A high degree of sonographically-visualised collapse of the inferior vena cava (IVC) during inspiration suggests a volume-responsive cardiac output. This inspiratory collapse is said to be due to a fall in intra-thoracic pressure. However, the IVC traverses the abdominal compartment and is therefore under the influences of hepatic weight, intra-abdominal pressure, and venous return of pooled splanchnic and lower extremity blood.
Diaphragmatic descent, which increases intra-abdominal pressure, may contribute to the respiratory change in IVC diameter. This was borne out in a volunteer study in which diaphragmatic breathing was compared with chest wall breathing. With diaphragmatic breathing there was a trend for a larger IVC collapse index (median 0.80, range 0.48–1.00 vs. 0.57, range 0.13–1.00, P = 0.053). The authors state:
“These findings suggest that during inspiration the IVC, in addition to responding to falling intra-thoracic pressure, may also be compressed with diaphragmatic descent and have implications regarding the use of IVC diameters to estimate the central venous pressure without knowing the manner of breathing, intra-abdominal pressure, or magnitude of diaphragmatic excursion.”
The take home message for me is that there is probably a more complex mechanism of IVC behaviour during respiration than is often taught, and that breathing pattern and abdominal issues may influence the IVC diameter and degree of collapse seen on ultrasound. This might not however negate the correlation between a high degree of collapse and fluid-responsiveness, which is what I’m looking for in my patients with shock or hypotension.
Incidentally the first author of this study is Bruce Kimura, a pioneer of focused echo in the emergency setting and author of a fantastic little book all about the parasternal long axis approach, which seems to be impossible to source on the web at the moment.

AIMS: Although the inspiratory ‘collapse’ of the inferior vena cava (IVC) has been used to signify normal central venous pressure, the effect of the manner of breathing IVC size is incompletely understood. As intra-abdominal pressure rises during descent of the diaphragm, we hypothesized that inspiration through diaphragmatic excursion may have a compressive effect on the IVC.
METHODS AND RESULTS: We measured minimal and maximal intrahepatic IVC diameter on echocardiography and popliteal venous return by spectral Doppler during isovolemic inspiratory efforts in 19 healthy non-obese volunteers who were instructed to inhale using either diaphragmatic or chest wall expansion. During inspiration, the maximal diaphragmatic excursion and popliteal vein flow were compared between breathing methods. The IVC ‘collapsibility index,’ IVCCI, was calculated as (IVC(max)-IVC(min))/IVC(max). The difference in diaphragmatic excursion between diaphragmatic and chest wall breaths in each subject was correlated with the corresponding change in IVCCI. Diaphragmatic breathing resulted in more diaphragmatic excursion than chest wall breathing (median 3.4 cm, range 1.7-5.8 vs. 2.2 cm, range 1.0-5.2, P= 0.0003), and was universally associated with decreased popliteal venous return (19/19 vs. 9/19 subjects, P< 0.004). The difference in diaphragmatic excursion correlated with the difference in IVCCI (Spearman’s rho = 0.53, P= 0.024).
CONCLUSION: During inspiration of equivalent tidal volumes, the reduction in IVC diameter and lower extremity venous return was related to diaphragmatic excursion, suggesting that the IVC may be compressed through descent of the diaphragm.

The effect of breathing manner on inferior vena caval diameter
Eur J Echocardiogr. 2011 Feb;12(2):120-3