Fluid responsiveness: PLR ΔVTI and IVC variation

ΔVTI(PLR), IVC distensibility · LVOT VTI (A5C) before and 60–90 s after passive leg raising; IVC in subcostal long axis for ventilated patients

Normal range & thresholds

• ΔVTI with passive leg raising ≥10–15% predicts a rise in cardiac output with fluid (pooled AUC ≈0.95 in meta-analysis) ^[121]; in spontaneously breathing patients a ≥12.5% rise in SV predicted response ^[168].
• IVC distensibility index >18% (passive, fully ventilated patients) predicts response ^[122]; respiratory variation >12% by another formula in ventilated septic patients ^[169].
• A small, collapsing IVC in spontaneous breathing indicates low RAP, not fluid responsiveness.

Pathophysiology

A heart is fluid-responsive when both ventricles are on the steep part of their Frank–Starling curves. PLR transfers ≈300 mL of venous blood into the central circulation reversibly — a fluid challenge that can be undone; the stroke-volume change it produces shows where the ventricle sits on its curve ^[121].

Raised by

• Hypovolaemia, vasodilation with under-filling
• Early sepsis before resuscitation

Lowered by

• Fluid-overloaded or failing ventricles on the flat part of the curve
• RV failure, tamponade physiology, severe PH (fluid may harm)

Technique & pitfalls

• Start semi-recumbent at 45°; tilt the bed so the trunk goes flat and legs rise to 45°; measure VTI at 60–90 s; return and confirm recovery.
• Keep the sample volume and angle identical before and after; average several beats.
• IVC distensibility = (max − min) / min in passive ventilation with tidal volume ≥8 mL/kg and sinus rhythm ^[122].
• Surviving Sepsis 2021 suggests dynamic measures over static ones to guide fluids ^[155].

Pseudo-change & artefact

• PLR is unreliable with intra-abdominal hypertension, and pain or movement causing sympathetic surges.
• A change of probe position between measurements invalidates ΔVTI.
• IVC indices fail with spontaneous breathing effort, low tidal volumes, raised abdominal pressure and RV failure.

Treatment thresholds

• Positive test + hypoperfusion → give a fluid bolus and reassess.
• Negative test → stop fluids; consider vasopressors or inotropes according to phenotype ^[153,155].
• Echo-guided fluid decisions are part of structured critical-care echocardiography ^[151,152].

Next step

• Next: after each bolus, repeat VTI; stop when the response ceases or congestion appears (lung B-lines, rising E/e′, plethoric IVC).

Drugs

Vasopressors raise venous return and can change the test result; diuretics and vasodilators reduce responsiveness to volume.

Reversibility

Changes with every intervention — test, treat, retest.