Latent Heat and its applications in anesthesia practice

• Heat capacity: The heat energy required to raise the temperature of a given object by one degree. (J.K−1 or J.°C−1)

• Specific heat capacity: The heat energy required to raise the temperature of one kilogram of a substance by one degree. (J.kg−1.K−1 or J.kg−1.°C−1)

• But not all heat energy results in a temperature change. 

• Latent heat: This is the heat energy that is required for a material to undergo a change of phase. (J) The heat is not utilised for raising the temperature, but for changing the phase.

• If heat is applied to matter, temperature increases until the melting or boiling point is reached. At these points the addition of further heat energy is used to change the state of matter from solid to liquid and from liquid to gas. This does not cause a change in temperature. The energy required at these points is referred to as latent heat of fusion andlatent heat of vaporisation, respectively.

• Specific latent heat is the heat required to convert one kilogram of a substance from one phase to another at a given temperature.

• As temperature increases, the amount of additional energy required to overcome the intermolecular forces of attraction falls until the critical temperatureof a substance is reached. At this point the specific latent heat is zero, as no further energy is required to complete the change in state of the substance.
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• Variable bypass vaporisers function by passing a small amount of fresh gas through the vaporising chamber, which is fully saturated with anaesthetic vapour. This removes vapour from the chamber. Further vaporisation from the anaesthetic liquid must occur to replace the vapour removed, which requires energy from the latent heat of vaporisation. This cools the remaining liquid, reducing the saturated vapour pressure and thus the concentration of anaesthetic vapour delivered, resulting in an unreliable device.

• Temperature compensation features help to overcome this problem; a copper heat sink placed around the vaporising chamber is one such example. Copper has a high heat capacity and donates energy required for latent heat of vaporisation, maintaining a stable temperature and reliable delivery of anaesthetic agent.

• Evaporation of sweat is another example. It requires the latent heat of vaporisation, which is provided by the skin’s surface, exerting a cooling effect upon the body.

• Evaporation from open body cavities can be a cause of significant heat loss from patients while under anaesthesia.

• These principles are also applicable to blood transfusion. Blood is stored at 5°C and has a specific heat capacity of 3.5 kJ·kg−1·K−1. If cold blood were transfused into a patient without pre-warming, the heat energy required to warm the blood to body temperature would need to be supplied by the patient, which would have a significant cooling effect.

APPLICATIONS

• Variable bypass vaporisers function by passing a small amount of fresh gas through the vaporising chamber, which is fully saturated with anaesthetic vapour. This removes vapour from the chamber. Further vaporisation from the anaesthetic liquid must occur to replace the vapour removed, which requires energy from the latent heat of vaporisation. This cools the remaining liquid, reducing the saturated vapour pressure and thus the concentration of anaesthetic vapour delivered, resulting in an unreliable device.

• Temperature compensation features help to overcome this problem; a copper heat sink placed around the vaporising chamber is one such example. Copper has a high heat capacity and donates energy required for latent heat of vaporisation, maintaining a stable temperature and reliable delivery of anaesthetic agent.

• Evaporation of sweat is another example. It requires the latent heat of vaporisation, which is provided by the skin’s surface, exerting a cooling effect upon the body.

• Evaporation from open body cavities can be a cause of significant heat loss from patients while under anaesthesia.

• These principles are also applicable to blood transfusion. Blood is stored at 5°C and has a specific heat capacity of 3.5 kJ·kg−1·K−1. If cold blood were transfused into a patient without pre-warming, the heat energy required to warm the blood to body temperature would need to be supplied by the patient, which would have a significant cooling effect.

OHM'S LAW

• The strength of an electric current varies directly with the electromotive force (voltage) and inversely with the resistance. So I = V/R or V = IR where V is voltage, I is current and R is resistance.

• The equation can be used to calculate any of the above values when the other two are known. When R is calculated, it may represent resistance or impedance depending on the type of circuit being used (AC/DC)

• Resistance: The opposition to flow of direct current. (ohms, Ω)

• Reactance: The opposition to flow of alternating current. (ohms, Ω)

• Impedance: The total of the resistive and reactive components of opposition to electrical flow. (ohms, Ω)

• The reactance of an inductoris high and comes specifically from the back electromotive force that is generated within the coil. It is, therefore, difficult for AC to pass.

• The reactance of a capacitoris relatively low but its resistance can be high; therefore, direct current (DC) does not pass easily.

THERMISTORS AND THEIR USE IN ANESTHESIA

🔻A #thermistor is a temperature-sensitive resistor whose resistance changes with temperature.

🔻Most temperature-sensitive resistors are constructed from a semiconductor material (carefully chosen metal oxides) and the resistance increases with a fall in temperature (they have a negative temperature coefficient)

🔻So they are known as negative thermal conductivity (NTC) thermistors.

🔻A Wheatstone bridge circuit is used to measure the resistance accurately.

🔻The main disadvantage of thermistors is the non-linear resistance versus temperature characteristic, although this can be compensated for using an appropriate calibration equation programmed into an electronic measurement system.

🔻Thermistors remain highly popular due to their cost, miniature size and convenience.

🔻Thermistor probes are commonly placed in the nasopharynx, oesophagus, rectum or bladder (integrated with a urinary catheter).

🔻They have excellent accuracy and their small mass means that there is a quick response to variations in temperature.

🔻True or False? 'A thermistor comprises a junction of dissimilar metals'

🔻Answer: False. Dissimilar junctional metals are thermocouples

TURBULENT FLOW AND CLINICAL APPLICATIONS

⚱️The flow pattern of a river running over rapids is very different to the steadily flowing river (laminar flow). Here, the water’s path of travel becomes far less predictable than for laminar flow. This is an example of turbulent flow. An intermediate example is water flowing near the bank of a steadily flowing river, which often tends to meander, turning round in gentle circles. This is an example of eddies, the forerunner to full-blown turbulence.

⚱️As flow is, by definition, unpredictable, there is no single equation that defines the rate of turbulent flow as there is with laminar flow.

⚱️But, in well controlled circumstances the point at which flow changes from laminar to turbulent flow can be estimated using the Reynolds number, Re, which is named after Osborne Reynolds (1842–1912) of Manchester University, an engineering professor.

⚱️The Reynolds number allows us to predict whether turbulent or laminar flow would occur in a given system. The Reynolds number is a dimensionless quantity, i.e. it has no units. It is defined as the ratio of inertial and viscous forces. 

⚱️A Reynolds number <2000, where viscous forces predominate, predicts flow to be laminar. Between 2000 and 4000, both laminar and turbulent flow are anticipated. Above 4000, flow is likely to be completely turbulent because inertial forces are dominant. Critical flow is the point above which turbulent flow commences, which occurs at a Reynolds number of around 2000.

⚱️Viscosity is the important property for laminar flow

⚱️Density is the important property for turbulent flow

⚱️Reynold’s number of 2000 delineates laminar from turbulent flow (Tim and Pinnock: Re < 1000 is associated with laminar flow, while Re > 2000 results in turbulent flow)

⚱️A high Reynolds number means that the inertial forces dominate, and any eddies in the flow will be easily created and persist for a long time, creating turbulence. In a given airway with a known gas and flow velocity, the likelihood of turbulent flow can be predicted from Re.

⚱️APPLICATIONS: Both laminar and turbulent flow exist within the respiratory tract, usually in mixed patterns. Turbulent flow will increase the effective resistance of an airway compared with laminar flow. Turbulent flow occurs at the laryngeal opening, the trachea and the large bronchi (generations 1–5) during most of the respiratory cycle. It is usually audible and almost invariably present when high resistance to gas flow is encountered

⚱️APPLICATIONS: The principal sites of resistance to gas flow in the respiratory system are the nose and the major bronchi rather than the small airways. Since the cross-sectional area of the airway increases exponentially as branching occurs, the velocity of the airflow decreases markedly with progression through the airway generations, and laminar flow becomes predominant below the fifth generation of airway

LAMINAR FLOW

# When watching a steadily flowing river, the flow of water may be seen to be fastest in the middle, while near the banks of the river the water flows more slowly. 

# This behaviour is also observed in fluid travelling slowly along a wide straight cylindrical tube, where the fastest velocity occurring in the centre of the tube and the slowest at the edge where there is friction between the wall of the tube and the fluid. This is known as laminar flow.

# Viewed from the side as it is passing through a tube, the leading edge of a column of fluid undergoing laminar flow appears parabolic. The fluid flowing in the centre of this column moves at twice the average speed of the fluid column as a whole. The fluid flowing near the edge of the tube approaches zero velocity.

#  #Hagen (in 1839) and #Poiseuille, a surgeon (in 1840) discovered the laws governing laminar flow through a tube. If a pressure P is applied across the ends of a tube of length, l, and radius, r. Then the flow rate, Q, produced is proportional to:
*The pressure gradient (P/l) *The fourth power of the tube radius *The reciprocal of fluid viscosity . This is often combined as: (see the figure for the equation)

where Q is flow, ΔP is pressure gradient, r is radius, η is fluid viscosity and l is length

# Also note: Viscosity is the important property for laminar flow, whereas density is the important property for turbulent flow. Reynold’s number of 2000 delineates laminar from turbulent flow