ICU Physiology in 1000 Words: IVC Collapse, Revisited – Part 1
Three years ago I wanted to share my physiology website heart-lung.org; I needed a topic to stoke some interest, so I sent a brief essay to Matt here at pulmccm.org. In it, I briefly described inspiratory IVC collapse and its relationship to volume status and volume responsiveness. With this, the series of ‘1000 Words’ was borne and a few people started to watch my moving Guyton Diagrams.
Rightly, I was delighted to read Dr. Marik’s very recent take on IVC collapse and thank Scott for posting my vodcast on the topic. The discussion also included a very thoughtful essay by Rory Spiegel.
As well, I’m placing my vodcast here; the animation is a simplified illustration of the intricate intersection of venous return and cardiac function – two broad hemodynamic processes that seesaw within waxing and waning thoracic and abdominal pressures.
PEEP & IVC Collapse
In the video above, the effect of positive end-expiratory pressure [PEEP] on IVC collapse is mentioned. PEEP has conflicting effects on all of the four general elements that determine IVC size and collapse – venous return, cardiac function, intra-thoracic pressure [ITP] and intra-abdominal pressure [IAP]. An often overlooked, but terribly relevant, investigation is that of Juhl-Olsen and colleagues .
Healthy volunteers had their volume status altered and IVC collapsibility was measured at varying degrees of PEEP and pressure support. While there are multiple avenues of discussion on this paper, a few interesting observations are noted below [see their figure 4]:
Healthy volunteers lying in the horizontal tended to increase the IVC collapsibility index with the application of 10 cm H2O of PEEP from a baseline of 0.15 to about 0.20. When PEEP rose to 20 cmH2O, the collapsibility index fell to an intermediary value between 0.15 and 0.20.
When placed in Trendelenberg, given 1 L of NS and administered 20 cmH2O of PEEP, their IVC collapsibility was also about 0.15; i.e. an equivalent collapse to their supine, volume unloaded physiology without PEEP.
How might these results be reconciled? A detailed evaluation of end-expiratory pressure and its effects on the 4 key determinants of IVC size and collapsibility [venous return, cardiac function, ITP and IAP] can provide a pathway for understanding. These will be discussed in more detail in part 2.
Juhl-Olsen, P., C.A. Frederiksen, and E. Sloth, Ultrasound assessment of inferior vena cava collapsibility is not a valid measure of preload changes during triggered positive pressure ventilation: a controlled cross-over study. Ultraschall Med, 2012. 33(2): p. 152-9.