The use of arterial catheters (A-lines) to monitor blood pressure in septic shock isn’t well supported by evidence. There’s never been an RCT showing that A-lines improve outcomes. It remains debated whether invasive blood pressure monitoring is even needed.1
Nonetheless, most clinicians believe that some patients benefit from continuous, invasive arterial BP monitoring (especially those on higher doses of vasopressors). The issue then becomes the best site to insert the A-line. Traditionally, the first-line choice has been the radial artery.
Implicit assumptions versus new concepts
Historically, emphasis has rested on the distinction between noninvasive versus invasive BP (e.g., cuff pressure vs. radial arterial pressure). Attention focused on whether noninvasive oscillometric BP monitoring is adequate. Meanwhile, it has been assumed that all invasive BP measurement sites are created equal:2
However, the anesthesiology literature suggests that this model is wrong. Specifically, the radial and femoral BP diverge among the sickest patients (e.g. patients undergoing cardiopulmonary bypass or liver transplantation).3 In such patients, the radial arterial pressure may be misleadingly low, with a potential to instigate excessive vasopressor and fluid administration.
We routinely do femoral A lines plus radial A-lines during liver transplants. Interesting to observe the discrepancies between the two measurements as vasopressor doses increase.
— Vivek Moitra, MD (@vmoitra) August 2, 2018
How reliable is the radial artery pressure in septic shock? This post will attempt to evaluate the evidence.
Relative accuracy of BP measurement at different sites: Primary literature
Noninvasive BP cuff vs. radial arterial catheter
Riley 2017 may be the best study to directly investigate this question.4 These authors prospectively compared noninvasive versus radial arterial pressure monitoring among 31 ICU patients with septic shock. All patients were on vasopressors, with about a quarter of the patients on multiple vasopressors. Using the radial arterial pressure as the gold standard, noninvasive BP had a bias of -2.5 mm and a 95% confidence interval ranging from -9 mm to +14 mm.
Ribezzo 2013 performed a similar prospective study comparing noninvasive versus invasive radial BP.5 This study utilized a mixed population of 50 ICU patients, of whom only 26% had septic shock. Using radial arterial pressure as the gold standard, noninvasive BP had a bias of -5mm with a 95% confidence interval between -19mm and +9mm:
This data may seem to be incompatible with Riley’s results above. However, most of the discordance occurs among patients with normal or elevated blood pressure. If we ignore patients with a MAP > 85mm and align this data with Riley’s data, the results are consistent:
Radial versus femoral arterial catheters
Dorman 1998 prospectively compared invasive radial vs. femoral artery pressures in 14 postoperative septic patients receiving at least 5 mcg/min norepinephrine (although most received much higher doses, with a mean rate of 86 mcg/min).6 On average, femoral artery MAP was 15 mm higher than that obtained from the radial artery! Femoral A-line insertion often prompted substantial reductions in vasopressor dose after concluding that the radial A-line pressure was underestimating central arterial pressures.
Mignini 2006 prospectively compared radial and femoral A-line pressures among 55 patients in a medical-surgical ICU.7 Most patients (40/55) were receiving significant vasopressor infusions (defined as >10 mcg/kg/min of dopamine or >0.1 ug/kg/min of norepinephrine/epinephrine). Using femoral MAP as the gold standard, the radial MAP was on average 3 mm lower, with a 95% confidence interval ranging between -11 mm to +5 mm. This study may not be perfectly generalizable to septic shock since these patients had a variety of different diagnoses (with only 17/55 patients receiving norepinephrine).
Compton 2008 prospectively compared radial and femoral A-line pressures among 25 medical ICU patients (19/25 of whom were admitted due to respiratory failure).8 Nearly all patients were on vasopressors, most often norepinephrine with an average dose of 0.3 mcg/kg/min. Compared to the femoral MAP, the radial MAP was on average 3 mm lower, with a 95% confidence interval ranging between -17 mm to +11 mm:
Galluccio 2009 prospectively compared radial and femoral A-line pressures among 24 patients in a mixed medical-surgical ICU.9 Most patients (62%) had septic shock. All patients were on vasopressors, most commonly norepinephrine (79%) with a mean dose of 0.26 ug/kg/min. Compared to the femoral MAP, the radial MAP was on average 4 mm lower, with a 95% confidence interval ranging between -12 mm to +4 mm. However, this confidence interval isn’t entirely accurate since the distribution of blood pressure deviations appears skewed (the radial BP is never higher than the femoral BP):
Kim 2013 prospectively compared radial and femoral A-line pressures among 37 patients with septic shock who were initially receiving >0.1 ug/kg/min norepinephrine.10 Compared to the femoral MAP, the radial MAP was on average 5 mm lower, with a 95% confidence interval ranging between -17 mm to +7 mm. Differences tended to decrease as patients were weaned off norepinephrine.
Don't trust radial arterial BP on high dose vasopressors. Femoral art line allows for marked dose reductionhttps://t.co/2cX1Zz6aVe pic.twitter.com/JujrA5TpUz
— Ognjen Gajic (@ogi_gajic) August 28, 2017
Traditional assumptions vs. evidence
To summarize the above studies:
- Cuff vs. radial A-line: The 95% confidence interval of the MAP measured using an oscollometric noninvasive BP cuff is roughly +/- 12 mm when compared to a radial A-line.
- Radial A-line vs. femoral A-line: The 95% confidence interval of the MAP measured with a radial A-line appears to range between ~5 mm higher and ~15 mm lower that measured with a femoral artery A-line. Among sicker patients on higher vasopressor doses, correlation may be worse.
Overall, the correlation between cuff and radial artery MAP isn’t hugely different from that observed between radial and femoral artery A-line pressures (figure above; in both cases, the 95% confidence interval spans ~20 mmHg). This seems to dispel the traditional belief that all invasive BP sites are created equal:
This creates a newer schema, with a focus on measurement of central arterial blood pressure (the pressure in the aorta and great vessels):
Evidence vs. physiology: Will the real blood pressure please stand up?
This uncovers a problem in how we’ve been measuring blood pressure for decades:
- Most evidence and experience about blood pressure involves radial pressures. Therefore, one could argue that to be consistent with this historical evidence basis, we should continue using radial pressures.
- From a physiologic standpoint, central pressures ought to be better. Central pressures best describe perfusion of critical visceral organs (heart, kidneys, etc.). Alternatively, the blood pressure in the hand is unimportant.
Ideally, RCTs would be performed to determine the impact of using femoral vs. radial pressures. Ideally, landmark studies on BP targets in septic shock should be replicated using central BP targets. Lacking such evidence, most writers assume that central arterial pressures are preferred. I agree with this assumption, but it’s worth noting that it is an assumption.
Evidence-based approach to invasive Bp monitoring?
The first question is who needs invasive BP monitoring. There’s little evidence about this. Many patients don’t seem to require invasive BP monitoring (patients who are clinically improving and perfusing well on low-dose vasopressor support). Such patients certainly require meticulous ICU-level monitoring, but this can be achieved noninvasively. If the patient is responding to therapy and improving clinically, then there is little rationale to add invasive BP monitoring.
Invasive BP monitoring is reasonable for patients who are responding poorly to therapy or requiring higher-dose vasopressors. In this situation, a femoral arterial catheter seems more logical for several reasons:
- If a cuff pressure is deemed insufficiently accurate, then a radial A-line is also insufficiently accurate. A radial A-line might not be dramatically superior to the cuff pressure in this situation. If definitive BP monitoring is needed for a septic patient on high-dose vasopressors, central arterial pressure is ideal.
- Femoral lines can generally be inserted faster among critically ill patients with hypotension and vasoconstriction. Alternatively, it’s possible to waste an enormous amount of time fiddling around with radial A-lines, delaying other critical interventions.
- Femoral arterial lines are more reliable over time and less likely to suddenly quit working.11
- With good technique and ultrasonography, femoral A-lines have a similar safety profile compared with a radial A-lines (with one possible exception being coagulopathy: femoral A-lines do carry a risk of retroperitoneal hemorrhage, whereas the radial site is easily compressible).
Love it. My 2 cents for fem art lines over radial
1. Central BP more accurate in really sick
2. Bigger target for Ultrasoud
3. Less painful than radial
4. Safer during cardiac arrest
5. Sure for VA ECMO (if you do that type of thing)— Haney Mallemat (@CriticalCareNow) August 2, 2018
In short, if you’re going to perform a procedure to measure the arterial pressure, you might as well perform the definitive procedure. The main drawback of a femoral arterial catheter may be infection, but septic patients will often improve within a few days allowing arterial line removal.
Fem A lines are the only good option in shock. Radial line BP unreliable, often underestimate central bp (up to 30% – will dig up reference). Dangerous. Seen a few pts almost killed by over-vasoconstriction. Levo dropped by >70% when fem line placed. Radials only ok for gases.
— Philippe Rola (@ThinkingCC) August 3, 2018
A few tips on safe placement of a femoral A-line
Intuitively it might seem that the femoral arterial line is a more invasive procedure that should be associated with greater complications. However, available evidence suggests that the femoral A-line is a safe procedure, with a similar complication rate compared to radial A-lines.12 13 14 Furthermore, meticulous placement with full sterility and ultrasound guidance makes the femoral A-line procedure safer today than it has been historically.15 Instead of running away from this procedure, we need to learn how to perform it safely.
Based on its ease of cannulation and low record of complications, the femoral artery has been called the vessel of choice for arterial access… some hospitals use femoral arterial lines almost exclusively -Roberts & Hedges’ 6th edition.
Negative example: How to damage the femoral artery
My guess as to how femoral arteries get badly damaged is as follows:
- The needle is inserted at a sharp angle, causing it to back-end through the vessel.
- The wire is forcibly advanced, causing it to kink several times within the soft tissue behind the vessel.
- Eventually, the wire must be removed. However, the kinked end of the wire prevents it from being smoothly withdrawn through the artery. Instead, the kinked wire lacerates both walls of the artery as it is dragged through them.
Complications may be minimized with some basic safeguards:
1) Positioning
- External rotation of the legs (“frog-leg positioning”) is often helpful.
- If there is a pannus it must be retracted (typically with tape) until the skin is pulled taut. If pannus retraction cannot be achieved, consider placing axillary arterial line (more on this below).
2) Use ultrasonography to identify the common femoral artery
The catheter should be placed within the common femoral artery for several reasons:
- It’s a larger target, so it’s easier to cannulate and wire successfully.
- As a larger artery, it’s less likely for it to become occluded if there is some minor trauma.
- A common femoral puncture allows the A-line to be converted into a site for intra-aortic balloon pump, ECMO catheterization, or interventional radiology procedures.16
Ultrasound should be used to identify the location where the common femoral artery splits into the superficial and deep femoral arteries. The femoral arterial catheter should be placed a few centimeters proximal to this split.17
3) Try to center the needle on the vessel
Using ultrasound guidance, insert the needle so that it punctures the center of the artery. If the needle punctures the edge of the artery this will produce blood return, but it may be difficult to advance the wire (the wire may impinge on the arterial side-wall). Your goal with cannulation isn’t merely to get the needle somewhere inside the artery – the goal is to get the needle into the center of the artery so that the wire threads easily.
4) Don’t insert the wire unless blood is spurting out of the needle
If the needle is positioned within the artery lumen, blood should be spurting out. If blood stops spurting, that means the needle has slipped into a suboptimal position (either outside vessel or hugging the vessel wall). Don’t insert the wire if blood isn’t gushing out. Stop, take a deep breath, evaluate needle position with ultrasound, and make small adjustments to the needle until blood starts spurting out again – then insert the wire.
5) Don’t force the wire
If the wire doesn’t seem to be advancing smoothly, don’t force it. This usually means that the wire is suboptimally placed. Forcing the wire will often cause the wire to kink – which risks vascular damage and also requires opening a new line kit. It’s safer and faster to remove the wire and try again.
Parting shot: The ultrasound-guided axillary arterial line?
Another nice option here is the ultrasound-guided axillary arterial catheter. This basically involves ultrasound-guided placement of a long catheter (use a femoral A-line kit) into the axillary artery close to the armpit.18 19 This is essentially “the femoral A-line of the arm,” with many of the same advantages as a femoral A-line:
- Placement of an axillary A-line is generally faster and easier than a radial arterial line.
- A long catheter placed via the axillary artery will lie in the subclavian artery and thus transduce central arterial pressure.20
The axillary A-line has some advantages compared to the femoral A-line. The greatest advantage is probably that an axillary A-line is easier to insert in patients with supermorbid obesity – if the patient’s hand is positioned behind their head, this will generally open up an ultrasonographic window allowing axillary A-line placement. One potential concern is that air bubbles could theoretically cause cerebral embolism, but this may be avoided by preferential use of the left axillary artery and flushing the A-line slowly.
Where should the axillary A-line fit within this rubric? It’s hard to say. There isn’t an enormous volume of evidence with axillary A-lines, which makes it difficult to clarify their precise role. Overall the axillary site seems like an excellent option that might become more widely utilized in the future.
- Historically BP monitoring has focused on the difference between noninvasive cuff monitoring vs. invasive BP monitoring, with the assumption that any site of invasive monitoring would be equivalent.
- Accumulating evidence shows that the radial A-line may inaccurately estimate central MAP in extremely sick patients. As such, a radial A-line may not represent a huge benefit over a noninvasive brachial BP cuff.
- Femoral A-lines offer several advantages in critical illness: accurate measurement of central BP, greater ease of insertion, and more reliable functioning. The femoral site may be preferable in most patients.
- Ultrasound-guided axillary A-lines are an emerging option which may allow safe insertion and accurate measurement of central blood pressure.
Related
- EMCrit Arterial Lines: Part 1 & Part 2
- The Dirty Double (PulmCrit)
- The anatomy of femoral vascular access (Taming the SRU)
Acknowledgement: Thanks to Dr. Gilman Allen for thoughtful comments on this post.
- PulmCrit wee: Why I like central lines for GI bleed resuscitation - March 13, 2024
- PulmCrit wee: Polypharmacy in the ICU – when in doubt, deprescribe - January 30, 2024
- PulmCrit hot take: VAP prophylaxis (PROPHY-VAP trial) - January 22, 2024
Great write up, makes sense that vasoconstriction caused by high dose pressors would affect peripheral art lines. Question about the proposed axillary line. It is also mentioned in anesthesia texts as being reliable and easy to insert, but how would you instruct a novice about avoiding median and ulnar nerves, which are superficial to the axillary artery? They are hard to identify, sometimes even to those of us regularly doing regional anesthesia, and during out of plane approach they would be easy to injure. Thanks
So if the radial is lower, and a good many patients have been maximized with radial a-lines to a goal MAP of 65mmHg in studies, do we need have pause if we are all in on femoral lines and titrate to 65mmHg?
This is an evidence-free zone as discussed in the blog. I think it makes more sense to use a central pressure as this is the pressure perfusing organs I care about (eg kidney).
I wonder about the risk of damaging nerves as well, particularly on the dominant hand. I’ve heard from anesthesia colleagues that, preoperatively, it’s not infrequent to have the patient tell you about tingling in the arm that leads them to stop or redirect, but in the ICU we’re unlikely to get this feedback. Any experience with anterior approach axillary art lines? Seems this would potentially be safer in regards to nerve injury.
Well there is some risk of nerve injury with either radial or axillary approaches. It’s hard to say exactly which site is safer. With good technique, I think the risk of nerve injury is low at either site.
Take a look at the supplemental video in Htet (ref #18). They had zero nerve injuries in a series of 159 cannulations using ultrasound to avoid the nerve. Agree this isn’t necessarily a procedure for the novice operator.
Interesting, in our facility, we are very gun shy about fem lines because of 2 main factors.
-supposedly MASSIVE increase in infection risk,
-people get scared and don’t have a good answer or conflicting answers on whether or not the pt can bend at the waist with one in.
Great article! What about vascular and ischemic complications when cannulating the axillary artery? Serms too risk to jeopardize the whole arm for an A line…
No mention of leveling, zeroing or most importantly, square waveform analysis when comparing the various types of A-lines. Square waveform analysis is imperative at regular intervals to validate integrity of transducer systems which often deteriorate over time. After 35 years of bedside ICU nursing, agree that radial art-lines often underestimate pressure readings as acuity and vasoactive medication dosing escalates.
NIBP devices have their pitfalls as well. Positioning, cuff size, patient movement, hemodynamic changes during cycling of cuff, proprietary algorithms etc. OR and ICU and transport All different environments with various influences on accuracy of monitored physiologic variables. ….
And using the NIBP – remember that a lot of information can come from a venous bloodgas and ETCO2.
Not an indication for a A line.
The radial and femoral MAP can differ in both vasoconstricted (as seen in anesthesia and ICU) and vasodilatory states (as often seen in anesthesia). A consultant I worked with used to squeeze the patients’ fist (on the side with the radial a-line) to show that the radial BP approached the femoral BP as the resistance increased.
I am a huge fan of the axillary arterial line when a radial fails or if the patient is sicker and I anticipate the patient needing the line for a longer period of time. Too many complications from the femoral arterial line; I have seen numerous pseudoaneurysms, one or two amputated legs (and another one that was threatened from a line I put in in residency), and even the death of one or two patients from RP hematomas from femoral arterial line placements. The same cannot be said of axillary lines. A study you did not include here that I… Read more »
Outstanding write up. I would make one small change. I think that central vs peripheral pressure is not really the issue, but large vs small vessel. The likely reason that radial arterial lines underestimate the BP in pts requiring high dose vasopressors is that the vessel is constricting around the line. This is less likely to happen if the line is placed in a large vessel. Personally, I prefer axillary to femoral, as too many patients are obese.
Great post Josh. I’d also throw in a plug for the potential value of brachial arterial lines. They’re usually very easy to place with ultrasound and a micropuncture kit, and also seem to produce a pressure more closely approximating central pressure than the radial. The complication rate may be slightly higher than radials, but the following review (the largest yet that I’ve found) suggests it to be quite low at 0.15%.
http://anesthesiology.pubs.asahq.org/article.aspx?articleid=2618830
To improve safety with femoral arterial lines, I have adopted the approach our IR/Interventional Cardiology/Vascular Surgery colleagues use: Initial common femoral artery access with a micropuncture set, then upsizing to the final catheter. I use a 4 french 10 cm sheath, typically as the final device and have had great success with this. Using this approach, there is less hematoma formation as the dilator and sheath are passed together (similar to a MAC or Cordis) rather than a separate pass for the dilator. Using the micropuncture needle as first access is felt to decrease the risk of pseudoaneurysms and nerve… Read more »
william
a standard groin aline is 20 gauge and there is no dilator. this is smaller than your 4F sheath. the a line kit needles are also 20gauge and similar to 23 gauge micropuncture set needles
I just use the micropuncture introducer as my fem aline. I have used the arrow fem kit after getting access with micropuncture but find that the introducer is a reliable and long lasting solution.
Doc, now I am just a nurse, but why not opt for brachial intra-arterial monitoring over radial? The few studies I have found are quite promising. I’ve lost BP accuracy in radial lines numerous times. Typically very sick patients who are either severely volume depleted, or have profound vasoplegia and are on multiple vasopressors (especially vasopressin or neo, which I assume is due to both being such strong alpha agonists). This can lead to improper titration of medications which has happened to me in the past. Moreover, these lines are often placed exactly where wrist restraints are applied. When patients… Read more »