Background and Goal of Study: When a patient's trachea is intubated, the normal warming and humidifying functions of the upper airways are bypassed. Hence, gas delivered to the patient needs to be artificially conditioned in order to preserve the mucociliary function of the respiratory tract. A circle breathing system can increase the humidity of inhaled gas, especially with the use of a low fresh gas flow (FGF). A heat and moisture exchanger (HME) can be used to further humidify the inhaled gas. The Dräger Primus anaesthesia workstation has a built-in hotplate to heat exhaled and inspiratory gases in the breathing circuit. The temperature and humidity of the inhaled gas of this anaesthesia machine have not yet been investigated in paediatric patients. The aim of this study was to compare the humidity of inhaled gas from a low-flow paediatric breathing system of the Primus workstation with or without a HME.
Materials and Methods: Twenty ASA I-II children (mean age: 4.5±2.3 years; mean weight: 19.1 ±6.2 kg) scheduled for general anaesthesia with tracheal intubation were randomized in two groups: without (Group I) or with (Group II) a HME into the breathing circuit. The children were ventilated using a paediatric circle system with CO2 absorber with a low FGF (1 L/min) of the Dräger Primus workstation. Temperature and relative and absolute humidity of the inhaled gas were determined close to the tracheal tube using a thermo-hygrometer. The measurements were made at 10, 20, 40, 60 and 80 min after tracheal intubation.
Results and Discussion: The use of an HME (Group II) provided significant higher mean temperature (30.3±1.1°C vs 27.0±1.2°C in the Group I) and mean relative (80±2% vs 65±4% in the Group I) and absolute humidity (25±1.5 mg H2O/L vs 17±1 mg H2O/L in the Group I) of the inhaled gas (P< 0.0001). During anaesthesia, a minimum moisture target of 23 mg H2O/L is recommended to reduce the risk of dehydration of the respiratory tract. Dry gases can also result in thickened mucous and obstruction of a small tracheal tube.
Conclusion(s): The low-flow paediatric breathing system of the Primus workstation does not meet the minimum requirements for humidity of inhaled gas during anaesthesia in children. The insertion of an HME increases the humidity of the inhaled gas, bringing it close to physiological values.
Acknowledgements: FAPESP (grant number 2011/13545-0).