Diagnostics and Therapeutics: Arterial Lines and Invasive Blood Pressure Monitoring

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Blood pressure monitoring is an important but frequently misunderstood cornerstone of emergency medicine. There are many opinions swirling around the complexities of accuracy, logistics, and practice patterns regarding invasive arterial blood pressure (IABP) monitoring and noninvasive blood pressure (NIBP) or cuff measurements. This post will not cover how to insert arterial lines (see this prior Taming the SRU post by Dr Baez), instead we will cover the utility, mechanics, and pitfalls of IABP.

TLDR 

  • NIBPs estimate blood pressure measurements based off proprietary algorithms 

  • The jury is still out on how accurate NIBP is compared to IABP but is less reliable with higher doses of norepinephrine, lower MAP value, higher BMI, and increased patient age.

  • Central arterial lines are more accurate in critically ill patient and fail less often than peripheral lines  

    • Axillary lines may be a good option to obtain central pressures in patients with high BMI 

  • Waveform morphology can help you identify a number of pathologic states 

  • Incorrect transducer placement and zeroing can overestimate MAP 

  • The fast flush test can help you assess if your system is appropriately damped and guide troubleshooting efforts

NIBP vs. IABP

How does a NIBP cuff work? 

These devices are typically called oscillometric, although cardiac waveforms are not technically oscillations. All noninvasive automatic cuffs use proprietary algorithms to analyze the waveform and produce an estimated blood pressure. They first estimate a mean arterial pressure (MAP) based on maximal oscillation waveform then use fixed ratios to estimate systolic and diastolic pressures (1). 

Increased arterial stiffness leads to overestimation of systolic, diastolic and mean arterial BPs. NIBPs are also less accurate with variable and slower heart rates. In patients with atrial fibrillation, the NIBP cuff often underestimates blood pressure when the heart rate is outside of normal ranges (2). Additionally, environmental factors such as movement, noise, vibrations can make these readings less accurate. 

But are they good enough? 

This has been the central question of many research projects with variable outcomes. Unfortunately there is a paucity of high quality evidence to guide our practice as many of these are observational studies.

One such study of 263 patients without hypertensive emergencies treated in a resuscitation unit found that 40% of patients had a MAP difference ≥ 10 mmHg between IABP and NIBP measurements. Nine percent of all patients had a clinically relevant difference in MAP leading to change in management, and found clinically relevant discrepancies were more likely when patients had higher lactate levels. Peripheral artery disease and kidney disease were associated with higher likelihood of MAP differences of ≥ 10 mmHg (3). A published abstract from the same clinical setting evaluated 183 patients with hypertensive emergencies. This study reported that 64% had a >10 mmHg difference between IABP and NIBP, while 38% had a >20 mmHg difference. Seventy-nine patients (43%) had a change in management after arterial catheters were inserted. Their findings suggested measurements were more discordant at higher systolic pressures (4).

A recent observational study was performed to pragmatically assess clinically meaningful differences in BP in a diverse critically ill cohort with shock. These authors assessed 50,000 BP measurements from over 1800 patients. There was agreement (less than or equal to 10%) in the majority of simultaneous noninvasive and arterial line BP measurements across a range of BP and severity of illness. There was a median difference of 6mmHg in MAP which was primarily driven by a lower arterial line diastolic BP and had an unclear overall significance particularly with regard to tissue perfusion. Disagreement was more likely with higher doses of norepinephrine, lower MAP value, higher BMI, increased patient age, and radial arterial line location. Disagreement did not correlate with SOFA score (5).

When I chose the topic of arterial lines, based on my personal experience of the unreliabilities of NIBP in the prehospital setting, I assumed that my takeaway would be that NIBP can’t be trusted and IABP is superior. While it is clearly not that simple, the lack of high quality evidence makes this even more complicated. The evidence we do have is inconsistent but suggests that NIBP is less reliable in patients who are old, very sick, hypertensive, or have a high BMI. My personal critical care Messiah, Josh Farkas, suggests that IABP monitoring is reasonable when patients are not improving with treatment or are requiring higher dose vasopressors. He astutely points out that in this situation “if a cuff pressure is deemed insufficiently accurate, then a radial A-line is also insufficiently accurate.” More to come on location below… 

Disadvantages 

The primary obvious disadvantage to IABP is the more robust training and time required to place an arterial line as opposed to placing a BP cuff. Arterial lines require specialized monitoring and nursing resources which are not available in all hospital settings. Arterial lines are painful for the patient which may in some cases lead to an increase in sedation required, they limit movement and participation in physical therapy, and pose a risk of digital ischemia (5). Interestingly, the frequently taught and accepted principle that arterial blood flow is protective against line associated infections has been called into question as multiple recent studies have found similar, nonsignificant differences in rates of colonization between central and arterial line catheters (6, 7). 

Central vs. peripheral 

Central pressures are transduced from arteries inside the chest or abdomen, most commonly the femoral artery but also includes the subclavian artery. Peripheral arterial measurements can be taken anywhere outside the thorax or abdomen but in practice and research is almost exclusively radial artery cannulation.

Accuracy 

Many studies have examined the accuracy of femoral versus radial arterial blood pressure measurements. While some studies of critically ill patients have found acceptable MAP differences of around 5mmHg (8) others have found more clinically meaningful MAP discrepancies of at least 10 to 15 mmHg (9, 10). In general, radial artery readings in patients with shock likely underestimate central pressure which can lead to increasing vasopressor dosing. Higher doses of vasopressors subsequently leads to even less correlation between central and peripheral measurements (11). 

Failure rates 

An ambispective, observational cohort study from 2012 - 2016 found that femoral arterial lines fail significantly less often than radial arterial lines with an absolute risk reduction of line failure of 20%. Femoral lines only made up 34% of the assessed lines but accounted for only 8.6% of failed lines. These authors reported a number needed to treat (femoral access over radial access) of four to prevent one premature line failure. They also found that when femoral lines did fail, they did so later in the patient’s clinical course. (12) 

Axillary 

While axillary arterial line placement has been uncommon, in the age of facile ultrasound operators it may become a more advantageous and accessible site particularly in patients with high BMIs. Placement is theoretically faster and easier than a radial arterial line. Placement of a long catheter puts the tip in the subclavian artery yielding the advantages of a central pressure. It should be avoided in patients who you suspect will be undergoing cardiac cath.

Benefits Beyond the Number (Waveform Interpretation)

Normal Arterial Waveform

The systolic phase begins with the opening of the aortic valve and represents left ventricular ejection. This is characterized by a rapid increase in pressure up to a peak followed by a rapid decline. Next comes the dicrotic notch which is widely believed to represent the closure of the aortic valve but the peripheral dicrotic notch owes more of its shape to the vascular resistance of peripheral vessels. The deformation in the waveform caused by aortic valve closure is called the incisura and typically is only seen in pressure tracings taken in the proximal aorta. The diastolic phase represents the run off of blood into the peripheral circulation and the shape is affected by the elasticity of the arterial system.

Abnormal Waveforms

Aortic Stenosis waveform

Cardiac output can be calculated from the waveform itself by some monitors. Pulse pressure variation which can suggest fluid responsiveness can also be calculated. Specific wave form morphologies might be diagnostic. For example, in aortic stenosis no matter the force of LV contraction, the pressure rise will not be rapid and the systolic upstroke will be less steep than usual. The dicrotic notch will also be slurred or lost. Conversely steeper upstroke can represent increased contractility. Pulsus paradoxus, waveform amplitude variation with respiratory cycles, can also be visualized on arterial waveform in tamponade. Pulsus alternans, beat to beat waveform amplitude variation, can be visualized in very low cardiac output states. Finally, a rapid decline in the diastolic phase can suggest a low systemic vascular resistance.

Troubleshooting 

Phlebostatic axis (Image courtesy of UNM Emergency Medicine)

Arterial line catheters must be attached to a normal saline bag placed in a pressure bag fixed at 300 mm Hg. Excessively long tubing or soft tubing can affect the accuracy of the measurements. The transducer should then be placed and zeroed at the phlebostatic axis. The phlebostatic axis is defined as the intersection of a vertical line from the 4th intercostal space to the horizontal mid axillary line. Zeroing and/or transducer placement not at the phlebostatic axis leads to significant overestimation of MAP (13).

Damping 

Damping is essentially the shock absorbers in the pressurized system. 

Underdamped System

Overdamping: Essentially more loss of energy, when there is air, kinks, or clot in the catheter system the higher frequency components don’t make it to the sensor and thus don't contribute to the waveform. This leads to an unnaturally smooth wave with a diminished or absent dicrotic notch. Will subsequently underestimate the systolic and overestimate the diastolic while preserving the MAP.

Underdamping: Steep systolic phase with a narrow peak and non-physiological oscillations during the diastolic phase. Can be caused from catheter whip within the vessel or seen in tachydysrhythmias. Leads to overestimation of the systolic and underestimation of the diastolic. MAP is not impacted. 

Fast flush is a way to assess the characteristics of your waveform. Squeezing the fast flush valve exposes the transducer to the 300mmHg in your pressure bag. This gives a box waveform as the pressure rises sharply and plateaus. As you release the valve the pressure drops quickly, in an appropriately damped system you will then see 1-2 short oscillations demonstrating the natural frequency of the system. In an overdamped system there are no oscillations and conversely in an underdamped system there will be more than 2 oscillations. 

Normal Fast Flush

POST BY Bonnie Snyder, MD

Dr. Snyder is a PGY-1 in Emergency Medicine at the University of Cincinnati.

EDITING BY ARTHUR BROADSTOCK, MD

Dr. Broadstock is a Clinical Instructor and Ultrasound Fellow in Emergency Medicine at the University of Cincinnati and an Assistant Editor of TamingtheSRU.

Cite As: Snyder, B. Broadstock, A. Diagnostics and Therapeutics: Arterial Lines and Invasive Blood Pressure Monitoring. TamingtheSRU. 2/19/2023

REFERENCES

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  2. Zhao X, Li J, Huang M, You N, Li J, Li R, Chen S, Liu T, Zeng J, Li X, Jiang W. The effect of heart rate on blood pressure measurement in patients with atrial fibrillation: a cross-sectional study. Hypertens Res. 2022 Jul;45(7):1183-1192. doi: 10.1038/s41440-022-00897-1. Epub 2022 Mar 25. PMID: 35338337.

  3. Keville, M. P., Gelmann, D., Hollis, G., Beher, R., Raffman, A., Tanveer, S., ... & Tran, Q. K. (2021). Arterial or cuff pressure: clinical predictors among patients in shock in a critical care resuscitation unit. The American Journal of Emergency Medicine, 46, 109-115.

  4. Gelmann, D., Tran, Q., Fairchild, M., Alam, Z., Engelbrecht-Wiggans, E., Hart, E., ... & Haase, D. (2021). 280 Arterial Line versus Noninvasive Blood Pressure Monitoring in Hypertensive Emergencies. Annals of Emergency Medicine, 78(4), S113-S114.

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  8. Galluccio, S. T., Finnis, M. E., & Chapman, M. J. (2009). Femoral-radial arterial pressure gradients in critically ill patients. Critical care and resuscitation, 11(1).

  9. Dorman, Todd MD, FCCM;  Breslow, Michael J. MD, FCCM;  Lipsett, Pamela A. MD;  Rosenberg, Jeffrey M. MD, PhD;  Balser, Jeffrey R. MD, PhD;  Almog, Yaniv MD;  Rosenfeld, Brian A. MD, FCCM. Radial artery pressure monitoring underestimates central arterial pressure during vasopressor therapy in critically ill surgical patients. Critical Care Medicine 26(10):p 1646-1649, October 1998. 

  10. Wisanusattra, H., Khwannimit, B. Agreements between mean arterial pressure from radial and femoral artery measurements in refractory shock patients. Sci Rep 12, 8825 (2022). https://doi.org/10.1038/s41598-022-12975-y

  11. Kim, Won Young*; Jun, Jong Hun†; Huh, Jin Won‡; Hong, Sang Bum‡; Lim, Chae-Man‡; Koh, Younsuck‡. Radial to Femoral Arterial Blood Pressure Differences in Septic Shock Patients Receiving High-Dose Norepinephrine Therapy. Shock 40(6):p 527-531, December 2013. | DOI: 10.1097/SHK.0000000000000064 

  12. Greer MR, Carney S, McPheeters RA, Aguiniga P, Rubio S, Lee J. Radial Arterial Lines Have a Higher Failure Rate than Femoral. West J Emerg Med. 2018 Mar;19(2):364-371. doi: 10.5811/westjem.2017.11.34727. Epub 2018 Feb 20. PMID: 29560067; PMCID: PMC5851512.

  13. Jacq G, Gritti K, Carré C, Fleury N, Lang A, Courau-Courtois J, Bedos JP, Legriel S. Modalities of Invasive Arterial Pressure Monitoring in Critically Ill Patients: A Prospective Observational Study. Medicine (Baltimore). 2015 Sep;94(39):e1557. doi: 10.1097/MD.0000000000001557. PMID: 26426625; PMCID: PMC4616871.