To our knowledge, this is the first study that compared the performance of two humidifiers and evaluated rainout and alarms. Our results show that DuoTherm generated significantly less rainout for the pediatric and the adult circuits as well as for different ambient temperature (20 to 26°C). Due to the reduction in rainout, the number of tube occlusion alarms were significantly lower for DuoTherm humidifier. As shown by several studies[17–19], rainout adversely affects the ventilator performance and can lead to ventilator shut down. Schwarz et al showed that rainout may also cause double triggering of the ventilator. Particularly in the neonatal units, this is particularly dangerous given the smaller diameter of the tubing.[18, 21]. Besides reducing rainout, DuoTherm also reduced the temperature alarms compared to MR850. The reduced alarms will potentially reduce the risk of alarm fatigue in health care workers. As shown by Lewandowska et al, who evaluated 389 nurses working in different intensive care units, the nurses felt overburdened with an excessive number of duties and a continuous wave of alarms. The authors concluded that alarm fatigue may have serious consequences, both for patients and for nursing personnel. One paper in 2016 reported that about 10% of patients were responsible for nearly 60% of alarms, and the authors recommended adjusting alarm sensitivity on a case-by-case basis. According to another study, nurses have a tendency to respond to alarms in patients who are known to be physiologically less stable and did not record most alarms.
Ventilators generate a large proportion of alarms in some critical care settings. In a study by Belteki, ventilators in the neonatal intensive care unit triggered 603 alarms per baby per day, an average of 10 per hour. Most of the alarms were related to inappropriate settings and were brief, but some were ignored by staff for prolonged periods of time. These alarms are audible and loud enough to disturb the baby, the parents, and the staff. Another study, this from Johns Hopkins Hospital, studied alarms generated by ventilators in adults. The study found an average of 6-8 alarms per hour generated by the ventilators, many of which resulted in a cascade of other notifications by telemetry, pagers, and telephone calls to nurses and respiratory therapists. About 5% of the alarms were related to the “Other” category which included circuit occlusions, but they did not specifically quantify rainout or condensation.
To reduce the influence of ambient temperature on circuit condensation, a HH device may maintain gas flow temperature with heated wires along the inspiratory limb. Circuits use sensors at the HH outlet and at the Y-piece near the patient to create a feedback mechanism to automate increases to the water temperature and/or heater wire duty cycle to help regulate gas temperature at the Y-piece. Importantly, the location in the circuit matters when assessing gas flow temperature, as a 2 to 4°C drop temperature may occur at the proximal end of the ET tube, a point that more closely reflects inhaled air dynamics.
In general functionality, devices such as the MR850 HH and the DuoTherm HH system are similar in design as they are both pass-over humidification systems. However, to improve rainout control and alarms, the DuoTherm heated breathing circuits add an additional outer corrugate that creates an insulating airgap between the breathing gas pathway and the patient room environment. Also, the DuoTherm expiratory heater wire provides constant output, but remains adjustable to the user, in contrast to that of the MR850, which mirrors the actions of the inspiratory limb.
The finding that more alarms and more rainout occurred at lower ambient temperatures was anticipated because of the increased cooling effect on the walls of the heated breathing circuit. Both active humidification systems in this study humidified the breathing gas at the same chamber outlet temperatures to nearly 100% relative humidity, so any drop in temperature can create condensation. The DuoTherm HH system likely performed better because of its ability to maintain the gas pathway temperature in comparison to the MR850 system, thereby reducing the amount of condensation that forms on the walls of the breathing circuit. The more consistent temperature may be due to a combination of the dual-wall design, the heater wire design, and the temperature management algorithms of the heater base.
Differences at higher ambient temperatures were less substantial because HH in such situations do not run at maximum capacity and can easily maintain gas-pathway temperatures. In these conditions, the smaller temperature gradient between the inside and outside of the breathing circuit results in very little condensation and the advantage of the dual-wall breathing circuit is diminished. Mid-level to sub-maximal flow rates (~20 to 50 L/m) also reduce the opportunity for the breathing gases to cool while in the breathing circuit, with subsequently less of a difference in the middle of the flow range.
One of the consequences of rainout in ventilator circuits is an occlusion alarm. Alarms prompt the care team to stop what they are doing to assess the patient. Consistent with lower rain-out, there were fewer alarms in the DuoTherm circuits compared to the MR850 circuits. Rainout alarms and patient temperature alarms are actionable alarms, since they indicate that the patient may be at risk for an adverse event. Many approaches described in the literature aimed to alter the team’s response to alarms.[8–10] With lower rainout, the critical care team can be less burdened with alarms and more able to respond to other physiologic alarms. A clinical assessment of true positives, false negatives, and alarm fatigue would aid in the assessment of potential benefits or harms.
Limitations of the testing included controlling the air currents within the test lab, which were minimized via the installation of baffling on the room’s air ducts, but the investigators noted that areas of the room were draftier than others. Also, documentation captured minor fluctuations in room temperature, although this is common with all HVAC systems. Other limitations include that lab testing does not always reflect device performance in clinical settings and the inherent variability of rainout testing. Minor limitations included sample size due to room space and ventilator availability, and the use of endpoints for testing rather than continuous monitoring during active ventilation trials. However, the collection of multiple samples enabled robust statistical analyses.