Chapter Outline
Manual Ventilators 49
Automatic Ventilators 51
Classification 52
Ventilation Modes 55
Positive End-Expiratory Pressure (PEEP) Valves 57
Alarms 58
New Features 58
Current Models 61
Jet Ventilators 62
1993 FDA Anesthesia Apparatus Checkout Recommendations 63
The maintenance of appropriate ventilation and oxygenation is an essential aspect of the provision of anesthesia. Numerous approaches to ventilatory support are available to the anesthesia care provider during the administration of anesthesia. The purpose of this chapter is to provide an in-depth overview of manual ventilators, automatic ventilators, jet ventilators, and the terminology used to provide ventilatory support. The principles associated with the application of PEEP and the overall capabilities of the newest generation of anesthesia ventilators will also be presented.
Manual Ventilators
Manual ventilators are the mainstay for the provision of ventilatory support in all care settings. Although they are not the ideal approach for continuous ventilatory support they are commonly used for the transition from spontaneous breathing to ventilatory support and for support during emergency airway management. In general, manual ventilators are composed of three parts: (1) A self-refilling bag; (2) A non-rebreathing valve; and (3) an oxygen/air inlet and oxygen reservoir ( Figure 5-1 ).
- 1.
The self-refilling bag: This component acts as a reservoir for the O 2 and/or air that is directed to the patient when the bag is compressed during inhalation. During exhalation, the bag automatically recoils to its inspiratory position or it passively expands as O 2 /air refills the reservoir. The bag may be made of silicone rubber, chloroprene rubber, butyl rubber, or polyvinyl chloride ( Figures 5-2 through 5-5 ). All rubber bags can be reused after sterilization. The polyvinyl chloride bags are for single use only. Most new designs are latex-free.
Figure 5–2
Non-disposable Military Ambu bag and face mask. This model is made of butyl rubber.
Figure 5–3
The Laerdal system in adult, child and infant sizes. The versions shown all have oxygen reservoir bags attached. The child and infant versions show overpressure safety valves fitted.
Figure 5–4
Ambu single-shutter valve.
Figure 5–5
Illustration of the body of the nonrebreathing valve in a manual ventilator. All consists of an inspiratory port that directs the gas from the bag to the patient, an expiratory port that allows the gas from the patient to leave the valve, and a patient port that serves as a connector for the airway devices (masks, endotracheal tubes, etc.).
- 2.
Non-rebreathing valve: This valve ensures that exhaled gas does not mix with the fresh gas entering the self-refilling bag and allows exhaled gas to enter the atmosphere. All manual ventilators allow the attachment of a positive end-expiratory pressure (PEEP) valve to the expiratory port to ensure that varying levels of PEEP can be applied during manual ventilation. During inspiration the non-rebreathing valve ensures that fresh gas from the self-refilling bag directly enters the patient without leaking and that the patient only receives gas from the bag during inspiration. With most manual ventilators, patients are able to breathe spontaneously directly from the self-refilling bag via the non-rebreathing valve. However, inspiratory effort may be excessive depending on the opening pressure of the non-rebreathing valve and the amount of PEEP applied to the system. If PEEP is applied, the patient must decompress the applied PEEP before they can spontaneously inhale gas from the self-inflating bag.
Non-rebreathing valves are formed by two parts: the body, and the unidirectional valve or valves. The body of the valve is usually T-shaped and consists of an inspiratory port that directs the gas from the bag to the patient, an expiratory port that allows the gas from the patient to leave the valve, and a patient port that serves as a connector for masks, endotracheal tubes, or other airway devices ( Figure 5-6 ). Unidirectional valves ensure that the patient inhales the fresh gas in the bag by closing the expiratory port during inspiration, and that the exhaled gas leaves the system without being mixed with the fresh gas by closing the inspiratory port during exhalation. More than one unidirectional valve may be used for this purpose. A number of designs of unidirectional valves are currently in use, all operating by similar principles: a spring, a duckbill, or a flap mechanism.
Figure 5–6
Spring valve operation during inhalation and exhalation phases.
Fig: Steven Mcpherson, Respiratory Care Equipment, 5th ed., 1995, USA, Mosby-Year Book Inc., pp130, Fig 6-2.A
Spring valve : With this design, a ball or a disc is attached to the spring. In its resting state or during exhalation, the spring seats the ball or the disc on the inspiratory port. In this position, the exhaled gas from the patient port is directed out of the expiratory port. During inhalation, when the bag is compressed, the fresh gas from the self-refilling bag moves the ball or the disc back, so that it blocks the expiratory port and fresh gas is directed to the patient port ( Figures 5-7 and 5-8 ).
Figure 5–7
The Ruben Valve is an example of spring valve mechanism.
Figure 5–8
Duckbill valve operation during inhalation and exhalation phases.
Fig: Steven Mcpherson, Respiratory Care Equipment, 5th ed., 1995, USA, Mosby-Year Book Inc., pp131, Fig 6-4.A
Duckbill valve: During inhalation, fresh gas flow from the self-refilling bag opens the duckbill valve, allowing the fresh gas to enter the inspiratory port. When the valve is open, it also prevents gas from entering the expiratory port. When fresh gas flow ceases, the duckbill valve returns to its closed state and exhalation begins. The exhaled gas flow from the patient is directed to the expiratory port maintaining the valve closed ( Figures 5-9 and 5-10 ).
Figure 5–9
The Laerdal valve
Figure 5–10
Flap valve operation during inhalation and exhalation phases.
Fig: Steven Mcpherson, Respiratory Care Equipment, 5th ed., 1995, USA, Mosby-Year Book Inc., pp136, Fig 6-11.A
Flap Valve: During inhalation, the opening of the flap valve by the fresh gas flow from the self-inflating bag closes the expiratory port and directs the fresh gas to the patient. At the end of inhalation, the flap returns to its original position, which allows exhaled gas to exit through the expiratory valve. Various designs of this valve are available by positioning the flap on its edge or centrally, or combining it with a diaphragm (e.g., diaphragm-flap valve, mushroom-flap valve, fish-mouth-flap valve) ( Figures 5-11 and 5-12 ).
Figure 5–11
Various designs of flap valves; Edge mounted flap valve, centrally mounted flap valve, diaphragm flap valve, mushroom flap valve, fishmouth flap valve.
From Dorsch JA, Dorsch SE, Understanding Anesthetic Equipment, 5th edition, 2007, USA, Lippincott Williams & Wilkins, pp 285, Fig 10-6(edge m), 10-7(centrally m), pp 286 Fig 10-10(diaphragm flap), pp 287, Fig 10-11(mushroom flap), 10-12(fishmouth flap).
Figure 5–12
AMBU re-usable adult A, and paediatric B, resuscitators.
- 3.
Oxygen/air inlet and oxygen reservoir: The gas inlet to the reservoir is generally located at the other side of the self-refilling bag from the non-rebreathing valve end. However, in some older designs the gas inlet is part of the non-rebreathing valve assembly. The gas inlet normally has a unidirectional valve, which opens during exhalation when the bag is refilling and closes during inhalation when the bag is compressed.
Oxygen may be delivered into the system in two ways; through a nipple as part of the inlet valve assembly or via an oxygen reservoir. Oxygen reservoirs are either bags (closed reservoir) or lengths of large bore (22 mm internal diameter) tubing (open reservoir) that allow the accumulation of oxygen during the inhalation phase and release the stored oxygen into the self-refilling bag during the exhalation phase when the bag is refilling ( Figure 5-13 ). The addition of an oxygen reservoir to the system significantly increases the achievable inspired oxygen concentration.
Figure 5–13
PEEP valves may be added by inserting the valve into the expiratory port of the manual ventilator.
Two security valves are placed between the reservoir and the gas inlet; one is a “pressure relief valve,” which opens at a threshold pressure, preventing the bag from being overfilled. This valve limits the exposure of the patient’s airway to high pressure. The other is the “air intake” valve; it opens when subatmospheric pressure is established in the self-refilling bag by its normal elastic recoil allowing air to enter and refill the reservoir. This preserves the ability to provide ventilation even when the oxygen tank is empty.
Positive end-expiratory pressure (PEEP) valves may be added to the system simply by inserting the PEEP valve into the expiratory port of the manual ventilator. Some of the newly designed models have built-in PEEP valves with adjusting dials ( Figures 5-14 and 5-15 ).
Figure 5–14
A PEEP valve is shown as attached to the expiratory port of a manual ventilator.
Figure 5–15
Mechanical thumb ventilator. Closing and opening one end of the t-shaped mechanism by the anesthesiologist’s thumb or a mechanical thumb delivers intermittent positive pressure to ventilate the patient.
From; Davey A, Diba A, Ward CS, Ward’s Anesthetic Equipment, 5th edition, 2005, UK, Elselvier Health Sciences. pp 247, Fig.11.4
Automatic Ventilators
Historically, anesthesia ventilators were designed to relieve the anesthesia care provider from continuously squeezing the bag, thus the design of the ventilator was quite simple. As technology advanced, and critical care management of patients improved, anesthesiologists increasingly have been required to provide anesthesia to critically ill patients with a variety of severe cardiopulmonary problems. As new airway devices such as the laryngeal mask were introduced into anesthesia practice, the need for ventilation modes beyond the standard volume control became critical. To meet these new demands, anesthesia ventilators have been designed to allow a comparable range of operation as seen on ICU ventilators. Most new anesthesia ventilators are equipped with a wide selection of modes, the ability to apply PEEP, and the capability of monitoring patients in the same manner as in the ICU.
Classification
Today’s anesthesia ventilators can be classified according to their application, power system, and cycling mechanism.
Application Type
Ward et al have classified anesthesia ventilators into four different groups: mechanical thumb ventilators, minute volume dividers, bag squeezers, and intermittent blowers.
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Mechanical thumb ventilators: This type of ventilator delivers pressurized gas from a cylinder or the wall outlet to the patient as a continuous flow through a simple T-piece breathing system. Either the thumb of the anesthesia care provider or a mechanical thumb (a pneumatic valve) rhythmically occludes and opens the end of the T-piece, which creates the intermittent positive pressure to ventilate the patient. The use of this early design ventilator is currently limited to ventilation of neonates or during emergencies since it requires a high fresh gas flow, which is wasted and lost to the atmosphere during exhalation resulting in higher costs of ventilation. In addition, ventilation is unregulated (i.e., the anesthesiologist has no idea of the pressures or volumes of gas delivered to the patient’s airways) ( Figure 5-16 ).
Figure 5–16
Minute volume divider type ventilator, working principle.
From; Davey A, Diba A, Ward CS, Ward’s Anesthetic Equipment, 5th edition, 2005, UK, Elselvier Health Sciences. pp 249, Fig.11.6
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Minute volume divider ventilators: In this simple design, a specific volume of fresh gas equal to the patient’s minute volume is delivered to the ventilator per minute. This volume is then divided by the number of the breaths, and the resultant tidal volume is delivered to the patient. Gas entering the ventilator is collected in a reservoir. The alternate opening and closing of two linked valves directs the gas from the reservoir to the patient and the exhaled gas from the patient to the atmosphere. These ventilators are very rarely found in U.S. anesthesia practice ( Figure 5-17 ).
Figure 5–17
Ascending (standing) and descending (hanging) bellows ventilators.
From Morgan GE, Mikhail MS, Murray MJ, Clinical Anesthesiology, 4th ed, 2006, USA, McGraw-Hill Companies Inc. Fig 4.12
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Bag squeezer ventilators: This application was inspired by the anesthesia care provider’s hands squeezing the breathing bag. Until recently, these ventilators were the most common type found on anesthesia machines.
Bellows Type
The most typical design of an anesthesia ventilator is “a bag in the bottle,” that is a bellows (bag) inside a cylinder. The bellows is filled with gas that moves to and from the patient, whereas the cylinder is filled with the driving gas that compresses the bellows, moving gas to the patient. The bellow’s movement is easily observed through the cylinder. The bellows may be ascending or descending according to its movement during exhalation.
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Ascending: Ascending bellows rise during exhalation. The bellows is attached to the system from its base; exhaled gas coming from the patient fills the bellows and generates the ascending motion. During inhalation, the driving gas surrounding the bellows pushes the bellows downward and the gas inside the bellows moves toward the patient. During exhalation, the expired gas refills the bellows and the next cycle starts. If there is any disconnection of the circuit, the bag does not fill providing an instant visual alert for the anesthesiologist. Because of this design, ascending bellows ventilators provide an obligate amount of PEEP, usually 2 to 3 cm H 2 O applied to the exhalation port of the bellows so that the exhaled gas will force the bellows to rise.
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Descending: Descending bellows fall during patient exhalation and ascend during inhalation. The bellows is attached to the system from the top and usually is weighted to facilitate its descending motion during exhalation. If there is a leak or disconnection of the system, the bellows will still descend during exhalation, and the negative pressure produced by the weight of the bellows allows room air to enter the system. Since the movement of the bellows appears normal despite the leak, the bellows movement may be deceiving to the anesthesiologist. Early models of anesthesia ventilators used descending bellows but due to the high risk of complications all ventilators produced after the 1980s have ascending bellows for improved safety. However, recent models of anesthesia ventilators (e.g., Drager Julian, Datascope Anestar) have again integrated descending bellows into their design but they have also added safety features: these include built-in end-tidal carbon dioxide monitors; software, sensors and monitors to identify bellows failure and activate alarms; and a negative pressure relief valve to prevent the development of negative pressure in the system ( Figure 5-18 ).
Figure 5–18
Double circuit pneumatic ventilator designs. (A:Datex-Ohmeda, B: Drager)
From Morgan GE, Mikhail MS, Murray MJ, Clinical Anesthesiology, 4th ed, 2006, USA, McGraw-Hill Companies Inc., pp 79 , Fig 4.29
Drive Mechanism and Circuit Design Type
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Double circuit pneumatically driven: In these ventilators, a pneumatic force created by the driving gas compresses the bellows so that the bellows empties its contents—exhaled gas that has been passed through the CO 2 absorbent, gas from flowmeters, and gas from the vaporizer—into the inspiratory limb of the breathing circuit. The driving gas may be oxygen, air, or both. This pressurized gas from the ventilator power outlet directly enters the space between the inner wall of the container and the outer wall of the bellows. To reduce oxygen consumption when 100% oxygen is the driving force, a venturi device may be used to entrain room air and conserve the amount of oxygen driving the ventilator ( Figure 5-19 and 5-20 ).
