Syringes and pressure

[RxBoy]

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I have a couple questions for the mathematically inclined anesthesiologists out there. I know syringe pressure calculations require physics formulas ect but I couldn't find an easy source for this question that I was thinking about today.

questions:
1) What negative syringe pressure is required to collapse a typical epidural vein?
2) What positive syringe pressure is required to overcome static CSF pressure?

The reason I ask the first question is because during aspiration of a catheter in a small epidural vein, conceptually too much negative pressure would collapse a vein and cause a false negative aspiration test. What syringe size or force should be applied to minimize this?

The reason I ask the second question is because I was thinking of a scenario in which a wet tap occurred and then another site was used and a catheter threaded in the epidural space. I know there is positive ICP pressure in the supine adult (normally 7–15 mmHg) but ICP becomes negative in the vertical position (averaging −10 mmHg). If this is the case then conceptually bolusing local through a syringe in the catheter could cause the local to migrate intrathecally into the previously punctured site (vacuum effect). So what rate should a bolus be bolused if at all?? Something else I postulated was not bolusing a labor epidural until the patient was supine, but again this theoretic and not clinical.

Lastly any articles someone could share regarding these topics would be greatly appreciated 🙂

 

[Idiopathic]

I have a couple questions for the mathematically inclined anesthesiologists out there. I know syringe pressure calculations require physics formulas ect but I couldn't find an easy source for this question that I was thinking about today.

questions:
1) What negative syringe pressure is required to collapse a typical epidural vein?
2) What positive syringe pressure is required to overcome static CSF pressure?

The reason I ask the first question is because during aspiration of a catheter in a small epidural vein, conceptually too much negative pressure would collapse a vein and cause a false negative aspiration test. What syringe size or force should be applied to minimize this?

The reason I ask the second question is because I was thinking of a scenario in which a wet tap occurred and then another site was used and a catheter threaded in the epidural space. I know there is positive ICP pressure in the supine adult (normally 7–15 mmHg) but ICP becomes negative in the vertical position (averaging −10 mmHg). If this is the case then conceptually bolusing local through a syringe in the catheter could cause the local to migrate intrathecally into the previously punctured site (vacuum effect). So what rate should a bolus be bolused if at all?? Something else I postulated was not bolusing a labor epidural until the patient was supine, but again this theoretic and not clinical.

Lastly any articles someone could share regarding these topics would be greatly appreciated 🙂

Just my personal experience.

1) i dont know, it probably is insignificant. if you have more than one orifice of a multi-orifice catheter in, then the pressure equalizing effect will make it even less likely. on those rare occasions when you get one orifice in the vein and the others in the epidural space, you may just have to rely on the test dose.

2) theoretically, you are right, and some people are more cautious than normal when changing sites after inadvertent wet tap. we thread intrathecal catheters in this situation, and i think that takes a lot of the guesswork out of that, although our L&D staff all understand how to handle them. i think the situation you describe could happen, but think its very unlikely to be clinically significant

 

[IlDestriero]

Think # 2 through again WRT ICP in the vertical and horizontal position. Remember where the puncture is, and what ICP is.
As for # 1, when I put my epidural in a vein (n=2 with a multiorifice cath), the blood slowly filled the catheter, no suction needed with a syringe. I've not had a + test dose indicating an intravascular injection. Obviously, I didn't test dose the bleeders.

 

[pgg]

I have a couple questions for the mathematically inclined anesthesiologists out there. I know syringe pressure calculations require physics formulas ect but I couldn't find an easy source for this question that I was thinking about today.

questions:
1) What negative syringe pressure is required to collapse a typical epidural vein?
2) What positive syringe pressure is required to overcome static CSF pressure?

I doubt there is any fixed answer. The pressure required to collapse a vein is going to depend on the diameter of the vein, pressure within the vein, and available flow of blood to that vein. (Pressure and flow being related but NOT always directly proportional numbers - ask anyone who's ever installed a sprinkler system without bothering to do the math.)

The reason I ask the first question is because during aspiration of a catheter in a small epidural vein, conceptually too much negative pressure would collapse a vein and cause a false negative aspiration test. What syringe size or force should be applied to minimize this?

When it comes to aspiration, syringe size is going to be irrelevant IF one pulls back hard enough to create a vacuum. Absent a vacuum with aspiration, or when pushing fluid in, the mechanical advantage of syringes will vary with size (distance plunger moves divided by volume). So when aspirating (but not enough to create a vacuum) X Newtons of force applied to a 3 cc syringe will transmit more force through the fluid than the same force to a 10 cc syringe.

Sort of ... The formula for calculating the mechanical advantage of a hydraulic system assumes postive pressure through a noncompressible fluid. Negative pressure, or aspiration is going to be different because of the potential to either create an actual vacuum or because bubbles (microscopic or not) will expand as pressure drops and the fluid won't be noncompressible/nonexpandable as assumed.

But in any case I can't really believe this is important. So long as you're aspirating gently and patiently I don't think it matters. It's the half-second hard-vacuum aspiration checks that I believe run the theoretical risk of collapsing a cannulated vein.

I know there is positive ICP pressure in the supine adult (normally 7–15 mmHg) but ICP becomes negative in the vertical position (averaging −10 mmHg).

ICP might be -10 in the cranium of a sitting/standing patient, but it won't be -10 way down in the lumbar spine where your needle is, 60 cm lower. (It won't be +60 cmH2O higher because it's not an open column, but it won't be -10.)

 

[proman]

I'm not mathematically inclined, but the point of aspirating on an epidural catheter isn't to test intravascular placement but intrathecal placement. CSF comes back easily with aspiration because it's a low viscocity fluid. Like IlD mentioned, sometimes blood fills the catheter without any suction but it's not reliable. Absent frank blood in the catheter, the test dose is really the only reliable way of assessing intravascular placement.

 

[Idiopathic]

I'm not mathematically inclined, but the point of aspirating on an epidural catheter isn't to test intravascular placement but intrathecal placement. CSF comes back easily with aspiration because it's a low viscocity fluid. Like IlD mentioned, sometimes blood fills the catheter without any suction but it's not reliable. Absent frank blood in the catheter, the test dose is really the only reliable way of assessing intravascular placement.

i dont really think this is accurate. i look for frank evidence of either blood or csf and that heightens my concern, but it isnt absolute.

there are other ways to check for intravascular placement. aspiration of the catheter has very low sensitivity and very high specificity and as such it is limited, but I believe there is a study of 1000 or so epidurals that states that absence of heme or CSF in the catheter with aspiration reliably excluded the likelihood of intrathecal/intravascular placement. ill look for the reference.

 

[Idiopathic]

ICP might be -10 in the cranium of a sitting/standing patient, but it won't be -10 way down in the lumbar spine where your needle is, 60 cm lower. (It won't be +60 cmH2O higher because it's not an open column, but it won't be -10.)

i think the point there was that you will have relative shifts in the pressure on the other side of the intact dura with positional change, and that these could theoretically create a localized vacuum effect with positional changes. obv a low ICP would equate with a higher relative pressure of the lumbar CSF and vice versa, so a bolus dose placed while a patient is vertical, in theory, could then get "pulled" into the csf through a hole in the dura when the patient is laid supine

 

[RxBoy]

Thanks for the replies... Very helpful. I think it would be an interesting study to see if any neurologic exam changes occurred during during a bolus using a 2nd placed catheter after wet tap vs. no wet tap. Maybe even a higher incidence of high/total spinal (although I doubt it).

But in any case I can't really believe this is important. So long as you're aspirating gently and patiently I don't think it matters. It's the half-second hard-vacuum aspiration checks that I believe run the theoretical risk of collapsing a cannulated vein.

If only you could see how many of my attendings do that half second hard vacuum aspiration and say negative aspiration. It erks me but who am as a resident to correct these attendings.

 

[RxBoy]

Side note: One thing I did learn from various article readings is that higher pressure "boluses" uses the multiport catheter much more efficiently then a slow infusion pump (which will typically only exit the closest orifice). No wonder why intermittent labor boluses seem to work better for pain control.

 

[proman]

i dont really think this is accurate. i look for frank evidence of either blood or csf and that heightens my concern, but it isnt absolute.

there are other ways to check for intravascular placement. aspiration of the catheter has very low sensitivity and very high specificity and as such it is limited, but I believe there is a study of 1000 or so epidurals that states that absence of heme or CSF in the catheter with aspiration reliably excluded the likelihood of intrathecal/intravascular placement. ill look for the reference.

You're probably thinking of the Norris study, Anesthesiology June 1998. It's a bit of a strange argument. The hypothesis was that epinephrine is not necessary because of the high rate of positive aspiration in IV catheters. However, they cite other studies that describe a low rate of positive aspiration in IV catheters. The difference appears to be the type of catheter, with single orifice catheter having a low rate and multi-orifice ones having a higher rate. Regardless, their study didn't really test the validity of epi because the catheters with positive blood aspiration were all removed. Their conclusion may be accurate: "These data do not suggest that false-negative aspiration is impossible, but rather that it is no more likely than a false-negative response to epinephrine or air". For a screening test, you want a high sensitivity and accept a lower specificity. I don't think aspiration accomplishes this.

 

[drf]

As a pain guy, I can say that I've had negative aspiration through catheters as well as 25g needles followed by very obvious intravascular spread on contrast injection.

I don't put a whole lot of trust in negative aspirations as a way to exclude intravascular placement,

 

[Arch Guillotti]

You're probably thinking of the Norris study, Anesthesiology June 1998. It's a bit of a strange argument. The hypothesis was that epinephrine is not necessary because of the high rate of positive aspiration in IV catheters. However, they cite other studies that describe a low rate of positive aspiration in IV catheters. The difference appears to be the type of catheter, with single orifice catheter having a low rate and multi-orifice ones having a higher rate. Regardless, their study didn't really test the validity of epi because the catheters with positive blood aspiration were all removed. Their conclusion may be accurate: "These data do not suggest that false-negative aspiration is impossible, but rather that it is no more likely than a false-negative response to epinephrine or air". For a screening test, you want a high sensitivity and accept a lower specificity. I don't think aspiration accomplishes this.

Negative aspiration doesn't exclude the possibility that you are intavascular or intrathecal. Almost every IV catheter I have ever out in has immediatey bled back prior to aspirating but I have seen ones that haven't. Also, (personally and anecdotally) I have seen catheters with a negative aspiration prove to be intrathecal.

 

[rsgillmd]

Negative aspiration of blood or CSF is useful, but as pointed out by others it needs to be more than just a half-second.

I find the 3 cc syringe to yield the most positive aspiration results for blood. However, I have also seen negative aspiration with a clearly positive test dose response when using a 3 cc syringe.

Many of the CRNAs at my current institution like to dose through the needle for labor epidurals, instead of doing a CSE (probably because they want to be lazy). I don't like to do that because I have seen high blocks result from undiagnosed dural punctures. Whether this is because they were not aspirating correctly or whether it is the syringe size (they typically use a 10 cc syringe when dosing through the needle), I don't know. It's also possible they nicked the dura enough to result in a high block, but clearly not enough to result in a positive aspiration.

The intravascular rate will depend on technique, but it will also depend on the catheter. When I was a resident I had one catheter migrate intravascular, and heard of only a handful of positive test doses. We were using Arrow catheters at that time.

At my current institution we use BD's catheters. Approximately 1-2% (probably closer towards 1%) will be intravascular even when no paraesthesias were elicited during the performance of the procedure, or even when a CSE showed flow of clear CSF in the spinal needle and without paraesthesia.

In residency I quit performing test doses because we were doing CSEs. We didn't bolus the catheters. The infusion served as a test dose of sorts. I have changed my practice with the higher percentage of intravascular catheters at my current institution.

I can only blame the patient population or the catheter. I pick the catheter in this situation.

I think I'm rambling. Good night.