Mechanical ventilation

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The main goal of mechanical ventilation is the support of gas exchange and fatigued respiratory muscles while the precipitating causes of acute respiratory failure are corrected. Before the advent of non-invasive ventilation, mechanical ventilation with endotracheal intubation was used for two categories of patients: those in life-threatening situations such as respiratory or cardiac arrest, extreme exhaustion, or major neurological alterations; those whose condition worsened despite conservative management. However, in the latter category, the early use of non-invasive ventilation can reduce the need for intubation from 75 to 25 per cent and possibly improve survival (Biochard.., efa/ 1995). Therefore, since mechanical ventilation with intubation increases morbidity and mortality, a trial of non-invasive ventilation should be performed in patients with respiratory failure and intubation reserved for those unable to co-operate or whose condition continues to deteriorate.

Non-invasive ventilation

The term 'non-invasive ventilation' applies to various techniques not implying endotracheal intubation or tracheotomy, and includes negative- and positive-pressure ventilation. Positive-pressure ventilation is clearly preferred in the acute setting. Pressure support administered with a face mask or intermittent positive-pressure ventilation through a nasal mask are the two most widely used methods. The former is probably the technique of choice in chronic obstructive pulmonary disease patients, as patient-ventilator interaction is better tolerated than with the latter ( Bro^hiEd,1et a[ 1995). The initial level of pressure support should be around 20

cmH2O, subsequently adjusted to patient tolerance and arterial blood gases. Trigger sensitivity should be maximum, to reduce inspiratory muscle work. The most frequent complications are patient discomfort and dyspnea, excessive air leaks at the junction between mask and face, gastric distension due to air ingestion, and pressure sores along the mask lining.

If pressure support fails, intermittent positive-pressure ventilation with a nasal mask should be attempted. The assist-control mode is preferable, with initial settings of 10 ml/kg tidal volume, 15 to 20 breaths/min respiratory rate, trigger level at maximum sensitivity, and an inspiratory-expiratory time ratio of 1:3. The complications are similar to those observed with pressure support, but air leaks around the nasal mask and pressure sores on the nose are added problems. Readily available masks can be unsuited to the patient's anatomy, but this difficulty can often be overcome by tailor-molding a mask from a special silicon paste.

Air trapping and intrinsic positive end-expiratory pressure (PEEP) due to incomplete emptying of the lung during expiration because of airflow obstruction is often present in these patients (Rossietal 1995). Hence, alveolar pressure is positive rather than atmospheric at the start of inspiration. Successful triggering of the delivery of either pressure support or a ventilator-controlled breath requires the generation of a spontaneous inspiratory flow, which implies that the alveolar pressure must become subatmospheric. More inspiratory muscle work is needed to achieve this if end-expiratory alveolar pressure is positive rather than atmospheric. Applying a PEEP of 5 to 10 cmH2O can reduce the added work required to trigger the ventilator (Fig 1).

Fig. 1 Effects of intrinsic PEEP on respiratory muscle work required to trigger the ventilator. A 2-cmH 2O drop in alveolar pressure Palv and pleural pressure Ppl below pressure at the mouth Pao is assumed to be necessary to induce an inspiratory flow sufficient for triggering. All pressures are expressed relative to atmospheric (0 cmH2O). (a) In the absence of air trapping, Palv and Pao are atmospheric before inspiration. The 2-cmH 2O decrease in Ppl and Palv generates the inspiratory flow. (b) Owing to air trapping, Palv is positive ('intrinsic PEEP'+ 10 cmH2O) before inspiration. If Pao is atmospheric, Ppl and Palv will have to decline by 12 cmH2O to initiate an inspiratory flow. This will result in considerable added inspiratory muscle work. (c) If a PEEP equal to intrinsic PEEP is applied by the respirator, a drop of only 2 cmH2O in Ppl and Palv will be required for inspiration to start, thus reducing respiratory muscle work.

At present, there are no clearly defined criteria for initiating non-invasive ventilatory support. Based on our experience, a patient whose condition is severe enough to warrant intensive care admission but who does not meet the criteria for immediate intubation discussed above should receive a trial of non-invasive ventilation.

Mechanical ventilation with intubation

The intensive care mortality in intubated severe chronic obstructive pulmonary disease (FEV 1 < 1 liter) patients varies between 10 and 30 per cent, with 1-year survival approximately 50 per cent. Prognostic criteria of a fatal outcome are scarce and their predictive ability is at best 70 to 80 per cent. Hence the decision to intubate or not must rest on patient history, in particular previous intubation with difficult weaning, and assessment of such difficult issues as quality of life before admission, nutritional state, age, and known FEV., in stable conditions. An endotracheal tube with an internal diameter of at least 8 mm reduces the work of breathing and allows flexible bronchoscopy to be performed if necessary. Most patients exhibit respiratory muscle fatigue at this stage. Controlled ventilatory modes are preferred for at least 48 h, or until the precipitating event is corrected; the goal is to rest the fatigued respiratory muscles before weaning can start. Sedation with a continuous infusion of a short-acting benzodiazepine, such as midazolam, combined with morphine is suitable. Muscle relaxants should be reserved for situations where ventilation is hampered by excessive rib-cage stiffness or respiratory muscle activity. Initially, a low respiratory rate (10 breaths/min) and tidal volume (8 ml/kg), a high inspiratory flow (60 l/min), and an inspiratory-expiratory time ratio of 1:3 should be used. There are two reasons for using this hypoventilation approach: first, the risk of air trapping is reduced; second, in the presence of increased bicarbonate levels often found in these patients, excessive alveolar ventilation resulting in normo- or hypocapnia can lead to severe alkalosis, with resultant arrhythmias or convulsions. Inspiratory pressures should be monitored to avoid barotrauma.

No consensus exists as to the optimal timing of tracheotomy in patients undergoing protracted ventilation. The low-pressure cuffs of modern endotracheal tubes cause much less damage than the older high-pressure type and can remain in place longer, thus avoiding the small added morbidity of tracheotomy. However, tracheotomy improves patient comfort by allowing eating, drinking, and phonation. Our policy is to perform tracheotomy after 21 days of mechanical ventilation.

The weaning process can be very difficult in chronic obstructive pulmonary disease patients, probably because of respiratory muscle fatigue. Many criteria have been published aimed at determining the optimal timing, clinical or laboratory parameters, and technique for weaning. However, the capacity for an individual patient to resume sustained breathing without any mechanical assistance ultimately depends on the outcome of trials of decreasing ventilatory support. As inspiratory muscle fatigue is probably central to the failure of these trials ( Rqussos... and...Za.ky.n.t.h.i.n..o..s ..1996), factors that decrease strength and endurance or increase the load placed on these muscles should be identified and corrected (Table. .1). Our approach, once the precipitating causes have been corrected, is to stop sedation while the patient is placed in the assist-control mode, followed by synchronized intermittent ventilation modes with pressure support levels of 30 cmH 2O or higher. Pressure support is then used for longer periods of time, with mechanical ventilation being reinstituted if signs suggesting respiratory muscle fatigue develop (respiratory rate of 30 breaths/min or more, intense accessory muscle activity, and acute rise in PaCO2 with decreasing pH). Subsequently, the level of pressure support is reduced. Once the patient can breathe at a pressure support level of 10 cmH2O, a T-piece trial is performed. There is no consensus as to the optimum duration of such a trial before extubation is decided. Indeed, the shorter the trial, the greater is the risk of reintubation, and the longer the trial, the greater is the risk of respiratory muscle fatigue due to the increased work of non-assisted breathing through an endotracheal tube. The optimum time is probably between 2 and 8 h. Our approach is to extubate after at least 2 h of T-piece breathing. In the case of postextubation respiratory distress due to increased secretions, dyspnea, or patient anxiety, non-invasive ventilation can be temporarily useful to avoid reintubation. Patients should be monitored in the intensive care unit for 24 h following successful extubation.

Table 1 Main factors contributing to respiratory muscle fatigue during weaning

Chapter References

Brochard, L., et al. (1995). Noninvasive ventilation for acute exacerbations of chronic obstructive pulmonary disease.

New England Journal of Medicine, 333, 817-22.

Derenne, J.P., Fleury, B., and Pariente, R. (1988). Acute respiratory failure of chronic obstructive pulmonary disease.

American Review of Respiratory Disease, 138, 1006-33.

Rossi, A., Polese, G., Brandi, G., and Conti, G. (1995). Intrinsic positive end-expiratory pressure. Intensive Care Medicine, 21, 522-36.

Roussos, C. and Zakynthinos, S. (1996). Fatigue of the respiratory muscles. Intensive Care Medicine, 22, 134-55.

Ziment, I. (1990). Pharmacologic therapy of obstructive airway disease. Clinics in Chest Medicine, 11, 461-86.

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