Cerebrovascular physiology – article alert #15

Sepsis and the brain

53- Cerebral vasoreactivity to acetazolamide is not impaired in patients with severe sepsis – Fülesdi et al.

Brain hypoperfusion

54- Cardiovascular risk factors promote brain hypoperfusion leading to cognitive decline and dementia – de la Torre

Dynamic cerebral autoregulation

55- Dexmedetomidine weakens dynamic cerebral autoregulation as assessed by transfer function analysis and the thigh cuff method – Ogawa et al.

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Going for the jugular: the end

Regular readers will remember my research interests. Thanks to good friends, it has now become essential for my research projects evaluating the influence of certain vasopressors on brain oxygenation to invasively measure the levels of oxygen in the brain (specifically in the blood coming from the internal jugular vein). Indeed, a recent paper from my Danish colleagues suggested that changes in brain oxygenation (measured non-invasively by near-infrared spectroscopy), following manipulation of blood pressure by norepinephrine, could be partly influenced by changes in skin perfusion. I had the opportunity to measure jugular venous oxygen saturation in healthy volunteers and patients during my postdoc (in Copenhagen with that same group) and this is definitely a “must” for the kind of research project I am working on.

In front of this evidence, I decided to make an amendment to my current vasopressor study in order to add jugular venous oxygen saturation to existing measures. But remember, I am not in Copenhagen anymore…I didn’t know if it would be difficult for this amendment to be accepted.

I thus have presented my amendment to our Scientific committee first and soon received a positive answer. It was pretty scientifically obvious that we needed that measure. Then, we have (for that specific meeting, I have asked the anesthesiologist who will insert these lines to be present) tried to convince the Ethics committee…

Well, I am happy to inform you that blood samples from the internal jugular vein will be taken in that vasopressor study! Yay! As I have already mentioned,  I am aware of only one lab in Canada that can insert catheters into the internal jugular vein in human volunteers. For the moment, I will be mostly interested in jugular venous oxygen saturation. However, I will now have the possibility of studying cerebral metabolism.

For those wondering what a catheter into the internal jugular vein looks like, well here it is (the catheter is advanced until the black arrow):

jugulaire

I am looking forward to seeing the results !!!

Cerebrovascular physiology – article alert #13

Brain blood flow and metabolism

45- Cerebral perfusion, oxygenation and metabolism during exercise in young and elderly individuals – Fisher et al.

Cerebral autoregulation

46- Effect of isoflurane on brain tissue oxygen tension and cerebral autoregulation in rabbits – Aksenov et al.

47- Continuous cerebral blood flow autoregulation monitoring in patients undergoing liver transplantation – Zheng et al.

Brain monitoring

48- Transcranial Doppler Pulsatility Index: What it is and What it Isn’t – de Riva et al.

Heart failure and brain blood flow

49- Resting and exercise cerebral blood flow in long-term heart transplant recipients Smirl et al.

Methods: Monitoring cerebral blood flow velocity with transcranial Doppler: hand-held vs. frame-held probe

When I was introduced to cerebrovascular physiology a couple years ago, the first piece of equipment I’ve worked with was transcranial Doppler. After having learned the technique to search flow velocity in the middle cerebral artery, the following was pretty clear to me: 1) a good quality Doppler signal was critical for subsequent interpretation and, 2) you first use a hand-held probe to search for the artery of interest and then switch to a frame-held probe (see figure below) to monitor, usually for a continuous period of time (for example, continuous infusion of vasopressors or endurance exercise), blood flow velocity.

TCD

That frame-held probe is appealing since, among other things, you need to keep the same angle of insonation throughout the study protocol in order to have reliable blood flow velocity data (because there is no way to know that angle of insonation with transcranial Doppler) especially in situations where the head is moving during data acquisition.

So, in the beginning of my training, the utilization of a frame-held probe was the way to go in cerebrovascular research since I had only worked with healthy volunteers/patients without head injuries…but what about these individuals for whom these head frames are most likely not tolerated ? Could we use hand-held probes and still be confident in our data acquisition ? What about stability and reliability of hand-held probes?

The main objective of an interesting study recently published by Saeed et al. was to compare cerebral blood flow velocity and cerebral autoregulation using a hand-held vs. frame-held probe.

Cerebral blood flow velocity (in the middle cerebral artery) was thus monitored with a frame-held probe or held in position by one investigator for 5 minutes (for each method) in 11 heatlhy volunteer. Hemodynamics was also monitored and cerebral autoregulation calculated for each period. These measurements were repeating in a subsequent visit. A total of 3 frame-held and 2 hand-held measurements were performed on both left and right sides at each visit.

The main findings of this study were:

1) no significant differences in cerebral blood flow velocity and cerebral autoregulation between the methods;
2) no significant difference between repeated measures on separate days for both cerebral blood flow velocity and cerebral autoregulation.
As highlighted by the authors:

(…) this is the first study to report that HH measurements of CBFv and ARI estimates are not statistically different from FH, which has important implications for bedside estimates of CBFv and ARI, particularly in traumatic brain injury (TBI) patients and neonates, where CBFv and ARI measurements form an important part of clinical assessment.

(HH: Hand-held; CBFv: cerebral blood flow velocity; ARI: cerebral autoregulation; FH: frame-held)

These results are very interesting and promising if one wants to investigate cerebral blood flow velocity with transcranial Doppler in patients who cannot tolerate a head frame. However, we have to note that a short period of recording was investigated in this study. It will be important to replicate this protocol with longer recording periods.

Reference

Saeed NP, Panerai RB, Robinson TG. Are hand-held TCD measurements acceptable for estimates of CBFv? Ultrasound Med Biol 2012 38(10):1839-44