## 1 Introduction  
Closed suctioning was originally introduced for hygiene reasons and as a method of avoiding desaturation and reduction in lung volume during suctioning. In a closed suctioning system (CSS), the catheter is a part of the ventilator circuit and there is no need to disconnect the ventilator. Continuing connection to the ventilator helps prevent loss of both positive end-expiratory pressure (PEEP) and lung volume [1, 2]. Thus, it may enable volume recruitment in the lung and avert a drop in oxygenation. CSS can thus reduce the risks of hypoxemia, atelectasis, and hemodynamic fluctuations [3].  
During open endotracheal suctioning and disconnection of a ventilator, patients may be exposed to a sudden unintended withdrawal of PEEP, which may induce repeated lung derecruitment and hypoxia [4; 5; 6]. However, CSS can prevent alveolar derecruitment and maintain appropriate oxygenation through a steady functional residual capacity (FRC) [3; 4]. Regional lung derecruitment after endotracheal suction has been measured by electrical impedance tomography. The results demonstrated that FRC decreased by 58 \(\pm\) 24% of baseline at disconnection and 22 \(\pm\) 10% further during open suctioning [7].  
This study investigates an important question: whether ventilator-related parameters are maintained during a closed suctioning procedure. These procedures induce an increase in airway resistance (R\({}_{\text{aw}}\)), resulting from the insertion of a suction catheter (SC) into an endotracheal tube (ETT) with a narrower area. During mechanical ventilation, the peak inspiratory pressure (PIP) is higher to overcome the increased resistance. PIP is a predictor for pulmonary barotrauma in the indications for mechanical ventilation [8; 9]. Barotrauma is a well-recognized complication of mechanical ventilation and frequently occurs in patients with a wide range of underlying pulmonary conditions [10].  
The primary disease in the decline of pulmonary mechanics is divided into obstructive and restrictive respiratory diseases. Mechanical ventilation support is often needed when pulmonary mechanics cannot function properly. Obstructive diseases can cause unstable airways that are prone to collapse, leading to difficulty in exhaling full tidal volume before the next breath, known as dynamic hyperinflation, which occurs in conditions such as asthma and chronic obstructive pulmonary disease (COPD) [9; 11]. Mechanical ventilation can alleviate this problem by reducing minute volume and extending expiratory time. In some cases, it is necessary to increase the external PEEP for ventilator synchronization [12; 13; 14]. However, little research has been carried out on the use of CSS in obstructive disease patients for endotracheal suctioning.  
Furthermore, restrictive diseases impair compliance in pulmonary mechanics. Acute respiratory distress syndrome (ARDS) features severe inflammation caused by impairment of lung compliance [15; 16]. Mechanical ventilation is an essential life support for these patients; it maintains the openness of the lungs, minimizing lung injury caused by repetitive alveolar collapse and overdistention [13; 17; 18]. Clinical management aims to significantly reduce end-expiratory lung volume via CSS to avoid repetitive alveolar collapse and oxygen desaturation during suctioning [4]. However, guidelines for clinical endotracheal suctioning practices are needed in regard to how to ventilate the critically ill, with or without ARDS [13; 19; 20].  
Therefore, clinical trials have focused on those patients who can benefit clinically from CSS. However, airway disease or lung tissue disease will affect real-time ventilator-related parameters during the CSS suction procedure, at a scale that is yet to be defined. This study aimed to determine the effects of endotracheal tube size, suction catheter size, different modes (VC-CMV and PC-CMV), and pulmonary mechanics (obstructive and restrictive) on ventilator-related parameters. Suction was performed on a simulated lung model with operating ventilation mode.