(1)
Hôpital Ambroise Paré Service de Réanimation Médicale, Boulogne (Paris-West University), France
Notions of the physical properties of ultrasound are not indispensable for the user (as we wrote in our 1992, 2002, 2005, 2010, and 2011 editions). Interested readers will find them in any ultrasound textbook.
We will discuss here the notions useful for understanding critical ultrasound. Every maneuver which favors simplicity will be exploited. Space will be used for explaining why only one setting is used; why, at the lung or venous area, only one probe orientation is favored; and how to easily improve the image quality.
Preliminary Note on Knobology. Which Setting for the BLUE-Protocol? Which Setting for the Other Protocols (FALLS, SESAME, etc.) and Whole Body Critical Ultrasound?
An ultrasound machine includes a various number of buttons, cursors, functions, etc. In our routine, we use only three functions:
1.
The gain
2.
The depth
3.
The B/M-mode
The sole use of these three buttons converts any complex unit into a simple stethoscope (since 1982).
The setting is a basic point. Our setting is not “Lung”, but “Critical Ultrasound.” This concept, which initiates the SESAME-protocol, allows us to see the heart, veins, and belly (and lung) with a single approach, a single probe [1]. Our setting is, briefly, always the same. No filter, no facility. The next chapter will develop this point.
Some revolutionary machines use this concept with electronic control (basic/expert level), which is fine, but we did the same for a lesser cost, with a simple piece of cardboard (or thick plastic) and a cutter for making holes and hiding those scary, useless buttons, respectively. Since 1982, these machines were suddenly transformed into user-friendly units. A genuine stethoscope, making novice users at ease.
We quite never touch the countless pre- and post-processing possibilities nor all modern facilities, mainly harmonics (see Chap. 2). Annotations are useless when the examination is not made by a radiologist (or technician) for a doctor: the spirit of critical ultrasound.
The B/M mode seems insignificant. Technical misconceptions can contribute in losing lives, especially for diagnosing pneumothorax in difficult conditions (i.e., the most critical ones precisely). We will see in Chaps. 8, 10, and 14 that the modern manufacturers are usually unable to provide a left image in real time, and a right image in M-mode: side by side and without freezing the real-time image. This configuration, easily found in the 1980’s technology, is a critical basis in lung ultrasound.
Read if you have time the interesting Anecdotal Note 1 of Chap. 28, proving that lung ultrasound could have been perfectly developed since the 1960s.
Step 1: The Image Acquisition
Whatever the unit (even with pocket machines), the mastery of the spatial dimension is probably the major difficult point of ultrasound. When the probe is moved, significant changes appear on the screen – very unsettling at the beginning. How to understand what happens on the screen should be mastered in priority. We travel through the third dimension. These changes will be integrated and become automatic with practice. The other step (interpreting the image) is much easier. The spatial control also makes the superiority of ultrasound, i.e., the possibility, by a slight change, of answering the clinical question. Even if we assume that in the current times physicians have all access to basic programs which explain this delicate step, the aim of CEURF is to simplify this step too.
For achieving this simplification, we will suppress movements we never do. Tilting the probe for instance. For anterolateral lung venous (belly, optic nerve, etc.) ultrasound, our probe is always perpendicular to the skin (Fig. 1.1). The two exceptions are (1) the heart, subcostal and apical views, (2) the posterior aspect of the lung in ventilated patients, where the probe tries to be as perpendicular as possible (see description of the PLAPS-point in Chap. 6). Being quite always perpendicular suppresses other movements, i.e., simplifies ultrasound (and is what we daily do).
Fig. 1.1
How we hold the probe, how we don’t. Left: Like with a fountain pen, the operator can stay hours without any fatigue, and the image is stable on the screen. The probe is applied at zero pressure, which is comfortable for the patient and mandatory for any venous analysis as well as the optic nerve. The probe is (reversibly) stable on the skin, not slippery using Ecolight, which decreases the energy needed for keeping it stable. The probe is perpendicular to the skin. It is applied longitudinally. Three main movements are arrowed. These blue arrows indicate the Carmen maneuver (this movement is done from left to right in this scan moving the skin on the underskin). If the probe was transversally applied, the Carmen maneuver would be from head to feet. The turning arrow indicates rotation of the probe (like screwdriving). The black arrows indicate a scanning looking like changing gears of an automobile (of major importance to the trainee for reaching the good position). Right: The pressure is not controlled (a very bad habit in venous ultrasound), and this position will generate fatigue. More severe, the hand is not stable; this will disturb the practice of a discipline based on the analysis of dynamics
Our microconvex probe has a sectorial scanning, displaying a trapezoidal image, the probe head being on top.
We assume that what is at the left, the right, the superficy, and the depth of the image is integrated. Note that for lung ultrasound, we adopted the radiological convention, head to the left, feet to the right, unlike the echocardiographists (roughly the only element that we took from the radiologic culture). Critical ultrasound should be homogenized: lung with head left, heart with head right makes no sense.
The operator must apply the probe on the skin, then search for the best image. For that, a good acoustic window must be found. This is really easy, never a problem for the lung, ironically. First, any perpendicular scan at any point of the chest wall provides the same basic image: the lung is “everywhere,” just below the skin. Second, the gas is not a hindrance here. This completely changes the traditional rules of ultrasound. At the heart, the abdomen, etc., we admit that this step is challenging (although countless tricks are available).
Once a structure is detected more or less, subtle movements of the probe will optimize the image.
How We Hold the Probe Basically
Critical ultrasound analyzes vital structures, i.e., permanent movements. The operator’s hand must be standstill (Fig. 1.1): the dynamic should be generated by the patient alone (never the operator’s hand). Figure 1.1 shows how we do not hold the probe. Uncontrolled movements of the probe create dynamics which bring nothing. Ecolight®, our contact product, allows to save energy usually lost for stabilizing a slippery probe (Chap. 2). We find critical to hold the (microconvex) probe like a fountain pen between the thumb and index fingers (+/− medium etc.), with the operator’s hand quietly applied on the patient’s skin. For many parts, we work at “Zero pressure”: the probe is applied to the skin until an image appears on the screen. This minimal pressure warrants absence of pain (or cardiac trouble when working onto the eyelid), absence of fatigue (in prolonged examinations), and absence of errors (too much initial energy will result in squashing veins).
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