NIRS

March 19, 2016

Cerebral & Somatic NIRS (Near InfraRed Spectroscopy) in shock states: tailoring therapy. (PART 1) #FOAMed, #FOAMcc

So I’d mentioned using NIRS to monitor and tailor therapy a few months ago, and promised a more in-depth discussion to come, so here we go.

For this not familiar with the technology or the concept, NIRS measures tissue saturation, predominantly venous. Hence physiologically it is akin to central/mixed venous gases, but localized. Cerebral NIRS found its foothold in the OR with carotid and cardiac surgery, but its use is now expanding. Given typical knowledge translation time of a decade, it should end up joining ETCO2 as a routine vital in monitored units, but probably not soon enough.

So in our unit at Santa Cabrini Hospital in Montreal, we’ve had this technology for about a year (the INVOS system), and have been studying its uses. In this time, three applications have stood out:

  1. Finding the “Sweet Spot” for vasopressors.
  2. Confirmation that therapeutic interventions are hemodynamically appropriate.
  3. Cardiac arrest: CPR adequacy, prognostication and detecting ROSC.

 

  1. Finding the “Sweet Spot” – I think (hope) that anyone reading this with professional interest understands that pressure does not necessarily equal perfusion.  With that in mind, adjusting vasopressors to a pressure makes little sense, and represents at best a guesstimate of perfusion, which is what we really are after. We can all agree, however, that a certain minimum pressure is required, but whether that is 65, 55 or 45 MAP no one can say for sure.  So the way I like to use it is to establish a baseline and watch the direction of the tissue saturation with vasopressor therapy. If the saturation begins to drop off, we may have reached a point at which excessive vasoconstriction is worsening tissue perfusion, and that inflexion point may represent the upper beneficial limit of the vasopressor – this may happen to be under 60 or 65 of MAP.  However, it is key to understand that this inflexion point is reflective of the current state of hemodynamics, such that a change in volume status or cardiac output, in one direction or the other, would likely change the position of this physiological point.  For example,  a volume depleted patient may reach a decreasing tissue saturation point at 55 MAP, but, once volume replete, may reach a higher MAP of 65 or above before a drop in saturation is seen.  Conversely, a patient whose best tissue saturations were around 65 MAP who suffers an MI and sudden drop in cardiac output may now see his perfusion compromised at that same MAP, which would now be achieved with a greater vasoconstriction, less cardiac output and consequently, poorer flow… I posted a case discussion which illustrates this.
  2. Confirmation that therapeutic interventions are hemodynamically appropriate – I feel this is really important. When a patient’s life is literally on the line, and knowing that our interventions are seldom without potential nefarious side effects, it is poor medicine to be introducing a therapy without having some form of monitoring – preferably multiple – that we are headed in the right direction, or at least not making things worse. Of course, we already do this – with BP, sat, lactate, CCO, ultrasound, ETCO2 – but I think using a realtime measure of tissue saturation adds to this. It is also my firm opinion that integrated, multimodality monitoring is necessary – at least until someone develops some form of mitochondrial monitoring which tells us that the cytoenergetics are sufficient to survive. Until then we are stuck with surrogate markers and many of them (e.g. lactate) are the result of complex processes that preclude them being a simple indicator of perfusion adequacy. For instance, when giving a fluid bolus/infusion – after having determined that the patient is likely fluid responsive AND tolerant – one should expect to see an increase in ETCO2 (other parameters being constant), an increase in CO, an increase in NIRS values. The absence of such response should make one reconsider the intervention, because without benefit, we are left only with side effects.

Here is a patient’s cerebral (top) and and somatic (thigh – bottom) and CO values. This patient had an RV infarct and was in shock.

IMG_7948IMG_7946

 

Following initiation of dobutamine, this is what occurred:

IMG_7951IMG_7949

Given that we cannot always predict the response to an inotrope – depending on the amount of recruitable myocardium, it is reassuring to see an improving trend. This enabled us to decrease the vasopressor dose significantly.

Note that, so far, and unless some good evidence comes out, I don’t use a goal value, and so far, I have not identified a value that is predictive of prognosis. However, downward trends usually bode very poorly. For instance, I had a severe chronic cardiomyopathy patient whose cerebral saturation was 15%!!!  But more surprisingly, she was awake, alert and hemodynamically stable. Adaptation.

Part 2 and the stuff on cardiac arrest coming soon!

Please, anyone using NIRS in shock, share your experience!

 

cheers

 

Philippe

September 6, 2015

NIRS-Assisted Resuscitation: Following the N=1 Principle. (Part 1) #FOAMed #FOAMcc #NIRS

So for anyone who has been reading any of my stuff, you know that I believe we best serve our patients by using the information in good evidence and blending it with bedside assessment (ultrasound being an integral part of physical examination) and physiology to come up with the best therapeutic approach to the one patient we are treating.  Contrast that to the blinded and naive belief that one protocol from one study is the best thing for all patients suffering from disease X.

So I am constantly looking for a way to fine tune resuscitation to the individual patient, given his/her particular cardiovascular function, volume tolerance (not just volume responsiveness), vasopressor tolerance (blue fingers probably mean blue livers and kidneys to some degree) and metabolic reactions.

One of my friends and mentors is Dr. Andre Denault. Absolutely incredible guy who is a complete triple threat of academia (internist/intensivist/anaesthetist massively published), experience and raw neuronal power. Throw in open-mindedness and humility (he actually once picked my brain about abdominal compartment syndrome pressure monitoring) and you have a lethal package. So after killing off the filed of intraoperative/critical care TEE (yes the leading textbook is his), he’s plunged into NIRS spectroscopy and is now dragging it from the OR to the ICU, and so in the last few years he’s put me on this trail with anecdotes and his (unpublished yet but coming) findings.

So a quick few words about NIRS (Near InfraRed Spectroscopy). The technology looks at hemoglobin saturation, and does so in a predominantly venous way. Hence this behaves similarly to a central or mixed venous gas, except at the local tissue level. A gross normal is >70% but this has to be interpreted in clinical context, along with the knowledge of simultaneous arterial saturation. Hence lower values (assuming normal arterial sats) will mean one of two things: increased demand or decreased supply. The demand issue is a clinical one. What we are looking for using this type of monitoring is decreased supply, e.g. cardiac output that is inadequate for current demands.

The information gleaned from our discussions was enough to make me get some loaner time with some devices, and that was enough to have a few clinical cases pique my interest.

Here is one:

In the ICU at Scarborough General in Toronto, I admit a lady with urosepsis on a stone-obstructed hydronephrosis. We get urology to slip in a double J, but she is still very norepinephrine-dependant. She isn’t intubated, lactate about 5 and on high doses of norepi to maintain MAP 60-65. Extremities are cold and mottled and there is mottling up to the thighs. She is awake and communicating. Over the next hour or so lactate rises to 5.5. I’m not liking this.

I put on the NIRS monitor with a cerebral lead and one on the thigh. They both read in the 50’s, somewhat suboptimal.

Bedside US reveals a good sized IVC with little variation. She’s well filled. Her LVEF is 50-60%, her RV is dynamic and with a normal RV/LV ratio. The hydronephrosis is improved on the affected side.

So I have a patient who’s adequately preloaded, without obstructive or systolic failure, who is on very high doses of norepinephrine with a rising lactate, despite source control and antibiotics. However, she doesn’t “look” that bad aside from the cold and blueish extremities…

So I decrease the norepinephrine. Systolic BP drops to 80…but…cerebral NIRS and tissue NIRS rise…now in the high 50’s. Patient remains awake and communicating. Drop norepinephrine some more. Systolic 75. Cerebral NIRS 62%, tissue 61%. Systolic 70. Cerebral 59%, tissue 57%. Back up to 75%. NIRS back up.  I finally settle on 75-80 systolic. NIRS settles in low 60’s. An hour later, lactate is 4, two hours later 2.5, then normalizes. Over that time span, the mottling gradually resolves and the urine output picks up. By the next morning she is off norepinephrine, and her BP sits around systolic 85-90 on its own. Turns out her usual BP is in the low 100’s.

I’m not sure how long – without the reassurance of improved tissue saturation – I would have been able to tolerate systolic BPs in the 70’s. Remember that lactate takes time to clear and urine takes time to make.

So this case reinforced my belief that not every patient’s needs are best met by an MAP of 65, and that targeting this may be harmful. It isn’t hard to imagine a scenario (which I may very well have pursued at a more junior stage) where further fluid resuscitation, coupled with insistence on a BP value may have resulted in iatrogenic fluid overload or Paul Marik’s “salt water drowning” (more commonly thought of as “ARDS”) and tissue ischemia/organ dysfunction (partly related to over-vasoconstriction) and who knows what outcome could have transpired… And very possibly, a bad outcome may have been blamed on severity of illness… Food for thought.

So one definitely possible use for NIRS is to find the “sweet spot” for the BP/vasopressor relationship.

More on NIRS in Part 2 in the next week or so.