Ventilatory support

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Mechanical ventilatory support is an external source of energy which reduces the work of breathing and compensates for apnoeas, hypopnoeas and hypoventilation. It can be provided during wakefulness as well as during sleep.

Indications

In general, long-term ventilatory support is indicated if respiratory failure is causing troublesome symptoms, potentially serious complications such as polycythaemia or pulmonary hypertension, or is likely to lead to these problems or to premature death.

It may also be indicated in asymptomatic high-risk patients, such as those with Duchenne's muscular dystrophy [14] or poliomyelitis, in order to delay or prevent complications from nocturnal respiratory failure. In neuromuscular conditions causing selective diaphragmatic weakness, such as motor neurone disease, assisted ventilation gives symptomatic relief to nocturnal breathlessness, even if there is no evidence for hypoventilation during sleep.

Ventilatory failure should be suspected when there are awakenings from sleep, early morning headaches, excessive daytime sleepiness, worsening breathless-ness and ankle swelling, in a patient with a high-risk disorder, such as following a thoracoplasty or with a kyphosis or scoliosis, especially if the vital capacity is less than 1-1.5 l.

Techniques

Non-invasive techniques are preferred to tracheos-tomy ventilation unless there is upper airway obstruction which prevents them from being effective, or the airway has to be protected because of a risk of aspiration, or ventilatory support is needed almost continuously.

The methods of ventilatory support are as follows. 1 Positive pressure ventilation. This can be achieved during sleep by the following methods.

(a) Tracheostomy ventilation. This invasive method of ventilation is surprisingly well tolerated but requires a higher level of care in the home than the non-invasive methods. Complications such as tube displacement, obstruction, impairment of swallowing and occasionally a tracheo-oesophageal fistula or tracheo-innominate artery fistula may develop.

(b) Mask and mouthpiece ventilation. The nasal masks used for ventilatory support are similar to those used for nasal CPAP (Fig. 11.3), but oronasal masks are occasionally required. If neither can be tolerated a mouthpiece or nasal seals are alternatives. The mask connects to either a pressure or volume preset ventilator. The former is more frequently used, and in neuromuscular and skeletal disorders it is common for the peak inspiratory pressure to be 20-25 cmH2O with a positive end expiratory pressure of 2-4 cmH2O to prevent alveolar closure. The inspiratory time is 0.8-1.0 s with an expiratory time of around 2 s. A sensitive trigger and short response time are preferable in view of the rapid respiratory rate adopted by these patients.

These systems are usually well tolerated, but several difficulties are recognized. Ulceration of the skin of the bridge of the nose is a common problem. The mask may become displaced during sleep, air-leaks around the mask or through the mouth may develop and upper airway symptoms such as a dry or blocked nose may develop. Functional upper airway obstruction may be seen and, if air enters the oesophagus rather than the trachea, abdominal distension with frequent belching, nausea and the passing of flatus may arise.

These complications can usually be overcome by careful attention to the mask and ventilator settings, but occasionally ventilation through a mouthpiece is required. This may lead to dental complications, and air-leaks through the nose. 2 Negative pressure ventilation. With these techniques the chest and abdomen are enclosed in an airtight rigid chamber from which air is evacuated by a ventilator connected to it through wide-bore tubing. Air is drawn in through the mouth and nose. Three types of negative pressure ventilation are available.

(a) Tank ventilation (iron lung). The patient lies supine on a mattress within the chamber which encloses the whole body up to the neck. Access to the subject is limited and this equipment is large, heavy and expensive, but effective. It is rarely used for long-term nocturnal respiratory support, for which a cuirass or jacket is usually preferable.

(b) Jacket ventilation. A framework of metal or plastic provides the rigidity and this is covered by an airtight garment from which air is evacuated by a negative pressure ventilator.

(c) Cuirass ventilation. The properties of rigidity and impermeability to air are combined in a single structure, the cuirass shell, which is usually individually constructed to fit the patient so that it encloses the anterolateral aspects of the rib cage and abdomen (Fig. 11.5). A cuirass is light and durable and, unlike a mask, does not lead to claustrophobia, but it can induce upper airway obstruction due to loss of the normal sequence of activation of the upper airway muscles during inspiration.

Mechanisms of action

The mechanisms of action of non-invasive ventilation in sleep hypoventilation are still uncertain (Fig. 11.6). It consolidates stages 3 and 4 NREM sleep, which is particularly important in relieving symptoms such as excessive daytime sleepiness, and probably in improving the respiratory drive. It also reduces the arterial

Fig. 11.5 Cuirass shell enclosing the patient's rib cage and abdomen with tubing connecting it to a negative presure ventilator.

Fig. 11.5 Cuirass shell enclosing the patient's rib cage and abdomen with tubing connecting it to a negative presure ventilator.

Pco2 while ventilation is applied, and this reduces the cerebrospinal fluid bicarbonate concentration and increases the ventilatory response to hypercapnia.

Non-invasive ventilation also improves the chest wall mechanics by increasing the respiratory excursion of the rib cage and the compliance of the soft tissues of the chest wall. It improves the metabolic environment in which the respiratory muscles function. Hypercapnia, acidosis, hypoxia and other endocrine and metabolic factors which impair contractility can be normalized. It also influences the cardiac output and blood supply to the diaphragm and other respiratory muscles, which is important since muscle fatigue is thought to be related to the balance between the metabolic activity and the quantity of blood flowing to the muscles.

Mechanical ventilation can also affect the intrinsic contractility of the respiratory muscles. Atrophy and functional impairment of respiratory muscles with myofibril damage can develop if these are completely unloaded and respiratory muscle activity abolished

Improvement in sleep architecture and less sleep fragmentation

Respiratory drive T

Improvement in sleep architecture and less sleep fragmentation

i

\

Non-invasive ventilation

\

r

Increased chest wall compliance

Respiratory muscle function Î

Increased chest wall compliance

Alveolar ventilation Î

CSF bicarbonate 4

Fig. 11.6 Mechanisms of action of non-invasive ventilation.

[15]. Most non-invasive ventilatory systems, however, only provide partial respiratory support, which avoids this complication, and if the ventilator is appropriately adjusted it can even become a respiratory muscle training or conditioning device.

Outcomes of treatment

The main outcomes of ventilatory support are as follows.

Quality of life. Nocturnal non-invasive ventilation improves the symptoms of respiratory failure and quality of life. It relieves breathlessness on exertion, improves sleep quality and relieves daytime sleepiness and early morning headaches [16]. Everyday activities such as shopping and cleaning can be carried out more easily. It may be possible to return to work which was previously too physically or mentally demanding.

Physiological changes. Nocturnal ventilation not only improves arterial oxygen saturation and transcutaneous Pco2 during sleep, but also normalizes the blood gases during wakefulness [17]. These improvements may be seen within 1-2 days of initiating treatment and the full effect is usually apparent within 1-4 weeks. Small improvements in vital capacity and maximal inspir-atory and expiratory mouth pressures and respiratory drive may be seen.

Survival. There are no controlled studies of survival in chronic respiratory failure due to neuromuscular and skeletal disorders. In a stable condition such as scoliosis the 1-year survival is around 90% and 5-year survival around 80% [18]. Those with a thoraco-plasty have a slightly worse outlook with survival at 3 years 75-85% and at 5 years over 65%, probably because of extensive pulmonary disease from the tuberculous infection [13].

Death may occur from a recurrence of chronic respiratory failure, despite ventilatory support, because of poor compliance, or due to an acute intercurrent illness, particularly a chest infection.

The prognosis is worse if the underlying neuromus-cular disease is progressive, as in motor neurone disease and Duchenne's muscular dystrophy, or if there is bulbar muscle weakness with a risk of aspiration pneumonia [19].

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