Fig. 9.1
The relative humidity of a gas depends on its absolute water content and gas temperature. At 37.0 °C and 100 % relative humidity, the respiratory gas has 44 mg/L absolute water content. If the gas is saturated (100 % relative humidity) at 30.0 °C, its water content will be only 30 mg/L. When the gas is then warmed to 37.0 °C, its relative humidity will fall to below 70 %
Air takes up energy when nebulized water or liquid water is converted into water vapor. Conversely, heat is generated in the process of rainout of water vapor (condensation). Hence, there are two components to the total energy content of air: a sensible and a latent heat content. The air temperature solely reflects the sensible heat while the water vapor mass reflects the latent energy component. It is important to understand that changing the air temperature alone without changes in water vapor mass constitutes a small change in total energy content compared to changing the humidity of the gas. The difference may approach an order of magnitude. Therefore, warming of frigid inspiratory air does little cooling to the upper airway lining compared to the amount of heat loss that incurs when this air is particularly dry. Conversely, if air is inhaled at 39 °C without being fully saturated with water vapor, it will quickly cool to core body temperature without significant heating of the airway lining. The following illustrates this issue quantitatively in an example: if air is saturated with water vapor at 37 °C in a humidifier chamber and subsequently dry heated to 39 °C in a heated breathing circuit, this dry heating adds almost no energy to the gas—it contains 143 J/g at 37 °C, 100 % relative humidity vs. 145 J/g at 39 °C, 90 % relative humidity. The ASTM humidifier standards state that there is no thermal issue if the gas contains less than 194 J/g of energy. If, however, air enters the airway at much higher than core body temperature and full saturation, there may be a risk of thermal injury to the airway.
There are fundamental differences between nebulized and vaporized water: nebulization creates a dispersion of small droplets of water in air. These particles may vary in size from about 0.5–5 μm. They are visible because they scatter light (clouds) and may carry infectious agents or other particulate matter. In contrast, vaporization generates a molecular, i.e., gaseous distribution of water in air. Hence, water vapor is invisible and unable to carry infectious agents. It 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 
= 47 mmHg/760 mmHg = 0.062, i.e., 6.2 %.
= 47 mmHg/760 mmHg = 0.062, i.e., 6.2 %.9.3 Clinically Relevant Humidification Principles and Devices for Invasive and Noninvasive Ventilation
Medical grade gases have virtually no water content at room temperature. There are three options to deliver water into inspired gas or directly into the airways:
1.
Vaporization of water using heated humidifiers or heat and moisture exchangers (“artificial noses”)
2.
Nebulization of water using jet or ultrasonic devices
3.
Periodic instillation of bolus water or normal saline solution into the endotracheal or tracheostomy tube as commonly done prior to suctioning procedures
9.3.1 Heated Humidifiers
For various physiological reasons, it is rational to deliver the inspiratory gas at or close to core body temperature and full saturation to endotracheally intubated infants. Heated humidifiers (Fig. 9.2) can achieve this goal. The respiratory gas is warmed inside the humidification chamber to a set target temperature, and water vapor is added from the heated water reservoir. 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) unless the vaporizing capacity of the device is too low relative to the size of the gas flow. The vaporizing capacity of the humidification chamber depends on its water surface area and on temperature. 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. 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 if the chamber temperature was set at or below 37 °C, 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. The inspiratory gas can rapidly cool down in unheated segments of the circuitry. This is promoted by the 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 circuit gas flow rates due to the longer contact time. Insulating unheated segments of the inspiratory circuit may partly obviate these problems. Rainout should also be avoided for other reasons: condensate gets easily contaminated, may be flushed down the endotracheal tube with risks of airway obstruction and nosocomial pneumonia, and may disturb the function of the respirator. 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. The gas will quickly cool down to core body temperature inside the adapter and the endotracheal tube.
Fig. 9.2
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). The water consumption rate of a humidifier chamber required to reach a target respiratory gas humidity can be calculated from the circuit flow rate. Observation of this water consumption rate can be employed as a simple test of the efficiency of a humidifier
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. The temperature probe close to the patient connection serves to monitor the respiratory 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. If the temperature probe is in the presence of a heated field, it may register a temperature higher than the actual respiratory gas temperature as a result of radiation or convection from the warmer environment near the patient. This may signal to the servo to decrease the heating output of the ventilator circuit and lead to loss of gas temperature and rainout. Such problem may arise when the incubator temperature is higher than the targeted gas temperature or from a radiation heat source. Insulating the temperature probe by a light-reflective patch or other material can improve the performance of the system. Another way to alleviate this problem is to place the 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. If cooler incubator temperatures are employed as usually used for older preterm infants, rainout will occur in the unheated segment particularly at low circuit gas flow rates. A circuit should then be used that is equipped with a heated wire along the entire length of its inspiratory limb. Another suitable type of circuit is that with two temperature probes, one outside the heated field and another one close to the Y adaptor. These circuits can perform well over a range of incubator temperatures both above and below the target respiratory gas temperature. This is because the heated wire servo control can be programmed to select the lower of the two recorded temperatures to drive the power output. The maximum heat output of any heated wire circuit may not be sufficient to meet target gas temperatures under extremes of room and incubator temperatures. Also, generic circuits have been on the market that may not be fully compatible with the humidifier and its power source. There has been a warning that covering heated wire circuits with drapes or other material for insulation may involve a risk of melting or charring of circuit components.
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