Automatic ventilators

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Automatic ventilators enable the provision of intermittent positive pressure ventilation (IPPV) to support ventilation in anaesthetized or heavily sedated patients. An automatic ventilator is an invaluable tool that provides repeated, controlled breaths, so allowing the anaesthetist to attend to other aspects of patient monitoring and support. This chapter deals with indications for IPPV, when to initiate IPPV, physiological effects of IPPV, classification of automatic ventilators and their practical use.

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6.3 Thoracic pressures and volumes during spontaneous ventilation. Note that interpleural pressure remains negative during the entire respiratory cycle – a key component of the ‘thoracic pump’ and the maintenance of cardiac preload.
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6.4 Invasive arterial blood pressure trace showing the variation in pulse pressure during the respiratory cycle.
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6.7 (a) The Pneupac ventiPAC (). (b) Detail of the Pneupac ventiPAC. A = inspiratory phase timer; B = expiratory phase timer; C = flow controller; D = pressure gauge; E = on/off switch. (c) A typical bag-in-bottle arrangement which would be attached to the Pneupac ventiPAC in volume-controlled time-cycled mode. (d) Working principles of the Pneupac ventiPAC. A = inspiratory phase timer; B = expiratory phase timer; C = flow controller; D = gas inlet.
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6.8 Nuffield Series 200 anaesthesia ventilator with Newton valve attachment (N), converting this versatile ventilator into a ‘mechanical thumb’ or pressure generator. B = expiratory phase time dial; D = pressure reading; E = on/off switch.
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6.9 (a) The Hallowell EMC Model 3000 veterinary ventilator. A = inspiratory flow controller; B = peak pressure controller; C = respiratory rate controller; D = respiratory hold button. (b) The Surgivet SAV 2500 Ventilator. (Courtesy of Tanya Duke-Novakovski, Western College of Veterinary Medicine, University of Saskatchewan, Canada)
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6.10 (a) The Vetronic ‘Merlin’ veterinary ventilator and controls. A = flow or volume control; B = inspiratory time control (seconds); C = expiratory time control (seconds); D = maximum airway pressure control (cmHO); E = assist threshold control (cmHO); F = flow rate/volume selector switch; G = assist mode on/off selector switch; H = LED indicator for expiratory port pressure (cmHO); I = LED indicator for inspiratory port pressure (cmHO); J = ventilate mode stop/run selector switch; K = alarm reset switch – non-latching, momentary action only; L = alarm flashing LED indicator; M = main status LCD screen; N = attachment of inspiratory limb of Y-piece; O = attachment of expiratory limb of Y-piece; P = one-way valve; Q = entry for fresh gas (connected to anaesthetic machine); R = exit for gas from ventilator (to scavenging system if used in non-rebreathing mode; to carbon dioxide-absorbent canister if used with a circle system); S = high-pressure relief valve; T = reservoir bag (of appropriate size for patient). (b) Colour coding of inspiratory and expiratory gas connections and Merlin breathing tubes allows easy and rapid set-up when circle systems are used. (c) Set-up for use of the Merlin ventilator with a circle system. (bc, Courtesy of Tanya Duke-Novakovski, Western College of Veterinary Medicine, University of Saskatchewan, Canada)
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6.11 PEEP valve for use with the Merlin ventilator. (Courtesy of Keith Simpson, Vetronic UK)
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6.13 (a) The Vetronic SAV03 veterinary ventilator – a pressure-cycled ‘mechanical thumb’ designed for patients less than 10 kg. A = display; B = expiratory phase time control; C = trigger set control. (b) The Vetronic SAV03 veterinary ventilator valve assembly. FG = fresh gas; L = limb; P = patient attachment.
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6.14 The Bird Mark 7 ventilator – a pressure-cycled machine which may drive ventilation systems for both small and large animals. A = triggering pressure control; B = pressure indicator; C = flow control; D = expiratory time control; E = inspiratory pressure control.
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6.15 (a) The Manley MP2, a classic if ageing machine. A = externally visible bellows; B = volume control; C = tidal volume catch for external bellows; D = sliding weight to control pressure of gas delivered; F and G = controls to switch ventilator to automatic mode. (b) Working principles of the Manley MP2. A = externally visible bellows; D = sliding weight; E = internal bellows; FGF = fresh gas flow.
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