From the Guidelines
Respiratory variation, IVC caliber, and RA pressure are not typically measured together as a single combined measurement, though they are related parameters often assessed during evaluation of cardiac function and volume status.
Key Points
- Respiratory variation and IVC caliber are typically measured using bedside ultrasound (echocardiography) [ 1 ].
- Right atrial (RA) pressure is most accurately measured via invasive catheterization.
- Clinicians often use IVC diameter and its respiratory variation as non-invasive surrogate markers to estimate RA pressure [ 1 ].
- A dilated IVC with minimal respiratory variation suggests elevated RA pressure, while a small, collapsible IVC suggests normal or low RA pressure [ 1 ].
- The relationship between IVC diameter, respiratory variation, and RA pressure exists because the IVC directly returns blood to the right atrium, so changes in RA pressure affect IVC dynamics [ 1 ].
- While these parameters are physiologically related and often interpreted together, they represent distinct measurements obtained through different techniques rather than a single combined measurement.
Measurement Techniques
- IVC diameter should be measured in the subcostal view with the patient in the supine position at 1.0 to 2.0 cm from the junction with the right atrium, using the long-axis view [ 1 ].
- The diameter of the IVC decreases in response to inspiration when the negative intrathoracic pressure leads to an increase in RV filling from the systemic veins [ 1 ].
- The collapsibility index can be used to quantify the relationship between IVC diameter and RA pressure [ 1 ].
Clinical Application
- Estimation of systolic PAP is based on the peak tricuspid regurgitation velocity (TRV) taking into account right atrial pressure (RAP) as described by the simplified Bernoulli equation [ 1 ].
- RAP can be estimated by echocardiography based on the diameter and respiratory variation in diameter of the inferior vena cava (IVC) [ 1 ].
From the Research
Measurement of Respiratory Variation, IVC Caliber, and RA Pressure
- The respiratory variation in inferior vena cava (IVC) diameter is used to predict fluid responsiveness in patients with acute circulatory failure 2.
- IVC caliber is measured to estimate central venous pressure (CVP) noninvasively, as it is a highly compliant vessel that does not constrict in response to hypovolemia 3.
- Right atrial (RA) pressure is estimated using Doppler recognition of low or normal central venous pressure from continuous flow from the inferior vena cava into the right atrium 4.
Relationship Between Measurements
- There is a limited ability to predict fluid responsiveness using respiratory variation in IVC diameter, particularly in spontaneously ventilating patients 2.
- A study found no significant correlation between CVP measurement and ultrasound assessment of the IVC, suggesting that they should be used as clinical adjuncts rather than stand-alone measures of volume assessment 5.
- The combination of IVC size, collapsibility, and Doppler flow pattern can enhance the sensitivity and specificity of predicting RA pressure 4.
Clinical Applications
- Spirometry is an essential clinical measurement for the diagnosis, differentiation, and management of respiratory illness, and can be used to assess lung health in smokers and those exposed to occupational and environmental hazards 6.
- Ultrasound assessment of the IVC and measurement of CVP can be used in the emergency center for early assessment of volume status in critically ill patients 5.