Non-invasive Ventilation in the Intensive Care Unit

Non-invasive ventilation (NIV)

· A method of providing ventilatory support without the need for tracheal intubation.
CPAP (Continuous positive airways pressure)
Bi-level positive airway pressure (BiPAP©)

Advantages of NIV compared to conventional ventilation

Compared with conventional ventilation, NIV is a relatively inexpensive and simple technique which prevents the need for conventional ventilation in some patients and improves survival. Major advantages include:

· Avoiding many of the complications associated with conventional ventilation (table 1).

· Avoiding the need for sedation.

· Easier communication with patient.

· Requires less intensive nursing care.

Indications for the use of NIV

NIV is commonly used for the treatment of respiratory failure from:

· Exacerbation of chronic obstructive airways disease (COPD)

· Pulmonary oedema

· Respiratory failure in immunocompromised patients. E.g. AIDS, malignancy

· Weaning from conventional ventilation and prevention of need for reintubation in high risk patients

· Chest trauma

· Asthma.

Contraindications to the use of NIV

These may be absolute:

· Non-compliant patient

· Unconscious patient / unable to protect airway.

Or relative:

· Facial fractures

· Excessive secretions

· Following oesophagectomy (risk of anastamotic breakdown due to increased pharyngeal/oesophageal pressures)

· Haemodynamic instability.

What type of mask to use?

· For acutely unwell patients with respiratory failure, it may be appropriate to start with a full face mask. This allows the use of higher pressure ventilation with less gas leakage.

· For patients who are relatively stable, a nasal mask or helmet may be better tolerated.

How to apply the mask?

· The patient may feel claustrophobic as the mask is applied and should be reassured. It may help if they are allowed to hold it on themselves.

· The mask is then held on the face by a harness which passes around the back of the head. When tightening the straps, it is important to find a balance between leaving the mask so loose that there is an uncontrollable leak and making it uncomfortably tight. Pressure relieving dressings may be used to reduce the chance of sores developing in sensitive areas such as the bridge of the nose.

What mode to use? (Respironics BiPAP© machines)

· CPAP is ideal for type I respiratory failure where CO2 elimination is not a problem.

· BiPAP is used to augment CO2 removal as well as improving oxygenation (type II respiratory failure). There are three mode options when using BiPAP:

1. Spontaneous – the machine will detect and support spontaneous breaths in patients with good respiratory drive (similar to pressure support).

2. Timed – the machine will provide mandatory breaths at a set frequency in patients with inadequate spontaneous respiration (similar to controlled mandatory ventilation).

3. Spontaneous / timed – the machine will support spontaneous breaths, but if the patient does not breathe for a set period, they will be given a mandatory breath.

Which initial pressure settings to use for BiPAP© spontaneous mode?

· Commonly the IPAP is set to 10 cmH2O and the EPAP to 5 cmH2O. The response to these pressures should determine future changes.

· Most machines can generate maximal pressures of 20-23 cmH2O. If higher pressures are required leakage around the mask is usually a problem, and conventional invasive ventilation is indicated.

What FiO2 to choose?

· Choose an initial FiO2 slightly higher to that the patient received prior to NIV.

· Adjust the FiO2 to achieve an SaO2 that you deem appropriate for their underlying disease. Generally SaO2 above 92% is acceptable.

· If a patient is hypoxic while breathing 100% oxygen on a CPAP circuit, their hypoxia will not improve if they are placed onto a BiPAP circuit (in spite of the increased ventilatory assistance) because the FiO2 will drop significantly.

· Similarly if a patient starts to work harder on a BiPAP circuit they may become more hypoxic due to a drop in FiO2 caused by increased gas flow through the breathing circuit.

How to monitor the patient’s response to NIV?

· The most useful indicator is how the patient feels. They should be able to tell you if they feel better or worse.

· Where available arterial blood gases (ABG) are useful to assess changes in oxygenation and CO2 clearance.

How to tell if NIV is not effective?

· Again, this is largely based on how the patient feels and ABG results.

· If the patient is getting increasingly tired, or their ABG deteriorating despite optimal settings, then they will probably need tracheal intubation and mechanical ventilation. It is important to recognise this as soon as possible so that management may be planned before the patient collapses.

How to intubate a patient previously on NIV?

· Patients are likely to be critically unwell at this point – seek experienced help.

· Assume the patient’s stomach is full, using a rapid sequence induction.

· Preoxygenate a thoroughly as possible, applying an FiO2 of 100% with or without CPAP.

· Expect and prepare for cardiovascular collapse on induction (have IV fluids running and vasopressors readily available).

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