Edi: It is the electrical activity of the diaphragm and can be thought of as a respiratory vital sign. The electrical nerve signal from the crural part of diaphragm are summated, filtered and processed to give the Edi signal. The Servo-n ventilator displays Edi as a waveform as well as numerically on a breath-by-breath basis. It is measured in microvolts (µV). When using the Edi to control all aspects of the ventilator breath, the patient determines inspiratory pressure (or volume), inspiratory and expiratory time and respiratory rate for each breath.
Edi peak (µV): It is the peak of the Edi waveform. It represents the work the diaphragm has to perform for each breath and is responsible for the size and duration of the breath. Edi peak is higher while awake than during sleep and lower in the post-prandial state than pre-prandial and feeding states. The reference range for Edi peak is 10.8±3.7 µV (Stein 2013).
Edi min (µV): It is the nadir of the Edi waveform. It represents the spontaneous tonic activity of the diaphragm which prevents de-recruitment of alveoli during expiration. Edi minimum is higher while awake than during sleep but similar among feeding states. The reference range for Edi min is 2.8±1.1 µV (Stein 2013).
Edi trigger (µV): It is the minimum increase in electrical activity from the previous trough that triggers the ventilator to recognize the increase in electrical activity as a breath, not just baseline noise. It is generally set at 0.5.
NAVA Level: NAVA level is a conversion factor that converts the Edi signal into a proportional pressure. The units of the NAVA level are cmH20/ µV. The Edi is multiplied by this NAVA level to determine airway pressure delivered by the ventilator for each breath. The peak pressure is determined on an ongoing basis every 60 msec, and continues to increase as long as the instantaneous Edi increases, as determined by the formula:
Peak pressure = NAVA level x Edi (peak-min) + PEEP Apnoea time: This represents the maximum amount of time the infant can be apnoeic before going into backup ventilation. This provides a minimum guaranteed rate which is different from the backup rate. For e.g. setting apnoea time at 2 seconds guarantees minimum rate of 30 breaths/min.
Determining appropriate NAVA Level
The Edi, in conjunction with the NAVA level, controls the NAVA ventilator support. The delivered pressure during NAVA is continually adjusted based on the neural feedback from respiratory centres. Increasing the NAVA level results in unloading of the respiratory muscles and is followed by an increase in inspiratory pressure, mean airway pressure and tidal volume while Edi remains constant. This is termed as first response wherein the respiratory muscles are unloaded insufficiently. Further increase in NAVA level results in reaching a ‘breakpoint’ where the respiratory muscle unloading becomes sufficient and the Edi signals starts decreasing and the inspiratory pressure reaches a plateau despite a further increase in NAVA level. This breakpoint is the appropriate NAVA level and is unique in each individual. Above a certain NAVA support level, the Edi signal becomes erratic, indicating over-assistance. The NAVA level is fluid and should be adjusted to clinical picture as the respiratory disease evolves. Example of breakpoint;