The most effective way to retain heat and humidity in the circuit is the combination of using a heat and moisture exchanger and low fresh gas flow. (Image source: Thinkstock)
The most effective way to retain heat and humidity in the circuit is the combination of using a heat and moisture exchanger and low fresh gas flow. (Image source: Thinkstock)

When a non-intubated patient breathes, inhaled air gets warmed and humidified in the nasopharynx and upper airway. This process is bypassed if the lungs have been intubated. Dry air can negatively affect the airway by impairing ciliary and mucus gland function and thickening secretions.  The minimum value of absolute humidity of inhaled gas necessary to prevent airway dehydration is 20 mg H2O per liter. Although it has been shown in adults that a circle breathing system with low fresh gas flow can maintain airway hydration, few studies have been done in children.  Due to the smaller minute ventilation in children, maintaining that level of absolute humidity may be difficult.

Dr. Gustavo P. Bicalho, Department of Anesthesiology, Botucatu Medical School, UNESP—Universidade Estadual Paulista, São Paulo State, Brazil, and colleagues evaluated the effect of heat and moisture exchangers under low and high fresh gas flow on the temperature and humidity of inhaled gases during pediatric anesthesia. The results of their study are published in this month’s edition of Anesthesia & Analgesia in the article titled “The Humidity in a Dräger Primus Anesthesia Workstation Using Low or High Fresh Gas Flow and With or Without a Heat and Moisture Exchanger in Pediatric Patients.”

Strategies for maintaining the inhaled gas humidity include using low fresh gas flow and placing a heat and moisture exchanger on the circuit.  The authors studied four groups of ten patients each undergoing elective abdominal or urological surgeries lasting ninety minutes or longer to determine the success of those methods of humidification.  In two of the patient groups, a high fresh gas flow rate of 3 liters/minute was used. A heat and moisture exchanger was placed between the Y-piece of the breathing circuit and the tracheal tube in one of those groups, while the exchanger was not applied in the other.  Low fresh gas flow was used in the remaining two groups, again with only one group using the heat and moisture exchanger. A Dräger Primus anesthesia workstation loaded with fresh soda lime was used for each case. The temperature and relative humidity of the gases were measured using a rapidly responding electronic thermo-hygrometer connected to the circuit via a T-piece.  These readings were recorded over 10 respiratory cycles after 10, 20, 40, 60, and 80 minutes of connection between the patient and the breathing circuit.

There were no differences between the groups with regard to mean operating room and core temperatures. The mean temperature of the inhaled gases was higher in the two heat and moisture exchanger groups compared with the groups in which the exchanger was not used. Relative and absolute humidity values of the inhaled gases were significantly higher in the groups who used the heat and moisture exchanger. The low-flow groups had higher humidity values than the high-flow groups. Regardless of fresh gas flow, the mean absolute humidity of the inhaled gases in the groups with a heat and moisture exchanger was always higher than 20 mg H2O per liter.  Without the use of the exchanger, the absolute humidity was always lower than the minimum threshold, even with low fresh gas flow.

The study shows that low fresh gas flow, combined with a heat and moisture exchanger, effectively retains heat and humidity in the breathing circuit. The clinical significance of these results, for example reducing postoperative pneumonia or intraoperative airway difficulties, remains unclear.