the acid required when Hb corrected to 50; more representative because Hb is a powerful buffer.
185
Diarrhoea
3 or more watery stools/day (WHO)
- osmotic
- secretory (infective/non-infective)
- inflammatory (infective/non-infective)
- dysmotility
186
Secondary brain injury
Arises when CMRO2 exceeds cerebral O2 delivery. Can result from increased CMRO2 or reduced delivery.
187
Contamination, colonisation and infection
Contamination: presence of microbes without proliferation.
Colonisation: microbial attachment and proliferation but no inflammatory response.
Infection: an inflammatory response due to the presence of organisms in a normally sterile tissue.
188
ARDS
An acute, diffuse, inflammatory lung condition, which results in endothelial dysfunction and non-cardiogenic pulmonary oedema. Exudative, proliferative and fibrosing stages.
189
Pyrexia/fever
Hyperthermia
Hyperpyrexia
Heat stroke
Pyrexia/fever: hypothalamic upregulation in response to a trigger; can be treated with antipyretics
Hyperthermia: failure of normal temperature regulation; not cytokine-mediated, will not respond to antipyretics
Hyperpyrexia: extremely high temperature (>41)
Heat stroke: core body temp >40C with CNS dysfunction (i.e. encephalopathy). May be exertional or classic (illness/drug effect).
Mild heat-related illnesses: heat oedema, heat syncope, heat exhaustion
190
Heterotopic ossification
The presence of bone in soft tissue where bone normally does not exist. A chronic complication of spinal cord injury and MSK trauma.
191
CRBSI/CLABSI
Catheter Related Blood Stream Infection
Bacteraemia originating from an infected intravenous catheter. Diagnosed from catheter tip culture or variation between catheter and peripheral blood cultures (e.g. quantitative >5:1 or qualitative positive >2h prior to paired peripheral sample).
Central Line Associated Blood Stream Infection
Bacteraemia in a patient who had a CVC in the preceding 48h. A research/surveillance tool which over-estimates true CRBSI.
192
Nocebo effect
The nocebo effect is when a person experiences harmful, unpleasant, or undesirable side effects after a placebo medical treatment. These effects are not chemically generated and are only due to a person's negative belief or expectation that the fake treatment or drug will produce bad side effects.
193
Driving pressure
Plateau pressure - PEEP
