📚 Guideline basis
ESICM 2025 recommendations on haemodynamic monitoring in shock; Surviving Sepsis Campaign 2021; Monnet & Teboul passive leg raising literature; Vieillard-Baron et al. Intensive Care Med 2019 (PMID 30877351); Muller et al. J Am Soc Echocardiogr 2020 (PMID 32147001).
Mechanisms
The two questions
Fluid administration involves two logically independent questions, and conflating them is the commonest error in shock resuscitation:
- Fluid responsiveness — will stroke volume increase by a meaningful amount (conventionally ≥ 10–15%) if preload is increased? This is a property of the position on the Frank–Starling curve.
- Fluid tolerance — will that increase in preload be paid for in pulmonary and systemic venous congestion? This is a property of ventricular compliance, pulmonary capillary permeability, and venous capacitance.
Roughly half of haemodynamically unstable ICU patients are fluid responsive. A patient can be simultaneously fluid responsive and fluid intolerant — the classic example being acute cor pulmonale, where fluid raises stroke volume marginally while worsening RV dilatation and organ congestion. Responsiveness is a permission, not an indication. The indication is evidence of hypoperfusion that fluid can plausibly correct.
Heart–lung interaction: the physiological basis of dynamic indices
During a positive-pressure inspiration in a passively ventilated patient:
- Increased intrathoracic pressure compresses the vena cavae and reduces RV preload.
- Increased transpulmonary pressure increases RV afterload.
- Pulmonary capillary blood is squeezed into the LA, transiently increasing LV preload and LV stroke volume.
- Two to three beats later (the pulmonary transit time), the reduced RV output reaches the LV, and LV stroke volume falls.
The resulting cyclical variation in LV stroke volume is large when both ventricles are on the steep part of the Frank–Starling curve, and small when either is on the flat part. This is the entire mechanism of pulse pressure variation, stroke volume variation, and respiratory variation of LVOT VTI.
Every validity condition for dynamic indices follows from this mechanism:
Condition required | Why | What happens if violated |
Controlled mechanical ventilation, no spontaneous effort | Requires a regular, predictable intrathoracic pressure swing | Irregular swings destroy the signal; false negatives and false positives |
Tidal volume ≥ 8 mL/kg predicted body weight | Smaller swings produce insufficient preload variation | False negatives — the commonest error in modern lung-protective ventilation |
Regular cardiac rhythm | Beat-to-beat variation must be respiratory, not rhythmic | AF makes the index uninterpretable |
Absence of severe RV failure | RV afterload variation dominates the signal | False positives — variation reflects RV afterload sensitivity, not preload reserve |
Closed chest, normal compliance | Open chest or very low compliance transmits pressure poorly | Signal attenuation |
Heart rate / respiratory rate ratio > 3.6 | Requires enough beats per respiratory cycle to sample the swing | Uninterpretable at high respiratory rates |
Because these conditions are absent in a large proportion of modern ICU patients — most are ventilated at 6 mL/kg, many have spontaneous effort or AF — dynamic indices are frequently inapplicable, and the passive leg raise has correspondingly become the default test.
Diagnostic Synthesis
Preload-responsiveness tests
Test | Method | Threshold | Best validated in | Fails when |
Passive leg raise + VTI | Semi-recumbent 45° → supine with legs at 45°; measure LVOT VTI at baseline and within 60–90 s | ΔVTI ≥ 10% | Broadly — spontaneous breathing, AF, low tidal volume, open chest | Raised intra-abdominal pressure; compression stockings; the patient cannot be moved (pelvic fracture, raised ICP) |
Respiratory variation of LVOT VTI / peak velocity | Record VTI across a respiratory cycle | ΔV(peak) ≥ 12% | Passive, ventilated, sinus rhythm, VT ≥ 8 mL/kg | All conditions above |
End-expiratory occlusion test | 15 s expiratory hold; ΔVTI | ΔVTI ≥ 5% (small threshold; needs precise measurement) | Ventilated patients tolerating a hold, including at low VT | Patient triggers during the hold |
Mini fluid challenge | 100 mL crystalloid over 1 min; ΔVTI | ΔVTI ≥ 6% | Most settings | Requires high-precision VTI measurement |
Fluid challenge | 250–500 mL; ΔVTI or ΔSV | ≥ 10–15% | The reference standard, but it is a treatment, not a test | Irreversible if the patient is intolerant |
IVC distensibility (ventilated) | (D_max − D_min)/D_min | ≥ 18% (dIVC) | Passive, ventilated, sinus rhythm, VT ≥ 8 mL/kg | Poor performance in ICU cohorts; large grey zone |
IVC collapsibility (spontaneous) | (D_max − D_min)/D_max | > 40–50% suggests responsiveness | Spontaneously breathing | Depth of effort, intra-abdominal pressure, PEEP |
SVC collapsibility (TEE) | (D_max − D_min)/D_max | ≥ 36% | Ventilated patients; better specificity than IVC | Requires TEE |
On the IVC: the IVC is the most-used and least-reliable index in this list. Its diameter is influenced by intra-abdominal pressure, PEEP, RV function, tricuspid regurgitation, and the depth of respiratory effort. Multiple ICU cohorts show diagnostic performance for fluid responsiveness that is only modestly better than chance, with a wide grey zone. Its legitimate uses are at the extremes — a small, fully collapsing IVC in a hypotensive spontaneously breathing patient, and a plethoric non-varying IVC as a marker of congestion (Chapter 13) — and as one input among several, never alone.
The passive leg raise deserves its primacy: it is reversible, requires no fluid, works with spontaneous breathing and atrial fibrillation, and works at low tidal volumes. Its technical requirements are strict: start semi-recumbent (not supine) so that splanchnic as well as lower-limb blood is mobilised; use the bed's movement rather than lifting the legs manually (to avoid pain and adrenergic stimulation); measure a real-time flow variable (VTI, pulse contour SV), never blood pressure alone; and measure within 60–90 seconds, because the effect dissipates.
Fluid tolerance assessment
Domain | Finding indicating intolerance |
Left-sided filling pressure | Elevated E/e′, TR velocity ≥ 2.8 m/s, restrictive mitral inflow (Chapter 8) |
Lung | ≥ 3 B-lines in ≥ 2 bilateral zones (Chapter 41) |
Right heart | RV:LV ≥ 0.6 with septal dyskinesia; acute cor pulmonale (Chapter 9) |
Venous congestion | Plethoric IVC ≥ 2.1 cm with < 50% collapse plus abnormal hepatic, portal, or renal venous Doppler (VExUS, Chapter 13) |
Systemic | Rising CVP without rising cardiac output; worsening oxygenation after fluid |
Therapeutic Logic
A defensible bedside sequence:
- Is there hypoperfusion? (lactate, capillary refill time, mottling, urine output, mentation). No hypoperfusion → no fluid, regardless of responsiveness.
- Is fluid plausibly the fix? In vasoplegic shock with a normal or high stroke volume index, the deficit is arterial tone, not volume.
- Is the patient fluid responsive? Passive leg raise with LVOT VTI is the default test.
- Is the patient fluid tolerant? Lung, right heart, and venous congestion assessment.
- Give a defined volume and re-measure. 250–500 mL, then repeat the VTI. If stroke volume did not rise, stop; do not repeat the same intervention expecting a different result.
- De-escalate deliberately. Once shock resolves, positive fluid balance becomes an independent harm. Echocardiographic congestion assessment guides removal as much as administration.
🛑 Critical pitfall: Using blood pressure as the read-out of a passive leg raise. Arterial pressure change is an insensitive surrogate for stroke volume change; PLR assessed by blood pressure alone has substantially lower diagnostic accuracy, and negative tests are frequently false.
🛑 Critical pitfall: Applying respiratory VTI variation in a patient ventilated at 6 mL/kg with spontaneous effort. The result is uninterpretable, and reporting "not fluid responsive" from it withholds indicated resuscitation.
🛑 Critical pitfall: A markedly variable IVC in a patient with severe RV failure and tricuspid regurgitation. The variation reflects RV afterload sensitivity, not preload reserve; fluid will worsen the RV.
🛑 Critical pitfall: Treating fluid responsiveness as an indication. A healthy volunteer is fluid responsive. Responsiveness answers "what would happen if", not "what should I do".
- 💡 Clinical pearl: The end-expiratory occlusion test is the most useful dynamic index at low tidal volumes, but its 5% threshold demands measurement precision that manual VTI tracing may not achieve; average multiple beats.
- 💡 Clinical pearl: Record the LVOT VTI envelope image before and after every fluid challenge in the record. It converts an impression into an auditable data point.
- 💡 Clinical pearl: In the patient with acute cor pulmonale, skip the responsiveness question. Volume loading a pressure-overloaded RV worsens output through septal shift even when the Frank–Starling curve says "responsive".
References
- Vieillard-Baron A, Millington SJ, Sanfilippo F, et al. A decade of progress in critical care echocardiography: a narrative review. Intensive Care Med 2019;45:770–88. PMID 30877351.
- Muller et al. Assessment of fluid responsiveness. J Am Soc Echocardiogr 2020. PMID 32147001.
- Monnet X, Marik PE, Teboul JL. Passive leg raising for predicting fluid responsiveness: a systematic review and meta-analysis. Intensive Care Med 2016;42:1935–47.
- Monnet X, Shi R, Teboul JL. Prediction of fluid responsiveness. What's new? Ann Intensive Care 2022;12:46.
- Evans L, Rhodes A, Alhazzani W, et al. Surviving Sepsis Campaign: international guidelines for management of sepsis and septic shock 2021. Crit Care Med 2021;49:e1063–143.