notizie sull'azienda Anesthesia Ventilators Key Principles Uses and Safety Explained

Imagine a patient on the operating table, their life sustained by a sophisticated machine—the anesthesia ventilator. Each breath delivered, each pressure adjustment, is critical to patient safety and postoperative recovery. But how does one choose a high-performance, reliable anesthesia ventilator to safeguard life? This article delves into every aspect of anesthesia ventilators, from their historical development to cutting-edge technology, working principles, and clinical applications, to help you make an informed decision.

In 1846, the earliest forms of anesthesia relied on simple vaporizers, requiring patients to breathe spontaneously to inhale anesthetic gases. Today, anesthesia ventilators have evolved into highly advanced, automated devices. From the HEG Boyle anesthesia machine developed by Coxeters in 1917 to the Pulmoflator automatic positive-pressure ventilator invented by Blease in 1945, and now to integrated anesthesia workstations with ICU-level ventilation capabilities produced by companies like Dräger and Datex-Ohmeda, anesthesia ventilators have undergone a remarkable transformation.

Modern anesthesia ventilators feature sophisticated computer control systems and multiple improvements to breathing circuits, enabling advanced ventilation support for patients in complex conditions. Below, we explore the classification, working principles, ventilation modes of newer ventilators, and improvements in breathing circuits, along with potential risks associated with ventilator use.

Anesthesia ventilators can be categorized in various ways, including by mechanism of action:

1. Mechanical Thumb Ventilators: These operate on the T-piece principle, generating intermittent positive-pressure ventilation by rhythmically occluding the T-piece. For example, the Sechrist ventilator uses a pneumatic valve instead of an anesthetist’s finger, with the valve’s cycling determined by settings on the ventilator control panel.
2. Minute Volume Divider Ventilators: These deliver pressurized gas to the breathing system, collected in a reservoir bag continuously pressurized by a spring, weight, or elastic recoil. They feature inspiratory and expiratory valves controlled by a "bistable" mechanism. All supplied driving gas is delivered to the patient. For instance, if the fresh gas flow to the patient is 10 L/min, this volume is delivered as minute ventilation but divided into tidal volumes based on ventilator settings (e.g., 10 breaths of 1 L or 20 breaths of 0.5 L). Examples include the East-Freeman, Flomasta, and Manley MP3 ventilators.
3. Bag Squeezer Ventilators: These are typically used with circle or Mapleson D systems. The bag can be squeezed pneumatically (placed in a chamber filled with driving gas) or mechanically (via a motor, gears, levers, springs, or weights). Examples include the Manley Servovent, Penlon Nuffield 400 series, Ohmeda 7800, and Servo 900 series.
4. Intermittent Blow Ventilators: These are driven by a gas source or compressed air at 45–60 psi. Driving gas is usually delivered undiluted to the patient but can be mixed with air, oxygen, or anesthetic gases via a Venturi device. Examples include the Pneupac and Penlon Nuffield 200 series.

Modern anesthesia ventilators can also be classified by power source, driving mechanism, circuit type, cycling mechanism, and bellows type.

Power sources include compressed gas, electricity, or a combination of both. Older pneumatic ventilators required only a pneumatic power source, while modern electronic ventilators need electricity or a combination of electricity and compressed gas.

• Double-Circuit: Bellows ventilators.
• Single-Circuit: Piston ventilators.

Double-circuit ventilators are the most common in modern anesthesia workstations. These feature a cassette-style bellows design, where pressurized driving gas compresses the bellows, delivering ventilation to the patient. Examples include the Datex-Ohmeda 7810, 7100, 7900, and 7000, as well as the North American Dräger AV-E and AV-2+.

Piston ventilators (e.g., Apollo, Narkomed 6000, Fabius GS) use a computer-controlled motor instead of compressed gas to deliver breathing gas. These systems have a single patient gas circuit rather than separate circuits for patient and driving gases.

Most anesthesia ventilators are time-cycled and provide controlled mechanical ventilation. The inspiratory phase is initiated by a timing device. Older pneumatic ventilators used fluidic timing, while modern electronic ventilators use solid-state timing and are classified as time-cycled and electronically controlled.

The direction of bellows movement during expiration determines their classification. Ascending (standing) bellows rise during expiration, while descending (hanging) bellows fall. Most modern anesthesia ventilators use ascending bellows, which are safer. In case of disconnection, ascending bellows collapse and do not refill, while descending bellows continue moving, potentially drawing room air into the breathing system. Some newer systems (e.g., Dräger Julian, Datascope Anestar) use descending bellows with integrated CO₂ apnea alarms for safety.

These ventilators consist of a bellows housed in a transparent rigid plastic chamber. The bellows acts as an interface between the breathing gas and the driving gas. During inspiration, driving gas (pressurized oxygen or air at 45–50 psi) is delivered into the space between the chamber wall and the bellows, compressing the bellows and delivering anesthetic gas to the patient. During expiration, the bellows re-expands as breathing gas flows in, and excess gas is vented to the scavenging system. Ascending bellows designs inherently create 2–4 cm H₂O of positive end-expiratory pressure (PEEP).

Piston ventilators (e.g., Apollo, Narkomed 6000, Fabius GS) use an electric motor to compress gas in the breathing circuit, generating mechanical inspiration. The rigid piston design allows precise delivery of tidal volume, with computer control enabling advanced ventilation modes like synchronized intermittent mandatory ventilation (SIMV), pressure control ventilation (PCV), and pressure support ventilation (PSV).

• Quiet operation.
• No inherent PEEP (unlike ascending bellows ventilators).
• Higher accuracy in delivered tidal volume due to compliance and leak compensation, fresh gas decoupling, and rigid piston design.
• Electricity powers the piston, eliminating the need for driving gas.
• Pressure sensors enable precise volume delivery.

• Loss of the familiar visual feedback of ascending bellows during disconnection.
• Quiet operation may make regular cycling less audible.

When using a ventilator, the adjustable pressure-limiting (APL) valve must be functionally removed or isolated from the circuit. The bag/ventilator switch accomplishes this. In "bag" mode, the ventilator is excluded, allowing spontaneous/manual ventilation. In "ventilator" mode, the breathing bag and APL valve are excluded from the circuit. Some newer machines automatically exclude the APL valve when the ventilator is turned on.

Fresh gas decoupling is a feature in some newer anesthesia workstations with piston or descending bellows ventilators. In traditional circle systems, fresh gas flow is directly coupled to the circuit, increasing delivered tidal volume. With decoupling, fresh gas is diverted during inspiration to a reservoir bag, which accumulates gas until expiration. This reduces the risk of volutrauma or barotrauma from excessive fresh gas flow. Examples include the Dräger Narkomed 6000 and Fabius GS.

Early anesthesia ventilators were simpler than ICU ventilators, with fewer ventilation modes. However, as critically ill patients increasingly undergo surgery, demand for advanced modes has grown. Modern anesthesia machines now incorporate many ICU-style ventilation modes.

All ventilators offer VCV, delivering a preset volume at constant flow. Peak inspiratory pressure varies with patient compliance and airway resistance. Typical settings:

• Tidal volume: 6–10 mL/kg.
• Respiratory rate: 8–12 breaths/min.
• PEEP: Start at 0–5 cm H₂O and titrate.

In PCV, inspiratory pressure is constant, and tidal volume varies. Flow is high initially to achieve set pressure early in inspiration, then decreases to maintain pressure (decelerating flow pattern). PCV improves oxygenation in laparoscopic bariatric surgery and is ideal for neonates, pregnant patients, and those with acute respiratory distress syndrome.

This newer mode combines PCV with a tidal volume target. The ventilator delivers uniform tidal volumes at low pressure using decelerating flow. The first breath is volume-controlled to determine patient compliance, and subsequent breaths adjust inspiratory pressure accordingly.

SIMV delivers guaranteed breaths synchronized with patient effort, allowing spontaneous breaths between mandatory breaths. It is useful in general anesthesia where drugs (e.g., anesthetics, neuromuscular blockers) affect respiratory rate and tidal volume. SIMV can be volume-controlled (SIMV-VC) or pressure-controlled.

PSV is useful for maintaining spontaneous breathing under general anesthesia, especially with supraglottic airways (e.g., laryngeal mask airway). It reduces respiratory work and offsets reduced functional residual capacity caused by inhaled anesthetics. Some ventilators offer apnea backup (PSV-Pro) if spontaneous efforts cease.

Examples include the Datex-Ohmeda S/5 ADU, which uses a microprocessor-controlled pneumatic double-circuit ascending bellows with a "D-Lite" flow/pressure sensor at the Y-piece, and Dräger’s Narkomed 6000, Fabius GS, and Apollo workstations, which use piston-driven single-circuit ventilators with fresh gas decoupling.

Disconnect alarms are critical and should be passively activated during use. Workstations should have at least three disconnect alarms: low peak inspiratory pressure, low exhaled tidal volume, and low exhaled CO₂. Other alarms include high peak pressure, high PEEP, low oxygen supply pressure, and negative pressure.

Common issues include breathing circuit disconnections, ventilator-fresh gas flow coupling (increasing tidal volume and peak pressure with high fresh gas flow), high airway pressure (risk of barotrauma or hemodynamic compromise), bellows assembly problems (leaks or malfunctions), tidal volume discrepancies (due to circuit compliance or leaks), power failures, and accidental ventilator shut-off.