Thorax 1990;45:753-758 753
Assisted ventilation 1 Series editor: John Moxham
Artificial ventilation: history, equipment and Thorax: first published as 10.1136/thx.45.10.753 on 1 October 1990. Downloaded from techniques
J D Young, M K Sykes
A brief history of artificial ventilators expedited research into powered mechanical ventilators. An artificial ventilator is essentially a device that replaces or augments the function of the Classification of artificial ventilators inspiratory muscles, providing the necessary The lungs can be artificially ventilated either energy to ensure a flow of gas into the alveoli by reducing the ambient pressure around the during inspiration. When this support is thorax (negative pressure ventilation) or by removed gas is expelled as the lung and chest increasing the pressure within the airways wall recoil to their original volume; exhalation (positive pressure ventilation). Negative pres- is a passive process. In the earliest reports of sure ventilators use a rigid chamber that artificial ventilation this energy was provided encloses either the thorax (cuirass) or the by the respiratory muscles of another person, whole body below the neck (tank respirator or as expired air resuscitation. Baker' has traced "iron lung"). The pressure in the chamber is references to expired air resuscitation in the reduced cyclically by means of a large volume newborn as far back as 1472, and in adults displacement pump, thus causing the lungs to there is a report of an asphyxiated miner being expand and contract. Negative pressure ven- revived with mouth to mouth resuscitation in tilation is fully discussed in article 5 of this 1744. In the eighteenth century artificial ven- series. These ventilators were used extensively tilation became the accepted first line treat- for poliomyelitis victims, and are still in use ment for drowning victims, though the use of for long term respiratory support or overnight bellows replaced mouth to mouth resus- support for patients with respiratory muscle
citation.2 Automatic artificial ventilators that weakness, Tank ventilators occupy much http://thorax.bmj.com/ did not require a human as a power source space, access to the patient is poor, and the took another 150 years to appear, being first neck seal can create problems. They are not suggested by Fell3 and then made available suitable for use in general intensive care units. commercially by Draeger4 in 1907. These There has been a recent revival in interest in were still devices for resuscitation, for the negative pressure ventilators in paediatric Draeger company at that time was noted for intensive care units to avoid the need for its mine rescue apparatus. endotracheal intubation.5 The introduction of artificial ventilators The basic classification of positive pressure on September 25, 2021 by guest. Protected copyright. into anaesthesia proceeded slowly until sur- artificial ventilators was first proposed by geons ventured into the chest. During Mapleson.6 Artificial ventilators are devices thoracic surgery on spontaneously breathing that control inspiration; expiration is usually patients the inevitable pneumothoraces and passive, and so the classification is based on mediastinal shift ("mediastinal flap") that the mechanism of gas delivery during inspira- occurred as the pleural cavity was opened tion. There are two types of machine. A flow caused a high mortality, which was generator produces a known pattern of gas substantially reduced when positive pressure flow during inspiration, and the lungs fill at a ventilation was used. A further boost to the rate entirely controlled by the ventilator and development of automatic artificial ventilators independent of any effect of lung mechanics. occurred in 1952, when a catastrophic A pressure generator produces a preset pres- poliomyelitis epidemic struck Denmark. sure in the airway and the rate of lung infla- There was a very high incidence of bulbar tion depends not only on the pressure gen- lesions and 316 out of 866 patients with erated by the ventilator but also on the res- paralysis admitted over a period of 19 weeks piratory resistance and compliance, which required postural drainage, tracheostomy, or determine the time constant of the lungs. respiratory support. By using tracheostomy With a square wave pattern of pressure the Nuffield Department of Anaesthetics, John and manual positive pressure ventilation the lungs fill in an exponential fashion. The Radcliffe Hospital, Danish physicians reduced the mortality from effects of the two types of ventilator on ins- Oxford poliomyelitis from 80% at the beginning of piratory flow and lung volume are shown in J D Young M K Sykes the epidemic to 23% at the end. The artificial figure 1. ventilation was entirely by hand, a total of In general, the flow generator ventilator is Address for reprint requests: Dr J D Young, 1400 university students working in shifts to used for adults and the pressure generator Nuffield Department of keep the patients ventilated. The fear that ventilator for children, or adults when control Anaesthetics, John Radcliffe Hospital, Oxford OX3 9DU. another epidemic might afflict Europe of peak airway pressure is important. The 754 Young, Sykes Thorax: first published as 10.1136/thx.45.10.753 on 1 October 1990. Downloaded from
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Figure 1 Lung volume and gasflow during one respiratory cycle with pressure andflowgenerator ventilators.
pressure generator is particularly useful in controlled by a pair of scissor valves and children where uncuffed endotracheal tubes monitored with pressure and flow sensors. are used and there is a leak of gas around the The control unit adjusted the scissor valves to
endotracheal tube during inspiration. A pres- ensure that the flow patterns measured by the http://thorax.bmj.com/ sure generator tends to compensate for this sensors corresponded to those selected by the leak by increasing the flow into the airway, operator. This method of control is termed a whereas with a flow generator a proportion of servo or feedback system and is very flexible. the tidal volume is lost. Potentially one machine could mimic all the A ventilator must also have a mechanism to previously described categories of ventilator. cause it to cycle between inspiration and expiration. Ventilators are usually subclas- Modern intensive care unit ventilators
sified as time cycled, pressure cycled, or Most modern intensive care unit ventilators on September 25, 2021 by guest. Protected copyright. volume cycled machines. Time cycled ven- use similar technology. A simplified diagram of tilators switch between inspiration and expira- such a ventilator is shown in figure 2. The tion after a preset time interval, pressure respiratory gas is held in a pressurised reservoir cycled ventilators switch when a preset airway and delivered to the patient via an inspiratory pressure threshold has been reached, and valve. The inspiratory valve and hence the volume cycled ventilators cycle when a preset inspiratory flow is controlled by the electronic tidal volume has been delivered. Cycling from control unit. The airway pressure and flow of expiration to inspiration is usually effected by gas into the patient are monitored by the a timing mechanism or by a patient triggering pressure and inspiratory flow sensors. The device that senses the subatmospheric pres- expiratory flow can also be monitored to check sure or the flow generated in the inspiratory for leaks and disconnection of the patient from tube by the patient's inspiratory effort. the ventilator. This design enables the ven- This traditional classification was devised tilator to be used as either a flow or a pressure during a period when ventilators were totally generator. For example, ifthe operator selects a mechanical, and driven either by compressed constant flow during inspiration the ventilator gas or by an electrically powered piston or will open the inspiratory valve until the flow bellows. By 1954, however, Donald had used sensor measures the required flow. If the an electronic trigger that initiated inspiration inspiratory flow decreases the inspiratory valve and in 1958 an electronic timing device was will be opened further to compensate, and vice incorporated in the Barnet ventilator.7 In 1971 versa. If the operator wishes the ventilator to the Siemens-Elema company introduced the act as a constant pressure generator, the ven- Servo 900 ventilator, which combined a sim- tilator will open the inspiratory valve until the ple pneumatic system with a sophisticated pressure sensor indicates that the desired pres- electronic measuring and control unit.8 Gas sure has been reached. The ventilator will then flow to and from the patient's lungs was maintain the airway pressure at the desired Artificial ventilation: history, equipment and techniques 755
manage and suffered complications.9 To min- imise these problems many ventilated patients are sedated, and in some cases paralysed with neuromuscular blocking agents (for example, To pancuronium). In the early 1980s paralysis of Thorax: first published as 10.1136/thx.45.10.753 on 1 October 1990. Downloaded from patient ventilated patients was very common in British intensive care units,1011 and sedation was used to dissociate the patients from their surround- ings. Over the last decade the amount of sedative and paralysing drugs given to inten- sive care patients has been reduced consider- ably. Paralysis ofventilated patients is not without risks.'2 The paralysed patient is unable to make any spontaneous respiratory effort, so a discon- nection from the ventilator is rapidly fatal. The patient may be aware ofhis or her surroundings Figure 2 A block diagram of a modern ventilator. The shaded area indicates the gas but is unable to communicate, and the risk of path during inspiration. pulmonary embolus is probably increased. Even sedative drugs are not without risk,'3 as shown by the substantial increase in mortality level by opening or closing the inspiratory in trauma patients given etomidate infusions. valve. The bulk of a modern ventilator is now This was subsequently shown to be caused by the electronic unit and the pneumatic units are suppression of adrenal activity by the drug. often very compact. Figure 3 is a photograph of Only a few clinicians now favour deep seda- a Servo 900D ventilator. The pneumatic sys- tion and paralysis,'4 the preferred approach tems are housed in the top section and the being a comfortable patient who can be easily bottom section contains only electronics. aroused and can communicate with the staff. The use of electronic feedback systems in To achieve this goal ventilators have to be more ventilators has many advantages. The moving flexible and adapted to the needs of the patient. parts within the ventilator are kept to a min- In its simplest form this is achieved by insert- imum, sophisticated alarm systems are possi- ing a valve into the ventilator tubing, which ble, the ventilators can be made physically allows the patient to breathe spontaneously small, and maintenance and repair are simple. from another gas source between the breaths If a new mode of ventilation appears to be delivered by the ventilator. This mode of useful clinically it can often be added to an ventilation is termed intermittent mandatory ventilation (IMV) and was initially used as a existing machine by altering the software in the http://thorax.bmj.com/ electronic unit-indeed, our ability to produce technique to wean patients from mechanical new modes of artificial ventilation far exceeds ventilation to spontaneous respiration. By our ability to test them clinically. 1987, however, over 70% of American inten- sive care units were using intermittent man- Interactions between patients and datory ventilation as their primary mode of ventilators ventilatory support."' It is not a truly interac- As early as 1929 it was observed that a patient tive mode ofventilation-the ventilator contin- ues to deliver the preset pattern of ventilation
who "fought" the ventilator was difficult to on September 25, 2021 by guest. Protected copyright. regardless of the patient's respiratory effort. The advent of microprocessor controlled ven- tilators has produced several "smart" modes of ventilation, where the ventilator adjusts the ventilatory support it supplies according to the patient's respiratory efforts. The commonest modes used are synchronised intermittent mandatory ventilation, mandatory minute ven- tilation, and inspiratory pressure support.
a Ventilatory strategies in common use
INTERMITTENT POSITIVE PRESSURE VENTILATION Each year millions of patients are ventilated during anaesthesia for surgical procedures that require muscular relaxation. Nearly all of these patients are ventilated with basic intermittent positive pressure ventilation by means of a simple mechanical ventilator. For these short periods of respiratory support these ventilators are perfectly adequate, for as the patient is both paralysed and anaesthetised there is no need for Figure 3 A Siemens Servo 900D ventilator. interactive ventilators. Weaning is accompli- 756 Young, Sykes
Figure 4 "Stacking" of sed with external intermittent mandatory ven- breaths during intermittent mandatory ventilation. tilation valves and used its internal valves and The arrow indicates where sensors to provide synchronised intermittent LA.
a mandatory breath starts Thorax: first published as 10.1136/thx.45.10.753 on 1 October 1990. Downloaded from 0 mandatory ventilation. Any spontaneous in- before the patient hasfully spiratory activity by the patient was sensed by expired. 0 the pressure sensor and then the expiratory
0 valve was closed and the inspiratory valve Eco opened. The flow of gas through the in- 0 spiratory valve was matched to the patient's inspiratory flow rate by opening and closing the inspiratory valve. This allowed the patient to
-j breathe spontaneously through the ventilator. When a mandatory breath was due the ven- Time tilator would wait until the patient began to inspire and then deliver the mandatory breath, synchronising the mandatory breath with the shed by reversing the muscle relaxation and patient's own inspiratory effort. lightening the anaesthetic. In intensive care All ventilators with synchronised intermit- units this mode of ventilation is used when the tent mandatory ventilation systems use some patient has to be heavily sedated and paralysed form of synchronisation between patient and to treat the primary condition (for example, machine, the exact details varying between tetanus) or when the patient is unable to make machines. All synchronised intermittent man- any respiratory movement (for example, severe datory ventilation machines are not equally Guillain-Barre syndrome). Intermittent effective, for the time taken for the inspiratory positive pressure ventilation can be achieved valve to open after the pressure sensor has been with almost any artificial ventilator as no triggered varies between machines. During the special refinements are needed. period between the beginning of the patient's inspiration and the opening of the inspiratory INTERMITTENT MANDATORY VENTILATION valve the patient inspires against a closed valve, As outlined above, intermittent mandatory increasing the work of breathing.'7 Machines ventilation describes a mode of mechanical with a short lag between the beginning of an ventilation that allows the patient to breathe inspiratory effort and the opening of the spontaneously through the ventilator circuit. inspiratory valve are preferred. Although At predetermined intervals a positive pressure subjectively patients usually appear more breath is provided by the ventilator totally comfortable with synchronised intermittent independently of the patient's spontaneous mandatory ventilation than intermittent
ventilatory pattern. The system is a simple positive pressure ventilation or intermittent http://thorax.bmj.com/ mechanical device that can be fitted to almost mandatory ventilation, there is no evidence any ventilator, though care has to be taken to that it reduces morbidity or mortality in ensure that the oxygen concentration is the intensive care units. same in the spontaneous breaths as during the breaths delivered by the ventilator and that the MANDATORY MINUTE VENTILATION inspired gas is appropriately humidified.'6 The synchronised intermittent mandatory Though simple it is not without disadvantages. ventilation systems described add a preset
There is no synchronisation between the number of breaths of a preset volume to the on September 25, 2021 by guest. Protected copyright. patient's respiratory efforts and the ventilator, patient's own respiratory efforts. If the and this can lead to "stacking" of breaths and patient's spontaneous breaths are providing high airway pressures. Figure 4 shows a plot of most of the required minute ventilation, the lung volume against time for a patient on mandatory breaths may be too large or too intermittent mandatory ventilation. The frequent. Alternatively, if the patient's own patient can take a spontaneous breath, and respiratory efforts diminish the mandatory before he has fully exhaled the ventilator breaths may not be enough to provide an delivers a breath. A very large tidal volume adequate minute ventilation. Ideally the ven- with high peak airway pressure results. To tilator should monitor exhaled volumes and overcome this problem ventilators were "top up" the patient's respiratory efforts if developed that were able to detect spontaneous needed. This is mandatory minute ventilation. breaths taken by the patient. In this mode of ventilation a preset minute volume is selected. If the patient's own res- SYNCHRONISED INTERMITTENT MANDATORY piratory efforts produce a minute ventilation VENTILATION equal to or greater than the preset value the To avoid "stacking" the ventilator must be able ventilator is not activated. If the preset minute to sense that the patient has taken a breath, and volume is not achieved the ventilator gives then avoid delivering a mandatory breath dur- some assistance to the patient, to bring the ing the period of the spontaneous breath. This minute ventilation back to the target value. level of sophistication was not easily achieved This system works well if the patient's res- before electronic control was used in artificial piratory rate is relatively normal. If, however, ventilators. One of the first machines to have a the patient has rapid, shallow breaths the synchronised intermittent mandatory ventila- efficiency of breathing will be reduced and the tion facility was the Servo 900B, a refinement mandatory volume may become inadequate. ofthe original Servo 900. This machine dispen- The method used to provide ventilatory Artificial ventilation: history, equipment and techniques 757
assistance depends on the design of the ven- UNUSUAL MODES OF VENTILATION tilator. The Ohmeda CPU1 and Engstrom There are several modes of ventilation that are Erica will provide synchronised mandatory still being evaluated clinically. In high breaths ofa preset tidal volume at an increasing frequency jet ventilation brief, frequently frequency as the patient's own respiratory repeated pulses of gas (up to 300 a minute) are Thorax: first published as 10.1136/thx.45.10.753 on 1 October 1990. Downloaded from efforts diminish. The Hamilton Veolar uses a directed from a high pressure nozzle down the different approach and provides increasing airway. The tidal volumes delivered are small, inspiratory pressure support (see below) as the airway pressures are low, and the patient can patient's own efforts decrease. Whichever breathe spontaneously while being ventilated.20 method is used, the machines ensure that the High frequency oscillation techniques oscillate patient always receives a predetermined minute gas in the airways using a piston pump or volume. diaphragm pump. The tidal volumes used are very small, much less than the anatomical dead INSPIRATORY PRESSURE SUPPORT space, so gas movement occurs by mechanisms The most recent mode of ventilation to be other than tidal exchange. Because of the large widely used is inspiratory pressure support. As size of oscillators needed for adults this tech- the patient initiates a breath the ventilator nique has been confined to use in infants. raises the airway pressure to a preset value. The There are also two extracorporeal systems in positive airway pressure provides some of the clinical use. In adults with acute respiratory energy needed to expand the lung and the failure extracorporeal membrane oxygenation efforts of the patient provide the rest. At the was abandoned after a large trial showed that it end of inspiration the positive airway pressure did not reduce mortality.2' In infants with is removed to allow unimpeded expiration. By neonatal respiratory distress, however, the selection of an appropriate level of airway method seems to be very effective. A remarka- pressure patients can be given only the res- ble reduction in mortality from this condition piratory assistance they actually require. The has been reported in America when extracor- patient also determines his or her own res- poreal membrane oxygenation has been ins- piratory rate. This is a relatively new mode of tigated.22 In adults a new approach has been ventilation and has not yet been fully assessed. pioneered by Gattinoni in Italy. He started This form of ventilation differs from the assist with the hypothesis that artificial ventilation mode commonly used in the 1960s. In the assist itself may exacerbate acute respiratory failure. mode the magnitude ofthe breath was preset by Tidal ventilation is required only to remove the adjustment ofcontrols on the ventilator and carbon dioxide, and oxygenation can be main- the breath was initiated whenever the patient tained in apnoeic patients if the carbon dioxide trigger was activated. In inspiratory pressure is removed by an extracorporeal circuit. By support a constant pressure is applied after the removing carbon dioxide with an extracor- patient triggers the ventilator so that the poreal artificial lung and reducing ventilation patient can determine the flow pattern and size to the minimum he has claimed to reduce http://thorax.bmj.com/ of breath. Inspiration is terminated when the mortality from acute respiratory failure.23 inspiratory flow ceases. Conclusion POSITIVE END EXPIRATORY PRESSURE The distinction between a ventilated patient Patients who are anaesthetised or comatose or and a spontaneously breathing patient is who have recently undergone abdominal sur- becoming increasingly blurred as more sophis- gery have a reduced functional residual ticated means of respiratory support are dev- capacity. The reduction may be sufficient to ised. In many cases "respiratory assistance" on September 25, 2021 by guest. Protected copyright. cause airway closure in dependent areas of the may be a more appropriate term than "artificial lung before the end of expiration, leading to ventilation." The efficacy of many of these underventilation ofthese areas and ventilation- ventilatory strategies has not been properly perfusion mismatch.'8 This in turn leads to assessed, in terms either of acceptability to the hypoxaemia. The functional residual capacity patient or of the effect on morbidity and can be maintained by leaving a constant stand- mortality. It is still true, however, that "the ing pressure on the lungs, keeping them type ofventilator used, provided that its design slightly inflated even at the end of expiration. is satisfactory, is of less importance than the This is positive end expiratory pressure or experience of the person using it."'8 PEEP. The use of PEEP, and how best to determine its optimum level, has been the subject of debate for many years. On the positive side, it causes an increase in arterial oxygen tension for the same inspired oxygen tension. On the negative side, the constantly 1 Baker AB. Early attempts at expired air respiration, intuba- tion and manual ventilation. In: Atkinson RS, Boulton raised intrathoracic pressure causes a diminu- TB, eds. The history ofanaesthesia. London: Royal Society tion in venous return to the heart, a decrease in of Medicine, 1987:372-4. 2 Kite C. An essay on the recovery of the apparently dead. cardiac output, and an increased risk of London: Dilly, 1788. pneumothorax. The controversy may now be 3 Goerig M, Filos K, Ayisi KW. George Edward Fell and the resolved, for a recent study showed that mor- development of respiratory machines. In: Atkinson RS, Boulton TB, eds. The history of anaesthesia. London: tality increased with the aggressive application Royal Society of Medicine, 1987:386-93. of PEEP and that the minimum level which 4 Rendell-Baker L, Pettis JL. The development of positive pressure ventilators. In: Atkinson RS, Boulton TB, eds. kept arterial oxygenation within acceptable The history of anaesthesia. London: Royal Society of limits was "best PEEP."'19 Medicine, 1987:402-21. 758 Young, Sykes
5 Samuals MP, Southall DP. Negative extrathoracic pressure and criteria used for weaning from mechanical ventilation. in the treatment of respiratory failure in infants and young Crit Care Med 1987;15:530-3. children. Br Med J 1989;299:1253-7. 16 Pybus DA, Kerr JH. A simple system for adminstering 6 Mapleson WW. The effect of lung characteristics on the intermittent mandatory ventilation (IMV) with the functioning of artifical ventilators. Anaesthesia Oxford ventilator. Br J Anaesth 1978;50:271-4. 1 962;31 :300-14. 17 Lemaire LF. Weaning from mechanical ventilation. In: Thorax: first published as 10.1136/thx.45.10.753 on 1 October 1990. Downloaded from 7 Mushin WW, Rendell-Baker L, Thompson PW, Mapleson Ledingham I, McA, ed. Recent advances in critical care WW. Automatic ventilation of the lungs. Oxford: Black- medicine 3. Edinburgh: Churchill Livingstone, 1988: wells, 1980:312-30. 15-30. 8 Ingelstedt S, Johnson B, Nordstrom L, Olsson S-G. A 18 Sykes MK, McNicol MW, Campbell EJM. Respiratory servo-controlled ventilator measuring expired minute failure. Oxford: Blackwells, 1976:320-2. volume, airway flow and pressure. Acta Anaesthesiol 19 Carroll GC, Tuman KJ, Braverman B, et al. Minimal Scand 1972;supple 47:9-28. positive end-expiratory pressure (PEEP) may be best 9 Drinker P, McKhann CF. The use of a new apparatus for PEEP. Chest 1988;93:1020-5. prolonged administration of artificial respiration. JAMA 20 Smith BE, Hanning CD. Advances in respiratory support. 1929;92: 1658-61. Br J Anaesth 1986;58:138-50. 10 Miller-Jones, CMH, Williams JH. Sedation for ventilation. 21 Zapol WM, Snider M'r, Hill JD, et al. Extracorporeal A retrospective study of fifty patients. Anaesthesia membrane oxygenation in severe acute respiratory failure. 1980;35:1104-7. JAMA 1979;242:2193-6. 11 Merriman HM. 'I'he techniques used to sedate ventilated 22 Toomasian JM, Snedcor SM, Cornell RG, Cilley RE, patients. Intens Care Med 1981;7:217-24. Bartlett RH. National experience with extracorporeal 12 Willatts SM. Paralysis for ventilated patients? Yes or No? membrane oxygenation for newborn respiratory failure. Intens Care Med 1985;11:2-4. Trans Am Soc Artif Intern Organs 1988;34:140-7. 13 Watt I, Ledingham I. McA. Mortality amongst multiple 23 Gattinoni L, Pesenti A, Mascheroni D, et al. Low-frequency trauma patients admitted to an intensive therapy unit. positive-pressure ventilation with extra-corporeal CO, Anaesthesia 1984;39:973-81. removal in severe acute respiratory failure. JAMA 14 Bion JF, Ledingham I. McA. Sedation in intensive care-a 1986;256:881-6. postal survey. Intens Care Med 1987;13:215-6. 24 Sykes MK, McNicol MW, Campbell EJM. Respiratory 15 Venus B, Smith RA, Mathru M. National survey of methods failure. Oxford: Blackwells, 1976:212. http://thorax.bmj.com/ on September 25, 2021 by guest. Protected copyright.