Non-invasive ventilation (NIV)
Intensive care and NIV exam revision for medical student finals, PLAB exams and MRCP PACES
Non-invasive ventilation is broadly divided into three categories: High-flow nasal oxygen, Continuous Positive Airway Pressure (CPAP) or Bi-level Intermittent Positive Airway Pressure (BiPAP). These methods of ventilation are used in spontaneously breathing patients. Click on the plus symbols below to expand the sections:
Respiratory support is needed when a patient has respiratory failure. Respiratory failure can be defined as type 1 or type 2 failure.
Type 1: Hypoxic respiratory failure, when the PaO2 is less than 8Kpa with a normal or low PaCO2. Type 1 failure is caused by diseases that impair alveolar function. e.g. pneumonia, pulmonary oedema and lung fibrosis.
Type 2: Hypoxic and hypercapnic respiratory failure, when the PaO2 is less than 8Kpa and the PaCO2 is greater than 6. Type 2 failure is common in chronic obstructive pulmonary disease but is also caused by neuromuscular diseases and conditions where there is a decreased respiratory drive.
If a patient requires respiratory support, this can be delivered on ICU as either non-invasive or invasive ventilation. Non-invasive ventilation can also be delivered on certain medical wards e.g. a respiratory ward or an acute medical unit.
Ideally a patient should be conscious and able to consent to treatment; patient’s wishes should be of the upmost importance and the potential for recovery and expected quality of life need to be considered. However, on medical wards NIV may be used in the presence of the contraindications if this decision is made by a senior doctor. It may be that NIV may be the only hope for survival. However these patients should not be sent to ICU, should have a DNAR in place and the decision to commence NIV in the presence of contraindications should be clearly documented.
Both CPAP and BiPAP can reduce cardiac output and cause haemodynamic instability so it is important to monitor pulse and blood pressure closely. It is also important to consider that whilst wearing an NIV mask the patient will have less oral intake and insensible losses will be higher. It is therefore imperative to monitor fluid balance and supplement this with IV fluids if necessary. If NIV is to be worn for more than 24-48 hours (which is usually the case) and the patient is struggling with oral intake then NG feeding should also be considered.
If the patient does not tolerate NIV it is important that every attempt is made to optimise their tolerance. Common problems are ill fitting masks, so other masks should be tried, skin necrosis can cause pain and so pressure dressings can be placed under the mask, bloating can be treated with pro-kinetics and an NG tube may help. Modification of the setting may also be required. However if the patient remains intolerant and wants to stop treatment it is important that they can be facilitated to make this decision. As a physician you need to counsel them on the risks and ensure they are competent to make this decision.
Hi-flow oxygen can be delivered via specialist nasal cannulae. The principle behind hi-flow oxygen delivery is that a high percentage of inspired oxygen can be delivered e.g. FiO2 of 60% but in addition the flow of oxygen e.g. 70L can deliver some positive end expiratory pressure (PEEP). The exact amount of PEEP delivered is not 100% predictable due to leakage but between 3-6 cmH20 can be delivered. Hi-Flow nasal cannulae can be used as a step up from conventional oxygen delivery methods to help treat Type 1 respiratory failure. Depending on your hospital, the patient may need to be admitted to HDU for this treatment to enable closer monitoring.
CPAP is delivered by means of a tight fitting mask over the nose or nose and mouth or it can be delivered via a whole head hood. It can also be delivered via a tracheostomy tube. The mask/hood is attached to a ventilator and delivers expiratory support which is analogous to PEEP in ventilated patients. Usual settings are 5, 7.5 or 10 cmH2O. An escalating pressure requirement may be an indication that invasive ventilation is needed. The aim of CPAP is to splint open airways, reducing alveolar collapse, enable alveolar recruitment and to increase the functional residual capacity to help reduce the work of breathing. These three mechanisms help to increase oxygenation and support respiration to prevent the patient from tiring.
BiPAP is also delivered by means of a tight fitting face mask but the ventilator delivers two different airway pressures. These are an inspiratory pressure and an expiratory pressure. The expiratory pressure (EPAP) is analagous to PEEP on CPAP and is usually set between 4-6 cmH20. The inspiratory pressure (IPAP) is a higher pressure which aims to augment the patient’s inspiratory effort. Common settings for this are 12 cmH20 which can then be escalated depending on the patient response. BiPAP is used to treat type 2 respiratory failure and is commonly used in exacerbations of COPD. BiPAP may be delivered on some medical wards but it is also used in ICU. BiPAP should not be used as a treatment in COPD until the patient has received optimal medical management. If following medical management the ABG results and patients clinical condition fail to improve then this BiPAP can be considered. The aim is to commence BiPAP at settings of 12 cmH2O/4cmH2O and escalate the IPAP. Some patients with COPD may need an IPAP over 20cmH2O. Setting changes should be guided by serial ABGs and the respiratory or ICU teams should be involved to help guide this.
With both CPAP and BiPAP the inspired oxygen (FiO2) can be titrated to achieve a target PaO2 or saturations. Close monitoring and serial blood gases are imperative for any patient being treated with non-invasive ventilation so any improvements or deteriorations in the clinical state can quickly be identified.
Prior to commencing non-invasive ventilation it is important that any contraindications to treatment are considered and that there is a clear escalation or de-escalation plan documented in the patients notes should the treatment fail. These plans should always be discussed with the patient (if able) and relatives. As a patient or relative the CPAP or BiPAP machine can be very frightening, so it is important that the rationale behind the treatment and a plan of what the treatment involves is clearly explained.
Prior to starting NIV the BTS recommend that treatment and escalation plans are clearly documented and discussed. They suggest that there are five categories in which a patient can fall. These are:
- In the event of failure of NIV in the first 4 hours of treatment, escalate to intubation and invasive ventilation if appropriate
- In the event of late failure of NIV treatment (between 4-48hours), escalate to intubation and invasive ventilation if appropriate.
- In the event of failure of NIV consider palliation if it is not appropriate to proceed to invasive ventilation
- Continue NIV if there is a response to therapy and seek specialist respiratory input with regards to duration of therapy and weaning.
- Inappropriate to commence NIV and palliate from the outset
In addition DNAR status should be considered prior to commencing NIV: having a DNAR does not exclude NIV as a treatment.
- Facial burns/facial trauma
- Recent upper GI surgery
- Fixed airway obstruction
- Undrained pneumothorax
- Patient is unable to protect their own airway
- E.g. moribund with low GCS or copious secretions
- Life threatening hypoxaemia
- Multiple comorbidities
- Confusion and agitation
- Patient refusal
- Bowel obstruction
- Haemodynamic instability