Efficiency of cold passover and heated humidification under continuous positive airway pressure

The use of humidifiers is recommended for the treatment of local side-effects associated with nasal continuous positive airway pressure (nCPAP) therapy to increase the humidity and temperature of the respired air 1–4. To date, however, only humidity data averaged over a lengthy period of time have been reported 5, 6. Humidifiers are thought to exert an effect by changing the inspired air. Separate measurement of humidity during inspiration and expiration appears to be particularly useful as, owing to the high expiratory humidity, averaging of humidity figures over lengthy periods cannot completely reveal the effect of humidification on inspired air. Therefore, the present authors studied the influence of humidification under continuous positive airway pressure (CPAP) using sensors with a small dead time, making it possible to determine the rapid changes in humidity in the alternation of inspiration and expiration. The authors aimed to compare the relative humidity (H_rel) of the inspired air between cold passover and heated humidifiers, and to measure the water loss of respiration under CPAP treatment. A secondary aim was to evaluate the impedance (Z) of the total respiratory system under the various humidification systems.

Methods

Materials

The subjects were connected via a nose mask (Special®, Gold Seal®, Respironics Inc., Pittsburgh, USA) to a CPAP device (fig. 1⇓). This device incorporates a surface contact humidifier, which was used for cold H (water temperature≤ambient temperature) or heated H (>35°C) (CPAP with integrated humidifier: Max II®, Medizintechnik für Arzt und Patient, Martinsried, Germany). The humidity of the air was measured with a capacitive hygrometer (custom-made, University of Göttingen, Germany, range for 5–99% rH 8) between the mask and Y‐adapter. This system incorporates a layer of plastic polymer, between two electrodes which, depending on the humidity, can adsorb water molecules. These bring about a change in the capacity, which is correlated with the humidity. The sensor is heated to prevent water condensation. The capacitive hygrometer has only a very low dead time, enabling changes in the humidity between inspired and expired air to be measured (time constant 0.23 s at 40°C, 0.18 s at 50°C and 0.14 s at 60°C). The nonlinearity of the sensor was ±0.1 mg·L⁻¹. The device was calibrated using a mirror-type dew point hygrometer (maximal error after calibration ±0.6 mg·L⁻¹) 8. Temperature was measured with a “Type K” thermosensor (range −200–1,300°C, tolerance 0.3%) in the mask and a second thermosensor in the capacitive hygrometer. The authors applied the following data sets, recorded for ∼1 min preceding each measurement of Z of the total respiratory system, for the evaluation of humidity and temperature. Expiration: 632 data sets under no H, 430 under cold H, 672 under heated H; inspiration: 633, 434, 660, data sets respectively. Z was determined every 5 min using impulse oscillometry (50 measurements under no H, 37 with cold H, and 60 with heated H (IOS, Jaeger, Höchberg, Germany); oscillating frequency 0–100 Hz). Impulse oscillometry was employed for the lower and upper airways 7, 9 and the forced oscillation technique proved to be useful in the diagnosis and automatic treatment of obstructive sleep apnoea syndrome (OSAS) 10–13. Flow was measured with a laminar flow element LFE-Typ PT (SI, Nördlingen, Germany; 0–150 L·min⁻¹, tolerance <0.1%). The ambient conditions were determined with the hygrometer HP 100A (Rontronic, Bassersdorf, Switzerland; temperature −20–60°C, tolerance 0.3°C, humidity 0–99% rH, tolerance 1.5% rH).

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Fig. 1.—

Schematic representation of the set-up used for the measurements. The measurement set-up comprises three modules: the impulse oscillometry for the measurement of impedance; the respiratory therapy unit; and the unit for data acquisition and processing. The subject is connected to the continuous positive airway pressure (CPAP) device and the humidifier via a nasal mask (1) and a tube system (2). The temperature of the respired air is measured within the mask (ν mask) and the hygrometer (ν hygr). Within the breathing tube there is another, small-calibre tube that transmits the pressure in the nose mask to the sensor in the CPAP device. The capacitive hygrometer for measuring the humidity in inspired and expired air (φ hygr) is located between mask and breathing tube. A valve serves to eliminate the expired carbon dioxide (3). In the impulse oscillometry unit (impulse oscillometry (IOS) device), an oscillating flow is generated and conducted, via a tube (4) and Y‐adapter (5), to the airways. The IOS device also picks up the pressure (pIOS) and flow (vIOS) signals recorded by the pneumotachograph (6) and filters out the signals produced by the oscillating flow. These signals are used to calculate the impedance. The transducer picks up all the measuring signals in the tube system and from the surroundings, and passes them, via an analogue-to-digital converter (A/D converter), to the computer (PC). pCPAP: pressure measured in the breathing tube; V′: flow measured in the breathing tube; φ amb: humidity of the ambient air; ν amb: temperature of the ambient air.

Results

H_rel of the inspired air with no H increased under cold H, but more so under heated H (table 1⇓). In the expired air, H_rel showed only mild, but significant, increases with both humidification modes. With no H, the difference in absolute humidity between inspired and expired air was 21.5±4.7 mg·L⁻¹. This was reduced under cold H to 18.8±4.6 mg·L⁻¹ (p<0.01) and under heated H to 12.3±5.1 mg·L⁻¹ (p<0.001 as compared with cold H and no H). Thus, the loss of water under no H was only slightly reduced under cold H, but was halved under heated H (38% decrease of the water loss under heated H compared to cold H) (table 1⇓). The Z of the total respiratory system increased under heated H (table 1⇓).

Discussion

In the present study the humidity of respired air for inspiration and expiration were measured separately for the first time. Earlier data were only average figures obtained over several respiratory cycles 5, 6. Although the inspired air was influenced by the humidifier, the figures reported were of the same order as those measured for expired air 6. The measurement of humidity can be considerably falsified by condensation of water on the sensors. This was likely with values of H_rel approaching 100% 8. To avoid this effect, the present study used a heated hygrometer. A significant increase in inspiratory humidity was found through the use of cold and heated H. However, a clinically relevant increase in H_rel was only seen under heated H. In expired air, small differences in H_rel were observed between the various humidification systems. The low level of expiratory H_rel appeared to be due to both the distance between the sensor and the nose and the reservoir function of the nose mask. The amount of water lost on respiring nonhumidified air decreased by 16% with cold H, but by 44% with heated H. Since separate measurements of inspiratory and expiratory H_rel were not possible previously, comparable data on water loss are not available. Despite pronounced differences, all studies show that humidifiers increase humidity. Heated H has a clearly greater effect than cold H 5, 6. Compared with CPAP under no H, Massie et al. 4 found that CPAP was utilised longer with heated H, but not with cold H.

Nasal obstruction is one of the major side-effects of continuous positive airway pressure treatment. Therefore, the possibility that the upper airways' impedance might be influenced differently by the various modes of humidification was investigated. The forced oscillation technique has proven useful in measuring the total airway system impedance under continuous positive airway pressure 10, 11. In the present study, it was measured with an impulse oscillometry device, selected because it does not require patient cooperation or, in contrast to posterior rhinomanometry, the introduction of a tube into the throat. Using posterior rhinomanometry, Richards et al. 3 obtained appreciably lower baseline measurements, which might have been due to differences in methodology. The present data showed no clinically relevant changes in impedance under the various humidification systems, and all measurements were carried out with the patient breathing quietly with the mouth closed. The small elevation in impedance under heated humidification compared to no humidification may have been due to the longer period of heated humidification (54.3±12.5 versus 32.8±10.8 min) and/or the kind of humidification system used. The present study provides no data on the long-term use of humidification systems and therefore, the conclusions presented cannot be related to long-term overnight use of humidification. Moreover, because the study was specifically designed to compare relative humidity of the inspired air under cold and heated humidification and to measure water loss in healthy subjects in the wake state and at a continuous positive airway pressure of 10 cmH₂O, the results cannot be translated to other specific situations (e.g. mouth leaks, different pressure levels and ambient conditions).