The expeditious and safe placement of venous catheters is vital to the practice of acute care medicine. Venous access includes the cannulation of both superficial peripheral and large central veins, depending on the particular clinical scenario. Traditionally, venous access is performed blindly by palpation or with the use of anatomic landmarks. More recently, ultrasound guidance has become standard of care for most central venous access, and using ultrasound for difficult peripheral access may reduce the need for central access.
Obtaining venous access in certain situations may be difficult secondary to patient and operator characteristics. These factors include obesity, intravenous (IV) drug use, history of multiple prior attempts due to chronic illness, severe intravascular volume depletion, venous thrombosis, coagulopathy, operator inexperience, and operator anxiety due to the patient's clinical condition. There are clinical scenarios in which a patient must receive IV access expeditiously such as for the administration of fluids or medications, hemodynamic monitoring, transvenous pacemaker placement or hemodialysis.
Complications of central venous cannulation include multiple failed attempts, arterial bleeding, pneumothorax, loss of the guide wire, and catheter-related bloodstream infections. Recent studies have demonstrated that ultrasound-guided central venous access decreases the number of unsuccessful attempts and the complications when compared to the landmark technique. In addition, ultrasound reduces the time required to obtain venous access, which is critical in ill patients. With the possible exception of subclavian access in certain situations, ultrasound guidance is recommended for placing central venous catheters.
Bedside ultrasound-guided venous access should be performed in the following patients:
- The difficult patient who requires peripheral venous access
- Patients requiring central venous access
Linear Array Probe with a Frequency of 7.5–10.0 MHz
A 7.5- or 10-MHz linear array probe should be used for vascular access. High-frequency probes provide higher-resolution images of superficial structures. Linear probes generate rectangular or square images, which makes it easier to follow the tip of the needle while visualizing the vessel.
The focus should be at the level of the blood vessel in order to maximize the lateral resolution of the image.
The depth should be adjusted so that the vessel of interest takes up approximately three-fourths of the screen. Most of the target vessels will be superficial; therefore, the depth should be decreased appropriately.
The machine should be used on the "vascular" setting for this procedure. This optimizes the resolution and clarity of the image.
The total gain may be increased to brighten the signal returning from the walls of the blood vessel, making it easier to identify. The far gain or time-gain compensation (TGC) may be increased alone in order to enhance the signal returning from deeper vessels and is especially helpful in obese patients.