Is the respiratory variation greater than 50% of the Inferior Vena Cava (IVC) caliber, with right atrial (RA) pressure measured simultaneously?

From the Guidelines

Respiratory variation > 50% in IVC caliber is typically associated with low RA pressure, and these measurements are related but assessed separately. The relationship between inferior vena cava (IVC) diameter and right atrial (RA) pressure is well-established, with IVC diameter and its respiratory variation used as a non-invasive estimate of RA pressure 1. When the IVC diameter collapses by more than 50% with inspiration, it suggests a low RA pressure, typically less than 5 mmHg 1.

Key points to consider in the assessment of IVC and RA pressure include:

• IVC diameter should be measured in the subcostal view, 1.0 to 2.0 cm from the junction with the right atrium, using the long-axis view 1.
• The collapsibility index of the IVC, which is the percentage decrease in diameter during inspiration, correlates with RA pressure 1.
• An IVC diameter < 2.1 cm that collapses > 50% with a sniff suggests normal RA pressure of 3 mm Hg, whereas an IVC diameter > 2.1 cm that collapses < 50% with a sniff suggests high RA pressure of 15 mm Hg 1.
• In clinical practice, the measurement of IVC respiratory variation and RA pressure are complementary, with IVC assessment providing a non-invasive estimate of RA pressure in settings where invasive measurements are not available or warranted 1.

The use of IVC respiratory variation to estimate RA pressure is particularly useful in critically ill patients, such as those with heart failure, sepsis, or requiring mechanical ventilation, to guide volume assessment and management 1. Therefore, a respiratory variation > 50% in IVC caliber, when considered in the context of RA pressure measurement, suggests low RA pressure and can be used to inform clinical decision-making.

From the Research

Respiratory Variation in IVC Diameter and RA Pressure

• The relationship between respiratory variation in inferior vena cava (IVC) diameter and right atrial (RA) pressure is complex, with multiple factors influencing IVC measurements 2.
• Studies have shown that IVC diameter and its respiratory variation can be used to estimate RA pressure, but there may be discrepancies between the values 3, 2.
• A study found that the higher mean RA pressure, presence of significant tricuspid regurgitation, larger body surface area, and younger age were independently associated with the IVC maximum diameter 2.
• Another study found that respiratory variation in IVC diameter has limited ability to predict fluid responsiveness, particularly in spontaneously ventilating patients 3.
• The use of IVC ultrasound to estimate RA pressure should take into account clinical context and other factors that may influence IVC measurements 3, 2.

Factors Influencing IVC Diameter and Respiratory Variation

• RA pressure is a major determinant of IVC diameter, with higher RA pressure leading to IVC distension 2.
• Other factors that can influence IVC diameter include significant tricuspid regurgitation, larger body surface area, and younger age 2.
• Respiratory variation in IVC diameter is influenced only by RA pressure, with higher RA pressure leading to decreased IVC collapsibility index (IVCCI) 2.
• The regression equation to estimate mean RA pressure from IVC maximum diameter and IVCCI is: estimated mean RA pressure = 3.7 + 0.62 × maximum IVC diameter / BSA - 0.07 × IVCCI 2.

Clinical Implications

• Clinicians should be aware of the factors that influence IVC diameter and respiratory variation when using IVC ultrasound to estimate RA pressure 3, 2.
• IVC ultrasound should be used in conjunction with other clinical and hemodynamic data to make treatment decisions 3, 2.
• Further studies are needed to fully understand the relationship between respiratory variation in IVC diameter and RA pressure, and to develop more accurate methods for estimating RA pressure using IVC ultrasound 3, 2.