Water Content of Delivered Gases During Non-Invasive Ventilation in Healthy Subjects

Water content of delivered gases during non-invasive ventilation in healthy subjects.

François Lellouche 1,2, Salvatore Maurizio Maggiore 1,3, Aissam Lyazidi 1, Nicolas Deye 1,4, Solenne Taillé 1, Laurent Brochard 1

Electronic supplementary material

Corresponding author: Dr François Lellouche

Institut universitaire de cardiologie et de pneumologie

Centre de recherche de l'Hôpital Laval, Université Laval

2725, chemin Sainte-Foy, G1V4G5, Québec, Canada

Phone: (418) 656.87.11 poste 3298

Fax: (418) 656.45.09

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Method

The study was conducted in three phases: an in vitro bench evaluation of HH performances with NIV settings, and two sets of in vivo measurements in healthy subjects during NIV and CPAP. Subjects were blinded for the humidification strategy used when comfort evaluation was conducted (CPAP study) and period order was randomized. The protocol was approved by the Ethics Committee of the Société de Réanimation de Langue Française (SRLF).

1) Bench evaluation of HH performances with NIV settings

Similar to what was previously reported during invasive ventilation (17), a bench evaluation of heated humidifiers set in NIV mode was conducted to assess the impact of temperature on HH performances. Briefly, we previously reported that, because of the regulation system of HH, the higher was the temperature at the inlet of the HH, the lower was the water content of the gas delivered to the patient. This assessment was repeated here with specific NIV settings on the HH as recommended by manufacturers. Inspired gas hygrometry was measured with the ventilator and airway circuit connected to a test lung under the following conditions known to influence temperature at entry in a HH, as previously described (17):

• Two ambient air temperatures were tested: normal, 22-24°C, and high, 28-30°C.

• Two ventilators delivering gas at two different temperatures were used: moderate (close to 30°C, Evita 4; Dräger Medical, Lübeck, Germany) and high output temperature (close to 40°C, T-Bird, Viasys Healthcare, Conshohocken, PA, USA) (17). The T-Bird was studied at FiO2 of 21 and 100% (thus varying the percentage of ambient air from 100% to 0%), and Evita 4 was set at 21% of FiO2. Wall medical gases contain around 3 to 5 mgH2O/L (personal data).

• Two levels of minute ventilation: low (RR: 25/min; Vt:400 ml = 10 L/min) and high (RR: 30/min; Vt: 600 ml = 21 L/min).

• The following heated wire heated humidifiers were tested: Aerodyne Ultratherm (Kendall, Tyco Healthcare, Mansfield, MA), MR 730 (Fisher&Paykel) and MR 850 (Fisher&Paykel) with compensation algorithm activated (software 560), The compensation algorithm allows an automatic increase of the humidification chamber temperature when the heater plate does not deliver enough energy to humidify gases as calculated by the system. The following settings were used(humidity chamber temperature / Y-piece temperature): MR 730: 31/34°C, 34/34°C, MR 850«NIV position» with compensation, Aerodyne 31/33°C, 33/33°C.

• Several other conditions were tested (no humidification, use of a long dry line).

Three psychrometric measurements were obtained for each condition at steady state after 3 hours. This technique uses two one-way valves to separate inspiratory and expiratory gases as previously described (18). Temperature and hygrometry of ambient air were measured each day and room temperature was maintained constant.

In total, 28 conditions were tested with heated humidifiers, in addition to the measurements of ambient air (n=45) and inspired gas without humidification (n=24).

2) Healthy subjects ventilated during NIV (Figure 1)

12 healthy subjects were ventilated with NIV using a bucco-nasal mask in the following conditions:
• 3 type of humidifications: heated humidifier (HH) set on “NIV” position (target temperatures are 31°C at humidification chamber and 34°C at Y-pieces) (MR 850® (Fisher&Paykel, Auckland, New Zealand) used without compensation system), heat and moisture exchanger (Hygrobac®, Tyco Healthcare) (HME) or no humidification (NoH).
• Three types of ventilation using an intensive care unit (ICU) ventilator or a turbine ventilator at two FiO2:

- an ICU ventilator (PB 840, Tyco Healthcare, Mansfield, MA, USA) with FiO2 set at 21% using wall dry medical gases

- a turbine ventilator (T-Bird) with FiO2 set at 21%, i.e., with gas taken from ambient air (12 to 15 mgH2O/L)

- a turbine ventilator (T-Bird) set at 50% of FiO2, i.e., with a mixture of medical gases and ambient air (8 to 10 mgH2O/L).

Each subject took part in the study over 3 different days, one day for each ventilator (PB 840, T-Bird 21% and T-Bird 50%). The order of the studied ventilators and of the type of humidification was randomized from predetermined randomization tables. Ventilation was delivered with pressure support at a level of assistance between 8 and 12 cmH2O to obtain an optimal comfort. No PEEP was applied.

• Each experiment was conducted with no or minimal leakage and in the presence of 50% of leaks: this was adjusted from the monitoring of expired (Vte) and inspired tidal volume (Vti), to reach a fraction of leaks (Vti-Vte/Vti) close to 0 or 50%). Leaks were created by placing a small tube (similar to a naso-gastric tube) between the mask and the volunteer skin to reproduce the clinical conditions.

• Last, experiments were repeated during normal minute ventilation and a period of hyperventilation around 20L/min. Hyperventilation was conducted only with the PB840 ventilator without leaks and lasted 5 minutes after a 15 minutes period of normal ventilation..

For each period, hygrometric measurements of inspired gases were performed after 15 minutes of ventilation using the psychrometric method (18). Temperature and hygrometry of ambient air were measured each day and room temperature was maintained constant.

In total, 21 conditions were tested.

3) Healthy subjects during CPAP

Six healthy subjects were given high flow CPAP (90 to 150 L/min) (Vital Signs, Inc., Littlehampton, UK) with bucco-nasal masks without leaks at 5 and 7.5 cmH2O of pressure, with FiO2 set at 21 and 50% and with different humidification strategies. Each period lasted 10 minutes with a “wash-out” period of 5 minutes between the different ventilation periods. Three humidification strategies were assessed: no humidification (NoH), HH with HC 100, a HH without heated wire, proposed for home ventilation (Fisher&Paykel, Auckland, New Zealand) set at maximal intensity (i.e. position 9) and HH with MR 850 (Fisher&Paykel, Auckland, New Zealand) set on “NIV” position (target temperatures being 31°C at humidification chamber and 34°C at Y-piece) with activated compensation system. The subjects were blinded for the humidification device used. At the end of each period, hygrometric measurements of inspired gases were performed and comfort related to mucosal dryness was assessed on a 0 (extreme mucosal dryness) to 10 (no mucosal dryness) visual scale. The question asked was «on this scale, how do you grade comfort related to dryness of nose and mouth». Comfort was assessed before humidity measurements.

Statistics

Results are expressed as mean ± standard deviation. Analysis of variance was first performed with Friedman test, and two-by-two comparisons were obtained by Wilcoxon signed ranks test. A value of p < 0.05 was considered significant.

Results

Healthy subjects during NIV - hyperventilation data

Hyperventilation was achieved with minute-ventilation above 20L/min in all subjects but did not modify the water content of inspired gases with the various strategies of humidification (Table E1, Figure E1).

Table E1: Healthy subjects breathing pattern with different humidification devices during normal ventilation and during hyperventilation.

RR: respiratory rate, VTe: expiratory tidal volume, VE: minute ventilation

Figure E1: Impact of hyperventilation on humidification during NIV

Absolute humidity (in mgH2O/L) of inspiratory gases during NIV during normal ventilation in comparison with hyperventilation. Data were obtained with the ventilator PB840.

HH: heated humidifier; HME: heat and moisture exchanger; NoH: no humidification

FigureE2: Problems associated with the regulation of the outlet chamber temperature in heated wire heated humidifiers with "NIV" settings.

In the most favorable situation (A), inlet chamber temperature is low and the gradient between the two temperatures is +1 °C, the heater plate must increase the gas temperature. In the unfavorable situation (B), inlet chamber temperature is low and the gradient between the two temperatures is -14 ° C, the heater plate stops to heat to attain the target temperature. With the settings recommended during NIV, the gradient is even more unfavorable than setting for intubated patients.

Figure E1: Impact of hyperventilation on humidification during NIV

FigureE2