Objectives: We sought to investigate factors that affect the difference between the peak inspiratory pressure measured at the Y-piece under dynamic flow conditions and plateau pressure measured under zero-flow conditions (resistive pressure) during pressure controlled ventilation across a range of endotracheal tube sizes, respiratory mechanics, and ventilator settings. Design: In vitro study. Setting: Research laboratory. Patients: None. Interventions: An in vitro bench model of the intubated respiratory system during pressure controlled ventilation was used to obtain the difference between peak inspiratory pressure measured at the Y-piece under dynamic flow conditions and plateau pressure measured under zero-flow conditions across a range of endotracheal tubes sizes (3.0-8.0 mm). Measurements were taken at combinations of pressure above positive end-expiratory pressure (10, 15, and 20 cm H 2 O), airway resistance (no, low, high), respiratory system compliance (ranging from normal to extremely severe), and

C ruise ships have long been associated with an increased risk for outbreaks of infectious diseases, illustrated by transmissions of respiratory pathogens and pathogenic microorganisms spreading by the fecal-oral route (1-5). The special circumstances on a cruise ship with crowded, confined spaces, where fresh air supply is sometimes limited, contributes to the risk for spreading airborne pathogens (6-9). An additional factor is that the passengers on cruise ships are in general of older age and therefore more susceptible to infections (10).The consequences of an outbreak of an infectious disease on a seafaring cruise can be massive (11)(12)(13).At every port, exchange of passengers occurs, leading to a new risk for introduction of a contagious disease. Besides the financial and commercial consequences, the distance to medical facilities is sometimes considerable, which hinders medical consultation and eventual hospitalization. Therefore, companies organizing seafaring cruises take extensive measures to reduce risks by appointing medically trained personnel, installing care facilities on board, and training personnel to be vigilant about presence of symptomatic passengers that might point to infectious diseases. In addition, prevention plans, outbreak protocols, and procedures for early contact with port health authorities, consistent with provisions of the Maritime Declaration of Health, Annex 8 of the World Health Organization International Health Regulations (14), should be installed (15,16).In contrast to the preparedness for seafaring cruises, only limited attention is given to those risks on river cruise ships (17), although there are many characteristics in common with the larger seafaring cruise ships. In general, river cruises are subject to less regulation concerning medical preparedness (expertise and facilities) because of proximity of shorebased facilities. It has been reported that the number of (river) cruises was increasing worldwide before the pandemic (18). Therefore, closer attention is justified.The outbreak of infection with SARS-CoV-2 on the seafaring cruise ship Diamond Princess in early 2020 gained worldwide attention (19), and many studies were directed at conditions on the ship and handling of viral spread among passengers (20-24). During the COVID-19 pandemic, additional outbreaks on boats and seagoing cruise ships were reported (12,17,25). Sekizuka et al. ( 26) reported a SARS-CoV-2 outbreak at a river-cruise ship sailing the Nile. However, there is only limited awareness of the risk for spread and handling of airborne pathogens on river cruise ships (27).