Which Equipment for the BLUE-Protocol 2. The Probe

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Hôpital Ambroise Paré Service de Réanimation Médicale, Boulogne (Paris-West University), France

The microconvex probe: a universal probe for the BLUE-protocol, the FALLS-protocol, the SESAME-protocol, and many others. A providence in cardiac arrest and routine daily tasks. Probably the probe of the future for critical ultrasound. Definitely one of the main paradoxes generated by critical ultrasound.

We could make this chapter very short and simple: just try our 1992 probe, or just see Fig. 3.1, a nice summary, and make an opinion. In daily practice, we start up our unit, take our probe, and scan the whole body, quietly or fast, in function of the emergency. We were a bit surprised by the length and complexity of writing of this chapter, which obliged us at making a deep analysis of the current situation. There is a lot to say for explaining that the usual three probes are, again, a use of the radiological or cardiological cultures without adaptation.

Fig. 3.1

Is it a vein? This slide, extracted from the CEURF courses, shows how one single probe can inform on superficial as well as deep veins, and besides the lung, heart, belly, even retroperitoneum and optic nerve. In two words, the whole body. Look at the image quality and the clinical information, and make your opinion

Intensivists who have not been educated in the spirit of holistic ultrasound are often cardio-centered (and use vascular probes for vascular access). Here, the reader should forget any preconception and carefully read this chapter. When he or she will have rendezvous with the SESAME-protocol (i.e., cardiac arrest, where each second matters), this reading will show particularly useful [1]. We use a probe which appears universal and should be that of the critical care physician. Since some institutions still have ultrasound machines with the traditional three probes, but not the universal one, we will demonstrate how one probe can be, paradoxically, superior to three others.

This chapter is sensitive, we know it may upset some key-opinion leaders, we know this may slow down the widespread of the message of a vision which will be soon or late a standard.

The Critical Point to Understand for Defining the “Universal Probe” in Critical Care: The Concept of the Providential (Optimal) Compromise

What is required from a probe? A correct vision of what is en face, this is all. Unfortunately, the price to pay for this is a shape non-suitable if too large, or a too small range, obliging to have several probes. In critical care, changing a probe costs time and money, and asepsis faults are quite unavoidable. Here, the smaller size in all dimensions is critically important: small footprint for avoiding any hindrance (dressings, nonlinear areas), small length for assessing the posterior part of the lung. None of the three usual probes of laptop machines answers to this requirement.

The users must realize two points, not really misconceptions, the term is too tough, but we did not find any other. Critical ultrasound analyzes first superficial fields; however, the first millimeters are not critically important. A very deep penetration (36 cm) is not critically important.

In critical ultrasound, superficial areas are first on focus. But not the first millimeters (the real raison d’être of the linear probes)

As a striking feature of critical ultrasound, the most critical data are extracted from the analysis of superficial areas usually (aorta and IVC being some exceptions). The lung (the most important part) first. But also most of the venous network (internal jugular, subclavian, femoral, etc.), peritoneum, and optic nerves are areas both superficial and relevant to analyze. The SESAME-protocol has distinguished the pericardium, a superficial structure, from the heart for this reason (and some others). As regards the heart, the ventricles are more superficial than the auricles, which are of lesser importance. In plethoric patients, deep abdominal analysis (pancreas, etc.) is often disappointing even with traditional ultrasound, and these patients are eventually referred to CT: our concept has taken this important detail into consideration.

This point is important: we need superficial, but not too superficial. The raison d’être of these vascular probes is the good quality of the first mms. Yet these 5 first mms, in the critically ill, are usually of no interest. We have paid attention in our daily practice in adults, to the critical zones. They are deeper than 1 cm (pleural line, most veins, etc.), rarely between 6 and 9 mm (some veins), and never below 5 mm. There is no critical target in this zone, apart from very rare and quite always not urgent cases (tracheal analysis in skinny patients, radial artery cannulation). In other words, those who are persuaded that these 5 first mm are important make a misconception (sorry for this word) which will be paid; see also below (for the same reason).

Regarding the far penetration, in most cases of investigations in critical holistic ultrasound, the lung is included and answers to questions usually pertaining to cardiac cultures. Here, attention is paid on basic items: the pericardium, LV, and RV. The fine analysis of the auricles can be done later usually, at opened hours.

How to Scientifically Assess This Notion of “Domain of Interpretability”? Our High-Level Compromise Probe

How can one probe be suitable for all areas? We just see since decades that it works (we tried to contact the Japanese engineer who made it in 1992). Figure 3.1 is a indisputable answer by the image.

When we write that some areas are “less well” seen than others, here comes the critical notion of the optimal compromise. Let us make an experience: take again the two radio sets used in Chap. 2, proving how modernity does not always provide improvements. Make again the manipulation (one in FM, one in LW). Do you hear well on the LW channel? Do you recognize the song? The words? If the answer is yes, clearly, you have demonstrated that LW has the required quality (while providing faster information). The difference is futile if you just want to hear radio, but critical if you want a fast information. Willing to have a better (and here, useless) quality yields dramatic regressions, i.e., the traditional three probes and their heavy issues: cost, ergonomy, dirt, imperfect quality for the lung, and huge waste of time in extreme emergencies or daily routine. Therefore, the only question the physician should ask is: “Does the quality of this given image remain within the domain of interpretation?” If the answer is yes, the user will have most of the time, for critical targets (the lung/veins first, then simply the heart and belly, i.e., the large majority of targets), the optimal quality: the winning choice. Our probe is more than a compromise; it is a providence: it is providential since quite all (93 %) of these critical targets will be seen with the “FM quality” (speed in addition), and a few (6 %) with the “LW quality.” See below how these numbers came.

Similarly, look at Fig. 3.2. It shows an animal. A dog or a cat? A cat definitely. The first image is heavy. If the question is to have a beautiful image which can be enlarged on a wall, it fits, thanks to its heavy weight. If the question was to distinguish this animal from a dog, the answer can be done, immediately, with a 600x lighter weight with no damage to the target recognition. Below a certain value (×1200), the recognition begins to be chancy. Far before this step, we achieved considerable decrease of weight. The same is applied to our universal probe. Our concept of the optimal compromise is critical as well for the management of cardiac arrest, where there is no option at all, as our daily routine work.

Fig. 3.2

Cat or dog? A simple manipulation. The 6 Mo image is heavy, for no advantage. We can decrease and decrease the weight without damage to the diagnosis: we still recognize the little cat, up to 10 Ko, i.e., 600 times lighter. At 5 Ko in gray scale, 1,200 times lighter, the risk of confusion just begins. Colored or gray scale, this is still a kitten. At 2 Ko, it is true, the resolution is not acceptable, one may confuse (with a scorpion, a stocky whale or any other image coming to your imagination). The weight of the 6 Mo image is the equivalent of the price to pay in terms of multiple probe equipment. Since we can do with light images without loss of safety for the patient, just imagine the impact for critical ultrasound

The figures dispatched in this textbook are not always perfect, but they always answer to the clinical questions. Maybe this probe will not show perfect images everywhere (retroperitoneum in fat people, the first mm of soft tissues).

This is what has initiated our concept of the optimal compromise. We will explain one of the main paradoxes in critical ultrasound, by considering the areas of interest (the lung, heart, etc.) in the function of their strategic importance (the lung comes first) and the frequency of assessment (the lung comes first), drawing a diagram (Fig. 3.3). To give an example, aortic dissection would have a high height (dangerous disease) and a narrow basis (rare event), i.e., a small surface in our graph. A pneumonia is highly lethal, and rather frequent: large surface. This diagram is indicative for the daily life of an intensivist.

Fig. 3.3

The concept of the optimal compromise. These four images, and the fifth, integrating the areas, indicate how we assess the universality of a probe. The severity of the diseases is multiplied by their frequency, for providing a clinical relevance. The heart appears as important (height) but not as frequent (width) as the lung, because pure cardiologic diseases are not the most frequent. Note the white, not full areas of images C, A, L, and M. Just some examples. In Image C, the white 2nd box, all cases with absent cardiac window decrease the power of this probe (we made here an optimistic white box). In Image A, the white 4th box corresponds to these postsurgical abdomens covered with stomies, dressings, etc.: large probes are a disadvantage. In Image L, 3rd box: all veins not assessable using the traditional vascular probe. In the bottom image summarizing the C, A, L, M images, see how restricted is the domain of the “vascular” probe, how universal is the one of our microconvex probe

This diagram shows that one probe (ours) is perfect for making 93 % of the daily work (with the “FM quality,” speed in addition) and also able to see, with inferior but anyway sufficient quality (the “LW quality”), the majority (say, 6 %) of the 7 % other points. This is the absolute future of the critical care. The user will accept to work, from time to time, with a quality inferior but perfectly suitable for answering the question. The concept of the optimal compromise, of vital relevance in cardiac arrest mainly, is illustrated in a self-speaking image (Fig. 3.2).

There is a paradox (one more): we could maybe say “Our target is not to have beautiful images, but diagnostic images,” but even though, it appears that our probe has a perfect image resolution for the most important targets (the lung, veins, belly, heart almost well); see by yourselves through this textbook and see again Fig. 3.1. The “almost” (for the heart) is perfectly suitable for us, even if cardiologists will probably disdain such a probe, instinctively. Let them think so.

Those who would like to see perfectly these few 7 % would be condemned to buy the three usual probes – in addition to the microconvex, irreplaceable for cardiac arrest (Chap. 31). For the roughly 1 % remaining field which we cannot explore, and is never urgent, don’t forget this precious DIAFORA option, defined in the previous Chapter (shortly: once a month, call radiologists with sophisticated, large machines full of probes for these very restricted applications). And it is good to work together in addition.

One anecdote from a nice friend, intensivist in India, summarizes a lot. He told us: “I see better those tracheas with a vascular probe.” Absolutely, he was right, the image was not bad. But please pay attention:

He was right for a not frequent application (one can ask a machine from outside).

We saw from our eyes the price he (many of us) had to pay everyday: the large machine, plenty of probes, an endless start-up time, cables making an inextricable mass (full of microbs usually), lying on the dirty ground (and the risk of the cable damage from the wheels, and the risk of sudden machine tipover), probes impossible to use rapidly and logically in the case of a cardiac arrest, and lateral stands which increase the width of these machines not to deal with the overall cost. All this for looking to a trachea.

He did not compare his vascular probe with our simple microconvex probe (Fig. 3.4). For sure, cardiac probes and abdominal probes will never be suitable. Even in very skinny patients, if our probe happens to miss the first mm, we share a very simple solution: making the area a bit far from the probe by using an acoustic device. It works like standard glasses for near vision. Let us call it temporarily “jellyfish,” from the French name (old radiologists should remember this device, impossible to find currently, but easy to replace by alternatives that are easy to purchase). We will see them later for not breaking the rhythm. In this Indian ICU or elsewhere, this will show the tracheal rings with a quality similar to the sound quality of the LW radio: sufficient. For these few “low-resolution” but sufficient image quality, we have a unique whole-body approach, optimal (FM) for critical targets and sufficient (LW) for the others, with a clean, compact unit. This also is a holistic ultrasound. We prefer to make our compromise at the (very) (relative) detriment of the trachea, used from time to time and quite always in nonemergency ambiance.