Anaesthetic ventilators

Overview & definition

The anaesthetic ventilator is the component of the anaesthetic machine that delivers the controlled mechanical ventilation to the intubated, apnoeic patient during the general anaesthesia. It is distinct from the intensive-care ventilator in its history, its design, and its integration: the anaesthetic ventilator is built into the anaesthetic machine, it drives the gas through the circle system (carrying the volatile agent and the oxygen), and it was, for decades, a simple pressure or volume generator rather than the sophisticated lung-protective device of the modern ICU. The modern anaesthetic ventilator has closed the gap and now offers many of the modes of the ICU ventilator, but the core principles remain those of the Primary physics.[1][2]

The function of the anaesthetic ventilator

The ventilator performs three tasks simultaneously during the controlled ventilation: it moves the gas into and out of the lungs at a set tidal volume and rate; it mixes and delivers the fresh gas (the oxygen, the air, the nitrous oxide, the volatile agent) through the breathing system; and it removes the carbon dioxide through the circle absorber. Unlike the ICU ventilator, which is a pure gas-delivery device driving compressed air and oxygen, the anaesthetic ventilator must preserve the accuracy of the volatile agent delivery and the economy of the low-flow anaesthesia while ventilating. This integration is the reason the anaesthetic ventilator has specific design constraints.[2]

The drive mechanism — bellows, piston, turbine

The ventilator generates the inspiratory flow by one of three drive mechanisms:[1][2]

• The bellows ventilator (the bag-in-bottle). A bellows (a collapsible container) holds the anaesthetic gas; the ventilator drives the bellows by compressing a driving gas (usually the oxygen or the air) around it within a sealed chamber (the bottle). The bellows is squeezed, expelling the anaesthetic gas into the patient. This is the classic anaesthetic ventilator design, used in most traditional machines.
• The piston ventilator. A motor-driven piston in a cylinder draws in and expels the gas. The piston delivers a precise tidal volume without a driving gas, and it is electrically powered. The piston ventilator is accurate and efficient and is used in many modern machines.
• The turbine ventilator. A high-speed turbine (a rotating impeller) generates the gas flow. The turbine-driven ventilator is compact, responsive, and can deliver the spontaneous and the supported modes well; it is the basis of the modern anaesthetic machines that approach the ICU ventilator in capability.[1]

The hanging versus the standing bellows

In the bellows ventilator, the bellows moves with the ventilation, and its orientation is an exam point and a safety feature:[2]

• The standing (ascending) bellows rises during the expiration (the returning gas fills it) and falls during the inspiration (the driving gas compresses it). A leak disconnects the bellows — it collapses and does not refill, so a standing bellows is a leak detector: if there is a leak or a disconnect, the bellows falls and may not rise, immediately signalling the fault. The standing bellows is the safer design and is now standard.
• The hanging (descending) bellows falls during the expiration and rises during the inspiration. A leak can be masked: the bellows continues to move even with a significant leak, because the weight of the bellows pulls gas in. The hanging bellows is therefore a poor leak detector and is now obsolete in the new machines.[2]

Volume-controlled ventilation

In the volume-controlled ventilation (VCV), the ventilator delivers a set tidal volume with each breath, regardless of the lung compliance and the resistance. The flow is constant (the decelerating flow is the pressure mode), and the inspiratory pressure is the dependent variable — it rises as the compliance falls or the resistance rises. The VCV guarantees the minute volume (the tidal volume times the rate), which makes it the default for the controlled ventilation in the theatre, but it delivers a higher peak pressure into a stiff lung, and the tidal volume may be distributed unevenly in the diseased lung (the preferential inflation of the more compliant regions).[1][6]

The practical points: the set tidal volume is not necessarily the delivered alveolar volume, because the circuit compliance and the gas compression (the compressible volume) steal a fraction of the set volume. The modern ventilator compensates for the circuit compliance so the displayed tidal volume reflects the patient's actual ventilation.[2]

Pressure-controlled ventilation

In the pressure-controlled ventilation (PCV), the ventilator applies a set inspiratory pressure above the PEEP for a set inspiratory time. The flow is decelerating (it is highest at the start and falls as the lung fills), and the tidal volume is the dependent variable — it depends on the compliance and the resistance. The PCV delivers a lower peak pressure for the same tidal volume (the pressure is held flat, not spiked), it distributes the gas more evenly in the diseased lung (the decelerating flow favours the slow-filling regions), and it is the preferred mode for the poor-compliance lungs (the obesity, the ARDS, the one-lung ventilation, the laparoscopy).[1]

The trade-off: the tidal volume is not guaranteed — if the compliance falls (the patient is moved, the surgeon leans on the abdomen), the tidal volume falls and the ventilation is reduced. The PCV requires the continuous monitoring of the expired tidal volume.[4]

Dual and adaptive modes — PCV-VG, pressure support, spontaneous

The modern anaesthetic ventilator offers the dual-control and the adaptive modes that combine the guarantees of the volume mode with the benefits of the pressure mode:[4]

• The pressure-controlled ventilation-volume guaranteed (PCV-VG, also called the pressure-regulated volume control or the adaptive flow). The ventilator delivers a decelerating flow (the pressure-control pattern) but adjusts the inspiratory pressure breath-by-breath to achieve the set tidal volume. It gives the low peak pressure and the even distribution of the PCV with the volume guarantee of the VCV. The randomised trial evidence shows it improves the intraoperative respiratory mechanics and may reduce the postoperative pulmonary complications versus the pure VCV.[4]
• The pressure-support ventilation (PSV). The patient breathes spontaneously, and the ventilator augments each spontaneous breath with a set pressure support. This reduces the work of breathing during the spontaneous ventilation in the theatre (the emergence, the LMA cases, the weaning).[1]
• The spontaneous modes. The modern anaesthetic ventilator can sense and support the spontaneous efforts (the pressure trigger or the flow trigger), allowing the smooth transition from the controlled to the spontaneous ventilation at the emergence.[1]

The ventilator controls

The principal controls on the anaesthetic ventilator:[2]

• The tidal volume (VT) — the volume per breath, set in the VCV and the PCV-VG, monitored in the PCV.
• The respiratory rate — the set number of the breaths per minute.
• The inspiratory pressure — the pressure above the PEEP, set in the PCV.
• The inspiratory-to-expiratory ratio (I:E ratio) — the ratio of the inspiratory time to the expiratory time, usually 1 to 2. The inverse ratio (the prolonged inspiration) is used for the severe oxygenation failure.
• The PEEP (the positive end-expiratory pressure) — the pressure maintained at the end of the expiration to prevent the alveolar collapse. The routine application of the low PEEP (5 cmH2O) is part of the lung-protective ventilation.
• The trigger — the sensitivity at which the ventilator detects a spontaneous effort (the pressure or the flow trigger), used in the supported modes.
• The inspiratory pause — a brief pause at the end of the inspiration, used to measure the plateau pressure (the true compliance pressure, distinct from the peak pressure). [1]

The circle-system interaction

The anaesthetic ventilator drives the gas through the circle system, and this interaction imposes specific considerations. The circle contains a large volume of gas (the tubing, the absorber, the bellows), so the circuit compliance and the gas compression absorb a fraction of the set tidal volume (the compressible volume loss, about 3 mL per cmH2O of the peak pressure for the adult circuit). The modern ventilator measures and compensates for this loss so the displayed and the delivered tidal volumes match. A circuit leak (the loose connection, the cuff leak) causes the bellows to under-fill; the standing bellows makes this obvious. The ventilator must also work correctly with the low fresh gas flows (the economy and the closed-circuit anaesthesia), and with the volatile agent in the circuit.[2]

Anaesthesia versus ICU ventilator — the key differences

The anaesthetic ventilator and the ICU ventilator differ in several important respects, summarised by the comparative reviews:[1]

• The integration. The anaesthetic ventilator is built into the anaesthetic machine and ventilates the circle system carrying the volatile agent; the ICU ventilator is a standalone device driving the compressed air and oxygen.
• The driving gas. The traditional bellows anaesthetic ventilator uses a driving gas (the oxygen); the ICU ventilator uses the proportional valves or the turbine.
• The modes. The ICU ventilator has long offered the full range of the spontaneous, the supported, and the adaptive modes; the anaesthetic ventilator historically offered only the controlled modes, but the modern turbine and the piston anaesthetic ventilators now approach the ICU capability.[1]
• The monitoring. The ICU ventilator monitors the full lung mechanics (the compliance, the resistance, the driving pressure, the P-V loops); the anaesthetic ventilator historically monitored only the airway pressure and the tidal volume, but the modern machines now display the advanced mechanics.
• The use. The anaesthetic ventilator is for the short-term, the intraoperative ventilation of the relatively healthy lung; the ICU ventilator is for the prolonged ventilation of the diseased lung.[1]

Lung-protective ventilation in the operating theatre

The evidence has established the lung-protective ventilation as the standard for the intraoperative ventilation, reducing the postoperative pulmonary complications. The protective bundle is the low tidal volume (6 to 8 mL per kg of the predicted body weight), the moderate PEEP (5 to 8 cmH2O), and the avoidance of the high driving pressure and the hyperoxia. The randomised trials show that the protective ventilation improves the postoperative pulmonary function and reduces the complications, especially in the abdominal and the thoracic surgery.[6][5]

The predicted (not the actual) body weight is used for the tidal volume, because the lung size scales with the height, not the total body weight. The obese patient is therefore at risk of the excessive tidal volume if the actual weight is used — a common error that raises the driving pressure and the complication rate.[5]

The driving pressure and the postoperative complications

The driving pressure (the plateau pressure minus the PEEP, or the tidal volume divided by the compliance) is the key mediator of the lung injury during the intraoperative ventilation. The large international cohort studies show that the driving pressure, more than the tidal volume or the PEEP alone, is associated with the postoperative pulmonary complications: a driving pressure above about 16 cmH2O is associated with the increased risk. The most recent randomised trials test the personalised driving-pressure-guided PEEP titration, which further reduces the complications compared with the fixed PEEP.[5][7]

The practical implication: in the volume control, watch the plateau pressure and the driving pressure, not just the peak pressure. If the driving pressure rises, reduce the tidal volume or the PEEP, or switch to the pressure-controlled or the adaptive mode that targets a low driving pressure.[5][4]

The pediatric considerations

The ventilation of the anesthetized child has specific features: the smaller tidal volumes are more sensitive to the circuit compressible-volume loss (a larger fraction of the set volume is lost in the circuit), so the circuit compliance compensation is essential; the uncuffed tubes (historically used in the small children) leak, and the volume mode must account for the leak; the rate is higher and the tidal volume is smaller (6 to 8 mL/kg); and the small airway and the low functional residual capacity make the rapid desaturation and the atelectasis prominent. The pressure mode (or the PCV-VG) is often preferred in the pediatric anaesthesia because it compensates for the leak and the varying compliance. The modern anaesthetic ventilator with the turbine or the piston drive and the sensitive trigger ventilates the small child accurately.[3]

[1] [1]

Red flags

[1] [1] [1] [1] [1]

References

1. [1]Jackson K, Zhu H, Yu X, Liu J, Hochman SE. Review of Anesthesia Versus Intensive Care Unit Ventilators and Ventilatory Strategies: COVID-19 Patient Management Implications AANA J, 2021.PMID 33501910
2. [2]Coisel Y, Constantin JM, Jung B, Jaber S. How to choose an anesthesia ventilator? Ann Fr Anesth Reanim, 2014.PMID 25138358
3. [3]Feldman JM. Optimal ventilation of the anesthetized pediatric patient Anesth Analg, 2015.PMID 25625261
4. [4]Liu Z, Wang Y, Chen X, et al. Effect of pressure-controlled ventilation-volume guaranteed mode on intraoperative respiratory mechanics, oxygenation and postoperative pulmonary complications: a systematic review and network meta-analysis Ann Med, 2025.PMID 41157953
5. [5]Neto AS, Hemmes SNT, Barbas CSV, et al. Association between driving pressure and development of postoperative pulmonary complications in patients undergoing mechanical ventilation for general anaesthesia: a meta-analysis of individual patient data Lancet Respir Med, 2016.PMID 26947624
6. [6]Severgnini P, Selmo G, Lanza C, et al. Protective mechanical ventilation during general anesthesia for open abdominal surgery improves postoperative pulmonary function Anesthesiology, 2013.PMID 23542800
7. [7]Futier E, Pereira B, Jaber S, et al. Personalized driving pressure-guided positive end-expiratory pressure in patients at risk of postoperative respiratory failure (IMPROVE-2): a multicenter, pragmatic, randomized clinical trial Intensive Care Med, 2025.PMID 40839096