What are the normal internal diameters of the Inferior Vena Cava (IVC) on Point-of-Care Ultrasound (POCUS) for assessing central venous pressure?

Normal Internal Diameters of IVC on POCUS for Assessing Central Venous Pressure

The normal inferior vena cava (IVC) diameter on POCUS is less than 2.1 cm with greater than 50% collapsibility during inspiration in spontaneously breathing patients, which correlates with a normal right atrial pressure (RAP) of 0-5 mmHg. 1

Standard IVC Measurements and Interpretation

Normal Values

• Diameter measurements:

  • Normal IVC diameter: <2.1 cm 2, 1
  • Dilated IVC: >2.1 cm 2, 1
  • Severely dilated IVC: >2.5 cm 3

• Collapsibility measurements:

  • Normal collapsibility: >50% during inspiration or sniff maneuver in spontaneously breathing patients 1
  • Reduced collapsibility: <50% with normal breathing 1
  • In mechanically ventilated patients: normal collapsibility is 15-20% 1

Correlation with Right Atrial Pressure

1. Normal RAP (3 mmHg, range 0-5 mmHg):

   • IVC diameter <2.1 cm
   • Collapsibility >50% 1

2. Intermediate RAP (8 mmHg, range 5-10 mmHg):

   • Intermediate IVC diameter and collapsibility values 1

3. Elevated RAP (15 mmHg, range 10-20 mmHg):

   • IVC diameter >2.1 cm
   • Collapsibility <50% 1

Proper Measurement Technique

1. Patient positioning: Supine position 1
2. View: Subcostal long-axis view 1
3. Measurement location: 1.0-2.0 cm from the junction with the right atrium 1
4. Measurement direction: Perpendicular to the long axis of the IVC 1
5. Respiratory cycle: Measure during both inspiration and expiration 1
6. Collapsibility index calculation: (Expiratory diameter - Inspiratory diameter) / Expiratory diameter × 100% 1

Special Considerations and Limitations

Factors Affecting IVC Measurements

• Right ventricular dysfunction and tricuspid regurgitation may influence IVC diameter independent of volume status 1
• Intra-abdominal hypertension can affect IVC measurements 1
• In mechanically ventilated patients, patient-ventilator dyssynchrony can affect measurements 1
• Young, healthy athletes may have dilated IVC even with normal right atrial pressures 1

Predicting Fluid Responsiveness

• IVC diameter distensibility index (IVC-DDI) >12% predicts fluid responsiveness with high predictive values 1
• IVC diameter ratio (IVC-DR) >1.43 predicts fluid responsiveness with 90% specificity 1
• In spontaneously breathing hypotensive patients, a dilated IVC (>2.5 cm) with minimal collapsibility (<50%) has 85.7% sensitivity and 86.4% specificity for predicting volume overload 3

Integration with Other Parameters

• IVC measurements should be integrated with other clinical and echocardiographic parameters for optimal assessment 2, 1
• Consider other echocardiographic signs of elevated RAP:
  • Right ventricular findings: RV/LV basal diameter ratio >1, flattened interventricular septum
  • Pulmonary artery findings: PA diameter >25 mm
  • Right atrial findings: End-systolic RA area >18 cm² 2, 1

Alternative Approaches

• When IVC imaging is challenging, femoral vein diameter (FVD) can be used as an alternative:
  • FVD ≤0.7 cm predicts low CVP with high accuracy (AUC = 0.97)
  • FVD ≥1.2 cm has 94% specificity for high CVP 4

By properly measuring and interpreting IVC diameter and collapsibility on POCUS, clinicians can reliably estimate central venous pressure and guide volume management decisions in various clinical scenarios.