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Non-invasive positive pressure ventilation

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VOL: 97, ISSUE: 26, PAGE NO: 52

Doug Sawkins, BSc, SRN, RNMS, ONC, is a senior charge nurse, Barnet General Hospital, Hertfordshire

Advances in the technology of applying continuous positive airway pressure, along with better informed medical and nursing staff, mean that the treatment of severe dyspnoea now includes the option of non-invasive positive pressure ventilation (NIPPV).

The British Thoracic Society (BTS) recommends NIPPV as a first-line treatment. Five years ago it would have been rare for patients in need of any form of artificial ventilation to be nursed on a general ward, but it is becoming increasingly common. Ideally, patients requiring NIPPV should be nursed on a respiratory ward, but this type of ventilation is often initiated in other clinical areas when a patient’s condition deteriorates unexpectedly. This shows how important it is for nurses in generalist areas to be skilled in the technique.

The equipment

The development of simple-flow generators for NIPPV stems from the work by Sullivan et al (1981) on patients with obstructive sleep apnoea. The treatment of choice is continuous positive airway pressure (CPAP). These generators deliver a constant pressure during inspiration and expiration, with a flow of air that prevents the occlusion of the airway during sleep - they are often described as pneumatic splints. With NIPPV the generators are more complicated. Instead of one constant pressure, two different pressures are set: a higher pressure for the inspiratory phase and generally a lower pressure for the expiratory period (Box 1).

NIPPV is also referred to as bi-level, bi-phasic, Bipap or Vpap ventilation, depending on the literature and the manufacturer of the generator. Volume-controlled ventilators were popular in the past, but since the development of microprocessor-controlled valves and other technical advances NIPPV via pressure-controlled ventilators is becoming the norm.

The generators can have up to three modes:

- Spontaneous: senses the breathing patterns of the patient and automatically alternates between inspiratory positive airway pressure (IPAP) and exhalatory positive airway pressure (EPAP), synchronising with the patient’s respiratory cycle. It is used if there are no signs of respiratory depression.

- Spontaneous/timed: as for spontaneous, but in addition it is possible to set a breaths per minute (BPM) back-up, usually just below the patient’s current respiratory rate. If the patient fails to achieve this rate the ventilator will automatically initiate enough cycles to maintain the set BPM.

- Timed: the ventilator delivers IPAP and EPAP cycles at a predetermined rate. It is not triggered by the patient’s respiratory effort. This mode allows for the inspiratory/expiratory ratio to be altered. The normal ratio is about 1:2.

There is some confusion over the terms CPAP and NIPPV and their application, which is compounded by both systems using similar masks and machines (Keilty and Moxham, 1995). A useful guide is to use CPAP for correcting hypoxia and NIPPV for the treatment of hypoxia with hypercapnia.

Indications for NIPPV

NIPPV is typically indicated for patients presenting with an acute or chronic airway disease where there is hypoventilation, severe dyspnoea, PO2 <6mmHg, PCO2 >6mmHg or PH <7.35. Other conditions include motor nerve disorders, neuromuscular disease, ventilatory muscle disease and abnormalities of the thoracic cage or symptom control, for example in patients with cancer of the lung.

The advantages of NIPPV are:

- Intubation is avoided;

- A bed in the intensive care unit is not needed;

- It is non-invasive (lower risk of infection);

- It requires a shorter hospital stay;

- Mortality rates are lower;

- It can be used at home.

The contraindications include:

- Patient non-cooperation/compliance;

- Head/facial injuries that prevent the application of the face mask;

- Pneumothorax;

- Excessive secretions;

- Vomiting;

- The patient’s inability to maintain his or her own airway.

Initiating ventilation

There is no doubt that patient cooperation and compliance is crucial to the success of NIPPV, and obtaining it has been described as an art. Patients will have severe dyspnoea and may be fighting for breath, have difficulty speaking and be exhausted from the effort of breathing. They may already have a degree of hypoxia (arterial blood gases are taken before starting ventilation). But these potentially insurmountable obstacles can be overcome by giving the patient calm, clear instructions and information with patience and tact. The doctor in A&E may not have the time to do this and a physiotherapist may not be available, but a nurse is available 24 hours a day. A clear explanation of what you are going to do is essential.

Tolerance of the treatment can be assessed by getting the patient to hold the mask against their faces and asking them to breathe with the machine at the pressures that have been set. Involving patients throughout the process empowers them and allows them some control over what is happening. If this is successful the mask can be fixed in place using a skullcap or straps. At first the optimal pressures may not be achievable and lower ones may have to be set until the patient can continue.

One may start with IPAP at 10-12cm H2O and EPAP at 3-4cm H2O, even if the ideal pressures should be higher. Most patients begin with a full-face mask, fitted securely to prevent leaks but not too tight for comfort. In patients with dentures, a poor seal around the mouth can sometimes be prevented if they keep them in. Skin protection is also necessary: the bridge of the nose can be protected by a wafer-type dressing. It cannot be emphasised enough that a comfortable mask and tolerable pressure settings are the key to success and taking the time to get this right in the initial stage is important.

Some compressors have the facility to control the speed of IPAP, which can help with compliance. NIPPV has been described as trying to breathe with your head out of the window of a speeding car. The flow rate of oxygen is prescribed and administered via the port on the mask or, if available, into the disposable filter attached to the generator. Two to four litres of oxygen may be sufficient to decrease hypoxia. Arterial blood gases should be reviewed 30-60 minutes after starting NIPPV. The aim of the treatment is to reduce acidosis, with adequate oxygenation but with no increase in PCO2.

Nursing care

Caring for patients using NIPPV, helping them by relieving their dyspnoea and reducing the severity of distressing symptoms can be one of the most rewarding aspects of nursing. But it is recognised that a great deal of time is needed to set up the system. Bott et al (1993) and Kramer et al (1995) found that caring for patients receiving NIPPV cost no more in nursing time than caring for those having conventional treatment.

Assessment

The following observations should initially be made every 15-30 minutes:

Respiratory rate

It is important that this is measured accurately. Tachypnoea should gradually lessen but signs of respiratory depression, which can include a very slow respiratory rate, must be detected quickly.

Pulse oximetry

This is a useful tool to measure alveolar ventilation. Set the low O2 alarm about 5% below the patient’s current SaO2 to prevent triggering the alarm continually. Remember that oximetry will not monitor CO2.

Patient’s level of consciousness

Check the patient’s awareness to detect any early signs of respiratory depression and CO2 retention. Asking two or three simple questions is usually enough.

The mask

Check it frequently to ensure that it is in the correct position and there is no leakage. Air leakage around the eyes can cause discomfort and can be avoided by adjusting the straps. A little leakage around the mouth is acceptable. Some masks have comfort flaps that can be fitted minimise leaks. Decide whether a skullcap or straps will achieve a better fit: this will depend on the patient’s hairstyle and head shape.

Skin protection

Nasal masks have spacers or polystyrene pads that fit against the forehead. Full-face masks need skin protection, such as a hydrocolloid dressing, on the bridge of the nose and on the forehead. The prevention of skin breakdown is vital as NIPPV is not a feasible treatment if this occurs. For long-term use, silicone and gel masks are available. Some patients have developed their own adaptations. One woman on long-term CPAP rolls a tissue into a cylindrical shape and fits it under the mask along her cheek, another uses a strip of towelling for a more comfortable fit.

Tubing

The tubing from the mask to the generator should not become kinked. Some tubing has exhaust ports near the connection to the mask. It is important to ensure that they do not open towards the patient, who would then feel a continual blast of cold air against his or her throat.

Sputum and vomit

Excessive sputum production can occlude any part of the circuit so frequent oral fluids, to make the sputum less viscose and tenacious, and physiotherapy will be necessary. There is also a possibility of the patient inhaling vomit, so the use of medicines that cause nausea must be avoided where possible and anti-emetics should be considered.

Eating and drinking

Nurses who are not familiar with NIPPV may be reluctant to remove the mask, fearing that the patient’s condition will deteriorate immediately. Although it is not possible to eat while wearing a full-face mask, drinks can be taken by using a straw and loosening one of the face straps. Most patients do not want to eat for the first few hours of NIPPV, but when they are ready to do so consider using a nasal mask rather than a full-face mask for that period only, or perhaps removing the mask altogether and continuing to administer O2 via nasal cannulas.

Weaning

The advantage of NIPPV is that it buys breathing space until conventional therapies, such as antibiotics, steroids and bronchodilators, take effect. There will come a time when the patient has improved enough to consider discontinuing the therapy. There are no hard and fast rules and each patient should be assessed on his or her progress. Either discontinue NIPPV for short periods, continue O2 and monitor the patient for increased dyspnoea and decreasing O2 saturation, or use NIPPV at night only if nocturnal hypoventilation is present.

Caring for people who need NIPPV can be stressful and frightening for nurses who have no experience or knowledge of this treatment, but as it becomes more common those working in acute areas can expect to do so. Many manufacturers are more than willing to provide training in the usage of their equipment, but for safety it is good practice to limit the range of machines used in any particular area.

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