Spontaneous modes :
PSV (pressure support ventilation) – pressure is set and tidal volume is variable.
VSV (volume support ventilation) – tidal volume is set and the peak pressure is variable.
NAVA (neurally adjusted ventilatory assist) – this one is tricky – the unit of measure is cmH2o per unit of microvoltage. In other words, if one were to set NAVA=1.0, the patient will get 1 cmH2o (1 centimeter of water pressure) for every microvolt the brain sends down the phrenic nerve.
If the NAVA level was set to NAVA=3.0, the patient will get 3cmH2o (3 centimeter of water pressure) for every microvolt the brain sends down the phrenic nerve.
However, when the brain feels it is adequately satiated with the size of the tidal volume and the stretch of the lung and acceptable pH and host of many other input receptors are satiated, the neural signal is tuned down.
Essentially, as one turns up the NAVA level (ex from NAVA=1.0 to NAVA=3.0), the more inverse the microvoltage signal from the brain to the diaphragm via the phrenic nerve (i.e. from 3microvolts to 1 microvolt) so that the tidal volume delivered is consistently the same (ex 250mls in both cases).
Notice that in the above two screen captures :
NAVA=4.5 & Edi=2.3 & Vt~221.
NAVA=2.5 & Edi=3.7 & Vt~225.