Anesthesia ventilators are crucial medical devices for supplying inhalational gases and anesthetic agents to a patient, while also providing mechanical ventilation, a life-sustaining approach for those undergoing general anesthesia with respiratory failure. While various gas delivery mechanisms, one in particular, ascending bellows, offers many benefits on its own, as well as when combined with low-flow anesthesia.
Before discussing the individual advantages of ascending bellows technology, let’s first look at the difference between ascending and descending bellows.
Ascending versus descending bellows
Closed-circuit ventilators can be a single- or double-circuit design. The single circuit utilizes a piston to move the gases, which requires electricity as the driving force. In a single circuit, the leak port is also always open to atmosphere. Therefore, pressure is generated based on the flow in the circuit and resistance through the leak port. This makes leak integral to the function of the ventilator.
On the other hand, in a double circuit design, the valves are within the ventilator. These types of designs consist of a primary, or driving circuit (bottle or box) and a secondary or patient circuit (a bag or a bellows or membrane chambers). The bellows-in-bottle ventilator is one type of double-circuit ventilator, which can be divided further into ascending or descending bellows based on their movement during expiration.
Ascending bellows rise during expiration, while descending bellows fall during expiration.1 Most modern anesthesia machines utilize ascending bellows because they are thought to improve patient safety in the case of a circuit disconnection. Descending bellows, which were believed to be a safety hazard, fell out of favor in the 1970’s and 1980’s.4
In anesthesia machines with ascending bellows, when a circuit disconnection occurs, the bellows will not fill, the bellows collapse due to loss of the pressure inside them and gravity, the low pressure alarm goes off and the collapsed bellows can be easily visualized.4
However, with descending bellows, if there is a complete circuit disconnection, the bellows will elongate when there is a loss of pressure and volume due to gravity. The bellows look full and continue to rise and fall, and the disconnection goes unrecognized4. Additionally, with descending bellows, a disconnect may not result in a large enough pressure change to trigger the low-pressure alarm.4
Advantages of Ascending Bellows Technology
There are many advantages to the ascending bellows circuit design, such as:
Immediate Leak Detection
Ascending bellows offers visible up/down movement of the bellows that shows proper ventilation and allows for immediate leak identification. This is crucial because leaks that are not promptly solved can potentially lead to hypoventilation, desaturation and patient awakening if volatile anesthesia is given.
This ability for leak detection is especially important in cases of:
- patient repositioning
- laryngeal masks
- thoracic surgeries
- neonatal and pediatric ventilation where the tube is uncuffed
While descending bellows is another type of double-circuit ventilator, it does not offer these same leak detection abilities. Unlike with ascending bellows, descending bellows do not collapse when a leak is present. Therefore, there is no immediate visualization of missed circuit pressurization/missed ventilation as there is with the ascending bellows. This means that the descending bellows cannot act to detect leaks or circuit integrity.3
Visibly See Ventilation
The visible up/down movement seen with ascending bellows provides reassurance that mechanical ventilation is being delivered, no matter the patient's condition. With the ability to visualize if ventilation stops, the care team can rest assured they would know and can then react appropriately to fix the issue and ensure patient safety.
This ability to visibly see ventilation is especially helpful for anesthetic providers choosing a lower fresh gas flow (FGF) rate, a decision that offers many benefits to the patient, facility, and environment.
With visible bellows, the physician is able to see, breath-by-breath, if the patient has been supplied the correct FGF for their current conditions.
Less Electricity Consumption
In the case of ascending bellows, ventilation is prompted by the driving gas and bellows movement, meaning there is no need for electricity to deliver the expansion of the bellows. This leads to lower electricity consumption.
Furthermore, by choosing air as the driving gas, gas costs for device utilization are minimal.
No Risk of Gas Dilution
With other vent technologies, room air can become entrained as the piston returns to the filled position, which could then cause the patient gases to become diluted with room air. This is even a possibility with descending bellows, but is not a concern with ascending bellows and decoupled circuit.
Ascending bellows are an ideal ventilation technique because of its ability to visibly see and monitor ventilation on a breath-by-breath basis. Other advantages of ascending bellows technology, such as immediate leak detection and decreased electricity consumption, make it an advantageous choice for supporting patient safety and financial outcomes, lowering environmental impact (no electricity use) and reducing operating costs outcomes.
- Baker, A.B. Low flow and closed circuits. Anaesth Intensive Care (1994) 22:341-2
- Jain, R., & Swaminathan, S. (2013). Anaesthesia ventilators. Indian Journal Of Anaesthesia, 57(5), 525. doi: 10.4103/0019-5049.120150
- GE HealthCare. (2006). Clinical Advantages of a Visible Standing Bellows During Low Flow Anesthesia. PDF file.
- Otteni, J. C., Beydon, L., Cazalaà, J. B., Feiss, P., & Nivoche, Y. (1997). Ventilateurs d'anesthésie [Anesthesia ventilators]. Annales francaises d'anesthesie et de reanimation, 16(7), 895–907. https://doi.org/10.1016/s0750-7658(97)89839-5
- “Anesthesia Machine Ventilators.” Anesthesia Key, 21 December 2016, https://aneskey.com/anesthesia-machine-ventilators-2/. Accessed 22 March 2023.
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