When it comes to physiology, there`s no doubt that Jon is the man, so I was really curious about his take on all this, which, no surprise, is definitely worth sharing, just in case everyone doesn`t go read the comments.
Wow; there is a lot to unpack here.
My first comment is that intra-renal venous flow [*not renal vein flow], hepatic vein flow, portal vein flow, etc, etc, etc [as well as IVC size and respiratory variation] are all ultrasonographic transductions of the central venous pressure …so I’ll give my boxed disclaimer that volume status and volume responsiveness cannot definitively and reliably obtained from this marker because the CVP is too complicated to make these physiological leaps.
Indeed. It is important to realize that, as Jon states below, that the angle for looking at the PV in this case is to assess congestion, rather than responsiveness or the ever-so-nebulous ‘status.’
Wait for it … volume tolerance and the CVP, is a bit more nuanced, i think. with a high CVP, you really have to ask yourself – **why** is the CVP elevated and go from there. if the CVP is elevated because of tamponade, its very different management from a high CVP from a massive PE or air-trapping versus a high CVP from volume overload.
Absolutely. Diuresing a pre- or full-fledged tamponade, PE or air-trapping could have disastrous consequences, i.e. PEA arrest!
There seems to be some confusion about *the renal vein* versus *intra-renal vein*. the lida trial is clear that it is intra-renal vein flow. i am not terribly familiar with *the renal vein flow, however, my hunch is that renal vein flow should always be biphasic [just as the jugular venous flow, SVC flow, IVC flow and hepatic vein flow are always biphasic] – that is a normal pattern close to the right atrium. normally the systolic inflow velocity is greater than the diastolic inflow velocity and there is fairly good data correlating reversal of systolic to diastolic venous flow ration to right atrial pressure [in the IVC and SVC].
Definitely the intra-renal vein should be the target here – not always easy in some patients, because the renal vein itself, especially the right (no crossover) really has an IVC pattern and won`t necessarily reflect the effect of intra-renal hypertension.
The pulsatility that evolves in the intra-renal vein as the CVP rises is beyond me, but the authors postulate that it has to do with the compliance of the vein at higher CVP and intra-renal interstitial pressure which makes some sense. But it is important to note that the compliance curves of an intra-renal vein and *the* renal vein are probably quite different.
Secondly, the pulsatility of the PV is a neat idea because of its relative ease of assessment. However, the pulsatility, presumably, is due to the PV encroaching the limits of its compliance curve – the PV, like the CVP – has an inflow and outflow pressure. It is highly likely that a pulsatile PV in a post-operative cardiac patient relates to an angry RV – but is this always true? What about the cirrhotic? What about differential partitioning of fluid into the splanchnic bed versus the lower body? What about differential expression of adreno-receptors between splanchnic arteries [beta and alpha] and splanchnic veins [mostly alpha]. My point is that there could be *other* inflow and outflow differentials that are affecting PV volume, compliance and therefore pulsatility that are not yet recognized. A cirrhotic on bomb dose phenylephrine/vasopressin may have their splanchnic venous volume recruited with blood expelled towards the liver, an engorged PV that is pulsatile – but is that RV failure? Is that a patient who needs to be decongested? I don’t know.
Thirdly, there are complex cardiac contributions to venous flow phase and vein pulsatility such as arrythmia – atrial compliance, etc. As the comment above notes – how might afib contribute to SVC or IVC venous inflow? It’s hard to know, but my hunch would be that afib itself would tend to reverse the normal S wave: D wave supremacy … that is, decrease the normal systolic inflow velocity relative to the diastolic inflow velocity. if the atrium is not emptied fully then its pressure with rise. if atrial pressure rises, when the atrium is pulled downward during ventricular systole, the S wave will be diminished. additionally, the more chronically dilated and poorly compliant the right atrium, the greater its pressure will be with the loss of atrial kick.
Fantastic points. Again, looking at POCUS metrics CANNOT BE DONE IN ISOLATION, from the rest of the POCUS and clinical data.
Lastly, the venous inflow pattern analysis approach to CVP estimation – i think – is better than IVC size and collapse because of how IVC size and collapse can also be affected by IAP, ITP/PEEP, etc. Because ITP affects systolic and diastolic inflow patterns similarly, that confound should be lessened. Nevertheless, as Dr. Denault mentions in the cases above – you have to treat the patient! This means integrating what the data is telling you in the patient in front of you. If in a certain clinical context the test results do not make sense, it’s probably a false positive or false negative test.
I dug up this gem from 30+ years ago. Excellent paper [https://www.ncbi.nlm.nih.gov/pubmed/3907280 – “Ultrasonic assessment of abdominal venous return. I. Effect of cardiac action and respiration on mean velocity pattern, cross-sectional area and flow in the inferior vena cava and portal vein”].
Ok that’s on my short reading list for the next 48h!
They show the venous inflow waveform for the IVC [presumably very similar to *the renal vein]; Afib *does* cause the S wave to become attenuated – so it would change the normal biphasic form to more of a monophasic form. In theory, giving a calcium channel blocker and slowing the patient down should improve this somewhat. They even have a brief discussion on portal vein pulsatility.
This venous inflow stuff is very interesting and potentially very applicable. @iceman tweeted out wave velocity patterns in the MCA during high ICP – indeed – an increase in ICP renders the flow more pulsatile and then there is loss of diastolic flow. Probably similar physiology for an intra-renal vein as intra-renal capsular pressure rises. A good sign that the kidney is under pressure!
Thank you Jon for some really excellent physiological points and the reminder that, in POCUS just as in clinical medicine, we cannot rely on one assessment, and that measure must be considered in the context of the factors affecting it. Otherwise, we are not truly tailoring our therapy to the patient, but only pretending to.