Non Invasive Ventilation
Indications
Indications for non-invasive ventilation are many. They include the following:
-Patients with an acute exacerbation of COPD in whom a respiratory acidosis persists despite maximum medical treatment uncontrolled oxygen therapy.
-Continuous positive airway pressure has been shown to be effective in patients with cardiogenic pulmonary oedema who remain hypoxic despite maximal medical treatment.
-Non-invasive ventilation may be indicated in those patients with a chest wall deformity neuromuscular disease who consequently go on to develop an acute or acute on chronic hypercapnic respiratory failure.
-Patients with decompensated obstructive sleep apnoea can benefit from non-invasive ventilation.
-Patients who have developed a type I respiratory failure due perhaps to an underlying pneumonia may benefit from non-invasive ventilation, and this may prevent the need for further invasive ventilation.
-Some patients, who are proving difficult to wean from invasive ventilation, may benefit from the application of non-invasive ventilation to help make the extubation process a successful one.
Contraindications
There are several contraindications to non-invasive ventilation which the practitioner needs to be aware of before considering this intervention for their patient.
-If the patient is unable to maintain their own airway perhaps due to a reduced GCS then non-invasive ventilation would be inappropriate.
-If the patient is at risk of vomiting non-invasive ventilation should not be used as this could result in aspiration.
-This there are copious respiratory secretions which the patient may or may not be able to clear effectively the use of non-invasive ventilation would be risky and is likely to make the patient uncomfortable and/or distressed.
-If the patient is so hypoxic as to be potentially life-threatening then the treatment of choice should be invasive rather than non-invasive ventilation.
-If the patient is confused or agitated the application of non-invasive ventilation is likely to make this worse and may make the patient very difficult to manage.
-Non-invasive ventilation would also be appropriate if the patient had recently undergone upper gastrointestinal surgery. The positive pressure flowing past the sutures may put the anastomosis at risk.
-If the patient has a known pneumothorax it is preferable to place a chest drain before starting non-invasive ventilation.
Mode/Settings used
Non-invasive ventilation is the delivery of positive airway pressure. This supports the patient’s breathing via either a face or nasal mask.
There are a number of different ways of delivering this type of ventilation to the patient, ranging from simple nasal delivery to a full hood which goes right over the patients head.
With continuous positive airway pressure ventilation or CPAP the positive pressure is only delivered during expiration. With bilevel positive airway pressure ventilation or BiPAP positive pressure is delivered during both inspiration and expiration.
CPAP is commonly used to treat hypoxaemia. The positive pressure used increases mean airway pressure and will help ventilate those parts of the lung which may be collapsed. It is this under ventilated area of the lung which, if it can be recruited, will help the patient’s respiratory function to improve.
The assistance provided by the positive pressure will also reduce the workload on the inspiratory muscles thereby reducing inspiratory workload, and the consequent increase in alveolar ventilation may result in a fall in PaCO2. With the acutely unwell patient there is likely to be an increased inspiratory rate, short inspiratory time and an increased minute volume. Because of this the CPAP machine must be capable of delivering high flows, sometimes in excess of 60 L per minute.
The amount of CPAP given is often determined by the valve which is placed externally to the CPAP machine itself. These usually come in the range of five, 7.5, and 10 cm H2O.
BiPAP is the mode of choice in the patient with a type II respiratory failure, that is one in whom the PaCO2 is above normal levels. In the patient with chronic COPD one needs to be aware what the normal level is for that person as it may be higher than what is considered normal for a well person. The addition of positive pressure during inspiration will help to reduce the levels of carbon dioxide which the patient is retaining.
The initial settings for the patient requiring this type of ventilation are 10 cm H2O for IPAP and 4-5 cm H2O for EPAP. The usual maximum target pressure is 20 cm H2O.
If oxygenation is also low then additional oxygen can be entrained by the circuit. Remember that with the COPD patient you are often aiming for oxygen saturations of only 88 to 92%. EPAP can be increased up to 6 cm H2O to help with oxygenation. In order to maintain the patient’s tidal volume when increasing the EPAP one should also increase the IPAP.
For the patient the use of non-invasive ventilation can potentially be an uncomfortable and distressing experience. Because of the need for positive pressure it is important that the mask or nasal cannula fit tightly all the pressure will just allow the gas to escape around the mask or cannula.
Nursing care therefore is very important in ensuring patient, concordance when using non-invasive ventilation.
The patient is at risk of:
-Pressure sores on the bridge of the nose.
-Irritation to the eyes due to leakage around the top of the mask.
-Skin irritation around the mask.
-Difficulties with communication, as it is very difficult for the patient to make themselves heard when wearing the mask.
-Irritation to the mucosa in both the nose and the mouth.
Measures should be taken by the staff to minimise the effects of all of these possible complications. These would include regular removal of the mask to relieve the pressure and allow for oral hygiene.
Also, where possible, the patient should be allowed breaks from the non-invasive ventilation so that they can eat and drink.
Guidelines for the management of tracheal intubation in critically ill adults
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