The use of positive end-expiratory pressure (PEEP) and alveolar recruitment manoeuvres (ARMs) has become a promising treatment option for perioperative alveolar collapse and subsequent hypoxaemia in equine patients. Unfortunately, only the newest generation of large animal ventilators, not using the bag-in-the-box principle, are able to automatically create sufficiently high PEEP levels for horses. To create a large range of PEEP in the commonly used bag-in-the-box ventilators and breathing circuits, threshold resistor valves can be added. These devices built up an adjustable PEEP by creating a force against the expiratory gas flow, thereby establishing a constant threshold pressure. Examples are the underwater seal, the spring-loaded threshold valve and the magnetic valve. However, there are some important aspects that need to be considered when using these devices. The location of both the threshold resistor and the PEEP measuring device within the circuit will determine, if PEEP can be detected within the entire breathing system. Furthermore, the time it takes to establish a stable PEEP within the system depends on the type of the threshold valve, and may be influenced by several other factors, e.g. the total gas volume in the circle, the type of ventilator used and the individual horse's lungs. In an in vitro study using an artificial lung the onset of PEEP was dependent on the fresh gas flow used the higher the fresh gas flow, the faster the onset of PEEP. However, in vivo, the onset of PEEP was independent of the fresh gas flow and was, compared to the in vitro study, significantly slower. It is therefore paramount to monitor the pressures within the circuit and carefully observe the time it takes to build up PEEP for each individual horse.