Intraop Fluid Management — GDT, SVV/PPV, Restrictive vs Liberal

Patients on the steep ascending limb increase SV with fluid (fluid responders); patients on the flat plateau do not (non-responders) — and adding fluid in the latter group only causes interstitial edema + AKI + pulmonary complications.

Goal-directed therapy uses dynamic indices to identify which side of the inflection point the patient is on, in real time, before each fluid bolus.

The endpoint is no longer 'maintenance + replacement of estimated losses' but 'titrate to optimize stroke volume + tissue perfusion.' Static indices (CVP, PAOP) do NOT reliably predict responsiveness — abandon them for fluid responsiveness assessment.

Stroke volume variation (SVV): the same concept derived from beat-to-beat SV by pulse contour analysis (FloTrac, LiDCO).

Threshold: PPV or SVV >12-13% predicts fluid responsiveness with sensitivity + specificity ~85%.

Mechanism: positive-pressure inspiration transiently reduces RV preload + increases LV afterload + decreases LV preload 2-3 beats later; the magnitude of the resulting SV variation reflects position on the Starling curve.

Higher variation = steeper curve = fluid responsive.

Open chest — eliminates the cyclical RV preload variation.

Cardiac arrhythmia (atrial fibrillation, frequent ectopy) — variation is from rhythm, not volume status.

Tidal volume <8 mL/kg (e.g., low-TV ARDS strategy) — variation too small to be reliable.

PEEP very high or very low — distorts the relationship.

Right ventricular failure — paradoxical responses.

Intra-abdominal hypertension.

Pediatric patients (compliance differences).

Open abdomen.

For these scenarios, use alternative methods: passive leg raise (PLR), end-expiratory occlusion test, fluid challenge with SV measurement, or echo-derived measures.

PLR: raise legs to 45° from supine (or move from semirecumbent to supine + legs up) — autotransfuses ~300 mL from lower extremities. ≥10% rise in cardiac output or stroke volume index within 60-90 seconds = responder.

Validated across mechanical ventilation status + rhythm.

Mini-fluid challenge: 100 mL crystalloid bolus over 1 min; ≥6% rise in SV = responder.

End-expiratory occlusion test: 15-second hold of mechanical ventilation; ≥5% rise in CO during the hold = responder.

Echo-derived IVC distensibility/collapsibility (mechanically ventilated patients: >18% distensibility = responder; spontaneously breathing: >50% collapsibility suggests volume depletion but with weaker predictive value).

Historical 'liberal' regimens delivered 12-15 mL/kg/hr crystalloid maintenance + repleted putative 'third space' losses (4-8 mL/kg/hr for major abdominal surgery) — based on flawed 1960s methodology.

Modern restrictive strategies: 1-3 mL/kg/hr balanced crystalloid maintenance + bolus titration to dynamic indices.

RELIEF trial (NEJM 2018, ~3000 patients abdominal surgery): restrictive vs liberal — restrictive group had MORE AKI + surgical site infection + RRT, surprising the field.

Interpretation: 'restrictive' as a fixed protocol can underfill responders — true GDT (titrated by responsiveness) outperforms either fixed strategy.

ERAS protocols now favor euvolemic GDT rather than dogmatic restriction.

Mechanism: large volumes of saline produce hyperchloremic metabolic acidosis + AKI signal (SMART + SALT-ED trials 2018 NEJM — balanced solutions reduced major adverse kidney events).

0.9% saline still used when alkalosis is desired (e.g., metabolic alkalosis from vomiting, hypochloremia).

Colloids (albumin, hydroxyethyl starch): SAFE trial (NEJM 2004) — 4% albumin vs saline in ICU showed no overall mortality benefit + harm in TBI subgroup.

HES is now LIMITED OR BANNED in many jurisdictions for AKI signal (CHEST, 6S, VISEP trials).

Albumin retains a niche in cirrhosis (SBP, large-volume paracentesis, hepatorenal) + selected refractory hypotension.

Intraoperative threshold often slightly higher (Hb 7-8 g/dL) given ongoing risk + difficulty assessing tissue perfusion.

Cardiac surgery + symptomatic CAD: threshold 8-9 g/dL (TRICS-III NEJM 2017 cardiac surgery non-inferior at 7.5).

Acute MI: 8-10 g/dL (REALITY 2021 JAMA non-inferior at 8).

Endpoints of resuscitation beyond Hb: MAP ≥65 mmHg (higher 80-85 in chronic hypertension + AKI), urine output ≥0.5 mL/kg/hr, lactate clearance, ScvO₂ ≥70%, capillary refill, narrowing arterial-venous CO₂ gap.

Use multiple endpoints, not a single number.