Although the inspiratory “collapse” of the inferior vena cava (IVC) under falling intrathoracic pressures has been used to signify normal central venous pressure, the effect of type of breathing (diaphragmatic vs. chest wall) on IVC size is unknown. As intra-abdominal pressure rises with abdominal compartment compression, we hypothesized that the diaphragmatic descent during inspiration has a compressive effect on the IVC.
We measured minimal and maximal intrahepatic IVC diameter on echocardiography and popliteal venous return by spectral Doppler during isovolemic inspiratory efforts in 11 healthy nonobese volunteers who were instructed to breathe comfortably through either diaphragmatic or chest wall expansion. At peak inspiration, the maximal amount of diaphragmatic excursion and popliteal vein flow were compared between breathing types. The echo index of IVC “collapsibility”, the IVCCI, was calculated as (IVCmax-IVCmin)/IVCmax. To examine the relationship of diaphragmatic excursion and IVC size, the difference in diaphragmatic excursion between diaphragmatic and chest wall breaths was correlated with the corresponding change in IVCCI.
In the setting of equivalent tidal volumes by spirometry, diaphragmatic breathing resulted in more diaphragmatic excursion than chest wall breathing (3.6+/−1.4 vs 2.4+/−0.9cm, p < 0.01). Popliteal venous return decreased in 11/11 subjects with diaphragmatic inspiration, as compared to 5/11 with chest wall inspirations (p < 0.05). An increase in venous return was observed only with chest wall inspiration. When diaphragmatic and chest wall breathing were compared in each patient, the difference in diaphragmatic excursion correlated (Spearman's rho = 0.68, p < 0.05) with difference in IVCCI.
During inspiration, the IVC may be compressed through descent of the diaphragm, reducing lower extremity venous return.
The compressive effect on the IVC has implications regarding the use of respiratory changes in IVC size to estimate central venous pressure in the absence of knowing type of breathing, intra-abdominal pressure or magnitude of diaphragmatic excursion.
Glynn Gilcrease, No Financial Disclosure Information; No Product/Research Disclosure Information