diagnosis of vasovagal syncope can be made after a suggestive history and positive tilt table testing . patient with vasovagal syncope usually develop bradycardia along with hypotension during head up tilt table testing ( hutt ) . in this case report we have discussed an unusual response to hutt due to improper filter application in cardiac monitor . a twelve - year old male , who was operated for ventricular septal defect closure and right ventricular outflow tract resection 5 years back , presented to us with two episodes of presyncope . his baseline head up tilt test ( hutt ) was uneventful but after provocation with sublingual nitroglycerine spray he developed presyncope . st elevation in the single lead ii ecg during the episode prompted us to repeat hutt with 12 lead holter monitoring . during repeat hutt , however , holter analysis showed left bundle branch block ( lbbb ) escape rhythm during the episode ( fig .  2 ) . there are two important lessons to be learnt . first , that single lead monitors , generally used for rhythm monitoring can be highly misleading for st segment evaluation . however due to improper filter application in the monitor settings , st elevation was exaggerated . appropriate filtering is done in 12 lead ecg machines to avoid unwanted noises , interference and signals . high pass filters are applied in the standard ecg machines to eliminate low frequency signals ( such as movement of patient or breathings ) which causes baseline wanders . as per guidelines , low frequency filter cut off of 0.05 hz is advised for non - linear filters to minimize the distortion of st segment in real time display such as cardiac monitors . showed that at 0.5 and 1.0 hz h high pass filter settings 60% and 100% of ecgs were suggestive of anteroseptal myocardial infarction , respectively , with maximum changes noticed in lead v2 . secondly , patient developed hypotension with blood pressure of 56/19  mm hg leading to presyncope . there seems to be sinus arrest or sinus pauses , with a wide qrs escape , probably a fascicular or ventricular rhythm . junctional  rhythm , since the qrs complex during the escape is very different from the qrs complex during sinus conduction . there are brief periods of sinus activity , conducted qrs complexes now look different from baseline . on holter monitoring patient developed fusion beat to lbbb during episode of presyncope ( fig .  2 ) . to conclude we have described an unusual response to hutt due to improper application of filter settings in single lead cardiac monitor .
tilt testing is a commonly used diagnostic tool to evaluate syncope of undetermined etiology . 
 we hereby describe a twelve year old male child who developed presyncope during provocative head up tilt testing along with st segment elevation in cardiac monitor during presyncope . 
 patient underwent repeat head up tilt testing along with holter monitoring which showed a left bundle branch block escape rhythm . hence development of st elevation was due to the improper filter application in single lead ecg monitor . 
 this case highlights the importance of filters in electrocardiographic monitoring .