An assessment of methods of humidification of inspired gas

Clinical and in vitro evaluation of membrane humidifier that does not require addition of water

It is well known that conventional bubbling humidifiers are capable of producing micro-aerosols contaminated with bacteria. We developed a unique humidifier, named a membrane humidifier, that does not require an external water supply. This new system obtains moisture from room air. We investigated the clinical and in vitro evaluation of the membrane humidifier. Ten patients with chronic pulmonary disease participated in the study. We evaluated the partial pressure of oxygen in arterial blood (PaO2) of 10 patients who used the new device. We conducted an in vitro study to determine whether the device could prevent the bacterial contamination of humidified-oxygen. We passed compressed air contaminated with Pseudomonas aeruginosa outside the hollow fibres of the membrane humidifier, and the humidified-oxygen passed inside the hollow fibres was sampled into nutrient broth periodically for 10 days. We also compared the relative humidity of oxygen humidified by a membrane humidifier with that of oxygen humidified by a bubbling humidifier. There was no significant difference between measured PaO2 while breathing oxygen humidified using a membrane humidifier and that while breathing oxygen humidified using a bubbling humidifier. Cultures of the humidified-oxygen passed through the hollow fibres were negative for bacteria. The membrane humidifier could produce good humidification. The new device appeared to prevent bacterial contamination, and may help to reduce the risk of infection in patients at hospital and home.

Comparison of hydrophobic heat and moisture exchangers with heated humidifier during prolonged mechanical ventilation

Inspired gases must be warmed and humidified during mechanical ventilation. In a prospective randomized study we compared the performance of a heated humidifier (HH) (Draegger Aquaport) and a heat and moisture exchanger (HME) (Pall Filter BB 2215). A total of 116 patients requiring mechanical ventilation (Servo 900 C Siemens) were enrolled into the study and were randomly assigned to 2 groups. Patients in group I were ventilated with a traditional breathing circuit with HH and patients in group II using a simplified circuit with HME. Pre-existing and hospital acquired atelectasis and pneumonia, occurrence of endotracheal tube (ET) occlusion and ventilatory parameters (respiratory rate, tidal volume) were studied. No statistical difference was found between groups for each parameter except the greater frequency of ET occlusions in the II group (0/61 vs 9/55) (p = 0.0008). Pall Filter (PF), a hydrophobic filter, humidifies the dry gases from the condensed water which is put down on the HME surfaces during cooling of saturated expired gases. This purely physical property is linked to the magnitude of the thermic gradient between the expired gases and the ambiant temperature. Performance impairment of PF in our study might be due to high ambiant temperature in the intensive care unit (usually around 28 degrees C) which reduces thermic gradient and water exchanges. We conclude that efficiency of PF may be weak in some conditions of ambiant temperature.

Inadequate humidification of respiratory gases during mechanical ventilation of the newborn

Proximal airway humidity was measured during mechanical ventilation in 14 infants using an electronic hygrometer. Values below recommended minimum humidity of adult inspired gas were recorded on 251 of 396 occasions. Inadequate humidification, largely due to inadequate proximal airway temperature, is commoner than recognised in infants receiving mechanical ventilation.

Pulmonary function in mechanically-ventilated patients during 24-hour use of a hygroscopic condensor humidifier

Hygroscopic condensor humidifiers (HCH) are reportedly capable of humidifying even the driest of ventilator source gases with at least 30 mg H2O/liter of ventilation. To assess the adequacy of the HCH during mechanical ventilation, we studied 26 patients over a 72-hour period (alternating 24-hour periods of humidification by a conventional cascade and the HCH). In these patients, we found no significant difference in static lung compliance, airway resistance, PaO2, and PaCO2 on either system. Additionally, estimates of sputum volume (over a four-hour collection period) and clearance of aerosolized 99mTc labelled DTPA (in five of these patients) also showed no significant differences between the two systems. We conclude that the HCH is capable of supplying necessary heat and moisture to most mechanically-ventilated patients for at least a period of 24 hours.

The efficiency of various humidifiers for respired gas

Eight different breathing gas humidifiers were tested by humidity measurements using two independent methods under simulated clinical conditions. The comparison considering the influence of tidal volume and inspiratory/expiratory ratios revealed great differences between the individual devices depending on the kind of humidification and functional principles. Only four of the humidifiers tested: Ultrasonic Nebulizer NB 108, Hydrotrop 200, Bennett Cascade and Nicholas Humidifier, gave a constant performance of more than 30.75 g H2O/m3 absolute humidity or 70% relative humidity, related to 37 degrees C. However, humidification and heating of the inspired gas could be ensured only by the last three devices. The humidifiers Bird 500 cm3 Micronebulizer, Ultrasonic Nebulizer NB 100 and Draeger Compressor 660, were strongly dependent on the respiration parameters and exceeded the required minimum of 70% relative humidity only under certain conditions. The Engstroem ER-200 Vaporizer had a low performance and was not suitable as a breathing gas humidifier for longterm ventilation.

Centralised system of humidification for respiratory treatment

A centralised system of hot water humidifiers for an intensive therapy unit has been described and its performance examined.

The Retec breathing assistor. An application of fluidics to apparatus for inhalation therapy

The Retec breathing assistor, with its gluidic valve, is an interesting method of providing assisted positive pressure inspiration, a resistive load to expiration and a capability for the administration of drugs by nebuliser. It requires some co-operation but very little training by patients, and is easily cleaned and maintained.

Continuous positive airway pressure breathing (CPAP). Apparatus for use in neonates or adults

CPAP is a technique of respiratory care which was originally described in the management of the respiratory distress syndrome of the newborn and later in the post operative management of the cardiac infant following surgery. It has potential value in the respiratory management of older children and adults. Apparatus is described suitable for the application of continuous airway pressure during spontaneous ventilation via endotracheal tube in either neonates or adults. The inspired oxygen content is adjustable and the fresh gas warmed and humidified. High and low pressure alarms are not considered necessary. A head-harness is described for the application of CPAP in neonates via twin nasal or nasopharyngeal tubes. This equipment may be obtained from Messrs. Lusterlite Products Limited, 56 Devon Road, Leeds 2.

A test system for the biological safety cabinet

A simple, cheap and readily available test system for biological safety cabinets is described. It depends on the containment of an aerosol of Bacillus subtilis spores generated in a BIRD micronebulizer and the measurement of air flows with an anemometer. The system was set up to survey new equipment but equally valuable results have been obtained from tests during use. New units were often badly installed and used equipment was poorly maintained. It is suggested that any department which has a need for a biological safety cabinet must be in a position to test its function.

Continuous positive airway pressure breathing in the postoperative management of the cardiac infant

Crew, A. D., Varkonyi, P. I., Gardner, L. G., Robinson, Q. L. A., Wall, E., and Deverall, P. B. (1974).Thorax, 29, 437-445. Continuous positive airway pressure breathing in the postoperative management of the cardiac infant. Continuous positive airway pressure with spontaneous ventilation was used in the postoperative period following palliative or corrective surgery for congenital heart defects in a group of children of less than 3 years of age. After stabilization of the cardiovascular state, continuous positive airway pressure breathing (CPAP) was shown to be a suitable alternative to continuous positive pressure ventilation (CPPV). A statistically significant increase in PaO₂ was observed on changing from CPPV to CPAP. A statistically significant decrease in PaO₂ and increase in pulmonary venous admixture was observed after discontinuing the positive airway pressure and allowing the patient to breathe at ambient pressure.

We would recommend CPAP as an intermediate manoeuvre in the withdrawal of ventilatory support as it introduces a smoothness and stability into patient management régimes which was previously lacking. Careful selection of apparatus is necessary as the airway pressure should be truly continuous and steady.

In neonates the dead space of the system should be reduced to a minimum; CPAP alternating with periods on CPPV may be necessary for some time after cardiovascular stability has been attained.

Equipment for inhalation anaesthesia for children

The modified T-piece is the most commonly used system for inhalation anaesthesia for children in Australia. Its advantages are that it is light-weight and convenient, having low dead-space and resistance. Artificial humidification of gases is recommended to prevent drying of the trachea and to reduce heat loss in anaesthetized infants.

The advantages of standard tapers in 15 mm and 22 mm sizes are discussed. These are interchangeable with other resuscitation and anaesthetic equipment. The Rendell-Baker Soucek face-masks used with a divided mask mount have smaller dead-space than cushioned face-masks. Endotracheal tubes made from PVC are preferred for children, provided they meet implantation test standards. Standard gas machines and ventilators may be used for small children, although specially designed ventilators are available.