12.2 Temperature Measurement, Humidity, Gas Analysis and SI Units

12.2 Temperature Measurement, Humidity, Gas Analysis and SI Units

Temperature Measurement

Units and Scales

The SI unit of temperature is the kelvin (K). A change of 1 K equals a change of 1 °C, and $T(\text{K}) = T(°\text{C}) + 273.15$. The triple point of water (273.16 K) is the reference point. Absolute zero is 0 K.

Types of Thermometer

Resistance thermometers and thermistors are usually used in a Wheatstone bridge circuit to measure small changes in resistance accurately.

Core Temperature Sites

• Pulmonary artery: the reference standard for core temperature.
• Distal oesophagus: probe in the lower third, behind the heart; accurate and practical during general anaesthesia.
• Nasopharynx: close to the internal carotid artery; reflects brain temperature but affected by leaks around the airway.
• Tympanic membrane: contact thermistors are accurate; infrared devices are variable.
• Bladder and rectum: acceptable but lag behind rapid changes such as rewarming after bypass.
• Skin and axilla: reflect peripheral temperature and underestimate core temperature.

Monitoring is recommended during anaesthesia lasting more than about 30 minutes, and active warming should keep core temperature above 36.0 °C (for example NICE guidance on inadvertent perioperative hypothermia).

Humidity

Fully saturated gas at 37 °C contains about 44 mg/L of water; at 20 °C it holds only about 17 mg/L. Cooling saturated gas therefore causes condensation (rain-out) in breathing circuits.

Why Humidification Matters

Normal breathing warms and humidifies inspired air in the upper airway so that gas reaching the isothermic saturation boundary (just below the carina) is fully saturated at 37 °C. Dry medical gases delivered through a tracheal tube bypass the nose, causing heat and water loss, thickened secretions, impaired mucociliary clearance, atelectasis and airway damage.

Measuring Humidity

• Hair hygrometer: a hair lengthens as humidity rises; works between about 30% and 90% relative humidity.
• Wet and dry bulb hygrometer: evaporation cools the wet bulb; the temperature difference indicates relative humidity.
• Regnault's hygrometer: cooling a polished surface until dew forms gives the dew point.
• Electronic sensors: humidity changes the capacitance or resistance of a sensing material.

Humidification Devices

Gas Analysis

Infrared Absorption Spectroscopy

Molecules containing two or more different atoms (carbon dioxide, nitrous oxide, volatile agents, water vapour) absorb infrared radiation at characteristic wavelengths. Carbon dioxide is measured at about 4.3 $\mu\text{m}$; volatile agents are identified using wavelengths around 8-12 $\mu\text{m}$. The amount absorbed follows the Beer-Lambert law.

• Side-stream analysers aspirate about 50-200 mL/min from the breathing system, which must be returned or scavenged; there is a short delay.
• Main-stream analysers place the sensor at the airway, giving an immediate signal but adding weight and dead space.
• Collision broadening: nitrous oxide broadens the carbon dioxide absorption band, causing slight over-reading of carbon dioxide unless corrected.
• Oxygen, nitrogen and other symmetrical diatomic gases do not absorb infrared, so other methods are needed.

Oxygen Analysis

Other Methods

• Piezoelectric analysis: a quartz crystal coated with oil absorbs volatile agent, changing its resonant frequency; it cannot identify which agent is present.
• Mass spectrometry: separates ionised gas molecules by mass-to-charge ratio; accurate for multiple gases but expensive.
• Raman spectroscopy: measures scattered light shifted in wavelength by gas molecules.
• Chemiluminescence is used to measure nitric oxide.

SI Units and Conversions

The seven SI base units are the metre, kilogram, second, ampere, kelvin, mole and candela. All others, including the pascal, newton, joule, volt and watt, are derived units.