The study was performed at the Respiratory Mechanics Lab (Ventilab) of the Fondazione Policlinico Universitario A. Gemelli IRCCS, Università Cattolica del Sacro Cuore in Rome, Italy.
Non-invasive CPAP and non-invasive positive pressure ventilation delivered in PSV mode were applied to a mannequin (LaerdalMedical AS, Stavanger, Norway) connected to an active test lung system (ASL 5000; Ingmar Medical, Pittsburgh, PA) set using a single-compartment model, an active inspiration simulated by a semi-sinusoidal pressure waveform (Rise Time 15 %, Pause 0 % and Release Hold 25 %) and the following mechanical properties of the respiratory system : resistance 5 cmH2O/l/s and compliance 40 ml/cmH2O.
nCPAP was applied via Helmet (H) (CPAP-Castar Starmed, Mirandola, Italy), PerforMax Full face mask (Philips Respironics, Murrysville, PA, USA) (RFF) and a modified full face snorkeling mask (MFF)(SEA VU DRY, Mares Spa, Rapallo, Italy), while non-invasive PSV was delivered through RFF and MFF. The Helmet used for this bench study is a transparent latex-free polyvinylchloride hood, joined by a rigid plastic ring to a soft collar and secured by two padded armpit braces at four hooks (two in the front and two in the back of the plastic ring). The helmet used was the size Small to attain a good seal and avoid air-leaks.
The modifed full face snorkeling mask differs from Performax full face mask for shape and design characteristic; it presents a complete separation between inspiratory and expiratory circuits with the following main features: hypoallergenic silicone mouth-and-nose pocket connected to a polycarbonate transparent main body; quick-release buckles for easy doffing and a polycarbonate Charlotte valve with an inspiratory and an expiratory channel. See figure 1 for details. MFF presents two parallel connections with a complete separation between inspiratory and expiratory limbs, while Performax Full face mask is characterized by a single limb connected to the Y piece. The measure of masks used were medium size for RFF and large size for MFF to attain a good seal and avoid air-leaks.
nCPAP (10 cmH2O) was applied at a simulated respiratory rate (RRsim) of 20 breaths per min (b/min) and a simulated level of inspiratory effort (Pmus) of 12 cmH2O, using a standard CPAP device delivering a ﬂow rate of 60 l/min with reservoir (Drager CF 800 Continous Flow CPAP System; Dragerwerk AG & Co, Lubeck, Germany).
NIV in PSV mode was delivered at 2 RRsim (20 and 30 b/min) and a Pmus of 12 cmH2O with the mechanical ventilator (Puritan Bennet 840; Covidien Health-Care, Mansﬁeld, MA) set in inspiratory pressure support (iPS) of 10 cmH2O, Positive End-Expiratory Pressure (PEEP) of 8 cmH2O, the fastest rate of pressurization, and a cycling-off ﬂow threshold of 25 and 50 % of the peak inspiratory ﬂow. We set the inspiratory flow trigger at the lowest value not determining auto-cycling: this threshold was 5L/min during all conditions tested. This setting was chosen for comparing the performance of these interface under condition of highest pressurization rate and fast or slow cycling-off criteria 11.
Air flow (V′) was measured with a pneumotachograph (Fleish No.1, Metabo, Epalinges, Switzerland), while airway pressure (Paw) was measured by a pressure transducer with a differential pressure of ±100 cmH2O (Digima Clic-1, ICULab system; KleisTek Engineering, Bari, Italy), placed distally from the pneumotachograph. Airflow (V’) and airway pressure (Paw) at the helmet inlet during the inspiratory phase were measured using a pneumotachograph (Fleisch n.2; Metabo, Epalinges, Switzerland) and a pressure transducer with differential pressure of ±100 cmH2O (Digima Clic-1; KleisTEK, ICU-Lab System, Italy) sited at the distal end of the inspiratory limb of the circuit. When the mannequin was ventilated through the RFF, the pneumotachograph and the pressure transducer were positioned at the Y-connection of the ventilator circuit, instead when we tested the MFF the pneumotachograph and the pressure transducer were positioned on the inspiratory channel. All these signals were acquired, amplified, filtered, digitized at 100 Hz, recorded on a dedicated personal computer, and analyzed with a specific software (ICU lab 2.3; KleisTEK Advanced Electronic System, Italy and Analysis Plus).
Each trial lasted 5 minutes; the breaths of the last minute (20 or 30 depending on the trial) were recorded and averaged for analysis.
The measured variables assessed during nCPAP were the maximum inspiratory deflection (∆Pawi, inspiratory drop) and the expiratory peak (∆Pawe), calculated as differences from the preset CPAP level.
During the NIV test, we evaluated the following variables: Ventilator inspiratory and expiratory time (mechanical TI and mechanical TE, respectively), and ventilator rate of cycling were all determined on the flow tracing. The inspiratory duty cycle (mechanical TI/Ttot) was calculated as the ratio between mechanical TI and the total mechanical breath duration (Ttot). Airflow (V′) and tidal volume (VT) delivered to the simulator, airway opening pressure (Paw), and inspiratory muscles effort were displayed online on the computer screen. The signals obtained with the ASL were transmitted to a PC host via 10/100MBit Ethernet, sampled and processed in real time by means of specific software (Lab View, Ingmar Medical). The signals obtained with the ASL were integrated with the signals from the ICULab system by using a specific application of the ICULab (ICULab 2.7, KleisTek). The numerical integration of flow over time determined the mechanical tidal volume (mechanical VT). The amount of tidal volume delivered to the simulator during its active inspiration (ie, the neural tidal volume, VTneu) was calculated as the volume generated from the onset of inspiratory muscle effort negative deflection to its nadir.
Interfaces performance was evaluated using the following parameters11-13:
1)Trigger pressure drop (ΔPtrigger), defined as the pressure swing generated by the simulator inspiratory effort in the airway during the triggering phase; 2) Inspiratory pressure–time product (PTPtrigger), defined as the area under the Paw curve relative to the time between the onset of inspiratory effort and the start of mechanical assistance; 3) pressure–time product at 200 ms from the onset of the ventilator pressurization (PTP200), as the index of pure pressurization performance14; 4)Pressure-time product at 300 ms (PTP300) defined as the integration of Paw over time during the first 300 msec and representing the speediness of the ventilator in reaching the preset level of pressure support; 5) Pressure-time product at 500 ms (PTP500), defined as the integral Paw area over insufflation time from the simulated effort onset, representing the ventilator capability of maintaining the pressurization; 6) PTP500 ideal index, expressed as a percentage of the ideal PTP, which is unattainable because it would imply a trigger pressure drop and an instantaneous pressurization of the ventilator.
Patient–ventilator interaction was evaluated by determining:
1) Pressurization time (Timepress), defined as the time necessary to achieve the pre-set level of pressure support from the baseline value; 2) Inspiratory trigger delay (Delaytrinsp), calculated as the time lag between the onset of inspiratory muscle effort negative swing and the start of the ventilator support (i.e., Paw positive deflection); 3) Expiratory trigger delay (Delaytrexp), assessed as the delay between the end of the inspiratory effort and the end of the mechanical insufflations (i.e., flow deflection); 4) Time of synchrony (Timesync), defined as the time during which inspiratory muscle effort and Paw are in phase (ideally 100%); 5) SimulatorVT/mechanicalVT, intended as the percentage of VT delivered during inspiratory muscle effort negative deflection; 6) The time during which simulator respiratory effort and ventilator assistance were synchronous, indexed to simulated inspiratory time (Timesync/Tineu) was also computed 15-17.7) Wasted efforts, defined as ineffective inspiratory efforts, not assisted by the ventilator; 8) Auto-triggering, namely a mechanical insufflation in absence of inspiratory effort.
Continuous data were expressed as mean ± standard deviation (SD). Categorical data were presented as number and percentage in brackets. All variables were compared with each interface used. Comparison were made by Student’s 𝑡-test and Chi test, as appropriate. The analysis of variance (ANOVA) for repeated measures was used to detect significant differences between the different experimental conditions. When significant differences were detected, post-hoc analysis was performed using the Bonferroni test; p values < 0.05 were considered statistically significant. Statistical analysis was performed using MEDcalc version 18.6.