Clinical Measurement Flashcards
What is humidity?
This is the amount of water vapour in the atmosphere.
What is absolute humidity?
Total mass of water vapour present in the atmosphere per unit volume. Measured in kg.m-3 or g.m-3
What is relative humidity?
This is the ratio of the amount of water vapour in air compared to the amount it would contain at the same temperature if the air was fully saturated. RH %
What is dew point?
This is the temperature at which the relative humidity of air exceeds 100% and water condenses out of vapour phase to form a liquid.
How is humidity measured?
The hygrometer
What does the humidity graph try to explain?
How a fixed amount of water vapour in the atmosphere can leave to variable RH depending on the current temperature.
Also explains the importance of the upper airway in a room fully humidified by the addition of 27g.m-3 of water vapour
Formulas for calculating heat loss during humidifying dry air or warming cold air?
See image attached
What is more relevant in the respiratory tract; Heat loss or drying of airway?
Heat loss is minimal but drying of airway is more relevant
What are the mechanisms of heat loss during surgery ?
Radiation (40 - 60%)
Convection (25 - 30%)
Conduction (5%)
Evaporation (15 - 50%)
Respiration (5 - 10%)
Energy consumed by the patient’s airway for warming and humidifying cold, dry gases?
About 10 Watts
Mechanism of heat loss via Radiation?
- Transfer of infrared radiation
- No direct contact with the cold body
- Proportional to the 4th power of temperature difference
Mechanism of heat loss via Convection?
- Loss via air movement across an exposed surface
- Warmed air is replaced with un-warmed air
Mechanism of heat loss via Conduction?
- Direct transfer of energy to an adjacent body
- Both bodies are in contact
Mechanism of heat loss via Evaporation?
- Loss of heat via latent heat of vaporization
- Liquid from body moves to vapor phase
Mechanism of heat loss via Respiration?
- Heat loss by humidification (8%)
- Heat loss by warming (2%)
Heat loss during surgery?
- Phase 1 - 37 - 35.5 degrees
- Phase 2 - 35 - 34 degrees
- Phase 3 - 34 degrees
Phase 1 - 37 - 35.5 degrees - Properties?
- Over a course of 1 hour
- Loss of core - periphery temperature gradient
- Redistribution of heat
- Phase 2 - 35 - 34 degrees - Properties?
- Over 2.5 hours
- Actual heat loss from body
- The slope may be altered by active or passive warming
- Phase 3 - 34 degrees - Properties?
- Thermo-regulatory mechanisms act
CVS implications of hypothermia?
- Arrhythmia
- Direct myocardial depressant effect
- Hypotension
- Peripheral vasoconstriction
Respiratory implications of hypothermia?
- Hyperventilation
- Respiratory alkalosis
- Left shift of the OHDC
Haematological implications of hypothermia?
- Coagulopathy
Renal implications of hypothermia?
- Increased renal blood flow
- Diuresis
Neurological implications of hypothermia?
- CNS depressant effect
GI implications of hypothermia?
- Impaired liver function
- Pancreatitis
- Ileus
Metabolic implications of hypothermia?
- Increased due to shivering
- Decreased metabolic rate with further decrease in temperature
What is the Bernoulli principle?
Decrease in pressure will lead to increase in flow velocity of a perfect fluid
What is the venturi effect ?
Introduction of constriction to fluid with increase of velocity. Pressure of the fluid falls
Define the law of conservation of energy ?
Energy cannot b created or destroyed but can only be changed from one form to another.
What is the Coanda effect ?
This is the tendency of a stream of fluid flow in proximity to a convex surface, to follow the line of the surface rather than its original course
What is Henry’s law?
The amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid.
What is Fick’s law of diffusion ?
The rate of diffusion of a gas across a membrane is proportional to the membrane area and concentration gradient across the membrane and inversely proportional to its thickness.
What is Boyle’s law (1st gas law)?
At constant temperature the volume of a fixed amount of perfect gas varies inversely with its pressure
PV=K
What’s is Charles’ law (2nd gas law)?
At constant pressure the volume of a fixed amount of perfect gas varies in proportion to its absolute temperature
V/T =k
What is Gay-Lussac’s law (3rd gas law)?
At constant volume the pressure of a fixed amount of perfect gas varies in proportion to its absolute temperature.
P/T =k
What is the perfect gas ?
- It obeys all three gas laws
- infinitely small in size
- Occupies no volume
- No force of attraction
- Hydrogen is the closest to a perfect gas
What is Avogadro’s hypothesis?
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules
What is the universal gas equation?
PV=nRT
Properties of Hagen-Poiseuille equation?
- Flow is proportional to the 4th power of the radius.
- Doubling radius is 2(4) - 16 fold increase in flow
What is turbulent flow?
- Fluid flow is unpredictable
- Multiple eddy currents
- Not parallel to side of tube
Calculation of Reynold’s number ?
Re=pvd/n
P = Density of fluid
V = Velocity of fluid
d = Diameter of tube
n = Viscosity of fluid
< 2000 laminar
> 2000 turbulent
What is surface tension?
Tendency of liquid surfaces to shrink into the minimum surface area possible
What is the clinical application of surface tension?
- Surface tension decrease the size of alveoli. Alveolar collapse following expiration
whats cells produce surfactant?
Type II pneumocytes
Whats is the chemical name of surfactant?
Dipalmitoylphosphatidylcholine (DPPC)
Whats is the function of surfactant?
Decreases the pressure applied to the alveoli thus decreasing surface tension
what is the LaPlace law for surface tension?
P = 4T/r
Pressure, Tension and radius
