Equipment for NIPPV in neuromusculo-skeletal disease

Anita K Simonds

Consultant in Respiratory Medicine

Royal Brompton & Harefield NHS Trust

Sydney Street, London SW3 6NP

For patients with neuromuscular, central neurological, or chest wall disorders there are special pathophysiological considerations when choosing a ventilator which are listed in Table 1 below.

Table 1.

Disorder Pathophysiological considerations

Neuromuscular eg. old polio, myopathy Often inspiratory & expiratory muscle

muscular dystrophy, motor neurone disease weakness

Tendency to poor cough.

Atelectasis, microatelectasis.

May be bulbar weakness, aspiration

Lungs usually compliant

Condition may be progressive

Mobility and hand control may be limited

Chest wall disease eg. scoliosis, thoracoplasty Chest wall compliance low.

fibrothorax, scleroderma May be additional pulmonary fibrosis

Cough usually preserved

Bulbar function tends to be normal

Large airway distortion can occur

Condition usually non-progressive

Central neurological disease eg. Congenital Absent or reduced ventilatory drive

hypoventilation syndrome, brainstem CVA, May be bulbar weakness, aspiration

cervical cord lesion and limb weakness. Can be

progressive

Tracheostomy ventilation (T-IPPV), negative pressure ventilation (nPV) or non-invasive

positive pressure ventilation (NIPPV)?

By and large, this is not a difficult question to answer. Individuals with severe bulbar weakness, who are 24 hour ventilator dependent will require T-IPPV. However, improvement in bulbar function can occur eg. following a CVA, and gradual recovery may take place in ventilatory capacity eg after low cervical spinal cord lesion, so continued reassessment to identify those who can be stepped down to a less invasive method of support is crucial. By the same token, patients with motor neurone disease (MND/ALS) or Duchenne muscular dystrophy (DMD) may develop progressive weakness and bulbar symptoms and require transfer for non-invasive to invasive ventilation.

It has been pointed out that 24 hour ventilator dependency is not an absolute indication for T-IPPV, as a proportion of these patients may be managed round the clock with NIV, nPV or a combination of techniques such as NIPPV, mouth ventilation and a pneumobelt. In some situations the use of NIPPV together with mechanical cough assistance may produce less pulmonary morbidity than T-IPPV. However, these judgements need to be made on a individual basis.

Negative pressure ventilation has been used for many years in small children and in the last few years, masks suitable for even neonatal use have become available. However, for babies there is little experience using long term non-invasive techniques, and so T-IPPV tends to be used especially if there are underlying conditions such as bronchopulmonary dysplasia, tracheo-bronchomalacia, congenital central hypoventilation syndrome (CCHS) or severe type 1 spinal muscular atrophy (SMA). Some children with less progressive disorders may be successfully transferred to mask ventilation when a few years old. Although applied extensively in the 1940s and 50s in patients with neuromuscular disase, particularly acute poliomyelitis, negative pressure ventilation is now use rarely in adults with acute or chronic ventilatory failure. In individuals with stable chest wall or neuromuscular disease nPV may be as effective as NIPPV, but efficiency is limited by the fact it may predispose the individuals to obstructive sleep apnoea, is not readily available and often inconvenient.

NIPPV

Non-invasive mask ventilation is therefore the preferred option in acute exacerbations of chronic lung disease and for long term home use in those with neuromusculo-skeletal disease, providing they do not meet the criteria for T-IPPV described above. The choice of ventilator lies between volume and pressure preset devices.

Volume or pressure ventilators

There is some confusion in terminology with some authors using the terms volume and pressure targeted ventilation, others using the term volume and pressure triggered ventilation. These terms are often used interchangeably, but strictly speaking some pressure ventilators are triggered in response to changes in flow and in some cases do not reach preset target levels eg.due to leak. In the interests of clarity, the terms pressure preset and volume preset will be used . Pressure preset machines include those which deliver bilevel pressure support. In the last few years models which incorporate both volume and pressure preset modes have been brought onto the market.

In theory, the performance of volume and pressure preset ventilators will differ in several important ways as listed below (Table 2). In practice, few comparative studies have been carried out, and many of these have used lung models which do not necessarily accurately represent lung and chest wall mechanics in vivo. Examples of the different types of ventilator are given in Table 3.

Table 2 Differences between volume preset and pressure preset ventilators

Characteristic	Volume preset ventilator	Pressure preset ventilator
Delivery	Delivers a constant tidal volume in the face of changing airways resistance and lung compliance.	Delivered tial volume will fall with increasing airways resistance or fall in lung compliance.
Leak compensation	Poor leak compensation	Good leak compensation
Addition of PEEP/EPAP	Can add PEEP, but many models do not incorporate this	EPAP available on bilevel pressure support machines
Peak airway pressure	Difficult to limit peak airway pressure	Can preset maximum IPAP which can be advantageous in patients with previous pneumothorax, bullous lung disease, hyperinflation or gastic distension
Size	Ventilators tend to be bulky	Usually smaller than volume preset models

Table 3

Examples of volume and pressure ventilators

Pressure Bilevel pressure support Volume

Nippy (B&D Electromedical) BiPAP (Repironics) PV 501 (Breas)

PV 401 (Breas) Harmony (Respironics) PLV 100/102 (Lifecare)

VPAP ST (Resmed) BromptonPAC (PneuPAC)

PV 102 (Breas)

Combination volume & pressure modes

PV 403 (Breas)

LTV 1000 (Pulmonetic Systems)

Achieva (Puritan Bennett)

High Dependency Unit ventilators

Vision (Respironics)

LTV1000 (Pulmonetic Systems)

May use other ventilators with suitable monitoring and alarm modules

Bilevel pressure support ventilation

This offers the potential advantages of comfort and the presence of positive pressure in expiration (EPAP). EPAP can:

· Prevent rebreathing of CO2 (minimum EPAP of 4 cmH2O recommended with expiratory systems such as whisper swivel valve)

· Stabilise the upper airway during sleep

· Recruit alveoli and thereby increase functional residual capacity

· Decrease a tendency to mico or macroatelectasis

· Reduce the inspiratory work required to trigger inspiration in patients with intrinsic PEEP

All these attributes, apart from the ability to reduce the work of breathing in the presence of autoPEEP (rare in neuromusculo-skeletal disase) may be especially relevant to the patients with neuromuscular disease.

Proportional assist ventilation (PAV) , available on the Vision ventillator (Respironics Inc.) has recently been introduced as a mode which is responsive to the patient’s respiratory effort and therefore may aid neuro-mechanical coupling and comfort. Further work is required to see whether this is the case for chest wall and neuromuscular patients. As the mode is contingent on respiratory effort, PAV should not be used without a back-up mode in patients with depressed respiratory drive (eg. during sleep).

Triggered, assist/control or controlled mode ventilation?

In triggered or assist mode the user is required to make a respiratory effort to generate a breath, whereas in assist/control mode (also known as spontaneous /timed mode) breaths can be triggered, but there is a back-up controlled automatic cycling rate which operates if the patient fails to trigger the machine for a predetermined period of time. Ventilators set in control mode deliver breaths regardless of patient effort. In most patients breathing is most comfortably and safely augmented using assist/control mode. Patients will usually trigger the ventilator during wakefulness, but many with neuromuscular and chest wall disorders are reliant on the ventilator working in control mode during sleep. Control mode may be helpful occasionally when there are major problems in reducing the PaCO2 level, or the patient suffers from primary alveolar hypoventilation syndrome.