How is Inferior Vena Cava (IVC) pressure assessed using ultrasound?

Ultrasound Assessment of IVC Pressure

The IVC should be measured in the subcostal view at 1.0-2.0 cm from the right atrial junction, with an IVC diameter >2.1 cm that collapses <50% with inspiration indicating elevated right atrial pressure of 15 mmHg, while an IVC <2.1 cm collapsing >50% suggests normal RA pressure of 3 mmHg. 1

Measurement Technique

Patient positioning and probe placement:

• Position the patient supine and use a phased array or curvilinear probe in the subcostal view to visualize the IVC as it enters the right atrium 2
• Measure the IVC diameter perpendicular to the long axis, approximately 1-2 cm from the cavoatrial junction 1, 2
• Obtain measurements in both inspiration and expiration using M-mode for accuracy 1

Respiratory assessment:

• Evaluate the IVC during a brief sniff maneuver, as normal quiet breathing may not elicit adequate collapse 1
• Calculate the collapsibility index as: (IVCe - IVCi)/IVCe, where IVCe is expiratory diameter and IVCi is inspiratory diameter 1

Interpretation for Right Atrial Pressure Estimation

The ASE/EACVI guidelines provide specific cutoffs for RA pressure estimation: 1

• Normal RA pressure (3 mmHg, range 0-5): IVC diameter <2.1 cm with >50% collapse during sniff
• Intermediate RA pressure (8 mmHg, range 5-10): When IVC diameter and collapse don't fit the normal or elevated pattern
• Elevated RA pressure (15 mmHg, range 10-20): IVC diameter >2.1 cm with <50% collapse during sniff

Clinical Applications

Right ventricular failure assessment:

• A dilated IVC (>2.1 cm) with minimal respiratory variation indicates RV pressure overload 1
• Combine IVC findings with RV/LV ratio >1.0 on apical 4-chamber view, paradoxical septal motion, and TAPSE <16 mm 1, 2
• In suspected massive pulmonary embolism, a normal-sized collapsible IVC effectively rules out obstructive physiology 1

Volume status assessment:

• Severe hypovolemia presents with a small, collapsing IVC (>50% collapse) accompanied by small cardiac chambers and intraventricular obliteration during systole 1, 2
• A dilated, non-collapsing IVC suggests volume overload or elevated right-sided pressures rather than hypovolemia 2

Cardiac tamponade evaluation:

• Evaluate IVC size to inform plausibility of tamponade physiology when combined with chamber collapse and clinical context 1, 2
• A non-dilated IVC usually rules out cardiac tamponade 2

Critical Limitations and Pitfalls

Mechanical ventilation significantly limits reliability:

• IVC assessment has limited reliability in mechanically ventilated patients due to altered intrathoracic pressure dynamics 2
• In ventilated patients, assess expiratory distension (distensibility index) rather than inspiratory collapse 2

Other confounding factors:

• High intra-abdominal pressure can falsely suggest IVC distension despite normal RA pressure 2
• Severe tricuspid regurgitation may affect IVC dynamics independent of volume status 2
• In normal young athletes, the IVC may be dilated despite normal pressures 1

Measurement challenges:

• Alternative transhepatic views can be used when subcostal views are inadequate, though percentage variation is more reliable than absolute diameter measurements 3
• Interpretation may be challenging in neonates or patients with umbilical central venous catheters 2

Integration with Systolic Pulmonary Artery Pressure

When calculating RV systolic pressure:

• Use the tricuspid regurgitation velocity by continuous wave Doppler and add the estimated RA pressure based on IVC assessment 1
• Report RV systolic pressure only when a complete TR Doppler velocity envelope is present 1
• Integrate other indices of RA pressure when IVC findings are equivocal to upgrade or downgrade the pressure estimate 1

Prognostic Significance

A dilated IVC without inspiratory collapse is associated with increased mortality independent of heart failure history, ventricular function, and pulmonary artery pressure, with hazard ratio 1.43 compared to normal IVC 4