Factors involved in site selection for arterial cannulation

This chapter is vaguely relevant to Section G7(iii)  of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the invasive and non-invasive measurement of blood pressure, including
limitations and potential sources of error". It is also vaguely related to Section X(ii), "describe the anatomy relevant to the insertion of an arterial line into a brachial, axillary, posterior tibial, dorsalis pedis, radial or femoral artery". None of the Part I questions have so far tested the candidate's ability to wisely select a correct insertion site, perhaps recognising that there are many different ways to skin a cat. However, Question 11.1 from the first Fellowship paper of 2021 asked for advantages and disadvantages of radial brachial and femoral arterial sites. 

There is no specific literature to guide decisionmaking in this area. Most of the material presented here is derived from "Chapter 3: Arterial line placement and care" from  Irwin and Rippe's Intensive Care Medicine (7th Ed), by  Lee-Llacer and Seneff. Site selection is guided as much by the effects we don't want as by the desired effects, and valuable reading about site-specific complications should include the excellent reviews by (Scheer et al, 2002; Frank et al,1983). . The chapter by Abide & Meissen (2021) from Interventional Critical Care is also superb.

In short:

However, everyone still ends up getting a radial arterial line. This is because the balance of risk and benefit favours the radial artery as the first choice for arterial catheterisation. Each site has its advantages and disadvantages, with respect to ease of nursing access, patient comfort, the accuracy of measurement and difficulty of cannulation; and rates of serious complications are similar for all sites. In the neonatal setting, the umbilical artery is a unique access point that also needs to be mentioned. Theoretically, any peripheral artery can be accessed for arterial pressure measurement.

Advantages, disadvantages and complications of each site

In summary:

Site	Radial	Brachial	Femoral
Advantages	Common, i.e. well practiced Low complication rate Easily accessible even during surgery Generous collateral circulation Easily compressed No collateral damage (not surrounded by delicate structures) Does not restrict patient movement	Large and proximal: more accurately reflective of central arterial pressure Much larger than the radial, and therefore easier to access and palpate Easily compressible	Largest and most proximal Most accurate reading of central arterial pressure (least affected by pulse wave amplification and peripheral vascular disease) Easily palpated  Distal enough to be accessible even during CPR
Disadvantages	Distal; pulse amplification makes the systolic and diastolic less accurate A highly mobile site; can easily kink or dislodge Small; may be difficult to access Subject to considerable anatomical variation Complications can be disastrous, especially if they result in a disability of the dominant hand	Difficult to access during surgery or CPR End-artery; i.e. no collateral circulation (theoretical increase in the risk of limb ischaemia) Right next to the median nerve More likely to kink and occlude with patient arm movement	Potential for retroperitoneal haematoma, which is not compressible Potential for through-and-through puncture of the femoral vein, resulting in an AV fistula Theoretically, a higher risk of infection Makes it challenging to mobilise patients
Complications	Cerebral embolization Peripheral neuropathy High risk of thrombotic complications	Median nerve damage Cerebral embolization	Retroperitoneal haematoma Abdominal visceral injury Arteriovenous fistula
	General complications for all sites Pain and swelling Accidental dislodgement Thrombosis Embolization Haematoma Haemorrhage Limb ischemia Catheter-related infection including bacteremia Iatrogenic blood loss from frequent sampling Pseudoaneurysm Heparin-induced thrombocytopenia (if heparin is used in the flush bag)

Scheer et al (2002) were able to compute the mean incidence of serious complications associated with arterial line insertion, for the two most common sites of insertion:

Site	Permanent ischaemic damage	Temporary occlusion	Sepsis	Local infection	Pseudoaneurysm
Radial	0.09%	19.7%	0.13%	0.72%	0.09%
Femoral	0.18%	1.45%	0.44%	0.78%	0.3%

Thus, though the radial artery has a far higher rate of thrombotic occlusion, the risk of serious ischaemic complications is decreased because of collateral circulation. The risk of local infection is similar for both sites, but the risk of catheter-related sepsis was significantly higher for the femoral insertion site.

Risk factors for complications from arterial cannulation include the following:

• Large tapered cannulas (> 20 gauge except at the large artery sites)
• Hypotension
• Coagulopathy
• Low cardiac output
• Multiple puncture attempts
• Use of vasopressors
• Atherosclerosis
• Hypercoagulable state
• Placement by surgical cutdown
• Site inflammation
• Intermittent flushing system
• Bactaeremia

Thrombotic occlusion is by far the most common complication at all sites. The radial artery and the dorsalis pedis artery as most prone to thrombotic complications. Scheer et al (2002) found that the rate of thrombosis and temporary occlusion was as high as 19.7% with radial arterial lines, as compared to 1.45% with femoral lines. Thrombosis can also occur following the removal of the line. The vessel recanalises after approximately 3 weeks. This temporary occlusion is usually without serious sequelae, owing to the generous collateral circulation at these sites; less than 1% of cases require surgical intervention. Most patients who develop clinically significant ischemia have some associated contributing cause, such as high-dose vasopressor therapy or severe peripheral vascular disease. The use of a heparinised flush solution is no longer recommended, as it does not appear to be protective against thrombosis and is associated with a risk of heparin-induced thrombocytopenia.

If clinically significant ischaemia is observed, immediate removal of the catheter will minimise the ensuing complications. If ischaemia persists following catheter removal, the following options can be explored:

• Anticoagulation with heparin (with conventional APTT targets)
• Thrombolysis (systemic or regional)
• Embolectomy
• Surgical bypass
• Cervical sympathetic blockade

Cerebral embolisation is a known complication of flushing the arterial line. Retrograde passage of small bubbles of gas into the arterial circulation is possible, considering that the pressure transducer is coupled to a bag with 300mmHg of pressure (i.e. enough to defeat systemic arterial pressure). 

Risk factors for clinically significant cerebral gas embolism include the following:

• Small patient (shorter vascular tree)
• Frequent flushes
• Position (air travels up towards the head in an upright patient)
• Injection site (brachial and radial lines are most at risk)

Iatrogenic blood loss is a significant risk of convenient arterial access. Generally, 2-3ml of blood is discarded for each arterial blood gas sample, and daily blood sampling for biochemistry can contribute a further 20-30ml. Smoller et al (1986) found that the total blood lost to phlebotomy in non-ICU patients was approximately 175ml over the whole period of their hospital stay. However in the ICU this increased to 300ml for patients without arterial lines. When the patients had an arterial line, this blood loss rate increased to over 900 ml, with implications for their transfusion requirements.