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Does the method of humidification within a CPAP circuit affect the work of breathing? A pilot study


It has been shown (for example [1]) that heat and moisture exchange filters (HME) can be as effective in humidification of a breathing circuit as a heated humidification (HH) system. Before considering the replacement of the present HH system with HME filters in the high-flow CPAP circuits, we wished to examine whether they would affect the work of breathing (WOB) through the circuit. We found no evidence in the medical literature that such an investigation had been carried out.


Six healthy male volunteers were studied. They were asked to breathe via a facemask through a high flow CPAP circuit at three different levels of CPAP: 0, 5 and 10 cmH2O. Either a HH system (MR210; Fisher-Paykel Healthcare, NZ) or a HME (Ventalink; Pennine Healthcare, UK) was used in the circuit. During tidal breathing over a 20 s period, the airflow through the circuit was measured with a Fleisch no.2 pneumotachograph and the pressure drop across the circuit was simultaneously recorded, both at 200 Hertz. The work of breathing for inspiration and expiration was calculated as the product of volume and pressure. Repeated-measures ANOVA was used for analysis.


The work of breathing per unit tidal volume attributable to the breathing circuit, shown in the Table, was not significantly affected by method of humidification for inspiration (P = 0.058) or for expiration (P = 0.343) although it increased for each level of CPAP (inspiration, P = 0.004 and expiration, P = 0.001).



This pilot study suggests a hypothesis that the use of a HME filter, compared to a HH system, may increase the WOB associated with a CPAP circuit. The results are not statistically significant and 25 subjects will be required to test this hypothesis according to power analysis. This study will be carried out on patients on the Intensive Care Unit.


  1. Martin , et al.: Heat and moisture exchange and vaporizing humidifiers in the intensive care unit. Chest 1990, 97: 144-145.

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Wills, J., Girling, K. Does the method of humidification within a CPAP circuit affect the work of breathing? A pilot study. Crit Care 6 (Suppl 1), P38 (2002).

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