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Volume 3 Supplement 1


Comparing two different modes of mechanical ventilation by the least square fitting method: nava versus PSV


The Least Square Fitting (LSF) method is a statistical approach used for evaluating respiratory mechanics [1]. It allows measurement of respiratory mechanics continuously at the bedside, even in presence of flow limitation [23], without the need for constant inspiratory flow rate, end-inspiratory hold and end-expiratory occlusion. These features allow the application of the LSF method to assisted ventilation modes, such as pressure support ventilation (PSV) [3] and neurally-adjusted ventilatory assist (NAVA).


We compared the LSF performance during PSV and NAVA. Our hypothesis was that the LSF works better during NAVA than during PSV, since NAVA algorithm allows a more accurate neuro-ventilatory coupling.


15 patients undergoing mechanical ventilation for acute respiratory failure were ventilated using randomly either PSV or NAVA. Data of resistance (Rrs), elastance (Ers) and total positive end expiratory pressure (PEEPtot) were obtained by fitting the equation Paw = Rrs x V´ + VT/Crs + PEEPtot during inspiration. The coefficient of determination (CD) of the applied equation was used to compare data obtained during NAVA and PSV, the higher being the CD, the better the quality of the data. These data were obtained at the beginning of mechanical ventilation (T0), and after 12 (T12), 24 (T24), 36 (T36), 48 (T48), 60 (T60) and 72 (T72) hours of mechanical ventilation.


Data obtained with LSF were statistically more reliable during NAVA than during PSV (Chi-squared test: p < 0.001). The CD level showed a higher value during NAVA (T0 median 0.9855), that was maintained constantly higher in time, than during PSV (T0 median 0.9288), in which the value of the CD progressively worsened by the hours of mechanical ventilation.


The LSF method of the LSF performs better during NAVA then during PSV. By the hours of mechanical ventilation the performance of the LSF method further worsens during PSV while remains constant during NAVA. Our data indirectly confirm more physiological patient-ventilation interactions during NAVA than during PSV.


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  1. 1.

    Gillard C, Flemale A, Dierckx JP, et al: Measurement of elffective elastance of the total respiratory system in ventilated patients by a computed method. Intensive Care Med. 1990, 16: 189-195. 10.1007/BF01724801.

  2. 2.

    Volta CA, Marangoni E, Alvisi V, et al: Respiratory mechanics by least square fitting in mechanically ventilated patients: applications on flow-limited COPD patients. Intensive Care Med. 2002, 28: 48-5. 10.1007/s00134-001-1147-8.

  3. 3.

    Iotti GA, Braschi A, Brunner JX, et al: Respiratory mechanics by least square fitting in mechanically ventilated patients: applications during paralysis and during pressure support ventilation. Intensive Care Med. 1995, 21: 406-413. 10.1007/BF01707409.

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Correspondence to V Cricca.

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  • Mechanical Ventilation
  • Acute Respiratory Failure
  • Little Square Fitting
  • Respiratory Mechanic
  • Pressure Support Ventilation