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Respiratory support and intensive care
Current Anaesthesia and Critical Care (1996) 7, 219-220
© 1996 Pearson Professional Ltd
Editorial
Respiratory support and intensive care
N. Soni
The days when intensive care and respiratory support were synonymous have hopefully passed. Nevertheless respiratory support is a fundamental part of management in the Intensive Care Unit. While the basic principles of respiratory support are not changing the technologies and clinical methodologies which may be applied in difficult cases are evolving rapidly. It should be remembered that the majority of ITU patients do not require sophisticated ventilatory methods. The impetus for evolution has come partially from trying to achieve a balance between methods of improving the efficacy of respiratory support and their associated complications and partly from the advances in ventilator technology, some of which are not necessarily medically driven. In the broadest terms, respiratory support facilitates gas exchange from atmosphere through lungs to blood and then on into the tissues. The mechanics of ventilators allow substitution for not only the mechanics of the chest wall but also the neurological components of respiratory drive both centrally and peripherally. It can also be useful for overcoming airways resistance. These are simple and usually effective mechanical functions. Current methods of respiratory support are far less effective in improving gas exchange at alveolar and capillary level. Indeed mechanical ventilatory effects on V/Q abnormality and haemodynamic function may be deleterious. Historically, simple objectives such as maintaining ventilation during the polio epidemic produced good results. As the objectives become increasingly sophisticated, involving mechanisms of gas exchange and oxygenation, these are far less easily achieved with such basic ventilatory support as is currently available. There
are three prevailing directions in the development of new techniques. The first is to minimize lung injury from the mechanics of ventilation, barotrauma and volutrauma. This encourages means of reducing pressures generally and trying to prevent alveolar collapse. The second is to optimize oxygen exchange by a range of means of altering ventilation to hopefully influence V/Q. The third is the progressive development of weaning strategies. Fundamental to developmental progress is the inescapable fact that conventional support uses positive pressure ventilation. This immediately imposes an abnormal physiological state which is significantly different from that during spontaneous breathing. 'Advances' in respiratory support are usually methods of overcoming or minimizing the impact of positive pressure ventilation. Where techniques are specifically geared to improving oxygen delivery there is inevitably a compromise between the pathophysiology of the disease and that of the treatment. As with any medical management where there is obvious room for improvement, respiratory support is frequently the victim to the phenomenon of vogue treatments. Each new advance in respiratory support goes through a well described pattern before acceptance. Initial caution is supplanted by overwhelming enthusiasm only to be tempered by disappointment and disillusionment and it sometimes seems that it is only at this stage that the physiology of the method becomes understood. The benefits and limitations of the technique are defined and it finds its niche in the respiratory armamentarium. It is important to note how difficult it is to produce studies to show 'significant' benefit from ventilatory techniques whether it be positive end-expiratory pressure, inverse ratio ventilation or weaning techniques and it is still a long way from evidence-based medicine. Inverse ratio ventilation, hailed as being a
Dr Neil Soni, Senior Lecturer, Charing Cross and Westminster Medical School, Magill Department of Anaesthetics, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK.
219
220
CURRENTANAESTHESIAAND CRITICALCARE
major breakthrough in severe acute lung injury, seemed to be the answer for every eventuality. Gradually as it has been examined more thoroughly it has found a permanent although somewhat limited place in respiratory support. 1 The advantages and disadvantages of the technique, with lower mean airway pressure but better sustained airway pressure could always have been predicted from the physiology. The benefits of improved alveolar recruitment, possibly enhanced V/Q and thereby improved oxygenation, are offset by haemodynamic impairment as a consequence of sustained positive intrathoracic pressure. This technique has not revolutionized respiratory support as we know it but it has enhanced our understanding of ventilatory methods and has given us an additional tool in dealing with difficult cases. Likewise weaning has always been a contentious issue) Fashions have come and gone. The era of episodes off ventilation each hour to train patients to breathe again was supplanted by IMV, intermittent mandatory ventilation where the patient could breathe by themselves. Then synchronized intermittent mandatory ventilation where there was less possibility of the patient either fighting the ventilator or getting a double dose of a spontaneous and ventilated breath at the same time. This has given way to means of assisting weak ventilatory effort, such as pressure support ventilation which aids inspiration or continuous positive airway pressure (CPAP) which may help some patients .3 There is rediscovery that the active patient with working muscle groups does better than the immobile patient and that breathing in the sitting position is easier than lying. There is also recognition that a patient who can sit out of bed, communicate and is cooperative is more likely to breathe well than if they are dazed, confused, half sedated and supine. All of these 'advances' in weaning are recognition of the normal physiology of spontaneous breathing, the importance of muscle function and in particular coordination, the integration of breathing with other organ systems and the general well-being of the whole patient.
There are regularly new developments in respiratory support. 4 In this symposium aspects of paediatric management have been included. This is a fertile area for ideas and several major innovations have started in paediatric intensive care units. Currently liquid ventilation is being evaluated and developed in that setting. It remains to be seen whether it will spread into adult practice. The other area of innovation is domiciliary ventilation. Techniques such as non-invasive nasal ventilation are applicable into weaning programs in ITU. There are patients in the wards, such as those with pneumocystis pneumonia, in whom a 'half way house' of non-invasive ventilation may avoid full ventilation. 5 There are increasing numbers of patients coming to ITU who have been on domiciliary ventilation programs. There are also those who may benefit from some form of respiratory support in the ward or at home on leaving the ITU. The intention of this symposium is to provide an overview of the prevalent methods available for respiratory support in ITU at the present time. It hopefully emphasizes that when new techniques appear the benefits and problems of techniques can usually be predicted from the physiology of the patient and of the technique.
References 1. Mancebo J, VallverduI, Bak E, et al. Volume-controlledventilation and pressure-controlledinverse ratio ventilation: a comparisonof their effects in ARDS patients. MonaldiArch Chest Dis 1994; 49(3): 201-207. 2. Esteban A, Alia I, Ibanez J, Benito S, Tobin M J. Modes of mechanical ventilation and weaning. A national survey of Spanish hospitals. Spanish Lung Failure Collaborative Group. Chest 1994; 106(4): 1188-1193. 3. Esteban A, Frutos F, Tobin M J, et al. A comparisonof four methods of weaning patients from mechanicalventilation. Spanish Lung Failure Collaborative Group. N Engl J Med 1995; 332(6): 345-350. 4. Tobin M. Mechanical ventilation. N Engl J Med 1994; 330(15): 1056-1061. 5. Gachot B, Clair B, WolffM, RegnierB, VachonE Continuous positive airway pressure by face mask or mechanicalventilation in patients with human irnmunodeficiencyvirus infection and severe Pneumocystis carinii pneumonia. IntensiveCare Med 1992; 18(3): 155-159.
© 1996 Pearson Professional Ltd
Editorial
Respiratory support and intensive care
N. Soni
The days when intensive care and respiratory support were synonymous have hopefully passed. Nevertheless respiratory support is a fundamental part of management in the Intensive Care Unit. While the basic principles of respiratory support are not changing the technologies and clinical methodologies which may be applied in difficult cases are evolving rapidly. It should be remembered that the majority of ITU patients do not require sophisticated ventilatory methods. The impetus for evolution has come partially from trying to achieve a balance between methods of improving the efficacy of respiratory support and their associated complications and partly from the advances in ventilator technology, some of which are not necessarily medically driven. In the broadest terms, respiratory support facilitates gas exchange from atmosphere through lungs to blood and then on into the tissues. The mechanics of ventilators allow substitution for not only the mechanics of the chest wall but also the neurological components of respiratory drive both centrally and peripherally. It can also be useful for overcoming airways resistance. These are simple and usually effective mechanical functions. Current methods of respiratory support are far less effective in improving gas exchange at alveolar and capillary level. Indeed mechanical ventilatory effects on V/Q abnormality and haemodynamic function may be deleterious. Historically, simple objectives such as maintaining ventilation during the polio epidemic produced good results. As the objectives become increasingly sophisticated, involving mechanisms of gas exchange and oxygenation, these are far less easily achieved with such basic ventilatory support as is currently available. There
are three prevailing directions in the development of new techniques. The first is to minimize lung injury from the mechanics of ventilation, barotrauma and volutrauma. This encourages means of reducing pressures generally and trying to prevent alveolar collapse. The second is to optimize oxygen exchange by a range of means of altering ventilation to hopefully influence V/Q. The third is the progressive development of weaning strategies. Fundamental to developmental progress is the inescapable fact that conventional support uses positive pressure ventilation. This immediately imposes an abnormal physiological state which is significantly different from that during spontaneous breathing. 'Advances' in respiratory support are usually methods of overcoming or minimizing the impact of positive pressure ventilation. Where techniques are specifically geared to improving oxygen delivery there is inevitably a compromise between the pathophysiology of the disease and that of the treatment. As with any medical management where there is obvious room for improvement, respiratory support is frequently the victim to the phenomenon of vogue treatments. Each new advance in respiratory support goes through a well described pattern before acceptance. Initial caution is supplanted by overwhelming enthusiasm only to be tempered by disappointment and disillusionment and it sometimes seems that it is only at this stage that the physiology of the method becomes understood. The benefits and limitations of the technique are defined and it finds its niche in the respiratory armamentarium. It is important to note how difficult it is to produce studies to show 'significant' benefit from ventilatory techniques whether it be positive end-expiratory pressure, inverse ratio ventilation or weaning techniques and it is still a long way from evidence-based medicine. Inverse ratio ventilation, hailed as being a
Dr Neil Soni, Senior Lecturer, Charing Cross and Westminster Medical School, Magill Department of Anaesthetics, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK.
219
220
CURRENTANAESTHESIAAND CRITICALCARE
major breakthrough in severe acute lung injury, seemed to be the answer for every eventuality. Gradually as it has been examined more thoroughly it has found a permanent although somewhat limited place in respiratory support. 1 The advantages and disadvantages of the technique, with lower mean airway pressure but better sustained airway pressure could always have been predicted from the physiology. The benefits of improved alveolar recruitment, possibly enhanced V/Q and thereby improved oxygenation, are offset by haemodynamic impairment as a consequence of sustained positive intrathoracic pressure. This technique has not revolutionized respiratory support as we know it but it has enhanced our understanding of ventilatory methods and has given us an additional tool in dealing with difficult cases. Likewise weaning has always been a contentious issue) Fashions have come and gone. The era of episodes off ventilation each hour to train patients to breathe again was supplanted by IMV, intermittent mandatory ventilation where the patient could breathe by themselves. Then synchronized intermittent mandatory ventilation where there was less possibility of the patient either fighting the ventilator or getting a double dose of a spontaneous and ventilated breath at the same time. This has given way to means of assisting weak ventilatory effort, such as pressure support ventilation which aids inspiration or continuous positive airway pressure (CPAP) which may help some patients .3 There is rediscovery that the active patient with working muscle groups does better than the immobile patient and that breathing in the sitting position is easier than lying. There is also recognition that a patient who can sit out of bed, communicate and is cooperative is more likely to breathe well than if they are dazed, confused, half sedated and supine. All of these 'advances' in weaning are recognition of the normal physiology of spontaneous breathing, the importance of muscle function and in particular coordination, the integration of breathing with other organ systems and the general well-being of the whole patient.
There are regularly new developments in respiratory support. 4 In this symposium aspects of paediatric management have been included. This is a fertile area for ideas and several major innovations have started in paediatric intensive care units. Currently liquid ventilation is being evaluated and developed in that setting. It remains to be seen whether it will spread into adult practice. The other area of innovation is domiciliary ventilation. Techniques such as non-invasive nasal ventilation are applicable into weaning programs in ITU. There are patients in the wards, such as those with pneumocystis pneumonia, in whom a 'half way house' of non-invasive ventilation may avoid full ventilation. 5 There are increasing numbers of patients coming to ITU who have been on domiciliary ventilation programs. There are also those who may benefit from some form of respiratory support in the ward or at home on leaving the ITU. The intention of this symposium is to provide an overview of the prevalent methods available for respiratory support in ITU at the present time. It hopefully emphasizes that when new techniques appear the benefits and problems of techniques can usually be predicted from the physiology of the patient and of the technique.
References 1. Mancebo J, VallverduI, Bak E, et al. Volume-controlledventilation and pressure-controlledinverse ratio ventilation: a comparisonof their effects in ARDS patients. MonaldiArch Chest Dis 1994; 49(3): 201-207. 2. Esteban A, Alia I, Ibanez J, Benito S, Tobin M J. Modes of mechanical ventilation and weaning. A national survey of Spanish hospitals. Spanish Lung Failure Collaborative Group. Chest 1994; 106(4): 1188-1193. 3. Esteban A, Frutos F, Tobin M J, et al. A comparisonof four methods of weaning patients from mechanicalventilation. Spanish Lung Failure Collaborative Group. N Engl J Med 1995; 332(6): 345-350. 4. Tobin M. Mechanical ventilation. N Engl J Med 1994; 330(15): 1056-1061. 5. Gachot B, Clair B, WolffM, RegnierB, VachonE Continuous positive airway pressure by face mask or mechanicalventilation in patients with human irnmunodeficiencyvirus infection and severe Pneumocystis carinii pneumonia. IntensiveCare Med 1992; 18(3): 155-159.