Non-invasive ventilation

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Non-invasive ventilation (NIV) is a form of mechanical ventilation. It can be used with a nasal mask, face mask, a full face mask, or a helmet. NIV does not protect the airways, and so the patient must be cooperative and protect their own airways. It can also not be used in severe gas exchange disorder. Some patients with chronic disorders may use a non-invasive ventilator continously or intermittently. If non-invasive ventilation is not successful in improving the condition of the patient, invasive ventilation is required.

The most common types or modes of NIV are continous positive airway pressure (CPAP) and bilevel positive airway pressure (BIPAP).

NIV involves the patient breathing "against" some pressure, like breathing against headwind. This can be uncomfortable, and many patients do not tolerate it.

Indications

Patients with OSA and obesity hypoventilation syndrome typically use a CPAP only at night. Those with neuromuscular disorders may use a BIPAP continously or intermittently during the day and night, depending on the severity.

Types

The most common types or modes of NIV are CPAP and BIPAP. Some ventilators can provide both types while some can only provide CPAP.

CPAP (continous positive airway pressure) provides, like the name suggests, a constant positive pressure in the airways throughout the whole respiratory cycle (both inspiration and expiration). This positive pressure is higher than the atmospheric pressure (usually 3 - 8 cmH2O), and it can be adjusted.

BIPAP (bilevel positive airway pressure) provides a positive pressure in the airways throughout the respiratory cycle, like CPAP, but the pressure is not constant throughout the cycle but rather depends on whether the patient is currently expiring or inspiring. The ventilator will detect whether the patient is expiring or inspiring and change the pressure accordingly. During inspiration, the machine will provide a higher airway pressure during inspiration (inspiratory positive airway pressure, IPAP) and a lower pressure during expiration (expiratory positive airway pressure, EPAP). Usual starting setting is IPAP 10 cmH2O and EPAP 5 cmH2O, usually written as 10/5 cmH2O.

Physiology

NIV provides many physiological benefits:

  • Positive airway pressure prevents alveoli from collapsing (atelectasis) at the end of expiration, thereby recruiting more alveoli
  • Increases the diameter of the smaller airways
  • Reduces work of breathing
  • Reduces left ventricular afterload

Contraindications

For a patient to use NIV, they must be awake and able to protect their own airways. If a patient is unconscious, vomiting, have upper airway obstruction, untreated pneumothorax, or is critically ill, NIV is contraindicated.

Complications

The sensation of breathing against pressure is uncomfortable, and so NIV can lead to anxiety and claustrophobia. If hypovolaemic, the patient may become hypotensive.