The work of breathing equals the product of pressure change across the lung and volume of gas moved. During inspiration, most of the work is done to overcome elastic recoil of the thorax and lungs, and the resistance of the airways and non-elastic tissues.

For years, negative pressure ventilators were the only non-invasive methods of assisting ventilation. It was later recognized that delivery of continuous positive airway pressure by close fitting nasal masks for treatment of obstructive sleep apnoea could also be used to deliver an intermittent positive pressure. This was followed by improvements in the interface and establishment of role of NIV in patients with COPD. The use of NIV has increased in the last decade in various conditions to avoid the complications of intubation.

Advantages of NIV

 Preservation of airway defence mechanism
 Early ventilatory support
 Intermittent ventilation
 Patient can eat, drink and communicate
 Ease of application and removal
 Patient can cooperate with physiotherapy
 Improved patient comfort
 Reduced sedation requirements
 Avoidance of complications of intubation
 Ventilation outside hospital setting possible

Disadvantages

 Mask is uncomfortable/claustrophobic
 Time consuming for medical and nursing staff
 Airway is not protected
 No direct access to bronchial tree for suction

Mechanism of Action

 Improvement in pulmonary mechanics and oxygenation
NIV augments alveolar ventilation and allows oxygenation without raising the PaCO_2.

 Partial unloading of respiratory muscles
NIV reduces trans-diaphragmatic pressure, pressure time index of respiratory muscles and diaphragmatic electromyographic activity. This leads to an increase in tidal volume, decrease in respiratory rate and increase in minute ventilation. Also overcomes the effect of intrinsic PEEP.

 Resetting of respiratory centre ventilatory responses to PaCO₂
In patients with COPD, the ventilatory response to raised PaCO₂ is decreased especially during sleep. By maintaining lower nocturnal PaCO₂ during sleep by administering NIV, it is possible to reset the respiratory control centre to become more responsive to an increased PaCO₂ by increasing the neural output to the diaphragm and other respiratory muscles. These patients are then able to maintain a more normal PaCO₂ throughout the daylight hours without the need for mechanical ventilation.



The carotid bodies are small structures, situated above the carotid bifurcation on each side. They are involved in the chemical control of breathing, by acting as chemoreceptors, being sensitive to levels of oxygen and carbon dioxide in the blood. There is evidence that suggests that carotid bodies are also sensitive to changes in blood pressure, blood flow and blood osmolarity.

Requirements for successful non-invasive support

 A co-operative patient who can control their airway and secretions with an adequate cough reflex. The patient should be able to co-ordinate breathing with the ventilator and breathe unaided for several minutes.

 Haemodynamically stable

 Blood pH>7.1 and PaCO₂ <92 mmHg

 The patient should ideally show improvement in gas exchange, heart rate and respiratory rate within first two hours.

Indications for NIV

 Acute respiratory failure
 Hypercapnic acute respiratory failure
 Acute exacerbation of COPD
 Post extubation difficulty
 Weaning difficulties
 Post surgical respiratory failure
 Thoracic wall deformities
 Cystic fibrosis
 Status asthmaticus
 Acute respiratory failure in obesity hypoventilation syndrome
 Chronic Respiratory Failure
 Immunocompromised Patients
 Patients 'not for intubation'.

 Hypoxaemic acute respiratory failure

The evidence is less convincing to show efficacy of NIV in hypoxaemic respiratory failure. Possible indications include cardiogenic pulmonary oedema, community acquired pneumonia, post traumatic respiratory failure and ARDS.

Selection Criteria

Acute Respiratory Failure

At least two of the following criteria should be present:

 Respiratory distress with dyspnoea
 Use of accessory muscles of respiration
 Abdominal paradox
 Respiratory rate >25/min
 ABG shows pH <7.35 or PaCO₂ >45mmHg or PaO₂/FiO₂ <200

Chronic respiratory failure (obstructive lung disease)

 Fatigue, hypersomnolence, dyspnoea
 ABG shows pH <7.35, PaCO₂ >55 mmHg, PaCO₂ 50-54 mmHg
 Oxygen saturation <88% for >10% of monitoring time despite O₂ supplementation

Thoracic Restrictive/ Cerebral Hypoventilation Diseases

 Fatigue, morning headache, hypersomnolance, nightmares, enuresis, dyspnoea
 ABG shows PaCO₂ >45mmHg
 Nocturnal SaO₂ <90% for more than 5 minutes
 

Contraindications

 Respiratory arrest/unstable cardiorespiratory status
 Uncooperative patients
 Unable to protect airway- impaired swallowing and cough
 Facial/oesophageal or gastric surgery
 Craniofacial trauma/burns
 Anatomic lesions of upper airway

Relative Contraindications

 Extreme anxiety
 Morbid obesity
 Copious secretions
 Need for continuous or nearly continuous ventilatory assistance

Choice of Ventilator

NIV can be given by conventional critical care ventilators or portable pressure or volume limit ventilators. When a critical care ventilator is used for applying NIV, the presence of variable leaks produces frequent alarming. Therefore a close monitoring of leaks is mandatory. NIV may be delivered more successfully using specially designed portable pressure ventilators. These provide a high flow CPAP or cycle between high inspiratory and low expiratory pressures (Bilevel positive airway pressure generators) These devices are sensitive enough for detection of inspiratory efforts even in presence of leaks in the circuits.

Interface

Interfaces are devices that connect the ventilator tubing to the face allowing the entry of pressurized gas to the upper airway. Nasal, oro-nasal masks and mouth pieces are currently available. Masks are usually made from a non-irritant material such as silicon rubber. It should have minimal dead space and a soft inflatable cuff to provide a seal with the skin. Face masks and nasal masks are the most commonly used interfaces. Nasal masks are used most often in chronic respiratory failure while face masks are more useful in acute respiratory failure.

Modes of Ventilation

All modes of ventilation can be used for applying non-invasive ventilation

CPAP

CPAP by nasal mask provides a pneumatic splint which holds the upper airway open in patients with nocturnal hypoxaemia due to episodes of obstructive sleep apnoea. It provides positive airway pressure throughout all phases of spontaneous ventilation. CPAP increases the FRC and opens collapsed alveoli. CPAP reduces left ventricular transmural pressure and therefore increases cardiac output. Hence it is a very effective for treatment of pulmonary oedema. Pressures are usually limited to 5-12 cm of H₂O, since higher pressure tends to result in gastric distension requiring continual aspiration through a nasogastric tube.

BIPAP

Bilevel positive airway pressure provides two levels of positive pressure. During exhalation, pressure is variably positive. Airflow in the circuit is sensed by a transducer and augmented to a preset level of ventilation. Cycling between inspiratory and expiratory modes may either be triggered by the patient's breaths or preset.

Volume limited ventilation

In this mode, ventilators are usually set in assist-control mode with high tidal volume (10-15 ml/kg) to compensate for air leaks. This mode is suitable for patients with obesity or chest wall deformity (need high inflation pressure) and in patients with neuromuscular diseases who need high tidal volumes for ventilation.

Proportional assist ventilation (PAV)

This is a newer mode of ventilation. In this mode the ventilator has the capacity for responding rapidly to the patients' ventilatory efforts. By adjusting the gain on the flow and volume signals, one can select the proportion of breathing work that is to be assisted.

Summary

The use of NIV has increased during the past few years. In acute exacerbation of COPD it is now considered the ventilator mode of first choice. For treatment of acute pulmonary oedema, CPAP alone is very effective. NIV reduces the chances of endotracheal intubation in hypoxaemic respiratory failure. It is also being used to facilitate weaning from invasive ventilation. NIV is first choice in patients with neuromuscular diseases and chest wall deformity. Central hypoventilation and patients of obstructive sleep apnoea not responding to CPAP are also acceptable indications.

Article on CCUK

Non-invasive ventilation on the ICU

References

[i] Non-invasive positive pressure ventilation for treatment of respiratory failure due to exacerbations of chronic obstructive pulmonary disease. Ram FS, Picot J, Lightowler J, Wedzicha JA. Cochrane Database Syst Rev. 2004;(1):CD004104

[ii] Non-invasive negative and positive pressure ventilation in the treatment of acute on chronic respiratory failure.Gorini M, Ginanni R, Villella G, Tozzi D, Augustynen A, Corrado A. Intensive Care Med. 2004 May;30(5):875-81.   

[iii] NIV
[iv]  Curves and Loops in Mechanical Ventilation

[v]  Autoflow

[vi]  Airway Pressure Release Ventilation: Theory and Practice P. Milo Frawley, RN, MS and Nader M. Habashi, MD

[vii]  ATC and PPS Breathing Support with Optimum Patient Comfort

[viii]  BIPAP: Two Steps Forward in Ventilation