neurally adjusted ventilatory assist ( nava ) is a new mode of assisted mechanical ventilation [ 15 ] . during nava
the ventilator is triggered by the electrical activity of the diaphragm ( eadi ) , assessed by a special gastric tube ( eadi - catheter ) .
pressure support is applied in proportion to the amplitude of the eadi , which represents direct control by the patients own respiratory center [ 15 , 7 ] . during nava
reliable positioning of the eadi - catheter is mandatory in order to trace a representative eadi signal from the diaphragm .
one method to predict the correct position of a gastric feeding tube is based on the measure from the nose to the ear lobe to the xiphoid process of the sternum ( nex distance ) . using the nex distance
the gastric feeding tube was placed trans nasally more than 10  cm into the stomach in 26% of 99 adult cadavers and five normal adult volunteers . for eadi - catheter positioning
the manufacturer suggests a modified nex distance which aims to predict the ideal position of the electrode array .
therefore we aimed to determine whether or not it is sufficient to place the catheter purely by using the modified nex distance .
this observational study was performed after ethics committee approval at the anaesthesiological icu , university of goettingen , germany .
nava was initiated with the intention to improve patient respirator synchrony and to facilitate respirator weaning .
patients were treated with a ventilator capable of nava ( servo - i , maquet critical care , solna , sweden ) .
patients were in supine position with the upper part of the body elevated in a 30 angle and gastric content was drained via the nasogastric tube .
afterwards the standard tubing was replaced by the eadi - catheter ( 16 french diameter , 125  cm long ; maquet critical care , solna , sweden ) , inserted nasally to a maximum distance of 80  cm . for eadi measurement this tube is mounted with an electrode array of nine electrode rings on the distal part of the catheter at intervals of 16  mm , starting 120  mm from the tip .
the position of the electrodes in relation to the heart and diaphragm can be estimated by evaluating the different leads for presence / absence of p - wave and qrs complex . during the placement procedure
eadi signal and electrical activity from raw leads were recorded at each step using special software ( nava tracker , maquet critical care , solna , sweden ) for offline analysis until ecg - signals disappeared .
the  optimal  catheter position ( opt ) was identified by checking the recording for the following three criteria : ( 1 ) stable eadi signal , ( 2 ) electrical activity highlighted in central leads of the catheter positioning tool , and ( 3 ) absence of p - wave in distal lead . within the catheter positions that fulfilled the above mentioned criteria we chose the one with the maximum eadi value as opt .
afterwards we calculated the insertion distance of the eadi - catheter ( nexmod ) by the formula proposed by the manufacturer ( maquet catheter packaging).\documentclass[12pt]{minimal } 
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 \begin{document}$$ { \text{nex}}_{\bmod } = \;\left ( { { \text{nex}}\ ; \times \;0 . 9 } \right ) + 1 8\left ( { { \text{for}}\;{\text{the}}\ ; 1 6\;{\text{f } } .\;{\text{eadi}}\hbox{-}{\text{catheter } } } \right ) $ $ \end{document}in the formula the measured nex distance is multiplied by a correction factor ( nex    0.9 for nasal catheter insertion ) in order to predict the distance to the crural diaphragm .
depending on the catheter size used , a constant is added to compensate for the varying electrode array localisation (  + 18  cm for the 16 f , 125  cm catheter ) . for nex measurement
the patient was placed supine and the outer canthus of the patient s eye was vertically aligned with the tragus of the ear to decrease errors resulting from head tilt .
patient characteristics of 26 enrolled patients are presented in table  1.table  1basic patient characteristics including gender , age , height , body mass index ( bmi ) , reason for icu admission ( reason adm . ) , severity off illness on admission evaluated by saps ii , positive endexpiratory pressure ( peep ) during eadi - catheter placement and heart rhythmpatientgenderage ( years)height ( cm)bmireason adm.saps iipeep ( cmh2o)heart rhythmnexmod ( cm)opt ( cm)diff .
( cm)1m5618237.7chd298sr70.2673.22m4119023.5tbi518sr66.6714.43f7616529.4cpr658sr56.7658.34f7915029.3as5211sr60.3600.35m3319027.7tbi487sr66.6660.66f7516030.1ards7910afib58.5612.57f7216025.4trauma666sr5466128m7317426.4tbi598sr65.7660.39m4119024.9ai548afib69.3711.710m6617429.4trauma328sr66.6681.411m6317026.0tbi5110sr61.2631.812m7817624.2chd388afib61.8664.213f6416224.8arf5910sr58.2571.214m8617526.1trauma638sr64.8694.215f8716324.1chf458sr5963416m5418923.8tbi207sr6567217f6416522.0sah588sr62.1652.918m6817029.4chd198sr63.9662.119m8217827.5chd2410afib71.1692.120m7816719.7as356afib61.2610.221m3518023.1sepsis4712sr68.4690.622f6416926.6sepsis9310afib63.9651.123f8115822.0chd758sr61.2664.824m7716529.4ich5310sr6367425m4018527.8trauma6412sr6969025  +  1m4017932.6chf72bvadmedian ( q25/q75)68 ( 55/78)170 ( 165/180)26 ( 24.1/29.4)52 ( 40/64)8 ( 8/10)63.9 ( 61.2/66.6)66 ( 65/68)1.8 ( 0/4)mean    sd65.3    16.0172.3    11.026.4    3.651.2    18.08.7    1.663.5    4.365.7    3.32.2    3.1the median exact catheter position estimated by nexmod was 64  cm compared to 66  cm detected by optimal catheter positioning ( opt ) based on a stable eadi signal , electrical activity highlighted in the central leads of the catheter positioning tool and the absence of p - waves in the distal leadm male , f female , chd coronary heart disease , tbi traumatic brain injury , cpr post cardiopulmonary resucitation , as aortic valve stenosis , ards adult respiratory distress syndrome , trauma severe trauma other than tbi , ai aortic valve insufficiency , arf acute renal failure , chf congestive heart failure , sah subarachnoid hemorrhage , ich intracerebral hemorrhage , afib atrial fibrillation , bvad biventricular cardiac assist device , sr sinus rhythm basic patient characteristics including gender , age , height , body mass index ( bmi ) , reason for icu admission ( reason adm . ) , severity off illness on admission evaluated by saps ii , positive endexpiratory pressure ( peep ) during eadi - catheter placement and heart rhythm the median exact catheter position estimated by nexmod was 64  cm compared to 66  cm detected by optimal catheter positioning ( opt ) based on a stable eadi signal , electrical activity highlighted in the central leads of the catheter positioning tool and the absence of p - waves in the distal lead m male , f female , chd coronary heart disease , tbi traumatic brain injury , cpr post cardiopulmonary resucitation , as aortic valve stenosis , ards adult respiratory distress syndrome , trauma severe trauma other than tbi , ai aortic valve insufficiency , arf acute renal failure , chf congestive heart failure , sah subarachnoid hemorrhage , ich intracerebral hemorrhage , afib atrial fibrillation , bvad biventricular cardiac assist device , sr sinus rhythm eadi - catheter placement was possible in all patients and enteral nutrition via the eadi - catheter was continued uneventfully during nava .
p - waves were absent in seven patients ( six due to atrial fibrillation ) . in one patient
( 25  +  1 ) , treated with an extracorporal cardiac assist device , there was no detectable eadi signal although the patient was able to trigger pressure support ventilation ( psv ) by his auxiliary respiratory muscles .
since cervical magnetic stimulation of the phrenic nerves did not result in any diaphragmatic response , bilateral injury was suspected and the patient excluded from further data analysis . in patient 19
the suspected malposition was detected at once by complete lack of stable eadi and ecg - signals and corrected immediately after verification by chest x - ray . in patient 24 ( 8 years post gastrectomy ) the catheter positioning tool did not highlight the electrical activity of the diaphragm comprehensibly although it was present . at nexmod
the remaining seven patients ( 28% ) had an inadequate signal quality , thus nava was impossible ( fig .  1 ) .
the nexmod position was identical with the opt position in four patients ( 16% ) .
in general we found the opt position caudal of the nexmod ( fig .  2 ) .
the median difference was 2  cm and the difference ranged from 3  cm too cranial to a position 12  cm too caudal ( p  <  0.01 ) .
patient 7 ( 12  cm difference ) had a short neck and low lying ears which led to the suspected diagnosis of a laurence  moon 
biedl  bardet  syndrom [ 9 , 10 ] and should be regarded as an outlier . at nexmod position the catheter positioning tool
p - wave was absent in the distal lead in nine cases ( 47% of patients with p - waves ) ( fig .  1 ) .
the criterion  highlighted electrical activity in central leads  narrowed the opt position down to an area of 27  cm . in consideration of  absence of p - wave  , the opt position could additionally be narrowed down to an area of 25  cm.fig . 
the different variables considered for optimal catheter placement ( opt ) ( eadi signal , highlighted eadi in central leads and no p - wave in lead one ) and the ventilators information on bad catheter positioning ( alarm :  check catheter position  ) are compared to the nexmod method , which is based on anatomical distance measure between xiphoid , earlobe and nose tip .
note : at nexmod nava was possible ( squares ) in 72% of patients whereas in seven patients ( filled circles ) nava was not possible at nexmod .
the highlighted eadi leads and p - wave interpretation indicated an eadi - catheter position that was too cranial in five patients . in six out of seven patients
alarm was active . from left to right : eadi signal : percentage of maximum eadi signal at different eadi - catheter insertion distances .
highlighted eadi in central leads : percentage of patients with highlighted eadi in leads two and three of the catheter positioning tool , normalized for opt position and displayed as mean of 25 patients .
check catheter position  : percentage of patients with alarm  check catheter position  at different catheter positions . no p - wave in distal lead : percentage of patients with no detectable p - wave in distal lead of the catheter positioning tool normalized for opt position , displayed as mean of 18 patients ; patients nexmod position : nexmod position of 25 patients in relation to their opt catheter position .
open squares nava was possible in these patients at nexmod position ; filled circle nava was not possible in these patients at nexmod positionfig . 
solid line shows mean of difference , dotted line shows mean    2 sd .  
the figure shows a mean difference of 2.2  cm between opt and nexmod and one outlier comparison of the different procedures of eadi - catheter positioning in 25 patients .
the different variables considered for optimal catheter placement ( opt ) ( eadi signal , highlighted eadi in central leads and no p - wave in lead one ) and the ventilators information on bad catheter positioning ( alarm :  check catheter position  ) are compared to the nexmod method , which is based on anatomical distance measure between xiphoid , earlobe and nose tip .
note : at nexmod nava was possible ( squares ) in 72% of patients whereas in seven patients ( filled circles ) nava was not possible at nexmod .
the highlighted eadi leads and p - wave interpretation indicated an eadi - catheter position that was too cranial in five patients . in six out of seven patients the  check catheter position
alarm was active . from left to right : eadi signal : percentage of maximum eadi signal at different eadi - catheter insertion distances .
highlighted eadi in central leads : percentage of patients with highlighted eadi in leads two and three of the catheter positioning tool , normalized for opt position and displayed as mean of 25 patients .
check catheter position  : percentage of patients with alarm  check catheter position  at different catheter positions . no p - wave in distal lead : percentage of patients with no detectable p - wave in distal lead of the catheter positioning tool normalized for opt position , displayed as mean of 18 patients ; patients nexmod position : nexmod position of 25 patients in relation to their opt catheter position .
open squares nava was possible in these patients at nexmod position ; filled circle nava was not possible in these patients at nexmod position bland 
solid line shows mean of difference , dotted line shows mean    2 sd .  
the figure shows a mean difference of 2.2  cm between opt and nexmod and one outlier
during nava , correct placement of the eadi - catheter is mandatory to deduce a reliable eadi signal for respirator control .
the modified nex distance successfully predicted the eadi - catheter insertion distance in 18 of 25 patients and was identical with the opt position in four patients . in combination with the catheter positioning tool , catheter placement was possible in all 25 patients .
former clinical investigations regarding eadi - catheter positioning refer to the work of beck , where the optimal catheter position was defined by an electromyographic signal from the central electrodes highest in center frequency and reduced in root mean square .
brander   additionally judged the correct position by ecg interpretation . in this study , different information provided by the catheter positioning tool were considered .
the anatomical proximity of the right atrium and diaphragm allows an ecg guided positioning of the eadi - catheter .
the progressively decreasing amplitude of the p - wave from the cranial to the caudal electrodes and its absence in the distal lead indicates a position caudal of the right atrium .
. a catheter position at which the signal for respirator control is taken from the center of the electrode array is advantageous , because displacement of the diaphragm during in- and expiration will most likely not exceed beyond the electrode array . the alarm  check catheter position
 appears in this case and proved to be a reliable indicator of a bad catheter position .
we found a median difference of 2  cm with the opt situated distal of the nexmod position .
although nexmod and opt positions were identical in only four patients , running the nava mode was possible in 18 patients at nexmod position .
the diaphragm forms a muscular  tunnel  of 23  cm covering the oesophagus .
this explains why we found stable signals over a certain range and could use nava at most nexmod positions .
however , in seven patients with a nexmod to opt difference greater than 3  cm , running nava was impossible at nexmod position due to impaired eadi signal detection .
thus the positioning tool did not only ensure the correct catheter position but also allowed running nava in those patients were nexmod failed to predict a good position .
the position of the diaphragm depends on the application of peep , body position and intra - abdominal pressure ( iap ) , respectively [ 1315 ] .
the patients included in our study were placed supine with the upper part of the body elevated in a 30 angle for catheter positioning and had a median peep level of 8  cmh2o .
pressure support ventilation with zero peep compared to a peep level of 10  cm h2o results in a 2  cm shift caudal regarding the middle part of the diaphragm .
although we did nt measure intra - abdominal pressure there was no clinical suspicion of such . the body position and application of peep may have contributed to the fact , that opt position was found distal of the nexmod position . in one patient missing eadi led to the diagnosis of bilateral phrenic nerve injury .
the patient with suspected laurence  moon  biedl  bardet  syndrom represented an obvious problem with nexmod calculation that was predictable due to the askew anatomy . in one patient ( post gastric surgery ) the catheter positioning tool did not provide reliable data , nevertheless we were able to treat the patient with nava .
thus gastric surgery might be considered as a relative contraindication for eadi - catheter placement .
positioning the eadi - catheter using nexmod gives a good approximation of the catheter position in most patients .
the additional tools for catheter positioning are needed to ensure an optimal position , especially in those patients were nexmod fails .
further studies based on a larger set of patients are required to enhance the accuracy of the nex formula taking into account peep , body position and iap , respectively .
 purposeduring neurally adjusted ventilatory assist ( nava ) the ventilator is driven by the patients electrical activation of the diaphragm ( eadi ) , detected by a special esophageal catheter . 
 a reliable positioning of the eadi - catheter is mandatory to trace a representative eadi signal . 
 we aimed to determine whether a formula that is based on the measurement from nose to ear lobe to xiphoid process of the sternum ( nex distance ) modified for eadi - catheter placement ( nexmod ) is sufficient for predicting the accurate catheter position.methodstwenty-six patients were enrolled in this study . the optimal eadi - catheter position ( opt ) 
 was defined by : ( 1 ) stable eadi signal , ( 2 ) electrical activity highlighted in central leads of the catheter positioning tool , and ( 3 ) absence of p - wave in distal lead . afterwards nexmod was calculated and compared to the opt finding.resultsat nexmod the eadi signal was suitable for running nava in 18 out of 25 patients ( 72% ) . 
 nexmod was identical with opt in four patients ( 16% ) . 
 nava was possible in all patients at opt . 
 median opt position was 2  cm caudal of the nexmod ranging from 3  cm too cranial to a position 12  cm too caudal ( p  <  0.01 ) . 
 in one patient excluded from further analysis eadi - catheter placement led to the diagnosis of bilateral injury of the phrenic nerves.conclusionseadi-catheter placement based on the nexmod formula allows running nava in about two - thirds of all patients . 
 the additional tools provided are efficient and facilitate the correct positioning of the eadi - catheter for neurally adjusted ventilatory assist . 
Introduction
Methods
Results
Discussion
Conclusion