Right probe position. The probe should be kept between the inch and the other fingers of the hand
57.4 Ultrasound Setting
Ultrasound setting is fundamental before starting to perform any US-guided procedure. Indeed, each type of setting is characterized by specific parameters, such as probe frequency, total gain, focus position, and number, which allow to obtain the best quality of ultrasonography images in respect of the anatomical region which should be studied. Usually, vascular setting is the ideal modality for central venous catheter placement [14, 15].
Once ultrasound setting is completed and the vein to be cannulated is visualized, total gain (image brightness in B-mode), depth (number of tissue centimeters under the probe), and focus position (depth corresponding to the one of the vein to be cannulated) might be modified in order to improve the quality of the ultrasonography image even further [14, 15].
57.5 Ultrasound Scan and Anatomy
Central vein to be cannulated may be visualized in a longitudinal scan (in plane – the long axis) where the probe is positioned in parallel to the longitudinal axis of the vein or in a transversal scan (out of plane – the short axis) where the probe is positioned perpendicularly to the longitudinal axis of the vein (Fig. 57.2a, b) [14–16]. In the transversal scan, both the artery and the vein appear as round anechoic images (Fig. 57.3). Usually, the vein presents thinner walls than the artery, collapses with a slight probe pressure in the absence of thrombosis, and dilates in case of increased venous blood flow (Valsalva maneuver, Trendelenburg position) [14–16]. Conversely, the artery does not collapse. In the longitudinal scan, both the vein and the artery appear as ribbon images with an anechoic lumen delimited by parallel and echogenic walls [14–16]. The use of color Doppler and pulsed Doppler may help clinicians in distinguishing the artery from the vein. It is important to remember that red and blue colors indicate only blood flow direction in respect of the probe (the red color indicates that flow is toward the probe, while the blue color indicates that the flow is leaving). Pulsed Doppler provides graphic and acoustic images of both arterial and venous flow. In particular, arterial flow is characterized by a Doppler wave with a typical systolic peak (Fig. 57.4) and an acute sound which are expressions of the cardiac systolic activity. On the contrary, venous flow is characterized by a Doppler wave which changes according to respiratory and cardiac activity and the hydration state.
(a). Longitudinal scan (in plane). Carotid artery (yellow arrow); internal jugular vein (white arrow); needle tip (red arrow); sternocleidomastoid muscle (green arrow) which appears edematous due to previous anesthetic administration (b). Transversal scan (out of plane). Carotid artery (yellow arrow); internal jugular vein (white arrow)
Transversal scan. Carotid artery (yellow arrow); internal jugular vein (white arrow); sternal part of sternocleidomastoid muscle (green arrow); clavicle part of sternocleidomastoid muscle (red arrow); jugular dimple (white star); trachea (orange arrow)
Pulsed Doppler image. Carotid artery (yellow star); internal jugular vein (white star); arterial flow (yellow arrow); venous flow (white arrow)
57.6 Preoperative Evaluation of the Patient
Before central venous catheter placement, even if ultrasounds are not used, it is important to perform an ultrasonography evaluation of the vein which should be cannulated [14 – 16]. Echo color Doppler allows clinicians to rule out the presence of thrombosis (Fig. 57.5) and possible vascular injury due to previous catheter placement (Fig. 57.6), as well as to evaluate anatomical characteristics of the vein (Fig. 57.7). Before the procedure, chest evaluation should be performed as well to confirm the presence of “pleural sliding” (Fig. 57.8). Indeed, after central venous catheter placement, it will be possible to rule out the presence of pneumothorax, a possible complication of the technique which is characterized by “pleural sliding” loss in real-time ultrasonography .
Internal jugular vein thrombosis. Carotid artery (yellow star); internal jugular vein (white star)
Vein thrombosis secondary to central venous catheter placement (green star)
Anatomical variation of internal jugular vein (white star); carotid artery (yellow star)
B-mode illustrating the pleural sliding (white arrows). The pneumothorax US pattern is characterized by pleural sliding loss which becomes evident in real-time US
57.7 Ultrasonography Techniques
There are two different techniques of ultrasound-guided central venous catheter placement [14–16]:
“Static” ultrasound-guided technique
“Dynamic” or “real-time” ultrasound-guided technique
In the first one, ultrasonography is used only to identify the vein to be cannulated and confirm the absence of thrombosis. The exact region should be marked with an indelible pen and the procedure is performed without ultrasounds [14–16].
In the “dynamic” ultrasound-guided technique, the probe is used during all the procedure in order to follow the needle trajectory in real time (clip) [14–16]. This technique has been shown to be superior to the first one . In the “dynamic” ultrasound-guided technique, it is possible to keep the needle free (ultrasound assisted procedure) or to allocate the needle in a system which does not allow to change its direction or angulation (ultrasound-guided procedure) [14–16]. According to our experience, the ultrasound assisted procedure should be preferred because it allows clinicians to correct needle direction and inclination, if necessary.