Portal de Periódicos da CAPES

IMV and Weaning

1980; Elsevier BV; Volume: 78; Issue: 6 Linguagem: Inglês

10.1378/chest.78.6.804

1931-3543

M. Henry Williams,

Poverty, Education, and Child Welfare

Once or twice a month a patient is transferred from the general intensive care unit to our respiratory intensive care unit for weaning from a respirator. The problem, monotonously similar among these patients, has resulted from the development of increasingly complicated respirators coupled with the growth of an independent respiratory therapy service. The solution, which is to remove the patient from the respirator, is surprisingly simple.Intermittent mandatory ventilation (IMV) is supposed to encourage patients to generate their own respiratory muscle activity between imposed mechanical inflations of the lung. In fact, many patients with COPD, particularly if elderly, simply do not make effective respiratory efforts when they are given this type of ventilatory support. As the frequency of imposed breaths is reduced, the Pco2 rises and the rate of IMV has to be once again increased so that weaning is thought to be impossible. If one observes such patients, it is obvious that the spontaneous respiratory efforts are extremely ineffective. The registered spontaneous tidal volume may be as small as 100 ml and there is generally paradoxic inward movement of the abdomen during inspiration suggesting lack of contraction of the diaphragm. It is quite clear that this type of respiration will not support life. It is as if the occasional imposed ventilation satisfies patients’ need to breathe enough that they will not make forceful, effective respiratory efforts. Instead, they learn to adopt an ineffective type of respiration. It would seem more appropriate to provide ventilation from the respirator as long as necessary and then, to allow the patient to breathe naturally and properly.If such patients are placed on a T piece with the appropriate inspired oxygen concentration and told to breathe spontaneously without any respiratory support they often will do so quite adequately. Sometimes it helps to encourage diaphragmatic activity by teaching the patient to protrude the abdomen during inspiration. At first it may only be possible to remove the patient from the respirator for a very short period before fatigue results, but this can be done for increasingly longer intervals until weaning is complete. With rare exception, patients deemed unweanable on IMV have been successfully removed from respiratory assistance within 24 to 48 hours by this old-fashioned technique.There are no accurate guidelines to make it possible to predict when a patient can be removed from a respirator. The way that patients breathe between assisted ventilations is no indication of how they will breathe when on their own. Maximal inspiratory pressure, respiratory dead space, expiratory flow rates and vital capacity may be far from what is considered adequate for spontaneous respiration, not only in these patients, but in many other patients with COPD who are actually ambulatory and self-sufficient. In fact, the only accurate criterion of weaning is weaning itself.IMV may have value in the management of some types of patients with respiratory insufficiency, particularly formerly healthy patients with acute pulmonary damage. It may, in some patients, make it possible to reduce ventilatory support and discontinue the use of a mechanical respirator without requiring that much time be spent with the patient. When weaning is accomplished by discontinuing respirator support, knowledgeable personnel must be present to observe the patient and reinstitute mechanical ventilation at the first sign of distress. However, the substitution of technology for personnel can greatly prolong the length of time that many patients spend on a respirator. Once or twice a month a patient is transferred from the general intensive care unit to our respiratory intensive care unit for weaning from a respirator. The problem, monotonously similar among these patients, has resulted from the development of increasingly complicated respirators coupled with the growth of an independent respiratory therapy service. The solution, which is to remove the patient from the respirator, is surprisingly simple. Intermittent mandatory ventilation (IMV) is supposed to encourage patients to generate their own respiratory muscle activity between imposed mechanical inflations of the lung. In fact, many patients with COPD, particularly if elderly, simply do not make effective respiratory efforts when they are given this type of ventilatory support. As the frequency of imposed breaths is reduced, the Pco2 rises and the rate of IMV has to be once again increased so that weaning is thought to be impossible. If one observes such patients, it is obvious that the spontaneous respiratory efforts are extremely ineffective. The registered spontaneous tidal volume may be as small as 100 ml and there is generally paradoxic inward movement of the abdomen during inspiration suggesting lack of contraction of the diaphragm. It is quite clear that this type of respiration will not support life. It is as if the occasional imposed ventilation satisfies patients’ need to breathe enough that they will not make forceful, effective respiratory efforts. Instead, they learn to adopt an ineffective type of respiration. It would seem more appropriate to provide ventilation from the respirator as long as necessary and then, to allow the patient to breathe naturally and properly. If such patients are placed on a T piece with the appropriate inspired oxygen concentration and told to breathe spontaneously without any respiratory support they often will do so quite adequately. Sometimes it helps to encourage diaphragmatic activity by teaching the patient to protrude the abdomen during inspiration. At first it may only be possible to remove the patient from the respirator for a very short period before fatigue results, but this can be done for increasingly longer intervals until weaning is complete. With rare exception, patients deemed unweanable on IMV have been successfully removed from respiratory assistance within 24 to 48 hours by this old-fashioned technique. There are no accurate guidelines to make it possible to predict when a patient can be removed from a respirator. The way that patients breathe between assisted ventilations is no indication of how they will breathe when on their own. Maximal inspiratory pressure, respiratory dead space, expiratory flow rates and vital capacity may be far from what is considered adequate for spontaneous respiration, not only in these patients, but in many other patients with COPD who are actually ambulatory and self-sufficient. In fact, the only accurate criterion of weaning is weaning itself. IMV may have value in the management of some types of patients with respiratory insufficiency, particularly formerly healthy patients with acute pulmonary damage. It may, in some patients, make it possible to reduce ventilatory support and discontinue the use of a mechanical respirator without requiring that much time be spent with the patient. When weaning is accomplished by discontinuing respirator support, knowledgeable personnel must be present to observe the patient and reinstitute mechanical ventilation at the first sign of distress. However, the substitution of technology for personnel can greatly prolong the length of time that many patients spend on a respirator.