Dynamic Indices of Fluid Responsiveness

1. Concept of Fluid Responsiveness

Fluid responsiveness = ability of the heart to increase stroke volume (SV) or cardiac output (CO) in response to a fluid bolus.

• Conventionally defined as:
• ↑ SV or CO ≥10–15% after a fluid challenge
• Only ~50% of hemodynamically unstable ICU patients are fluid responsive
  → indiscriminate fluids → fluid overload, ARDS, AKI, ↑ mortality

👉 Hence the shift from static indices (CVP, PAOP) to dynamic indices

2. Static vs Dynamic Indices (High-yield Comparison)

3. Physiological Basis of Dynamic Indices

Dynamic indices exploit heart–lung interactions during positive pressure ventilation.

Key Mechanisms:

1. Inspiratory phase:
• ↑ Intrathoracic pressure
• ↓ Venous return → ↓ RV preload

2. After 2–3 beats:

• ↓ LV preload → ↓ SV

3. If patient is preload responsive:

• These cyclic changes are exaggerated

👉 The greater the respiratory variation, the more likely the patient is fluid responsive

4. Main Dynamic Indices (Core Exam Content)

A. Pulse Pressure Variation (PPV)

Definition

Respiratory variation in pulse pressure during mechanical ventilation.

PPV (%)=(PPmax +PPmin )/2PPmax −PPmin ×100

Where:

• Pulse pressure = SBP − DBP

Interpretation

Advantages

• Well validated
• Easy if arterial line present
• Strong evidence base

Limitations ❗ (Very Important for Exams)

PPV is unreliable in:

• Spontaneous breathing
• Atrial fibrillation / arrhythmias
• Low tidal volume ventilation (<6 ml/kg)
• Low lung compliance (ARDS)
• High PEEP
• Open chest
• RV failure / pulmonary hypertension

B. Stroke Volume Variation (SVV)

Definition

Respiratory variation in stroke volume, measured by pulse contour devices.

SVV (%)=SVmean SVmax −SVmin ×100

Interpretation

Devices Used

• FloTrac/Vigileo
• LiDCO
• PiCCO
• Most advanced hemodynamic monitors

Pros

• Continuous
• Directly measures SV variation
• Better than PPV in some settings

Cons

• Same limitations as PPV
• Device-dependent accuracy
• Affected by vascular tone

C. Systolic Pressure Variation (SPV)

Definition

Difference between maximum and minimum systolic BP during one respiratory cycle.

SPV=SBPmax −SBPmin

Components

• ΔUp: Inspiratory increase in SBP
• ΔDown: Expiratory decrease in SBP (clinically more important)

Interpretation

Exam Pearl 🧠

ΔDown reflects preload dependency, not ΔUp

D. Pleth Variability Index (PVI)

What it is

Respiratory variation in pulse oximeter plethysmographic waveform.

• Non-invasive surrogate of PPV
• Derived from Masimo monitors

Interpretation

Limitations

• Poor signal quality
• Vasoconstriction
• Hypothermia
• Low perfusion states

5. Passive Leg Raising (PLR): Dynamic Test (Most Important)

Principle

• Transfers ~300 ml venous blood from legs + splanchnic circulation
• Acts as a reversible fluid challenge

How to Perform Correctly (Exam Favorite)

1. Start from semi-recumbent position
2. Lower trunk to horizontal
3. Raise legs to 45°
4. Measure CO / SV change within 30–90 sec

Interpretation

Why PLR is GOLD STANDARD

✔ Works in:

• Spontaneous breathing
• Arrhythmias
• Low tidal volume ventilation
• ARDS

✔ Reversible
✔ No fluid overload risk

Must Measure FLOW, Not Pressure ❗

Valid measurements:

• Echocardiography (LVOT VTI)
• Pulse contour CO
• Esophageal Doppler

Invalid:

• BP alone
• CVP change

6. Dynamic Indices: Conditions for Validity (Very High Yield)

All PPV/SVV/SPV require:

7. Dynamic Indices vs Fluid Challenge

8. Current Guideline Perspective

• Surviving Sepsis Campaign:
• Recommends dynamic measures over static
• ESICM / SCCM:
• PLR + CO measurement preferred
• CVP no longer recommended to guide fluids

9. Practical ICU Algorithm (Exam-friendly)

1. Shock patient → assess perfusion
2. If mechanically ventilated:
• Use PPV / SVV (if criteria met)

3. If spontaneous breathing / arrhythmia:

• Use PLR with CO monitoring

4. Give fluids only if responsive
5. Reassess frequently

10. Key points 

• Dynamic indices assess position on Frank–Starling curve
• High PPV ≠ hypovolemia → only predicts responsiveness
• PLR is the most universally applicable test
• Static pressures ≠ preload
• Always interpret in clinical context