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Introduction: Resting parameters of the lower esophageal sphincter are customarily measured in a "landmark" frame at the onset of a high resolution impedance motility (HRIM) study. We hypothesized that measurement at the completion of the study may give more representative values. We assessed the utility of a dual-landmark frame approach in HRIM interpretation and its effect on patient diagnostic classification.
Methods: 50 consecutive HRIM studies were analyzed independently by 4 experienced esophageal fellows. The overall and intra-abdominal length of the lower esophageal sphincter (LES), mean LES resting pressure (LESP), integrated relaxation pressure (IRP), intra-bolus pressure (IBP), mean upper esophageal sphincter pressure (UESP), and Chicago Classification diagnoses were recorded for each HRIM reading. Each of these parameters was measured using the conventional method of a landmark frame at the onset of the study and again using a similar 30-second frame at the end of the 10-swallow study. Previous data have shown excellent intra-class correlation coefficients among these 4 readers for HRIM parameters, EGJ diagnosis and esophageal body diagnosis. Wilcoxon signed-rank test was used to analyze the concordance of the measurements based on the early or late landmark frames.
Results: There were no differences in LES overall length, intra-abdominal length, IBP or IRP comparing early vs. late landmark measurement. Both lower and upper esophageal sphincter resting pressure were significantly lower when measured at the end of the study (table). These findings resulted in re-classification of LESP in 12% of studies, 3 each from hypertensive LES to normal and normal to hypotensive. In 8% (n=4), IRP measurement changed from normal (<14.7 mmHg) to abnormal, reflecting functional outflow obstruction. Manometric evidence of a hiatal hernia that was present in an early landmark frame was not seen in the late landmark frame in 14%.
Conclusions: A decrease in both upper and lower esophageal sphincter resting pressure with time is observed during the routine course of HRM studies. This decrease affects classification of sphincter parameters in a meaningful subset of patients. A dual-landmark frame approach should be considered when routinely analyzing HRIM studies.
High Resolution Impedance Manometry Diagnostic Variables, Dual-Landmark Frame Approach

Diagnostic Variable	Difference (Mean)	95%CI	P-value*
LES length (cm)	-0.03	(-0.23 - 0.18)	0.8205
Intra-abdominal LES length (cm)	0.03	(-0.34 - 0.39)	0.2238
LESP (mmHg)	-3.86	(-5.93 - -1.79)	0.0006*
IBP (mmHg)	0.07	(-1.2 - 1.36)	0.2745
IRP (mmHg)	0.01	(-0.93 - 0.95)	0.3616
UESP (mmHg)	-17.2	(-26.0 - -8.50)	<0.0001*

LES: Lower Esophageal Sphincter; LESP: Mean LES Resting Pressure; IBP: Intra-bolus Pressure; IRP: Integrated Relaxation Pressure; UESP: Mean Upper Esophageal Sphincter Pressure; Mean: The average differences between the late and early landmark frame measurements; CI: Confidence Interval; *Wilcoxon signed-rank test P-value <0.05