Non-Invasive Ventilation

Ventilation is controlled by setting the volume desired for each breath or by setting a pressure that will be delivered to the airway to assist with breathing.

Changes in volume and pressure are directly proportional to lung and chest wall compliance.

Compliance = V / P

NON INVASIVE VENTIALTION

When a volume mode is used, tidal volume provided by the ventilator will be fixed.

If the compliance of the lung is very low, then a high amount of pressure will be needed to deliver the set volume.

If there is a leak in the system machine, then actual tidal volume delivered may be lower than prescribed, as the machine cannot know how much air is delivered to the patient and how much is lost.

In Pressure mode, pressure will be fixed and tidal volume vary with compliance.

Same pressure may generate adequate volumes in a patient with good chest wall compliance, but would generate inadequate tidal volumes in a patient with poor chest wall compliance.

The machine compensate for leak until set pressure is reached.

NIV-CPAP – continuous positive airway pressure

NIV – PS – pressure support

Modes of non invasive ventilation

NIV – CPAPGives a continuous positive pressure throughout the respiratory cycle.

NIV – PSGives additional support during inspiration

and a continuous but lower pressure during expiration

In NIV-PS MODE, with BiPAP machine, the inspiratory support is termed the IPAP (inspiratory positive airway pressure), while the expiratory support is termed the EPAP and the difference between IPAP and EPAP is the amount of pressure support provided.

In NIV-PS and NIV-CPAP, flow (Q) will depend on set pressure, patient’s respiratory drive, airway resistance and presence or absence of leak.

Q = P / R, where P is the pressure gradient between airway and alveoli as developed by the patient (negative pleural pressure) and ventilator (positive airway pressure) and R is airway resistance.

In spontaneously triggered breaths, the ventilator can be triggered by either a change in pressure or by a change in flow.

Flow triggering is more sensitive than pressure triggering and reduces the work of breathing in spontaneous modes.

Ventilator triggering in NIV

The trigger for stopping ventilator assistance during inspiration can be either a decrease in flow to a percentage of the maximal flow rate (usually 25% of the maximal flow rate) or a set flow rate.

Ventilator cycling