Gas pre-warming for improving performances of heated humidifiers in neonatal ventilation

Heat and moisture exchangers (HMEs) and heated humidifiers (HHs) in adult critically ill patients: a systematic review, meta-analysis and meta-regression of randomized controlled trials

BACKGROUND

The aims of this systematic review and meta-analysis of randomized controlled trials are to evaluate the effects of active heated humidifiers (HHs) and moisture exchangers (HMEs) in preventing artificial airway occlusion and pneumonia, and on mortality in adult critically ill patients. In addition, we planned to perform a meta-regression analysis to evaluate the relationship between the incidence of artificial airway occlusion, pneumonia and mortality and clinical features of adult critically ill patients.

METHODS

Computerized databases were searched for randomized controlled trials (RCTs) comparing HHs and HMEs and reporting artificial airway occlusion, pneumonia and mortality as predefined outcomes. Relative risk (RR), 95% confidence interval for each outcome and I ² were estimated for each outcome. Furthermore, weighted random-effect meta-regression analysis was performed to test the relationship between the effect size on each considered outcome and covariates.

RESULTS

Eighteen RCTs and 2442 adult critically ill patients were included in the analysis. The incidence of artificial airway occlusion (RR = 1.853; 95% CI 0.792-4.338), pneumonia (RR = 932; 95% CI 0.730-1.190) and mortality (RR = 1.023; 95% CI 0.878-1.192) were not different in patients treated with HMEs and HHs. However, in the subgroup analyses the incidence of airway occlusion was higher in HMEs compared with HHs with non-heated wire (RR = 3.776; 95% CI 1.560-9.143). According to the meta-regression, the effect size in the treatment group on artificial airway occlusion was influenced by the percentage of patients with pneumonia (β = -0.058; p = 0.027; favors HMEs in studies with high prevalence of pneumonia), and a trend was observed for an effect of the duration of mechanical ventilation (MV) (β = -0.108; p = 0.054; favors HMEs in studies with longer MV time).

CONCLUSIONS

In this meta-analysis we found no superiority of HMEs and HHs, in terms of artificial airway occlusion, pneumonia and mortality. A trend favoring HMEs was observed in studies including a high percentage of patients with pneumonia diagnosis at admission and those with prolonged MV. However, the choice of humidifiers should be made according to the clinical context, trying to avoid possible complications and reaching the appropriate performance at lower costs.

Performances of heated humidifiers in mechanical ventilation: a preliminary intra-breath analysis

A correct humidification of gases delivered to mechanically ventilated patients is essential to avoid some complications. Heated Humidifiers (HHs) are widely used in long lasting ventilation to treat gases delivered to patients. Their performances depend on some parameters, such as environmental conditions and ventilatory settings. The aim of this work is to experimentally assess the influence of minute volume (MV), respiratory frequency (f) and the ratio between duration of inspiration and expiration (I:E) on HHs performances in terms of relative humidity (RH) at the humidification chamber outlet. The main novelty of this work is the assessment of RH oscillations during artificial ventilation. Results show marked oscillations of RH during a single breath (ripple of 20% in the worst case); oscillations decrease if f and I:E increase, on the other hand they increase with MV. Since the variation of gas temperature during a respiratory act can be neglected, the RH oscillations are related to the vapour content in the delivered gases. These results further support the hypothesis which asserts that HHs performances could be improved by using a control strategy taking into account ventilatory settings.

Measurement of condensed water mass during mechanical ventilation with heated wire humidifiers: experiments with and without pre-warming

Heated wire humidifiers (HWHs) are employed in mechanical ventilation with the objective of heating and humidifying the gases delivered to the mechanical ventilator. They use a control based on the adjustment of gas temperature at the chamber outlet. The condensation occurring within the breathing circuit is one of the most important concerns related to this control strategy. In the present study we focused on the measurement of the condensation amount within the breathing circuit during the employment of a commercial HWH (MR850, Fisher & Paykel). The measurement of the condensed vapor mass, performed during 7 h of ventilation, provides more objective information than the visual-based scale used in literature. Moreover, two solutions were proposed to minimize the condensation in the breathing circuit tract downward the heated chamber: i) a flexible insulating pipe was used to cover the mentioned breathing circuit tract, and ii) the air delivered by ventilator was heated before it passes through the chamber at different inlet temperature Ti obtained by employing pre-warming. To assess the improvement obtained by these two solutions, experiments have been carried out with and without their employment at two minute volumes. Results show that: i) insulation and pre-warming allows minimizing the condensation (e.g., at 8 L·min(-1) the mass of condensation after 7 h of ventilation decreases from 9.3 g to 2.5 g by using insulation and T(i)=27 °C); ii) the condensation mass decreases with T(i) (e.g., at 8 L·min(-1) the mass condensation was 2.5 g at T(i)= 27 °C and 1.1 g at T(i)= 30 °C); and iii) the amount of condensation linearly increases with time of ventilation.