8.8 Humidity + Temperature

Question 1

Why is humidification important?

Answer 1

1. Respiratory tract effects
   Without humidification, gases can dry and
   keratinise part of the bronchial tree,

reducing ciliary activity and impairing mucociliary clearance.

over time, inflammatory changes can occur in the pulmonary epithelium,
causing mucus plugging,
atelectasis, and superimposed chest infection. A

ll of the above will impair gas exchange.

Patients undergoing prolonged anaesthesia, those with pre-existing
pulmonary diseases, and those at extremes of ages are particularly at risk.

Degree of humidification
High humidity is uncomfortable. Low humidity increases the risk of static
sparks.

Question 2

How is humidity expressed?

Answer 2

Absolute humidity

Relative humidity

Question 3

Absolute humidity

Answer 3

Absolute humidity

Mass of water vapour that is present per unit volume of gas

SI unit g/m3 or g/L

Temperature dependent: At 20°C, it is 17 g/m3; at 37°C, it is 44 g/m3.

Question 4

Relative humidity

Answer 4

Relative humidity

Ratio of the mass of water in a given volume of air to the mass of water in
the same volume were it to be fully saturated.

Expressed as percentage.

Question 5

What are the methods of humidification?

Answer 5

Passive
Heat and moisture exchange filter

Active
Hot water bath humidifier
Cascade humidifier
Nebulisers—gas-driven or ultrasonic

Question 6

How can humidity be measured?

Answer 6

Relative:

1. Hair hygrometer
2. Wet and dry bulb hygrometer
3. Regnault’s hygrometer

Absolute:

1. Transducers
2. Mass spec

Question 7

Hair hygrometer

Answer 7

• Mounted on wall of operating theatre
• Direct reading of relative humidity
• Hair gets longer as humidity increases
• Hair length controls pointer moving over a scale
• Accurate for relative humidity between 30%–90%

Question 8

Wet and dry bulb hygrometer

Answer 8

• Consists of two thermometers,
  one in air and the other submerged in water.
• Temperature of mercury in bulb on thermometer
  in the air is in equilibrium with its surrounding—
  read true ambient temperature.
• Thermometer submerged in water reads lower temperature due to
  cooling effect from evaporation and loss of latent heat of vapourisation.
• Difference between temperatures is related to rate of evaporation of
  water, which depends on ambient humidity.

Question 9

Regnault’s hygrometer

Answer 9

• Consists of silver tubing containing ether.
• Air is blown through the ether, thereby cooling it.
• This initiates condensation on the shiny outside surface.
• Temperature at which condensation occurs is known as the dew point
  (i.e. temperature at which the ambient air is fully saturated).
• Relative humidity = saturated vapour pressure (SVP) at dew point
  SVP at ambient temperature

Question 10

Transducers

Mass spectrometry

Answer 10

Transducers
* Depends on change in electrical resistance or capacitance of a
substance when it absorbs water vapour from the atmosphere.

Mass spectrometry
* Humidity can be measured by light absorption technique based upon the
reduction of ultraviolet light transmitted when water vapour is present.

Question 11

What is heat?

Answer 11

State of energy an object has in relation to the kinetic energy of its molecules
or atoms.

Heat will transfer down a temperature gradient from a warm object to a cooler one.

Heat energy is measured in Joules (J).

Question 12

What is temperature?

Answer 12

Measure of the thermal state of a substance.

It is a property of a system and determines whether heat
can be transferred to or from an object.

Standard international (SI) unit for temperature measurement is Kelvin (K).

It is based on the triple point of water
(the temperature at a specific pressure at which water exists in all three phases).

1 unit Kelvin = 1/273.16 of thermodynamic triple point of water.

Change in temperature of 1 K is equivalent to change in temperature of 1°C.

Question 13

How is hypothermia graded?

Answer 13

Hypothermia is defined as body temperature below normal (36.7–37.0°C).

Mild hypothermia: 34–36.5°C

Moderate: 27–34°C

Profound: < 17°C

Question 14

What are the consequences of hypothermia?

Answer 14

1. Cardiovascular
2. Neurological
3. Resp
4. Renal
5. GIT
6. Haematological

Question 15

Cardiovascular

Answer 15

• Increased myocardial oxygen demand and consumption due to shivering
• Arrhythmias, sinus bradycardia
• Vasoconstriction, poor peripheral perfusion, increased systemic vascular
  resistance
• ECG: J waves in lead II if < 32°C, AV block, fibrillation

Question 16

neurological

Answer 16

• Decreased cerebral blood flow
• Reduction in cerebral metabolic consumption by 7% per °C
• Neuroprotection via decreased neurotransmitter release
• Impaired conscious state at 33°C
• Coma at 30°C
• Decreased train of four ratio

Question 17

Respiratory

Answer 17

• Increased VO2 with shivering (up to 300%)
• Increased PVR,
  V/Q mismatch,
  impaired hypoxic vasoconstriction
• Decreased ventilatory drive,
  decreased bronchial tone,
  increased dead space
• Increased gas solubility

Question 18

Renal

Gastrointestinal tract

Answer 18

• Decreased renal blood flow and glomerular filtration rate due to
  decreased cardiac output
• Decreased blood flow, metabolic and excretory functions
• Decreased gut motility

Question 19

Haematological

Answer 19

• Decreased platelet function
• Coagulopathy as clotting enzymes are temperature dependent
• Increased fibrinolysis
• Increased haematocrit
• Left shift of Hb-O2 dissociation curve

Question 20

Endocrine and metabolic

Answer 20

• Decreased basal metabolic rate by 5%–7% per °C if not shivering
• Metabolic acidosis
• Acute rise in potassium with rewarming
• Decreased insulin, causing hyperglycaemia

Question 21

How is heat lost in the operating theatre?

Answer 21

Vasomotor tone is reduced during general anesthesia causing vasodilatation,

which increases heat loss.

Factors related to surgery and anaesthesia
(e.g. exposure of body cavity, irrigation and infusion of cold fluids, operating room
temperature) will contribute to hypothermia.

Radiation 40%
* Heat loss by infrared radiation from exposed portions of the body to
neighbouring objects that are not in direct contact.

convection 30%
* Air layer adjacent to the body becomes less dense and rises as it
becomes warmed. This produces a convection current carrying heat
away from the subject.

• Laminar flow ventilation in operating theatre exacerbates the convection
  current.

evaporation 20%
* Evaporation of sweat from skin or body fluids from mucosal or tissue
surfaces results in heat loss due to latent heat of vapourisation.

Respiration 10%
* Via inspiring dry gases

Question 22

How can heat loss be reduced?

Answer 22

Radiation: By increasing the room temperature and covering exposed
surfaces with warm blankets

Convection: By forced air warming devices and fluid warmers

Evaporation: By covering the patient

Respiration: By humidifying the inspired gases; intravenous fluids should be
warmed during infusion

Question 23

How is temperature measured?

Answer 23

Nonelectrical
* Gas expansion thermometers (e.g. Bourdon gauge dial)
* Liquid expansion thermometers (e.g. mercury, alcohol)
* Bimetallic strip dial thermometer
* Chemical thermometer

Electrical
* Thermocouple—Seebeck effect
* Resistance thermometer (e.g. platinum wire)
* Thermistor

Infrared
Tympanic membrane thermometer