Heat and temperature

Learning objectives.

By reading this article you should be able to:

• •Describe the differences between heat and temperature.
• •Explain the mechanisms of heat loss during general anaesthesia.
• •Detail the electrical basis and differing devices used for measuring temperature.
• •Recall temperature scales and the laws of thermodynamics.
• •Illustrate the mechanisms and devices used to regulate heat during surgery.

Key points.

• •Heat and temperature principles are found in many aspects of anaesthesia practice.
• •Maintaining a patient's temperature is more efficient than restitution of heat loss.
• •Understanding the physics of heat and temperature allows rational management of warming devices and the patient's temperature.
• •Temperature is measured by electrical means; there are guidelines for the perioperative use of temperature measurement devices.

Temperature and thermal energy dynamics are important concepts in the natural sciences, forming core properties in both physics and chemistry. For the anaesthetist, temperature has particular relevance via the gas laws that govern foundational principles in ventilation, gas exchange and the properties of inhalation agents. In addition, many biochemical processes and pharmacological properties relevant to the anaesthetist are intimately affected by changes in temperature. Devices intrinsically linked to temperature are ubiquitous in perioperative care, from forced air warmers to vaporisers, vacuum-insulated evaporators, and heat and moisture exchange filters. As clinicians we require a robust understanding of the physics of heat and temperature, how it may be measured and how changes in temperature can occur and be regulated.

In clinical practice, alterations in temperature can directly impair outcomes associated with anaesthesia. Hypothermia, although not only detrimental to a patient's experience, has been associated with:

• (i)Increased rates of surgical site infections (although this association has been questioned by more recent reviews)^1,2
• (ii)Increased rates of myocardial injury and infarction³
• (iii)Disturbance of coagulation systems, with increased rates of bleeding and transfusion even with mild hypothermia (<1°C)⁴
• (iv)Altered drug metabolism and pharmacokinetics

These negative outcomes have been detailed a recent review of perioperative hypothermia in this journal.⁵ Hyperthermia may also be associated with negative sequelae. For example a 1.5°C increase in temperature causes a 25% increase in the body's basal metabolic rate (BMR). Through altered cardiovascular control and increased arterial CO₂ concentrations, a significant decrease in cerebral perfusion and oxygen extraction occurs, leading to poorer outcomes in neurosurgical patients or those at risk of cerebral injury.

Physics of heat and temperature

Heat as energy flow

Heat describes the transfer of energy between objects of differing temperatures. An object can gain heat or lose heat, but it is not an intrinsic quality or property of the object itself.⁶ Heat is measured in the Systeme Internationale (SI) unit of joules (J). The joule is a derived base unit and is a measure of the amount of energy transferred or work performed, which may also be electrical or mechanical in nature. The rate of heat exchange is expressed in watts (W), with 1 W equalling a change of 1 J s⁻¹.

Temperature

Temperature is an intrinsic physical property of a substance that is related to the kinetic energy of the molecules within the substance. This property is often referred to by the qualities of being either hot or cold. A variety of approaches have been used to explain the complex basis of temperature and the thermodynamics principles that govern its behaviour, including classical, quantum and statistical mechanics. As an abridged explanation, above a temperature of absolute zero, atomic particles will be in motion. This motion, which can either be oscillatory, translation, vibrational or rotational in nature, will relate to the energy state occupied by each subatomic particle and atom. From this microscopic view, a more familiar macroscopic understanding can be formed by calculating the average of these energy states using a proportional factor: the Boltzmann constant, a defining constant in physics.⁷

This understanding of temperature, being the mean kinetic energy of atomic particles, can readily be appreciated when considering a gas. Each particle within a volume of gas will be in random motion with a differing speed and direction. By calculating a probability distribution of these velocities, a direct proportional relationship can be demonstrated between temperature and kinetic energy. This kinetic energy can be translated to the wall of the container, leading to the development of pressure. The ideal gas equation PV = nRT provides the details of these relationships, where P denotes pressure, V is volume, n is the number of moles of substance, R is the gas constant and T is temperature.

Temperature gradients and heat

If there is a temperature difference between two systems or objects, then heat will flow between them, along a gradient from hot to cold, until thermal equilibrium is reached. This is regardless of the size of the two objects or the total amount of energy they each contain. A large iceberg holds a large amount of thermal energy because of its immense size, but as this energy is widely dispersed it has a low temperature. If a pot of boiling water is poured onto the iceberg, the water will cool, and the iceberg warm along their temperature gradient, despite the vast difference in the total energy content of the two substances. It is therefore temperature gradients that govern the flow of thermal energy.

The four laws of thermodynamics are fundamental laws of physics and describe the nature of the relationship between heat, temperature and work in a thermodynamic system. It is these laws that lead to the observed properties of heat, such as its unidirectional flow.

• 1.Zeroth law of thermodynamics. The law simply states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
• 2.First law of thermodynamics. When energy is gained or leaves a system (either as work, heat or matter), the system's internal energy is changed in accordance with the principle of the conservation of energy. Thus, energy is neither created nor destroyed, only converted to another form.
• 3.Second law of thermodynamics. The entropy of an isolated system never decreases. Entropy is a physical property of disorder or randomness. A high entropy implies a high degree of disorder. For example a glass vase has a low entropy (highly ordered), but the same molecules as the original sand granules have a high entropy (highly disordered). This law forms the basis for the unidirectional movement of thermal energy, as systems overall never gain order.
• 4.Third law of thermodynamics. The entropy of a thermodynamic system approaches a constant value as the temperature approaches absolute zero. This can be conceptualised as follows: as a system approaches a temperature of absolute zero, there is no longer kinetic energy fuelling the movement of atoms, which come to occupy a ‘ground state’.

Specific heat capacity

The specific heat capacity of a substance is the heat energy required to raise the temperature per unit of mass, measured in joules per kelvin per kilogram. Heat capacity exists because of the ability of a substance to store heat energy in forms other than kinetic energy. This differs slightly from the term heat capacity, which does not consider the relative mass of an object and is simply the heat required to produce a unit change in temperature. The effect of differing specific heat capacities can be illustrated by imagining two similarly sized objects, one stainless steel and one plastic, placed outside on a warm day. Although both are exposed to the same amount of solar energy, the steel object will become hotter more rapidly than the plastic object. With steel's specific heat capacity being one quarter of polyethylene plastics (∼400 J kg⁻¹ K⁻¹ compared with 1600 J kg⁻¹ K⁻¹), four times less energy is required per degree Kelvin temperature increase.

It should be noted that our perception of temperature in this scenario can vary significantly, based on the separate property of thermal conductivity. Thermal conductivity quantifies the ability of a material to conduct heat away to a separate object and is measured in watts per metre-Kelvin. This property explains why if both bowls were left for long enough to reach thermal equilibrium with the surrounding environment, the steel bowl would still feel hotter despite being the same temperature. Steel has a thermal conductivity roughly 10,000 times greater than that of plastic. Thus, despite a greater amount of energy required to increase the temperature of the plastic object, this energy will be much more readily transferred to your skin and temperature receptors when touching the steel.

Water has a high specific heat capacity relative to many other materials, being 4184 J kg⁻¹ K⁻¹ at 20°C. With the human body predominately composed of water, a loss of 1°C in temperature requires a considerable amount of energy to restore.

This same principle also illustrates why forced air warmers may take significant time to rewarm a patient that has cooled after induction of anaesthesia. Forced air warmers can produce a relatively large temperature gradient above core body temperature (up to 10°C) and even greater compared with the cooler peripheries. Air having a low mass, specific heat capacity (approximately 1000 J kg⁻¹ K⁻¹) and thermal conductivity (approximately 0.03 W m⁻¹ K⁻¹), carries a comparatively low amount of thermal energy, which is conducted relatively slowly.

Latent heat

Commonly taken as referring to the latent heat of vaporisation, latent heat instead relates to the heat involved in any phase change of a material. Unlike heat capacity, where heat transfer leads to an increase in temperature, in latent heat the energy transferred instead forms or breaks the bonds that hold molecules in proximity.

In a solid state, at an intrinsic temperature, all the energy added will go towards breaking the bonds between molecules to create a liquid. As such the temperature of the solid, for a period, will not increase despite the addition of heat. The amount of additional energy added is called the latent heat of fusion. Similarly, the latent heat of vaporisation represents the energy transfer required to turn liquid to gas. This process is bidirectional and exhibits the same characteristics when these bonds are re-established (Fig. 1).

Fig 1.

Graph of energy against temperature for the state change of a substance.

For a state change to occur, the energy required to break these bonds has to be exchanged from somewhere. If heat is not actively added, it can be extracted from the surrounding environment, leading to a localised loss of heat energy and subsequent cooling. Touching a rapidly discharging oxygen cylinder will demonstrate this effect. In a volatile anaesthetic gas vaporiser, there is a large block of copper to act as a heat sink; otherwise the volatile anaesthetic agent will cool as it vaporises. As saturated vapour pressure decreases with temperature, less vaporised volatile agent will lead to an unstable vaporiser output.

Mechanisms of heat flow

Four mechanisms are commonly reported for heat loss from the body: radiation, convection, conduction and evaporation. Fundamentally, only two underlying physical mechanisms exist, radiation and conduction. Conduction requires physical proximity for heat flow to occur, whereas radiation can occur at a distance. Convection and evaporation represent special subtypes of conduction, where the initial localised conductive heat transfer has subsequent remote transmission by a separate mechanism.⁸

Radiation

Radiation represents the bulk of heat loss from the human body (50–60%) but will vary markedly depending on the surrounding temperature gradients.⁹ Thermal radiation, as electromagnetic waves, are emitted from any substance above absolute zero. The kinetic interaction of charged particles results in the emission of a photon, with the amount of radiation emitted governed by the Stefan–Boltzmann equation. The equation takes the difference of the fourth power of an object's temperature from the fourth power of the environmental temperature and multiplies it with an object's emissivity and surface area. As it relies on the fourth power, by far the most important factor in this equation is the temperature differential. Emissivity is a dimensionless unit of measure that indicates an object's effectiveness at emitting and absorbing thermal radiation. A black body is an idealised material that has an emissivity of 1. Skin is an excellent emitter and absorber of thermal radiation, with an emissivity of between 0.97 and 0.99.¹⁰

Conduction

Conduction describes the transfer of heat within a substance by direct contact to another object. This occurs because of the movement of atomic particles within a material. Conduction takes place with all states of matter but is most effective in solids, owing to their closely fixed structure increasing the frequency of such interactions. The quantity of heat conduction is calculated by Fourier's law. By this law, conduction over a given surface area is directly proportional to the temperature gradient multiplied by the thermal conductivity of a substance. Thermal conductivity is the measure of a material's ability to conduct heat and is generally highest for metallic solids and lowest for gases. Conduction is typical a minor contribution to body heat loss, accounting for less than 5% of daily losses.

Convection

Convection is the transfer of heat via a combination of conduction and advection. Advection describes the bulk motion of fluids, be they gas or liquid. For heat to be lost via convection, it must first be conducted to the surrounding fluid molecules, which in turn disperses this energy via fluid currents. Convection is directly proportional to the temperature gradient and interacting surface areas, along with a coefficient factor that considers a fluid's physical characteristics and flow dynamics.

Skin temperature, when conscious, is approximately 34°C. When ambient temperature is lower than this, heat will be conducted from the skin to the surrounding air. This process can be markedly enhanced by air flow and is proportional to the square root of air velocity. Convection can play a significant role in heat loss from the body, which is highly variable based on the degree of skin exposed and air flow velocity. An exposed person in a room with minimal air flow will lose 15% of their heat via convection.

Evaporation

Evaporation is responsible for 20% of heat loss at rest. Because of the latent heat of vaporisation of water, 2430 J of heat is lost per gram of water that evaporates.¹¹ This equates to approximately 1°C decrease in temperature for every 100 ml water that evaporates. A person undertaking routine activities will lose between 600 and 700 ml water per day from insensible losses and respiratory humidification. Evaporation is unique amongst the physiological heat loss mechanisms as the only process that does not require the ambient temperature to be lower than that of the human body. The maximum evaporative heat loss in humans is approximately 2 L h⁻¹, or up to 730 W of heat loss, any excess sweat produced will slide off the body. Atmospheric humidity and local temperature can markedly alter the effectiveness of this mechanism.

Measurement of temperature in clinical practice

There are three common temperature scales: Celsius, Fahrenheit and Kelvin.

Celsius (centigrade) is defined as a thermometric scale on which the interval between the freezing point of water and the boiling point of water is divided into 100°C, with 0°C representing the freezing point and 100°C the boiling point.

Fahrenheit is now defined by the same physical properties of water – that is 32°F at the freezing point and 212°F at the boiling point.

Kelvin is the SI unit of temperature. This was previously related to the absolute temperature of the triple point of water. As part of a 2019 redefinition of SI units, the Kelvin scale is now definitionally fixed to the Boltzmann constant. The Boltzmann constant is equal to 1.380649 × 10⁻²³ J K⁻¹. In terms of temperature change, a difference of one Kelvin is equal to 1°C. Zero Kelvin equates to absolute zero. Water freezes at 273 K and boils at 373 K.

Temperature measurement is usually focused on that of our patients. However, temperature measurement and thermal control systems are ubiquitous in the theatre environment. Examples encountered in anaesthetic practice include:

• (i)Forced air warming devices, having at least two temperature sensors to prevent overheating.
• (ii)Rapid infusion devices that have infrared temperature monitoring to maintain temperature within 1°C between 30°C and 40°C.
• (iii)Vaporisers such as the GE Medical Aladdin cassettes, which have dual thermistors.

Thermometry

The ideal thermometer would have the physical properties of being robust, small, minimally or non-invasive, reusable and inexpensive. It would provide continuous measurements that would be precise, accurate, reproducible and rapid. It would exhibit a linear or highly predictable response and have no requirement for calibration. These requirements are often counterposed to each other in reality, as an accurate and rapid response time is unlikely if the device operates in a linear and highly precise manner. Table 1 compares commonly used thermometers with the ideal properties listed above.

Table 1.

Comparison of different types of thermometers and their advantages and disadvantages compared with an ideal thermometer.

Type of thermometer	Advantages	Disadvantages
Thermistor	Continuous Rapid response Accurate and sensitive to small temperature changes Small, robust devices Inexpensive	Non-linear response Requires calibration and may not be reproducible (can develop hysteresis and drift) Single use and invasive
Thermal resistor thermometer	Accurate and sensitive to small temperature changes Linear measurement	Intermittent and slow response Larger device Invasive
Infrared thermometer	Rapid and non-invasive Simple to use and reusable	Variable accuracy Requires calibration in usage Intermittent and expensive
Thermocouple	Continuous Rapid response Accurate and sensitive to small temperature changes Small and can be non-invasive devices Inexpensive	Complicated Small voltage changes require amplification and processing
Mercury or alcohol thermometer	Accurate and reliable Inexpensive and simple devices Reusable	Slow response Intermittent Invasive and potential dangers with toxicity

Thermal measurement is based on non-electrical or electrical phenomena to quantify temperature. Non-electrical systems are no longer commonly used in modern operating theatre environments.

Electrical thermometers

Thermistors

Better understood as thermal resistors, thermistors are a type of passive electrical component whose electrical resistance varies in a non-linear, but predictable manner with changes in temperature. They are commonly used in perioperative temperature monitoring and are found in devices such as pulmonary artery catheters and oesophageal temperature probes.¹²

There are negative and positive temperature coefficient thermistors. Negative coefficient thermistors have an inverse relationship between temperature and resistance, whereas positive coefficient thermistors have a direct relationship. Negative thermistors are most common. Thermistors are constructed from semiconductor powders of metallic oxides. They are accurate, cheap and robust. They are good at measuring small changes in temperature, but because of their non-linear response they are less accurate at temperature extremes.

Similar devices are resistance thermometers. Unlike thermistors, these devices use pure metals whose resistance changes linearly with temperature. Resistance thermometers are very accurate but have slow response time, especially to small temperature changes. The requirement for a relatively large bulk of metal to measure a significant change in resistance, makes them less desirable in clinical settings.

Thermocouples

Thermocouples are electrical thermometers that rely on the thermoelectric phenomenon called the Seebeck effect to measure temperature. The Seebeck effect explains a direct and linear relationship between temperature and electrical voltage at the junction of two dissimilar metals. The voltage changes are in the range of 40–50 mV per degree Celsius. There are multiple different constituent metal combinations used in thermocouples with all using two dissimilar metals or alloys to make a closed circuit of an unknown ‘hot’ junction and a reference ‘cold’ junction. Thermocouples are robust, simple, cost-effective and able to measure temperature over a wide range. A pair or multiple thermocouples can be electrically connected in series to create a device called a thermopile.

Infrared thermometers

These consist of an optical system, photodetector, signal amplifier, a processor and a display output. Infrared electromagnetic waves are focused on a thermopile photodetector via an optical lens. Infrared thermometers most commonly measure temperature via the tympanic membrane or forehead but can be used on any part of the body surface. Tympanic membrane temperature has been shown to have a predictably close association with other sites of core temperature measurement.¹³ Tympanic devices, however, are subject to disturbance in infrared wave collection such as by debris in the ear canal. Forehead temperature is easier to measure but is on average 2–3°C lower than oral temperature and has many confounders.¹³ This discrepancy becomes even more marked for more peripheral tissues, which can be up to 10°C lower than the core.

Infrared thermometers offer the advantage of non-contact measurement, which is rapid and simple to use. All infrared devices are subject to inaccuracies related to assumptions in their signal processing, especially around the emissivity of skin.¹⁴ Emissivity cannot be readily measured and must be assumed. The presence of sweat (water is a very good absorber of infrared), foreign material and altered skin perfusion can all render this assumption incorrect. Interference can also occur, from ambient environmental scatter or variation in the target temperature. Infrared sensors are specifically calibrated to focus radiative energy from a lens at specific distance and angle; thus absorption of radiation can be significant altered by an incorrect angle of incidence and distance from the probe to the target.

Clinical applications, including heat loss and anaesthesia, prevention and active heating methods

Normal temperature regulation

The maintenance of body temperature depends on a constant balance between heat production and loss. Heat production is principally dictated by the body's BMR, representing approximately 70% of daily energy expenditure, at 80–100 W in a sedentary adult. Within the thermoneutral zone (typically 25–29°C when a person is naked and 18–22°C when clothed), heat production via basal metabolic production is balanced with heat loss.

Effects of anaesthesia

Anaesthesia is associated with a triphasic pattern of temperature decline, with the most rapid and largest decline in temperature occurring in the first phase.¹² Peripheral vessels can produce an eight-fold increase in vasoconstriction with decreasing temperature. Vasodilation with anaesthesia blunts this mechanism, leading to convective heat transfer from the core to peripheral tissues. The rapidity of this redistributive phase is explained by high specific heat capacity of blood and large flow rates in vasodilated skin vessels (up to 30% of cardiac output).

The second and third phases of heat loss result from the lowering of the BMR, which can be expected to decrease by 15–40% after induction of anaesthesia. Radiative heat loss in this context far exceeds the requirement of the body to maintain normothermia, in the order of 133 W at an ambient temperature of 18°C. With the addition of conductive and evaporative heat loss (approximately 10 and 17 W, respectively), it becomes clear why heat loss continues with anaesthesia. Evaporation is generally a minor mechanism for heat loss under general anaesthesia, but insensible losses under surgical conditions can be significant.¹⁵

Heating devices and thermodynamic principals

A variety of different heating devices are commercially available, which may either be passive or active in nature. Passive devices include items such as gowns or blankets, along with specialised devices such as space blankets. Space blankets are constructed from plastic materials coated with a highly reflective metallic agent. The metallic component leads to up to 97% of thermal radiation being reflected, effectively minimising this form of heat loss. Furthermore, the poorly permeable plastic maintains the body's boundary layer, decreasing convection and evaporative heat loss. These devices are highly effective at preventing hypothermia but lack the ability to calibrate or adjust.

Active devices include air conditioning systems and specialised radiative heaters frequently utilised in trauma rooms and burns theatres. Devices commonly used specifically in anaesthetic care include intravenous fluid warmers, forced air warmers and conductive devices.

Forced air warmers

Forced air warmers use a powered air flow unit and specialised blanket to distribute heated air. These devices can be very effective at preventing and treating hypothermia.¹⁶ Usually, a covered or clothed body will form a protective boundary layer of insulating warm air that minimises convective heat loss. In theatres, air changes approximately 20–50 times per hour, leading to constant disruption of this boundary layer. Forced air warming devices effectively maintain this boundary layer despite the high degree of ambient air flow. This pocket of warm air not only minimises convective loss but also acts to decrease radiative and conductive losses by reducing the nearby temperature gradient. Distance has an inverse relationship; the further the heat must travel, the slower the rate of heat transfer. As such, a forced air warming device is likely to have maximum effect when applied to as large a surface area and as close to the patient as possible.

It would be preferable to prevent heat loss in the first instance. The prewarming of patients before induction decreases the temperature gradient between the core and peripheries such that the redistribution of central heat is reduced.¹⁷

Intravenous fluid warmers

Fluid warming devices can be divided into two subtypes: low- and high-flow rate devices. Low flow rate systems present the challenge of maintaining temperature within a fluid that is slowly infused, whereas higher flow rates are limited by the brief period for heating to occur and the need to minimise resistance to fluid flow. A variety of heat exchange methods are used, including counter-current exchangers, water baths, heating blocks/elements and convective air systems. No one technique has been conclusively demonstrated as superior to another. To prevent haemolysis, the American Association of Blood Banks limits heating devices to a maximum of 42°C.¹⁸ This limited temperature gradient between body and transfused fluids means that such devices are normally preventative – rather than effective – hypothermia treatment tools. Instead, they mitigate the deleterious effects of unheated fluid infusion, where a 1 L bag of room-temperature intravenous fluid decreases core temperature by 0.25°C.

Conductive devices

Conductive devices work on the principle of direct conductive heat transfer. Commonly used devices include electrical resistive heating mats or circulating water devices. With the limitation of being less effectively able to address radiative, evaporative or convective heat losses, these devices have a modest effect on temperature with multiple studies indicating they are no better or inferior to forced air warmers.^19,20

Thermochemical blankets have now also entered clinical practice and work by conduction from thermal pads which undergo an exothermic reaction when exposed to air. These devices appear to have similar efficacy to forced air warmers.²¹

Summary

An understanding of the physics of temperature and heat loss provides a foundation to help comprehend core principles in gas exchange, biochemistry and pharmacology, along with allowing a rational approach to managing a patient's temperature. Knowing that restoring temperature loss is difficult because of the high heat fluxes required means that every effort should be used to maintain temperature in the first instance. Having lost heat, the knowledge that surface area, conductivity and temperature gradient are critical to heat transfer allows the clinician to apply these basic concepts to maximise heat gain and reduce heat loss during anaesthesia. The measurement of temperature is reliable and simple in our patients, and there are guidelines for its measurement. Outcomes are known to be poorer in the patient who is cold, and so vigilance over temperature should be core to our practice.

Declaration of interests

The authors declare that they have no conflict of interest.

Biographies

Gavin Sullivan FRCA FANZCA AFRACMA is a senior staff specialist at John Hunter Hospital, with an interest in management, simulation, teaching and generalist anaesthesia.

Matthew Spencer B.Med B.Eng (Hons) M.Med FANZCA is a visit medical officer anaesthetist working in Sydney with a background in engineering and head and neck anaesthesia.

Matrix codes: 1A03, 2A07, 3I00

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