I recently had an interesting discussion with my colleague, Dennis, regarding whether we measured negative pressures on the venous line when using Vacuum Assisted Venous Drainage (VAVD) during the conduct of perfusion at our particular clinical site. I was frank in my answer (no pun) and said- it wasn’t something that I have ever done, nor was it anything (measurement) that I have seen monitored by any of my colleagues at any of the 50 odd perfusion sites I have actually pumped at in the two and a half decades. That doesn’t imply or suggest an endorsement pro-or-con regarding the validity of the issue, or lack of concern regarding paying attention to perfusion safety one way or the other, for this particular issue- it just was an honest answer- “yes I understand the significance of measuring negative pressures, however- I really don’t see my peers engaged in it”.
The obvious reasons to measure negative pressures and have any sort of control of them is to:
- Subvert the potential for extraction of gaseous emboli (air) into solution-
- Protect the same issue from occurring across the fibers of a membrane oxygenator-
- Reduce air-blood interface issues-
Obviously, the next step in my mind is to put it to the test and compare the impact of the negative pressure incurred because of the gravitational gradient we rely on as perfusionists to assure adequate venous drainage (the elevated height of the phlebo-axis of the patient on the OR table versus the lower vertical position of the venous reservoir = a negative pressure from the patient thus creating the same effect as siphoning gas from an automobile into a container. That process should be a measurable event.
The second aspect to be considered and subsequently eliminated, is the impact of positive pressure delivery devices such as vents and suckers that deliver air/blood/pressure to the reservoir- and must be considered as significant Konstant when employing a closed system reservoir (which is prerequisite for VAVD). Typically, the way I assess this, is prior to bypass, turning on my vacuum and asessing any changes associated with my pump suckers and vents rolling at 50%. The assumption would be- that due to the positive pressure delivered from the vents and suckers, that would reduce the negative pressure asserted by your vacuum assist device. Pretty simple to anticipate, and thus make adequate adjustments.
My target for a negative pressure has always been -40mHg or less depending on the patient and venous return. Keeping in mind that the primary objective is to adequately decompress the heart, as opposed to reaching a desired number, endpoint, and ignoring the obvious caveats or indicators that create more hazard then the hoped for benefit (venous chatter/entrained air in the venous line due to a loose suture on the venous cannula- or air coming out of solution.
My colleague suggested that not only should we be cautious and reconsider relying on “cheap vacuum regulators laying in some bin” but that we should reevaluate the additive impact of negative pressures generated in tandem with both vertical gradients as well as the numbers we were dialing up to with our vacuum regulators.
I can’t say that I disagree with that point. The argument is very cogent. However, in the light it was presented, I can’t say that it is a standard of care issue- YET. First of all, because I have never been at an institution that measured these values (and that is 50+), and second of all it basically was a suggestion that pretty much every suction regulator that well over 90% of the hospitals I have worked at depend on- are either unreliable, or inaccurate. Both implications are somewhat disturbing, because we as a profession have relied on these for most of our careers.
The question I present to our readers are the following:
- How often do you use VAVD
- Do you worry about gaseous micro-emboli
- How do you test the accuracy of your negative pressure
- Do you feel it represents a potential for delivering air to the patient
- Would you advocate for an Industrial “scientifically calibrated” device over the current run-of-the-mill vacuum regulators.
There will be a follow up on this post, and the plan is to document negative gradient pressures, and the subsequent additive impact of a VAVD suction device added to the mix ?
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