IVC, Venous Congestion & Fluid Responsiveness

5.1 IVC — thresholds embedded in main text

Spontaneous breathing

  • IVC less than 1.5 cm with collapse greater than 50%: collapsed or small; low CVP; possible hypovolaemia.

  • IVC 1.5–2.5 cm with collapse 20–50%: normal or intermediate; indeterminate; correlate clinically.

  • IVC greater than 2.1 cm with collapse less than 50%: formal elevated RAP estimate; standard formal echo RAP approximately 15–20 mmHg.

  • IVC greater than 2.5 cm with collapse less than 50%: plethoric or marked POCUS finding; avoid empiric fluids.

  • IVC greater than 2.5 cm with collapse less than 20%: clearly plethoric; high suspicion of volume overload or impaired RV filling.

  • IVC greater than 3.0 cm with minimal collapse: markedly plethoric; late warning; possible iatrogenic congestion.

  • IVC greater than 3.5 cm: marked plethora in some protocols; not a universal formal RAP cutoff.

Collapsibility index, spontaneous breathing

  • Formula: (Dmax − Dmin) / Dmax × 100.

  • Greater than 50%: low CVP.

  • 20–50%: indeterminate.

  • Less than 20%: high CVP.

  • For fluid responsiveness, a collapsibility threshold of approximately 40–41% or greater suggests fluid responsiveness.

  • Reported specificity is approximately 80–97%.

  • High collapsibility increases the likelihood of responsiveness but does not guarantee it.

  • Low collapsibility reduces the likelihood but does not absolutely exclude responsiveness.

Technique for spontaneous breathing

Measure during quiet spontaneous tidal breathing. Dmax is at end-expiration; Dmin is during inspiration. If a quiet tidal inspiration does not elicit a clear Dmin, a brief controlled sniff may be used. Do not use sustained maximal inspiratory strain or Valsalva-like effort. In fully mechanically ventilated patients without spontaneous breathing activity, IVC collapsibility is not the appropriate index; use IVC distensibility or mean-based variability instead.

Integration with diameter

Fluid responsiveness is more likely when the IVC is relatively small, commonly less than 2.1 cm, with collapse greater than 50%, suggesting lower RAP. Fluid unresponsiveness is more likely when the IVC is relatively plethoric, commonly greater than 2.1 cm, with collapse less than 50%. Very low collapsibility, less than 10–12%, is particularly indicative of fluid unresponsiveness. Published thresholds for spontaneous-breathing IVC variability are not uniform and have ranged approximately 8–50% depending on population, ventilatory conditions, and technique. Diagnostic accuracy in spontaneously breathing patients is less robust than in fully ventilated patients; integrate with other dynamic and static haemodynamic assessments.

Mechanical ventilation

Do not use inspiratory collapse for RAP.

  • IVC distensibility, Dmin-based: (Dmax − Dmin) / Dmin × 100.

    • Common general controlled-ventilation threshold: 15% or greater.

    • Validation-specific range: approximately 12–21%.

    • A 18% threshold remains appropriate in selected fully ventilated septic shock protocols; it is a validation-specific operating point, not the only standard.

    • Validity conditions: tidal volume 8 mL/kg or greater, stable ventilator settings, no spontaneous effort.

    • Invalidity: high PEEP, low tidal volume, spontaneous effort, RV dysfunction, intra-abdominal hypertension.

  • IVC variability, mean-based: (Dmax − Dmin) / [(Dmax + Dmin) / 2] × 100.

    • Common threshold: 12%.

    • Same validity and invalidity conditions as Dmin-based distensibility.

A 15% value labelled “distensibility” refers to the Dmin formula and is a common operational threshold. Do not interchange 15% Dmin-based distensibility, 18% septic-shock-specific distensibility, and 12% mean-based variability.

Formula warning

Collapsibility index, Dmin-based distensibility index, and mean-based variability are not interchangeable. State which formula was used. Do not mix thresholds from different formulas.

5.2 VExUS — Venous Excess Ultrasound

VExUS integrates four bedside ultrasound sites. Each site is scored 0 or 1.

Sites

  • IVC: normal diameter with expected respiratory variation versus plethoric or dilated IVC with reduced respiratory variation.

  • Hepatic vein: systolic predominance versus S/D less than 1, diastolic attenuation or reversal, or atrial reversal.

  • Portal vein: continuous antegrade flow versus increased pulsatility, attenuated or reversed diastolic flow. Abnormal portal pulsatility is commonly defined as pulsatility fraction greater than 50%.

  • Intrarenal vein: continuous antegrade diastolic flow versus loss, attenuation, or reversal of diastolic flow.

Exact portal and intrarenal Doppler thresholds can vary by protocol; state institutional algorithm.

Conventional VExUS grading

  • Grade 0: IVC normal and all venous Doppler normal. No systemic venous congestion.

  • Grade 1: isolated IVC abnormality; hepatic, portal, and intrarenal Doppler normal. At risk for venous congestion.

  • Grade 2: exactly one venous Doppler abnormality. IVC may be normal or abnormal. Possible congestion.

  • Grade 3: two or more venous Doppler abnormalities. IVC may be normal or abnormal. Probable congestion.

Examples

  • Plethoric IVC only: VExUS 1.

  • Normal IVC plus abnormal hepatic vein only: VExUS 2.

  • Abnormal IVC plus abnormal hepatic vein only: VExUS 2, because only one venous Doppler site is abnormal.

  • Abnormal hepatic vein plus abnormal intrarenal vein: VExUS 3.

  • All four sites abnormal: VExUS 3.

If an institution uses an additive 0–4 point system, state this explicitly; it is not identical to conventional categorical VExUS grading.

Interpretive caveats

  • VExUS is a likelihood scale, not a linear severity scale.

  • Grade 2 should be interpreted cautiously as possible congestion with higher false-positive susceptibility; a single venous Doppler abnormality may be influenced by non-congestive factors. Grade 2 is not equivalent to Grade 3.

  • VExUS is not a simple volume-status score. It reflects the interaction of venous return and cardiac function.

  • The same VExUS grade can arise from elevated RAP, RV failure, elevated PVR, LV filling pressure elevation, high intra-abdominal pressure, pericardial constraint, mechanical ventilation, arrhythmia, or altered venous capacitance.

  • Important confounders include spontaneous breathing versus mechanical ventilation, PEEP and tidal volume, atrial fibrillation or other arrhythmias, significant tricuspid regurgitation, RV systolic or diastolic dysfunction, LV diastolic dysfunction, pericardial effusion or tamponade, elevated intra-abdominal pressure, hepatic or renal parenchymal disease, probe positioning, and respiratory timing.

  • In sepsis and mixed ICU populations, VExUS–outcome associations are less consistent than in some cardiac surgery cohorts.

  • Lung ultrasound score and VExUS answer different questions. Discordant patterns are clinically meaningful.

5.3 Fluid responsiveness — dynamic indices

IVC collapsibility

  • Patient state: spontaneous breathing.

  • Key threshold: approximately 40–41% or greater.

  • Reported specificity: approximately 80–97%.

  • Validity: quiet respiration, good window.

  • Invalidity: PEEP, arrhythmia, RV failure, pericardial disease.

IVC distensibility, Dmin-based

  • Patient state: controlled mechanical ventilation, no spontaneous effort.

  • Key threshold: commonly 15% or greater; validation-specific range 12–21%; 18% in selected septic shock protocols.

  • Validity: tidal volume 8 mL/kg or greater, stable settings, no spontaneous effort.

  • Invalidity: high PEEP, low tidal volume, spontaneous effort, RV dysfunction, intra-abdominal hypertension.

LVOT VTI or peak aortic velocity respiratory variation

  • Patient state: controlled mechanical ventilation.

  • Key statistic: sensitivity approximately 79% reported in mechanical ventilation circulatory failure.

  • Validity: no arrhythmia, stable haemodynamics.

  • Invalidity: atrial fibrillation, spontaneous breathing, poor compliance, low tidal volume, RV failure.

Passive leg raising with LVOT VTI or stroke volume

  • Any patient state, especially when IVC assumptions are not met.

  • Positive response: 10–15% or greater increase in stroke volume, cardiac output, or LVOT VTI.

  • A 10–15% response is associated with excellent predictive performance.

  • Validity: supine position, legs raised approximately 45 degrees, maintained 30–60 seconds.

  • Invalidity/caution: lower limb fracture, DVT, pelvic or hip injury, severe oedema.

SVC collapsibility

  • TEE-based, not standard POCUS.

  • Threshold: greater than 39% in septic shock; specificity approximately 84% in mechanical ventilation circulatory failure.

  • Validity: TEE available.

  • Limitation: limited TTE access.

Formula warning

Percent variation = 100 × (max − min) / [(max + min) / 2].
Distensibility index = 100 × (Dmax − Dmin) / Dmin.
Collapsibility index = 100 × (Dmax − Dmin) / Dmax.
These are not interchangeable. State the formula. Do not apply a threshold from one formula to a value calculated with another.

5.4 Multimodal low-output/hypovolaemia pattern — sepsis

Aortic VTI less than 16 cm plus mitral E less than 67 cm/s plus SVC collapsibility greater than 39% is highly suggestive of hypovolaemia or fluid responsiveness in septic shock.

5.5 Fluid tolerance — harm-limiting markers

  • New or increasing B-lines: developing interstitial oedema; reduced fluid tolerance.

  • VExUS grade 2–3: systemic venous congestion; limited capacity for further volume.

  • IVC greater than 2.5 cm plus poor collapse: plethoric; avoid empiric fluids.

  • E/A shift from less than 1 to greater than 2 during resuscitation: approaching preload ceiling; further fluid of limited benefit.

  • Replacement of A-lines by B-lines is a practical bedside stop signal for further fluid unless the clinical question changes.

Responsiveness is not tolerance. A patient may be fluid responsive and have limited fluid tolerance.

5.6 Passive leg raising — practical points

  • Reversible autotransfusion approximately 300 mL.

  • Positive response: 10–15% or greater increase in stroke volume or LVOT VTI.

  • The response is transient; interpret in context.

  • Cautions: lower limb DVT, hip or pelvic fracture, severe oedema.

  • PLR is a preload test, not a fluid bolus.

5.7 IVC/venous-specific pitfalls

  • IVC diameter is a number, not a diagnosis.

  • Collapsibility index is less reliable in mechanical ventilation.

  • Plethoric IVC in heart failure is expected; in a young patient with sudden dyspnoea and leg swelling, consider PE.

  • Collapsible IVC in sepsis does not guarantee fluid responsiveness.

  • VExUS grade 2 is possible congestion, not definitively severe congestion.

  • VExUS reflects venous return plus cardiac interaction, not simple fluid overload.

  • VExUS is affected by COPD, PEEP, mechanical ventilation, obesity, tamponade, and CVC.

  • Formula-specific thresholds: do not mix.

  • IVC greater than 2.1 cm with collapse less than 50% is the formal elevated-RAP criterion.

  • IVC greater than 3.5 cm is not a universal formal RAP cutoff.

  • Using 18% as the only standard Dmin-based distensibility cutoff; the common general threshold is 15%, with 18% as a septic-shock-specific operating point.

  • Interchanging Dmin-based distensibility with mean-based variability.

  • Applying Valsalva-like effort for IVC collapsibility measurement.

  • Using a linear probe as the standard transducer for adult subcostal IVC assessment.

  • Ignoring PEEP, spontaneous effort, low tidal volume, RV failure, intra-abdominal hypertension, and arrhythmia.