Fluid responsiveness

Question 1

Assessing Volume Responsiveness

Answer 1

≈ Only ~50% of shocked patients are fluid responsive.

Conventional static parameters = poor predictors
* CVP and PAOP unreliable (ventricular compliance, pressure–volume relationships).
* ScvO₂ (single measurement) not useful for resuscitation target.

Question 2

Dynamic Parameters

Answer 2

1) Systolic Pressure Variation (SPV)
2) PPV ( Pulse pressure variation)
3) Plethysmographic Waveform Variability (PWV)
4) IVC Collapsibility
5) Passive leg Raise (PLR)
6) Mean Systemic Pressure (Pms)

Answer 3

Systolic Pressure Variation (SPV)
->SPV = Diff. between highest systolic and lowest systolic during one controlled breath has 2 components:
* Δup −Diff. between highest systolic pressure- Refrence systolic pressure taken during expiratory pause
* Δdown - Diff between lowest systolic pressure and Refrence systolic
* Δdown strongly predictive of volume responsiveness.
* Mechanism: PPV → ↑ intrathoracic pressure → Transiently ↑the SBP but, quickly, due to ↓ RV preload & ↑RV afterload→ ↓ RV stroke volume → ↓ LV filling → ↓ LV stroke volume-> ↓SBP.

Question 4

Pulse Pressure Variation (PPV)

Answer 4

=>Reading with the maximum variance and minimum variance are taken into account.
Calculated with formula:
PPmax-[PPmin]/[PPmax+PPmin/2] x100
* Reflects cyclic PP changes with ventilation.
* Change >13% considered significant.
* Reliable indicator of volume responsiveness when:
* Patient in sinus rhythm
* Controlled mechanical ventilation
* Adequate tidal volume-8ml/kg

Answer 5

Limitations / Situations where PPV is unreliable

1). Spontaneous breathing

Patient effort alters intrathoracic pressure → false PPV values.

2). Low tidal volume ventilation

Common in ARDS (6 ml/kg) → insufficient preload change → PPV falsely low.

3). Cardiac arrhythmias

Irregular stroke volume → PPV becomes uninterpretable.

4). RV/ LV failure or pulmonary hypertension

Large PPV may reflect RV dysfunction rather than preload responsiveness.

5). Open chest conditions

Thoracic pressure transmission altered → unreliable PPV.

6). High intra-abdominal pressure

Reduces venous return independent of volume status.

7). Reduced lung compliance - ARDS, pulmonary fibrosis

8). Arterial waveform damping, kinks, clots.

Transmission of airway pressure to pleural pressure reduced → PPV underestimates preload dependence.

Question 6

Evidence behind PPV as a marker of fluid responsiveness

Answer 6

Pulse pressure variation has been validated as a predictor of fluid responsiveness in mechanically ventilated ICU patients.

Evidence includes:
* Michard et al., 2000 – prospective study in septic shock demonstrating PPV >13% predicted fluid responsiveness with high sensitivity and specificity.
* Multiple subsequent prospective studies confirming dynamic indices outperform static preload markers.
* Meta-analysis (Yang et al., 2014) of 22 studies (807 ICU patients) showing AUC ≈0.94 with sensitivity ~0.88 and specificity ~0.89.

However, its clinical applicability is limited because strict conditions are required (controlled ventilation, tidal volume ≥8 ml/kg, sinus rhythm, no spontaneous breathing).

Question 7

=>Plethysmographic Waveform Variability (PWV)
=> Inferior Vena Cava (IVC) Collapsibility
=>Passive Leg Raise (PLR)

Answer 7

=>Plethysmographic Waveform Variability (PWV)
* Uses maximal/minimal pulse oximeter waveform amplitude during apnea.
* Shown comparable to PPV for predicting fluid responsiveness.

⸻
=>Inferior Vena Cava (IVC) Collapsibility
* Bedside ultrasound of IVC diameter variation with respiration.
* Reflects volume status but less sensitive than arterial-side indices.
* Still used due to noninvasiveness.

⸻

=>Passive Leg Raise (PLR)
* Reversible “auto-bolus” ≈ 300–500 mL fluid.
* Performed with patient supine and legs raised to 45°.
->Fluid responsive if:
* ↑ CO/SV as measured by COP pulse contour analysis / TTE by ≥10%, or
* Measurable ↑ BP / ↑ pulse contour output.
* Useful in spontaneously breathing patients and those with arrhythmias.

Question 8

Initial Treatment of Hypovolemia / Hypovolemic Shock

Answer 8

• Give 30 mL/kg low-chloride crystalloid if fluid responsive.
• Albumin may be added in septic shock or large-volume resuscitation needs.
• Use clinical endpoints to judge ongoing fluid needs:
  
  • MAP
  • Lactate
  • Urine output
  • CVP trends (not absolute values)
  • Central venous O₂ saturations
  • Dynamic indices (PPV, SVV, PLR)