Ventilators
Classification of ventilators
There are many ways of classifying ventilators (Table 8.1).
1. The method of cycling is used to change over from inspiration to exhalation and vice versa:
a) volume cycling: when the predetermined tidal volume is reached during inspiration, the ventilator changes to exhalation
b) time cycling: when the predetermined inspiratory duration is reached, the ventilator changes to exhalation. The cycling is not affected by the compliance of the patient’s lungs. Time cycling is the most commonly used method
c) pressure cycling: when the predetermined pressure is reached during inspiration, the ventilator changes over to exhalation. The duration needed to achieve the critical pressure depends on the compliance of the lungs. The stiffer the lungs are, the quicker the pressure is achieved and vice versa. The ventilator delivers a different tidal volume if compliance or resistance changes
d) flow cycling: when the predetermined flow is reached during inspiration, the ventilator changes over to exhalation. This method is used in older design ventilators.
2. Inspiratory phase gas control:
3. Source of power – can be electric or pneumatic.
4. Suitability for use in theatre and/or intensive care.
5. Suitability for paediatric practice.
6. Method of operation (pattern of gas flow during inspiration):
a) pressure generator: the ventilator produces inspiration by generating a constant and predetermined pressure. Bellows or a moderate weight produce the pressure. The inspiratory flow changes with changes in lung compliance (Table 8.2)
b) flow generator: the ventilator produces inspiration by delivering a predetermined flow of gas. A piston, heavy weight or compressed gas produce the flow. The flow remains unchanged by changes in lung compliance, although pressures will change (see Table 8.2). These ventilators have a high internal resistance to protect the patient from high working pressures.
7. Sophistication: new ventilators can function in many of the above modes. They have other modes, e.g. SIMV, PS and CPAP (see pp 224–225).
a) minute volume dividers: fresh gas flow (FGF) powers the ventilator. The minute volume equals the FGF divided into preset tidal volumes thus determining the frequency
b) bag squeezers replace the hand ventilation of a Mapleson D or circle system. They need an external source of power
c) lightweight portable: powered by compressed gas and consists of the control unit and patient valve.
Characteristics of the ideal ventilator
1. The ventilator should be simple, portable, robust and economical to purchase and use. If compressed gas is used to drive the ventilator, a significant wastage of the compressed gas is expected. Some ventilators use a Venturi to drive the bellows, to reduce the use of compressed oxygen.
2. It should be versatile and supply tidal volumes up to 1500 mL with a respiratory rate of up to 60/min and variable I : E ratio. It can be used with different breathing systems. It can deliver any gas or vapour mixture. The addition of positive end expiratory pressure (PEEP) should be possible.
3. It should monitor the airway pressure, inspired and exhaled minute and tidal volume, respiratory rate and inspired oxygen concentration.
4. There should be facilities to provide humidification. Drugs can be nebulized through it.
5. Disconnection, high airway pressure and power failure alarms should be present.
6. There should be the facility to provide other ventilatory modes, e.g. SIMV, CPAP and pressure support.
Manley MP3 ventilator
This is a minute volume divider (time cycled, pressure generator). All the FGF (the minute volume) is delivered to the patient divided into readily set tidal volumes (Fig. 8.1).
Components
1. Rubber tubing delivers the FGF from the anaesthetic machine to the ventilator.
2. Two sets of bellows. A smaller time-cycling bellows receives the FGF directly from the gas source and then empties into the main bellows.
3. Three unidirectional valves.
4. An adjustable pressure limiting (APL) valve with tubing and a reservoir bag used during spontaneous or manually controlled ventilation.
5. The ventilator has a pressure gauge (up to 100 cm H2O), inspiratory time dial, tidal volume adjuster (up to 1000 mL), two knobs to change the mode of ventilation from and to controlled and spontaneous (or manually controlled) ventilation. The inflation pressure is adjusted by sliding the weight to an appropriate position along its rail. The expiratory block is easily removed for autoclaving.
Mechanism of action
1. The FGF drives the ventilator.
2. During inspiration, the smaller bellows receives the FGF, while the main bellows delivers its contents to the patient. The inspiratory time dial controls the extent of filling of the smaller bellows before it empties into the main bellows.
3. During expiration, the smaller bellows delivers its contents to the main bellows until the predetermined tidal volume is reached to start inspiration again.
4. Using the ventilator in the spontaneous (manual) ventilation mode changes it to a Mapleson D breathing system.
Problems in practice and safety features
1. The ventilator ceases to cycle and function when the FGF is disconnected. This allows rapid detection of gas supply failure.
2. Ventilating patients with poor pulmonary compliance is not easily achieved.
3. It generates back pressure in the back bar as it cycles.
4. The emergency oxygen flush in the anaesthetic machine should not be activated while ventilating a patient with the Manley.
Penlon Anaesthesia Nuffield Ventilator Series 200
This is an intermittent blower ventilator. It is small, compact, versatile and easy to use with patients of different sizes, ages and lung compliances. It can be used with different breathing systems (Fig. 8.2). It is a volume-preset, time-cycled, flow generator in adult use. In paediatric use, it is a pressure-preset, time-cycled, flow generator.
Fig. 8.2 The Penlon Nuffield 200 ventilator. (Courtesy of Penlon Ltd, Abingdon, UK (www.penlon.com).)
Components
1. The control module, consisting of an airway pressure gauge (cm H2O), inspiratory and expiratory time dials (seconds), inspiratory flow rate dial (L/s) and an on/off switch. Underneath the control module there are connections for the driving gas supply and the valve block. Tubing connects the valve block to the airway pressure gauge.
2. The valve block has three ports:
a) a port for tubing to connect to the breathing system reservoir bag mount
b) an exhaust port which can be connected to the scavenging system
3. The valve block can be changed to a paediatric (Newton) valve.
Mechanism of action
1. The ventilator is powered by a driving gas independent from the FGF. The commonly used driving gas is oxygen (at about 400 kPa) supplied from the compressed oxygen outlets on the anaesthetic machine. The driving gas should not reach the patient as it dilutes the FGF, lightening the depth of anaesthesia.
2. It can be used with different breathing systems such as Bain, Humphrey ADE, T-piece and the circle. In the Bain and circle systems, the reservoir bag is replaced by the tubing delivering the driving gas from the ventilator. The APL valve of the breathing system must be fully closed during ventilation.
3. The inspiratory and expiratory times can be adjusted to the desired I/E ratio. Adjusting the inspiratory time and inspiratory flow rate controls determines the tidal volume. The inflation pressure is adjusted by the inspiratory flow rate control.
4. With its standard valve, the ventilator acts as a time-cycled flow generator to deliver a minimal tidal volume of 50 mL. When the valve is changed to a paediatric (Newton) valve, the ventilator changes to a time-cycled pressure generator capable of delivering tidal volumes between 10 and 300 mL. This makes it capable of ventilating premature babies and neonates. It is recommended that the Newton valve is used for children of less than 20 kg body weight.
Bag in bottle ventilator
Modern anaesthetic machines often incorporate a bag in bottle ventilator.
Components
1. A driving unit consisting of:
a) a chamber (Fig. 8.3) with a tidal volume range of 0–1500 mL (a paediatric version with a range of 0–400 mL exists)
2. A control unit with a variety of controls, displays and alarms: the tidal volume, respiratory rate (6–40/min), I/E ratio, airway pressure and power supply (Figs 8.3 and 8.4).
Mechanism of action
1. It is a time-cycled ventilator.
2. Compressed air is used as the driving gas (Fig. 8.5). On entering the chamber, the compressed air forces the bellows down, delivering the fresh gas to the patient (the fresh gas is accommodated in the bellows).
3. The driving gas and the fresh gas remain separate.
4. The volume of the driving gas reaching the chamber is equal to the tidal volume.
Problems in practice and safety features
1. Positive pressure in the standing bellows causes a PEEP of 2–4 cm H2O.
2. The ascending bellows collapses to an empty position and remains stationary in cases of disconnection or leak.
3. The descending bellows hangs down to a fully expanded position in a case of disconnection and may continue to move almost normally in a case of leakage.
