Time to initiate weaning
The weaning process is started when the patient's general condition is stable and the likelihood of success is judged to be good. Weaning is unlikely to be successful and must not be started if the conditions in Table.3 are not satisfied. Potential causes of weaning failure should be excluded. Patients with heart failure or chronic obstructive pulmonary disease may experience cardiopulmonary stress induced by weaning. This weaning-initiated left ventricular dysfunction should be recognized, and appropriate diuretic, vasodilator, or bronchodilator therapy given. The optimum level of arterial PCO2 to maintain prior to weaning is unknown. Zealous correction of hypercarbia in patients with ventilatory dysfunction shortly before weaning may subsequently increase the patient's work of breathing. With patient-assisted ventilation, the patient will usually maintain a satisfactory PCO2 level. Weaning is best started in the morning, when a full complement of staff is available.
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Table 3 Conditions necessary before weaning is initiated
The choice of weaning mode is determined by the attending clinician. Intermittent mandatory ventilation and pressure support ventilation are frequently used in combination. Both may be gradually decreased, or the level of one may be gradually decreased while the other is kept constant. One may be used as the predominant mode. For example, intermittent mandatory ventilation may be used and pressure support ventilation kept constant at a low level to overcome breathing circuit resistance. Alternatively, pressure support ventilation may be used with a constant low intermittent mandatory ventilation rate (e.g. 2-4 breaths/min). Mandatory minute volume ventilation, incorporating intermittent mandatory ventilation or pressure support ventilation, may be used if available. Despite proper use of a weaning mode, patients may sometimes feel more comfortable with another mode or combination mode. It seems reasonable to try another predominant mode on an empirical basis if the patient appears unsettled.
There are good reasons to introduce CPAP with whichever weaning mode chosen. The beneficial effects of CPAP stem from an increase in functional residual capacity and compliance, which potentially improve gas exchange and decrease work of breathing. Low levels of CPAP (e.g. below 5 cmH 2O) can reduce inspiratory work without increasing intrinsic positive end-expiratory pressure (auto-PEEP) and dynamic hyperinflation.
With all patient-assisted modes, triggering sensitivity and inspiratory flow rate must be adjusted to give the best co-ordination between patient and ventilator. Less effort is required for the patient to initiate inspiration with flow triggering (in ventilators with this facility) than with pressure triggering. Attention should be paid to the endotracheal tube size, humidifier and circuit resistance, equipment dead-space, demand valve, and fresh gas flow rate to minimize inspiratory work. Circuits which offer high resistances to air flow must be identified and rejected.
Whatever the mode or combination of modes used, unassisted breathing through a T tube or CPAP circuit can be the final stage of the weaning process. Bi-positive airway pressure can be tried as an intermediate stage between intermittent mandatory ventilation/pressure support ventilation and CPAP breathing for patients who are particularly difficult to wean. The CPAP level is gradually decreased to 2 cmH 2O, which approximates to intrinsic PEEP. Extubation at this low level of CPAP (without decreasing CPAP to zero) is proposed by some, as better effects on lung volume and oxygenation are claimed.
The optimal pace of weaning from mechanical ventilation is not established and has to be individually assessed. Respiratory muscles must not be overburdened, as fatigued muscles may require 10 h or more to recover. Relatively abrupt weaning may be applicable for patients with readily reversible forms of respiratory failure. However, graded weaning is necessary for sicker patients and those who recover more slowly. A more rapid weaning pace with earlier extubation must be weighed against a higher likelihood of reintubation. Adequate ventilatory support must be provided between weaning periods, particularly at night to permit sufficient sleep.
Close observation and good bedside care are essential during the weaning process. The patient should be positioned upright in bed or, if able to be moved out of bed, positioned sitting up in a chair. Mobilization is to be encouraged. Skilled physiotherapy, clearance of respiratory tract secretions, and monitoring of respiratory variables must be performed regularly. The use of pulse oximeters and CO 2 analyzers has reduced the need for frequent arterial blood gas sampling, but these tests should not be neglected. Stimulation of the respiratory center with analeptic agents is unlikely to be useful and cannot be recommended for routine use. Theophylline has been reported to improve diaphragmatic contractility, but this effect is small and evidence of clinical usefulness is lacking. The progress of the patient must be continually assessed. Criteria to terminate weaning do not exist. The attending nurse and doctor must appraise the level of fatigue that necessitates halting the weaning trial without waiting for absolute exhaustion to occur. Signs of cardiorespiratory distress must be recognized early.
Attention must be given to alleviating the patient's physical and emotional stress. Some patients may become extremely agitated during weaning. Pain may be present. Sedative agents and analgesics should be given cautiously. A continuous low-dose infusion of an opioid is often useful, supplemented if necessary by benzodiazepines. Regional local analgesic blocks can be applied where appropriate. Good communication, reassurance, and encouragement by staff (particularly to take periodic deep breaths) contribute to psychological support. A non-stressful environment and adequate rest and sleep between breathing trials should be part of the therapeutic aims.
Adequate nutrition must be given to provide energy for weaning, but excessive calories and high carbohydrate loads lead to excessive CO 2 production and lipogenesis, and must be avoided. For the majority of patients, 2000 to 3000 kcal/day will be appropriate, but the optimal nutritional regimen for weaning from mechanical ventilation is unknown.
Concurrent with providing nutrition, potential problems of sepsis, anemia, electrolyte imbalance, endocrine dysfunction, acid-base disorders, and unwanted effects of medications must be addressed. Potassium, calcium, magnesium, and phosphate are important minerals for muscle function, and deficiencies must be promptly corrected.
After extubation/separation from ventilator
Weaning from mechanical ventilation can occupy up to 41 per cent of the total time of ventilatory support. Nonetheless, extubation does not mark the end of the weaning process. The patient must continue to be closely monitored for 24 to 48 h following separation from the ventilator, when rapid deterioration can occur. Patients who were difficult to wean may benefit from continuing ventilatory support by non-invasive means after removal of their endotracheal tubes. Non-invasive ventilation or mask/nasal CPAP or bi-positive airway pressure may sometimes obviate the need for reintubation in those who initially deteriorate. There is no urgency to extubate patients with tracheostomies. Oxygen therapy, physiotherapy, airway toilet, and supportive care remain vitally important.
Some weaning techniques and strategies have been reported to facilitate or shorten the weaning process. They include respiratory muscle training, biofeedback, and use of an exclusive weaning team. The validity of these claims has not been universally accepted. Fundamental principles, such as close monitoring by staff and attention to the many factors that can adversely affect the process, are more important. An organized approach for each patient should be followed.
Brochard, L., et al. (1994). Comparison of three methods of gradual withdrawal from ventilatory support during weaning from mechanical ventilation. American Journal of Respiratory and Critical Care Medicine, 50, 896-903.
Esteban, A., et al. (1995). A comparison of four methods of weaning patients from mechanical ventilation. New England Journal of Medicine, 332, 345-50.
Knebel, A.R., Shekleton, M.E., Burns, S., Clochesy, J.M., Goodnough Hanneman, S.K., and Ingersoll, G.L. (1994). Weaning from mechanical ventilation: concept development. American Journal of Critical Care, 3, 416-20.
Slutsky, A.S. (1994). Consensus conference on mechanical ventilation—January 28-30, 1993 at Northbrook, Illinois, USA. Part 2. Intensive Care Medicine, 20, 150-62.
Yang, K.L. and Tobin, M.J. (1991). A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. New England Journal of Medicine, 324, 1445-50.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.