Foundations & Indications for FLIP

The technique was first described by McMahon and Gregersen in the late 1990s, applying impedance-based luminal measurement to anorectal and oesophageal models. The probe was commercialised as EndoFLIP (Crospon, later Medtronic) in the early 2010s, with the EF-322N (8 cm balloon) catheter dominating EGJ work and the EF-325N (16 cm) introduced for full-oesophageal panometry. The therapeutic sibling — EsoFLIP — was approved for controlled radial dilation in 2017. Today, more than 600 published clinical studies underpin the technique, and major societies (AGA, ASGE, ESNM) have integrated FLIP into their motility guidelines.

A FLIP catheter carries 17 ring electrodes spaced 1 cm apart, forming 16 paired impedance segments. The balloon is filled with a conductive saline solution. As current flows between each adjacent electrode pair, the device measures impedance — which, by Ohm's law in a fluid of known conductivity, is inversely proportional to cross-sectional area (CSA). A solid-state strain-gauge pressure transducer sits inside the balloon tip and reports intra-bag pressure simultaneously.

For each fill volume (typically 30, 40, 50, and 60 mL), the system reports CSA at the narrowest segment and the corresponding intra-bag pressure. The distensibility index is simply the ratio: DI = CSA (mm²) / pressure (mmHg). A normal EGJ at 60 mL fill yields CSA ≈ 90–160 mm² and pressure ≈ 25–35 mmHg — giving DI ≈ 3–6. A tight, non-distending achalasic EGJ at 60 mL might yield CSA ≈ 30 mm² and pressure ≈ 40 mmHg, giving DI ≈ 0.75 — well below the 2.0 threshold.

CSA alone is misleading — a wide opening at very high pressure can look "open" but mean nothing for swallowing. Pressure alone misses the patient with a chronically dilated, non-contractile oesophagus. DI fuses both: it asks, for a given pressure load, how much does the sphincter actually let through? That single ratio outperforms either component in predicting clinical outcomes after POEM, Heller, fundoplication, and pneumatic dilation. The Carlson 2015 validation cohort showed DI correlated with symptomatic outcome (r = 0.71) while CSA and pressure individually correlated at r = 0.42 and r = 0.38 respectively.

FLIP earns its bench space because it answers questions other tests cannot. The validated indications are:

(1) Confirm achalasia when HRM is inconclusive or normal but symptoms persist. (2) Subtype achalasia in conjunction with FLIP topography panometry. (3) Distinguish EGJ outflow obstruction from mechanical or motility mimics. (4) Guide intraoperative POEM — adjust myotomy length to a target DI of 6–9. (5) Tailor fundoplication tightness during Nissen/Toupet/Dor (target DI 2–3.5). (6) Assess incisural distensibility during sleeve gastrectomy to predict post-op reflux. (7) Select refractory gastroparesis patients for G-POEM using pyloric DI < 9–10. (8) Deliver therapeutic dilation via EsoFLIP with real-time CSA feedback in achalasia and post-fundoplication dysphagia.

HRM measures peristalsis and oesophagogastric junction relaxation during a swallow — it is unmatched for primary motility diagnosis. FLIP measures distensibility during volumetric distension at rest — it captures EGJ opening physics under load. Barium imaging measures emptying. Each answers a different question.

In practice: start with HRM. If HRM gives an unambiguous Chicago v4.0 phenotype, FLIP is optional. If HRM is borderline (IRP 10–15 mmHg), inconclusive, or in conflict with the clinical picture, FLIP at the same endoscopy session resolves the ambiguity in > 70% of cases. Timed barium oesophagram is added after therapy to assess emptying durability.