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Procedural Skills (SLO6)
This was previously featured in an exam
This was previously featured in an exam
Question 55 of 127
You are running a skills based teaching session for medical students and focusing on difficult peripheral venous access. Which of the following is the best method to differentiate between arterial and venous access when performing difficult peripheral venous access?
Answer:
- Identification of structures on ultrasound
- The difference between veins and arteries can be determined by compressibility - veins compress, arteries do not.
- Furthermore, the shape of the vessels is different - arteries tend to be circular in transverse view, whereas veins are often oval.
- Also, their flow dynamics differs, and if colour flow is available, it can be utilised to determine this - with the probe angled so that the flow is marginally towards or away from the probe, the colour flow convention is: blue = away and red = towards (B.A.R.T).
Ultrasound Guided Vascular Access
Procedural Skills (SLO6)
Last Updated: 9th March 2021
Ultrasound aids placement of central lines as well as peripheral lines, both in terms of speed of access and reduction in complications. The aim is twofold:
- To identify relevant anatomy and pathology prior to cannulation
- To use ultrasound to actually guide the process of cannulation
Indications
- In central access (internal jugular), ultrasound should be used at all times, unless time-critical intervention mandates otherwise (e.g. in cardiac arrest).
- In femoral access, ultrasound is a very useful adjunct.
- In peripheral access (e.g. basilic), ultrasound may be used when conventional access fails e.g. in a ill patient who is shut-down, or in an intravenous drug user whose veins are damaged.
Identification of structures
- The difference between veins and arteries can be determined by compressibility - veins compress, arteries do not.
- Furthermore, the shape of the vessels is different - arteries tend to be circular in transverse view, whereas veins are often oval.
- Also, their flow dynamics differs, and if colour flow is available, it can be utilised to determine this - with the probe angled so that the flow is marginally towards or away from the probe, the colour flow convention is: blue = away and red = towards (B.A.R.T).
Clinical anatomy
- Neck anatomy
- The right side of the neck is usually selected to avoid the theoretical risk of damage to the thoracic duct, which lies on the left. The thoracic duct ascends through the mediastinum and enters the left internal jugular vein. While injury to the thoracic duct is unlikely, it can produce a chylothorax.
- Traditionally the approach was defined by finding the apex of the triangle formed by the confluence of the sternal and clavicular parts of the sternocleidomastoid muscle. With ultrasound guided cannulation, the exact approach can be determined by direct visualisation of the anatomy. The approach is still within the triangle, but an optimal site can be readily selected.
- Position the patient in the 30 degrees head-down position, with little neck rotation and no extension. This maximises venous filling, and reduces the risk of air embolism.
- Groin anatomy
- In the groin the location of the femoral vein is straightforward i.e. medial to the femoral artery. Its depth varies and ultrasound location aids speed and efficiency of cannulation.
- The inguinal ligament is an important landmark as keeping immediately below it will avoid attempts at cannulating the great saphenous vein.
- Position the patient in the 5 degree head-up position. This ensures filling of the vein without undue pressure.
- Basilic vein anatomy
- Peripheral cannulation can be attempted at any site but technical expertise in cannulation of the basilic vein is extremely valuable as this vein is almost always patent.
- It tends to lie at least 5 mm below the skin, and may be deeper. It is found in the recess created by the medial border of the biceps muscle.
- The technique is most facilitated by the upper limb being held extended and externally rotated, or by the hands being beneath the patient's head.
Technical considerations
- A linear, high frequency probe should be selected. The machine should be set to a depth of around 5 cm for central access, 4 cm for femoral and 3 cm for basilic cannulation. The time gain compensation (TGC) should be flat. In all cases avoid zoom or the skin surface will not be seen.
- There are two key technical issues when introducing a needle into tissue with the hope of seeing it on the screen:
- Parallelism: This is a function of the physical characteristics of a linear probe. The transducer array will effectively only 'see' a needle if it sits within the tolerances of the lateral and slice thickness resolution. This means that the operator needs to keep the needle within the very narrow slice thickness of the probe, typically 5 mm or less.
- Angle of approach: If the angle of approach of the needle into the tissue is too steep, the reflected sound does not return to the probe, and consequently no image is seen. Angles less than 35 degrees to the skin are needed to see the needle, and the smaller the angle, the better the image.
- Ultrasound can be used in one of two ways when carrying out invasive procedures:
- Static ultrasound assisted: where anatomic structures are identified and an insertion position identified with ultrasound and marked on the skin. The cannulation then proceeds as it would without ultrasound.
- Real-time ultrasound guided: where the ultrasound transducer is placed in a sterile covering and the procedure is performed with simultaneous ultrasound visualisation of the cannulation. In most cases, this is the recommended approach.
- Be cautious in significantly hypovolaemic patients as negative pressures result in flat veins, with increased technical difficulty. However, using ultrasound, central vein cannulation is not difficult in the hypovolaemic patient using the 30 degrees head down patient position.
- All invasive procedures should employ standard sterile techniques to diminish the risk of infection. For venous access using real time ultrasound, a sterile probe cover should always be used.
- Venous cannulation can be carried out using a single or dual operator technique. It is recommended that during training, initially the trainee works with the trainer in the dual operator technique with the ultrasound carried out by the trainer and the cannulation by the trainee.
- Ultrasound guided vascular access requires two views:
- The short axis transverse view allows recognition of the target vein, and differentiation from the artery by virtue of its compressibility. However, this view only allows the cross section of the needle to be visualised by the ultrasound beam and may lead to errors in depth perception. In addition the acoustic shadow can lead to confusion. Therefore, this view is not optimal for real-time guided cannulation.
- The long axis longitudinal view allows the operator to trace the entire path and angle of the needle from the entry site at the skin and into the vessel. Therefore, after obtaining the transverse view, the operator should rotate the probe to view the vein in longitudinal section (and check to see if it is still compressible) before starting cannulation.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
