arterial BP Flashcards
How to calibrate an invasive arterial BP monitoring system?
-Factory calibration: at time of manufacture
-static calibration
* Zero calibration/Zeroing: to atmospheric pressure; eliminates offset drift (zero drift)
* High-pressure calibration: transducer connected to an aneroid manometer using sterile tubing through a 3-way tap and the manometer pressure is raised to 100 and 200 mmHg; the monitor display should read the same pressure as is applied to the transducer; eliminates gradient drift
* Leveling: transducer placed at level of right atrium (phlebostatic axis) to ensure hydrostatic pressure of blood is not included in the BP recording
* Phlebostatic axis: 4th ICS MAL
* or at external auditory meatus if head-elevated( beach chair position)
-Clinical: compare with NIBP; IABP SBP usually 5-10 mmHg higher, DBP 5-10 mmHg lower, MAP should be similar
Describe the methods of temperature measurement
-Methods:
1. Non-electrical:
* Liquid expansion thermometry: Mercury thermometer, Alcohol thermometer * Dial thermometer: eg Bourdon gauge, bimetallic strip
2. Electrical: generally shorter-response time and greater site selection
* Infrared thermometry* (*some sources classify as non-electrical or entirely separate category) * Resistance wire thermometer
* Thermistor
* Thermocouple
basic principle temp measurement
Non electrical
Mercury thermometer n Alcohol thermometer
* Glass tube filled w mercury or alcohol
* Liquid expansion thermometry; temp increase causes liquid to expand => reading taken at maximum point the fluid has reached
Dial thermometer: Bourdon gauge
* Gas expansion thermometry; temp increase causes gas to expand (Charles’ law) => moves pointer on a scale
Dial thermometer: Bimetallic strip
* 2 strips of metals w differing linear expansion coefficients arranged in a coil
* Uncoils as temp changes; degree of uncoiling shown by a pointer on a scale
Electrical
Thermistor
* Small bead of metal oxide semiconductor, galvanometer, +/- Wheatstone bridge
* Resistance decreases* exponentially** as temperature increases
* *negative thermal conductivity thermistors eg copper oxide, cobalt oxide, manganese oxide
* **non-linearly; but the response is made linear electronically over clinical ranges of body temperature
Thermocouple
* Two dissimilar metals eg copper and * constantan w two junctions; a reference junction (kept at constant temp) and * measuring junction (acts as the probe); a * galvanometer *
* Seebeck (aka thermoelectric) effect: at junction of 2 dissimilar metals a voltage is * produced, magnitude of which is linearly proportional to the temperature
difference at the junction
Infrared thermometer eg tympanic membrane
* Small probe, detector (consists of a thermopile = a series of thermocouples), pyroelectric sensors
* Stefan-Boltzmann law: as body temp increases, amount of IR radiation emitted will also increase (radiant power ∝ temp4)
* IR thermometers determine temp of an object from a portion of this thermal radiation (“blackbody radiation”) emitted by the object => signal converted to electrical signal
Resistance wire thermometer
* Platinum wire, galvanometer, +/- Wheatstone brdige
* Linear increase in resistance of a metal (eg platinum) w increasing temperature
list advantages and disadvantages of temperature measurement method
Mercury thermometer, Alcohol thermometer
Adv
* Cheap
* Easy to use
* Don’t require power
supply
Disadv
* Less accurate vs electrical methods
* Slow response time; 2-3 mins to thermal equilibrium
* Can’t be made small; eg for body cavities
* Mercury: Risk of breakage and subsequent toxicity
* Mercury: Can’t measure very low temps (Mercury
solidifies at -39C)
* Alcohol: Can’t measure very high temps (alcohol
boils at 78C)
Dial thermometer: Bourdon gauge, Bimetallic strip
Adv
* Simple, robust, cheap
Disadv
* Poor accuracy
* Can’t be made small; eg for body cavities
Infrared thermometer eg tympanic membrane
Adv
-Fast response (~3 secs)
Disadv
-Non-continuous (intermittent) readings Errors: ear wax, probe not aimed at tympanic membrane
Resistance wire thermometer
Adv
-Most accurate; over wide range 0-100C (not just for body temp measurement)
Sensitive: can measure changes in temp +/- 0.0001C (used to calibrate other devices)
Change in resistance linear across temp range 0-100C
DisAdv
-Slow response time Expensive
-Bulky
-Fragile (not-robust) Need power supply
Thermistor
Adv
* Can be made compact, robust and very small (eg in PAC)
* Fast response <0.2 secs
* Very accurate: 0.1°C
* Can be chemically sterilized
* Relatively cheap (vs resistance
thermometer)
* Continuous measurement
Disadvantages
* Need regular calibration; liable to drift
* May not tolerate extremes of temp (eg sterilisation)
* Need power supply
* Change in resistance is non-
linear across temp range (but manufactured to be linear over clinical ranges)
Thermocouple
Adv
-Can be made compact, robust * and very small
-Fast response <0.2 secs * Very accurate: 0.1°C
-Relatively cheap (vs resistance thermometer)
-Change in resistance is fairly linear across temp range
* Continuous measurement
Disadv
-Need regular calibration; liable to drift
-Need power supply
what is core body temperature?
where are the sites
Core body temperature (CBT):
* Temperature of internal organs within the body; regulated to 37C +/- 0.5C
Sites for CBT measurement:
1. Pulmonary artery: gold standard
2. Nasopharynx
3. Distal oesophagus: lower 25%
4. Bladder: when urine output is normal
5. Tympanic membrane: correlates well w brain temperature
6. Rectal: usually 0.5-1°C than CBT (d/t bacterial fermentation); altered by faeces; significant lag time (not considered acceptable in some sources)
