What is the purpose and procedure of the passive leg raising (PLR) test in assessing volume responsiveness in patients?

Passive Leg Raising Test for Assessment of Volume Responsiveness

The passive leg raising (PLR) test is a validated, non-invasive method for assessing fluid responsiveness in critically ill patients by temporarily mobilizing approximately 300 mL of blood from the lower extremities to the thorax, increasing preload without changing intravascular volume. 1

Purpose of the PLR Test

The PLR test serves as an "internal fluid challenge" that helps clinicians:

• Predict whether a patient will respond to fluid administration (fluid responsiveness)
• Avoid unnecessary fluid administration in non-responders
• Guide resuscitation decisions in shock states
• Provide a reversible alternative to actual fluid boluses

Physiological Basis

When a patient's legs are raised:

• Approximately 300 mL of blood is mobilized from the lower extremities to the central circulation
• This increases venous return and cardiac preload
• The effect is temporary and reversible (unlike actual fluid administration)
• Mean systemic filling pressure increases significantly, similar to volume expansion 2

Procedure for Performing PLR Test

1. Initial Position:

   • Place patient in semi-recumbent position (head elevated 45°)
   • Obtain baseline hemodynamic measurements

2. PLR Maneuver:

   • Lower the head of the bed to flat position
   • Passively raise both legs to 45° angle
   • Maintain this position for 1-2 minutes

3. Measurement:

   • Measure changes in stroke volume, cardiac output, or their surrogates
   • The maximum hemodynamic effect occurs within 30-90 seconds
   • Return patient to original position after measurements

4. Interpretation:

   • Positive response: ≥10-12% increase in stroke volume or cardiac output indicates fluid responsiveness 1, 3
   • Negative response: <10% increase suggests the patient will not benefit from additional fluid

Monitoring Methods During PLR

The accuracy of the PLR test depends on the method used to measure hemodynamic changes:

1. Most Accurate Methods:

   • Transthoracic echocardiography measuring left ventricular outflow tract velocity time integral (LVOT VTI)
   • Non-invasive cardiac output monitors
   • Esophageal Doppler monitoring (sensitivity 97%, specificity 94%) 4

2. Less Accurate Methods:

   • Arterial pulse pressure (sensitivity 60%, specificity 85%) 4
   • Common carotid artery blood flow parameters (poor correlation with cardiac output changes) 5

Clinical Applications

The PLR test is particularly valuable in:

1. Spontaneously breathing patients where dynamic indices like pulse pressure variation are unreliable 1, 4

2. Patients with cardiac arrhythmias where heart rate variability affects other dynamic parameters 4

3. Critically ill patients requiring volume assessment (only ~46-50% of ICU patients given fluid boluses are actually volume responsive) 6

4. Postoperative patients with suspected hypovolemia (only ~54% respond to fluid boluses) 1

Limitations and Contraindications

• Not reliable in patients with:

  • Intra-abdominal hypertension or abdominal compartment syndrome 1
  • Severe head injury (risk of increased intracranial pressure)
  • Lower limb fractures or amputations
  • Compression stockings (must be removed first)

• Technical considerations:

  • Requires appropriate monitoring equipment
  • Operator skill affects accuracy, especially with echocardiographic measurements
  • Position change must be maintained for adequate time

Clinical Decision Algorithm

1. Identify patient with potential fluid need:

   • Signs of hypoperfusion or shock
   • Hypotension (SBP <90 mmHg or MAP <70 mmHg)
   • Tachycardia, oliguria, or other signs of inadequate tissue perfusion

2. Perform PLR test with appropriate monitoring:

   • Measure stroke volume/cardiac output before and during PLR
   • Calculate percent change from baseline

3. Interpret results:

   • If ≥10-12% increase: Patient is likely fluid responsive
   • If <10% increase: Patient is likely not fluid responsive

4. Clinical decision:

   • For responders: Administer fluid bolus (typically 500 mL)
   • For non-responders: Consider vasopressors or inotropes instead of fluids

Evidence Quality and Consensus

The Society of Critical Care Medicine guidelines strongly support the use of PLR testing for fluid responsiveness assessment 1, 3. A meta-analysis showed that an increase in cardiac output after PLR strongly predicts fluid responsiveness with a positive likelihood ratio of 11 (95% CI, 7.6-17) and specificity of 92%, while no increase reliably identifies non-responders (negative likelihood ratio 0.13; 95% CI, 0.07-0.22; sensitivity 88%) 1, 3.

Common Pitfalls to Avoid

• Inadequate monitoring: Using only blood pressure changes instead of stroke volume/cardiac output
• Improper technique: Not starting from semi-recumbent position or not allowing sufficient time
• Misinterpretation: Using inappropriate thresholds for defining a positive response
• Contraindicated scenarios: Using in patients with intra-abdominal hypertension
• Overreliance: Using PLR as the sole determinant for fluid administration without clinical context

The PLR test represents a valuable tool in the hemodynamic assessment arsenal, allowing clinicians to make more informed decisions about fluid administration in critically ill patients.