Background: the accurate recording and precise analysis of the ECG traces is crucial for a good clinical interpretation, especially in outpatient assessment before surgical interventions, in whom the ECG is the first and often unique cardiological evaluation. The 2007 AHA/ACC/HRS recommendations for the standardization and interpretation of the electrocardiogram suggests an High-frequency filtering cut-off of at least 150Hz for all adolescents and adult ECGs to eliminate potential invalidations of any amplitude measurements used for diagnostic classification. However the mostly used 40Hz filtering allow a better graphic resolution of the traces. Aim of this study is to compare the 150 and 40Hz high pass filtering in a population afferent to the hospital and candidate to surgical interventions.
Methods: each patient underwent to a 12-lead ECG in double high frequency filtering (150Hz and 40 Hz), analyzed by two blinded cardiologist. The baseline characteristics and ECG signal differences were collected also in terms of subjective quality perception (grading between 1 as poor quality and 3 good quality).
Results: a total of 1582 patients were analyzed (42% males), 97.5% was in sinus rhythm with an average heart rate of 68.2±11.5 bpm, RBBB and LBBB was present in 7.4% and 2.5% respectively; 2.2% of patients was in atrial fibrillation and 7% had a 1st degree AV block at baseline ECG. ST-T anomalies were seen in 33.7% and 11.6% had Q waves > 1mm in at least one lead; 1.1% had pacemaker’s spikes visible. Analyzing the trace’s differences at 150Hz and 40Hz, the study population did not show any statistical difference in terms of ST and T wave abnormalities, presence of significative Q waves and visible pacemaker spikes (p=0.26; 0.79; 0.74; 1 and 1, respectively). However the QRS amplitude (manually measured adding the maximal positive and negative QRS deflection in precordial leads) demonstrated a significative difference between the groups (p < 0.0001), this finding was reflected also in Left ventricular hypertrophy diagnosis that was significatively different between the two traces (P < 0.0001) in favour of 150 Hz, and a difference in J point elevation diagnosis in favor of 40 Hz (P=0.007). As expected, in 40 Hz ECG there was an average reduction in QRS amplitude of 2±2.1 mV and 25 patients (0.01%) were not diagnosed with left ventricular hypertrophy, obviously the subject with borderline QRS amplitude for Sokolow criteria. There was instead a significative difference in favor of 40 Hz traces in terms of perceived quality (P < 0.0001). Of note, in the percentage of poor quality traces (0.6% Vs 10.1% respectively) a 45% of the traces was judged not readable by the physician (4 Vs 71 traces).
Conclusions: the 40Hz ECG filtering permits an accurate ECG analysis. It may underestimate the left ventricular hypertrophy in a small percentage of patients at a cost of a significative amelioration in perceived traces’ quality.