Influence of Two Different Interfaces for Noninvasive Ventilation Compared to Invasive Ventilation on the Mechanical Properties and Performance of a Respiratory System: Equipment
CPAP and NPSV were performed with a helmet (Starmed Castar R; Mirandola; Modena, Italy) or a face mask (King Systems Corporation; Noblesville, IN) put on a glass head that was connected to a lung model (Fig 1). The Castar R Helmet (size medium) has an internal volume of 7.5 L with inflated cuffs. When the head is inserted into the helmet, the internal volume is reduced to approximately 2.4 L due to the volume of the glass head used in this study. Two underarm laces attached to a ring at the lower site of the helmet should prevent the helmet from lifting when it is inflated. A plastic collar fitting around the neck prevents leakage during ventilation. Inspiratory and expiratory tube connectors are fitted in the upper part of the helmet.
The standard face mask (size medium, 125-mL internal gas volume) has an inflatable cushion fitted tightly to the head by dedicated rubber head straps. The cushion was inflated with 10 to 20 mL of air to adhere tightly to the glass head. When the mask is put on the head, the internal gas volume is reduced to approximately 100 mL.
For invasive ventilation, the lung model was connected to the ventilator via an endotracheal tube (Portex, 7.5 mm; Portex Ltd.; Kent, UK). CPAP and PS ventilation were performed using a conventional ICU ventilator used in our ward capable of invasive ventilation and NIV (Evita 4; Drager Medical; Lubeck, Germany). Link
Gas flow was measured with a pneumotachometer (Fleisch II; Fleisch; Lausanne, Switzerland) at the inspiratory side of the helmet (Fig 1). The ventilator and lung model were connected by standard disposable ventilator tubes (B&B Beatmungsprodukte GmbH; Neunkirchen, Germany). Flow signals were stored on a personal computer using an analog-digital converter, and the signals were integrated to obtain volume during off-line evaluation. The pneumotachometer was calibrated by a mass flowmeter (TSI 4040 D; TSI Inc.; Shoreview, MN). Airway pressure (Paw) was measured at the inspiratory side before the helmet with differential pressure transducers (Sensortechnics; Puchheim, Germany), adjusted meticulously during zero flow conditions before each measurement.
Figure 1. Illustration of the experiment. Left: Lung model 1: modified LS1500 (Drager Medical), capable of simulating spontaneous breathing. Pressure transducers were installed at the entrance of the helmet in order to measure DelayTRlGGER and DelayPEEP. Right: Lung model 2: Lung model (TTL 5600I; Michigan Instruments) to measure desynchronization between the simulated patient and the lung. Pressure transducers were installed at the entrance of the helmet and at the connection to the lung compartment.