Humidification in NICU - the concepts

1. HUMIDIFICATION
IN NICU
Dr Padmesh V
DNB (Ped), DM (Neonatology)

2. • Ambient air is cooler and contains less water.
• Alveolar air is fully saturated with water vapor at
core body temperature.
• This gradient in heat and water vapor pressure is
maintained along the nose and upper airways.

3. • The inspired air gains heat and water vapor from the
upper airway lining which is partly recovered when the
expired gas loses heat and water condenses back to the
airway surface. This recovery occurs because the upper
airway temperature remains below core body
temperature during expiration.
• Breathing is associated with a net heat loss and water
loss because the expired air temperature is higher than
ambient temperature under normal circumstances.

4. Alveoli fully
saturated with
water vapor at
core body temp
Ambient air
is cooler,
with less
water vapor
INSPIRATION EXPIRATION
Upperairway
Upperairway
Temperature
of expired air
is higher than
ambient
temperature

5. • The term “absolute humidity” (AH) is defined as the
amount of water vapor per gas volume at a given
temperature. (mg/L)
• “Relative humidity” (RH) is the actual water vapor
content of the gas volume (in milligrams) relative to
the water vapor content (in milligrams) of this same
gas volume at saturation at the same temperature.
• There is a fixed relationship between AH, RH, and
temperature.

6. • Absolute humidity is the amount of water vapour
per litre of gas, expressed in mg/l.
• Relative humidity is a measure of how much water
vapour is in a gas compared to its capacity to hold
water vapour. It is measured as a percentage.

7. • Water vapor in air will exert a gaseous pressure which
amounts to a partial pressure of water of 47 mmHg when
air is fully saturated with water vapor at 37 °C. This
corresponds to a water vapor mass of 44 mg of water per
liter of gas.
• The partial pressure of water vapor at saturation depends
solely on the temperature.
• The fraction of water vapor pressure at saturation, 37 °C,
and 760 mmHg ambient pressure is therefore
FH2O = 47 mmHg/760 mmHg = 0.062, i.e., 6.2 %.

9. 1. Heated Humidifiers
2. Heat and Moisture exchangers
(Artificial noses)

10. • Heated Humidifiers:
• Should deliver inspiratory gas at or close to core body
temperature and full saturation to endotracheally
intubated infants.
• Respiratory gas is warmed inside the humidification
chamber to a set target temperature, and water vapor is
added from the heated water reservoir.

11. • Heated Humidifiers:
• A heated wire inspiratory circuit tubing is then used to
maintain or slightly raise the gas temperature so as to
prevent water rainout before the gas reaches the infant.
• At a set humidifier chamber temperature of 37 °C, the gas
will absorb a maximum of 44 mg/L of water which
corresponds to full saturation (100 % RH).

12. • Heated Humidifiers:
• Position of three temperature probes of a heated wire humidification system for infants. The
user sets the target temperature to be reached at the endotracheal tube adaptor. This
temperature is commonly set at or slightly above 37.0 °C. The temperature inside the
humidifier chamber must be high enough to vaporize an amount of water near the absolute
water content of gas saturated at 37 °C (44 mg/L).
• Flow rate: 5L/min

13. • Recording the water consumption of the chamber
over time is a simple test to check for sufficient
vaporization.
• Because most infant ventilators use a continuous
constant circuit flow of known size, the absolute
and relative humidity delivered at the chamber
outlet can be calculated from the humidifier’s water
consumption rate.

14. • Any decrease in gas temperature along the way from the
humidification chamber to the Y adaptor will induce
condensation if the gas was saturated at the chamber
outlet.
• This implies that any rainout in the tubing indicates a
moisture loss of the respiratory gas. The gas will then reach
the infant underhumidified.
• It must be emphasized that rainout in the inspiratory limb
of the ventilator circuit does not indicate proper
humidification in such situations. To the contrary,
condensation will necessarily be associated with
underhumidification.

15. As temperature decreases, amount of water vapor the gas can hold also decreases.
So if temp of inspiratory limb decreases, water will be lost from the gas, which will
cause rain out, which ultimately causes under-humidification.

16. • The inspiratory gas can rapidly cool down in
unheated segments of the circuitry. This is
promoted by
– large outer surface area of small-diameter tubings
(particularly when corrugated),
– by drafts around the tubing (air conditioned rooms), and
– by a low room temperature.
• The decrease in temperature will be larger with smaller gas
flow rates due to the longer contact time.
• Insulating unheated segments of the inspiratory circuit may
partly obviate these problems.

17. • Therefore, in a regular application for an intubated subject,
a heated humidifier can be set up with:
– a chamber temperature of 37 °C (in order to saturate the
gas with 44 mg/L of water).
– To avoid loss of moisture in the inspiratory limb of the
heated circuit, the target gas temperature at the Y
adaptor can be set at 39 °C so that the gas arrives with
slightly less than full saturation.

18. • The technology required for preterm infants is slightly
more complex because the ventilator circuit passes
through two different environments:
– the room and
– the incubator or radiant warmer.
• Temperature probe close to patient connection
monitors the inspiratory gas temperature.
• It is commonly part of a servo control which aims at
maintaining the set gas temperature at the Y adaptor
by controlling the heated wire circuit’s power output.

19. • If temperature probe is in a heated field, it may register
a temperature higher than actual inspiratory gas
temperature as a result of radiation or convection from
the warmer  Servo decreases heating output of the
ventilator heated wire circuit  Loss of gas
temperature and rainout.
Note that the temperature probe
controls servo of heated wire circuit and
not the heating chamber of humidifier

20. • Insulating the temperature probe by a light-reflective patch or other
material can improve performance of the system.
• Another way to alleviate this problem is to place temperature probe
just outside the heated field and use an unheated extension adaptor
tubing to carry the gas through the heated field to the infant. The
extension tube does not need to incorporate heated wires because its
temperature is maintained by the heated field.
• Using a circuit with two temperature probes, one outside the heated
field and another one close to the Y adaptor.
two temperature probes

21. • Note: Expiratory limb should pass down-hill from
the patient manifold to prevent water trickling back
into the patient
• Minimum Humidification Requirements:
• International standards recommend an absolute
humidity level of inspired gas >33 mg/L in patients
whose supraglottic airway is bypassed.
(44 mg/L, 33 mg/L !)

22. 1. Heated Humidifiers
2. Heat and Moisture exchangers
(Artificial noses)

23. • Artificial Noses (HME)
• Heat and moisture exchangers (HMEs) are designed
to recover part of heat and moisture contained in
expired air.
• A sponge material of low thermal conductivity
inside clear plastic housing absorbs heat and
condenses water vapor during expiration for
subsequent release during inspiration.
• Some HMEs are additionally coated with
bacteriostatic substances and equipped with
bacterial or viral filters (HMEF).

24. • Artificial Noses (HME)
• HMEs may be an alternative to heated humidifiers
because:
– Simplification of ventilator circuit
– Passive operation without requirement of
external energy and water sources
– No ventilator circuit condensate
– Low risk of circuit contamination
– Low expense

25. • Artificial Noses (HME)
• Use of heat–moisture exchangers, or artificial
noses, in neonates should be discouraged because
they:
– increase ventilation requirements,
– increase CO2 levels,
– lower body temperature, and
– increase artificial airway blockage.
May be used only for short term:
eg: neonatal transport.

26. • Side effects of inadequate humidification:
• Even if all other forms of infant warming are provided,
ventilation with nonhumidified gases is a major reason
for hypothermia in neonates.
• Inadequate humidification of the respiratory tract may
reduce mucociliary clearance:
– airway obstruction by secretions,
– gas trapping
– VAP
• Dry gases  Severe lung injury.

27. • What happens if hot air without humidity is given?
• The increased temperature of a gas shifts the isothermal
boundary (point at which gas completes equilibrium to
body temp & humidity levels) to a point closer to airway
opening.
• At first glance, this seems beneficial because less mucosa is
exposed to humidity deficit. However, because the effect of
a given humidity deficit is concentrated on a smaller area of
the mucosa, there is potential for a greater degree of
damage.

28. • What happens if hot air without humidity is given?
• Moreover, use of higher airway temperatures means that,
even with lower humidity, there is relatively less
opportunity for exhaled humidified air from within the lung
to recondense some of its humidity. The result is an
increase in the humidity deficit .
• Thus, potential for adverse effects with use of heated wire
circuit is exacerbated by inadequate humidity levels.