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Related Subjects: |Respiratory Failure |Non invasive ventilation (NIV) |Intubation and Mechanical Ventilation |Critical illness neuromuscular weakness |Multiple Organ Dysfunction Syndrome |Haemodialysis |Dobutamine

Patients need to be conscious and have sufficient respiratory drive to benefit from NIV. Those who are drowsy and who have poor respiratory drive may need Intubation and ventilation.

Introduction

• Normal physiology involves using external intercostal muscles and descent of the diaphragm to create a negative intrathoracic and airways pressure relative to atmospheric pressure, sucking air in to reach the alveoli. The process of ventilation however involves pushing air in which is the most practical way to oxygenate lungs therapeutically but this alters normal physiology. This can be done invasively by full intubation and ventilation, or non-invasively using close-fitting face masks or hoods and a ventilator device that provides positive pressure.
• Positive airway pressure can then be given continuously (CPAP) throughout the respiratory cycle or varied with the cycle (e.g. BiPAP). In some NIV can help to avoid complications of intubation and invasive ventilation, to improve outcomes (e.g. reduce mortality rates, decrease hospital length of stay), and to decrease the cost of care. It is mainly used in those with Type II RF. The machine can automatically detect episodes of inspiration and expiration. NIV is usually used to improve oxygenation and increase minute volume, thus reducing PaCO₂.
• Continuous positive airway pressure (CPAP): is used mainly for Type 1 RF/Alveolar oedema and is the application of continuous positive pressure whilst the patient continues to initiate and generate breaths. Prevents collapse of airways, recruiting more alveoli for gas exchange. The positive pressures are maintained even on expiration. Work of breathing is reduced. Can be accompanied by high FiO₂. Pressures used are 5-20 cm H ₂O. Adjust FiO₂. The main use is cardiogenic pulmonary oedema at typical pressures of 5-15 cmH₂O. A trial can be given for 1-2 h to see improvement in O₂ sats, heart rate, etc. Primarily helps hypoxaemia rather than CO ₂ retention and can help avoid intubation. The patient wears a tight-fitting mask which is essential to maintain the positive pressure. Outside of the acute setting, it is useful for those with obstructive sleep apnoea because it acts as a pneumatic splint maintaining a patent airway throughout the respiratory cycle.
• Bi-level positive airway pressure (BiPAP): is used mainly for Type 2 RF and pH < 7.35. BiPAP usually refers to the application of positive pressure ventilation similar to CPAP except that pressures are changed for inspiration and expiration. Needs a tight-fitting face mask and a ventilator that is capable of delivering two levels of pressure: inspiratory (high pressure, e.g. 8 cmH₂O) and expiratory (?pressure, e.g. 4 cmH₂O).

Advantages of NIV over Invasive ventilation

• Reduced risk of infection
• Reduced length of hospital stay (it can be used at home).
• Reduced costs
• NIV and COPD related Type 2 RF: reduces intubation rate and mortality

Application

• NIV can be used in both Type I and II RF, e.g. exacerbations of COPD and cardiogenic pulmonary oedema.
• An effective seal is necessary and the patient needs to be able to tolerate the mask as well as the feeling of breathing out against resistance.
• Intolerable for some. Positive pressure aids the inspiratory phase of breathing, which is active.
• Reduces respiratory muscle work, maintains alveoli patency during expiration which improves the ventilatory process.
• Increased alveolar recruitment and so more air comes into contact with blood in pulmonary
• Ventilatory support that does not require intubation has proven useful in giving sufficient respiratory support to reduce the number of patients needing intubation.
• The lungs are ventilated under positive pressure using a close-fitting face mask or a hood to maintain a seal This helps to keep the airways open.
• There are two main types once with a fixed pressure called continuous positive airways pressure (CPAP) and one with an inspiratory and expiratory pressure (BIPAP)

Types

• CPAP: Keeps fixed positive pressure through inspiration and expiration. It can be used for sleep apnoea to keep airways open and for type 1 respiratory failure e.g. pulmonary oedema or pneumonia
• BIPAP: There is a different pressure in inspiration and expiration. The active ventilation is during IPAP and EPAP allows recruitment of alveoli and CO₂ removal. Used mainly for Type 2 RF with acidosis.

Benefits

• These are mainly the fact that it reduces the need for invasive ventilation and all the potential complications.
• Therefore it improve patient outcomes and reduces hospital and ICU length of stay.
• It has been shown to be effective for those with acute on chronic respiratory failure with exacerbation of COPD.

Indications for NIV

• pH <7.35 and relative hypercarbia
• ACPO and ARF in the absence of shock or acute coronary syndrome requiring acute coronary revascularization
• Immunosuppressed patients with acute respiratory failure
• High risk recurrent acute respiratory failure after planned extubation (not indicated post extubation for low risk patients)
• Weaning from mechanical ventilation particularly in patients with a background of COPD
• Acute respiratory failure following lung resection surgery or post abdominal surgery
• Asthma
• Acute respiratory failure in selected "not for intubation" patients
• Acute deterioration of disorders associated with sleep hypoventilation such as neuromuscular and chest wall restrictive disorders and obesity hypoventilation syndrome
• Palliation for symptom relief in combination with opioids and benzodiazepines to treat breathlessness. A medical team decision will be made when NIV is deemed no longer beneficial to the patient management

Side effects

• CPAP and BIPAP change raise intrathoracic pressure and this reduces venous return and therefore cardiac output so there may be a fall in BP.
• The positive pressure air can cause air in the stomach and discomfort and this can lead to vomiting. A closely applied mask can cause skin damage.

Contraindications

• Life threatening hypoxaemia <60 mmHg on FiO₂ 100% needing intubation and ventilation
• Reduced GCS, CPAP in acute lung injury (ALI)
• Respiratory arrest
• Untreated pneumothorax
• Life-threatening dysrhythmias
• Inability to protect own airway
• Copious, unmanageable respiratory secretions
• Facial burns/trauma/recent facial or upper airway surgery preventing mask fit
• Lack of trained staff to monitor and manage NIV

Settings

• Initial settings for bi-level positive airway pressure (BPAP)
  • Inspiratory positive airway pressure (IPAP) of 10cmH₂O
  • Expiratory positive airway pressure (EPAP) of 4-5cmH₂O = pressure support (PS) level of 5-6 cm H₂O.
• IPAP can be increased to 20 cm in small steps over time
• Initial settings for continuous positive airway pressure (CPAP): 5cmH20 increasing to 10 cm
• Optimal non-invasive positive pressure ventilation (NIV) is the lowest pressure and lowest Fi0₂ that achieve Sa0₂ of 90% or Pa0₂ of 60mmHg without further clinical deterioration
• If the patient does not clinically improve within four hours of starting NIV the decision to intubate and ventilate is to be made
• Patients are to be encouraged to sit out of bed as tolerated. When in bed they are to be positioned in an upright position to facilitate chest wall expansion
• Oral feeding is to be initiated if the patient is able to tolerate small periods off NIV.

Indications for Intubation and Ventilation

• Indications for endotracheal intubation and ventilation
• Protection of the airway and/or need to remove secretions.
• GCS <9, airway obstruction, respiratory fatigue or drowsiness.
• Apnoea with cardiac/respiratory arrest.
• Hypoxaemia (PO₂ <8 kPa) despite high FiO₂ +/- NIV.
• SaO₂ <90% despite CPAP with FiO₂ >0.6.
• Control CO₂ (hyperventilate to PCO₂ for ICP).
• Respiratory rate >35/min or <10/min.
• FVC <15 ml/kg or 1 L or <30% predicted.
• Tidal volume <5 ml/kg or inadequate inspiratory force <25 cmH₂O.
• Surgery to head and neck or involving muscular paralysis.

Home NIV may be considered for those with Chronic but stable respiratory failure due to

• May be used overnight or constant
• Severe spinal deformity
• Neuromuscular disease e.g. muscular dystrophy
• Central alveolar hypoventilation
• Cystic fibrosis
• Motor neurone disease

References