Arterial Blood Pressure Monitoring

Introduction

• Arterial blood pressure monitoring involves the placement of an indwelling catheter in an artery, allowing for continuous monitoring of arterial blood pressure. This catheter also permits vascular access for arterial blood sampling.
• Invasive arterial blood pressure monitoring is useful to anesthesia providers during procedures or surgeries where there is a risk of sudden blood loss or periods of hypotension and hypertension. The continuous nature of arterial blood pressure measurements allows for more rapid intervention and vasopressor titration in patients at higher risk of damage due to hypotension.¹
• The radial, brachial, and femoral arteries are the most commonly utilized vessels for monitoring. Of these, the radial is the most often used, partially because of the collateral ulnar blood supply to the hand, which helps prevent ischemia. The femoral artery is preferred in low-flow states due to its easy access. However, cannulation of the femoral artery should be done distal to the inguinal ligament to minimize the risk of hemorrhage into the pelvis or retroperitoneum.^1,2

Insertion

• When cannulating the radial artery, place the hand in a dorsiflexed position and attempt insertion as distally as possible. If the attempt is unsuccessful, one can move proximally to try again.²
• The umbilical artery is a convenient site for arterial line placement in newborns.¹
• Different techniques can be used to insert an arterial catheter.² A guidewire is commonly used to aid in arterial catheter insertion.
  • Separate guidewire approach:
    • The Seldinger technique involves puncturing the artery with a hollow needle, inserting a guidewire through the needle, removing the needle, and then inserting a catheter over the guidewire.
    • The modified Seldinger technique differs in that the artery is first cannulated with an angiocatheter over a needle. The needle is then removed, and a guidewire is inserted through the angiocatheter. This is followed by the insertion of the arterial catheter over the guidewire.
    • The key distinction between the two is that the modified Seldinger technique uses an angiocatheter to maintain vessel access after needle removal, whereas the classic Seldinger technique relies on the needle for guidewire passage.
  • Integrated guidewire approach: This approach utilizes a kit that includes a guidewire inseparable from the catheter kit. It is similar to the modified Seldinger technique.
  • Ultrasound-guided approach: In this approach, the arterial catheter is placed under ultrasound guidance.
  • A 2020 meta-analysis revealed that ultrasound-guided catheter insertion was associated with a higher incidence of first-attempt success, fewer mean attempts, a shorter mean time to success, and a reduced risk of hematoma compared to traditional palpation techniques.³
• After placement, the arterial catheter is connected to a tubing filled with crystalloid fluid, allowing the force generated by arterial blood flow to be transferred to a pressure transducer, which is typically attached to a nearby intravenous pole. The transducer converts changes in pressure to voltage changes, which are displayed in the arterial pressure trace.¹
• The tubing is attached to a pressurized bag of fluids that continuously flushes the cannula at a slow rate to maintain patency of the line.¹
• For this measuring system to work properly, the fluid inside the system must be able to detect and respond to the range of frequencies contained in the arterial pressure pulse wave. Additionally, the transducer must be zeroed at the level of the vessel of interest before being connected to the patient to ensure accurate measurements. Frequently, the vessel of interest is the aortic root, and the level of the right atrium is used as the reference point for zeroing. However, during brain surgery, the circle of Willis is often the vessel of interest, and the transducer must be zeroed at the base of the brain. This is depicted below in Figure 1.^1,2
• Zeroing involves opening the stopcock towards the atmosphere while activating the zeroing function on the monitor. Once the pressure tracing reads 0, the stopcock is closed to the atmosphere. This is depicted below in Figure 1.^1,2

• A zero line connected to the transducer can be used instead of moving the height of the transducer. The line is filled entirely with fluid, and the free end is attached to the patient at the height of the vessel of interest. Again, a zeroing maneuver must be completed before use. This maneuver involves opening the stopcock towards the zero line while activating the zero function on the monitor. The hydrostatic pressure is set to 0, and the stopcock is closed.²

Arterial Line Readings

• The arterial pressure waveform is illustrated in Figure 2 below. The arterial pulse wave appears 160-180 milliseconds after the R wave, which indicates the start of systole in an ECG. During this delay, the electrical depolarization signal spreads through the left ventricle, isovolumetric contraction occurs, the aortic valve opens, and blood flows through the aorta and into the peripheral vessels, at which point the pressure pulse wave is generated. The peak corresponds to the systolic arterial pressure (SAP). The Dicrotic notch approximates the time of aortic valve closure; it can be absent in cases of severe aortic stenosis or regurgitation. As blood flows into the peripheral vessels, the notch becomes slurred and softened as waves reflected off the aortic valve take longer to arrive in the distal circulation. The nadir of the waveform represents the diastolic arterial pressure (DAP).^1,4

• Slurring of the slope of the systolic upstroke can indicate aortic stenosis, as the acceleration of blood flow is decreased.⁴
• The SAP is proportional to the work done by the left ventricle to generate an adequate stroke volume. Inotropy, rate, and peripheral resistance affect SAP, but it is most dependent on stroke volume. Decreases in SAP may help to identify cardiogenic or hypovolemic shock. Furthermore, SAP is used to calculate the shock index, which is clinically valuable to detect hypovolemic shock, predict prognosis in patients with trauma, and determine if fluid resuscitation should be initiated.
• As blood flows peripherally, the cross-sectional area of the vessel decreases and resistance increases. This can result in pressure waves being reflected, producing a second systolic upstroke, called the anacrotic notch. As one continues to move distally, reflected waves become more prominent and move further into systole. Thus, there are changes in the systolic pressure as one moves further from the aorta because of the accumulation of reflected pressure waves. This is depicted below in Figure 3.⁴

• DAP correlates with vascular tone. Thus, systemic vasodilation can result in a low DAP. DAP is not currently used in the diagnostic criteria for septic shock. However, the loss of vascular tone (detected by a drop in DAP) can aid in initiating vasopressor therapies early.⁵
• MAP readings are also continuously displayed by arterial lines, calculated from the area under the pressure curve. Target MAPs are individualized to the patient and their clinical course, but a MAP of 65-70 mmHg is a frequently used target to ensure adequate organ perfusion.^4,5
• Small devices attached to arterial cannulas measure heart rate and estimate stroke volume. Accounting for the distension of an artery is crucial to correctly estimating stroke volume. During systole, inflow into the artery is less than outflow because a portion of the blood flowing into the artery is stored in the expanding compliant vessel. During diastole, inflow into the artery is zero, and outflow is enhanced by the contracting vessel. Based on the ΔV = ΔP/R flow equation, devices use data from the systolic and diastolic phases in complex algorithms to calculate the stroke volume. The monitor then calculates the product of heart rate and stroke volume to estimate the cardiac output (Figure 4).¹

Inaccurate Readings

• Optimal quality of the arterial waveform is crucial for accurate blood pressure measurements. The performance characteristics of the measurement system are determined by several factors including¹
  • Natural frequency of the system (frequency of pressure pulse oscillations)
  • Damping coefficient (decay of the oscillating waveform)
• Two main types of artifacts can result from an inappropriate dynamic response- underdamping and overdamping.²

• To test the damping properties of the system, a fast-flush test or square wave test can be performed. This involves rapidly flushing the system and counting the number of oscillations after the square wave (Figure 5).
  • In a properly functioning system, there are 1.5-2 oscillations after the square wave.
  • In an underdamped system, there are more than 2 oscillations after the square wave
  • In an overdamped system, there are less than 2 oscillations after the square wave.

Clinical Considerations

• Arterial lines should not be inserted in locations where there is infection, thrombosis, active Raynaud’s phenomenon, or vessel anatomy abnormalities.²
• Rapid changes in blood pressure readings must be verified and clinically correlated before intervention. Sudden inappropriately high blood pressure readings may result from a change in the patient’s position without adjusting the transducer’s position. Sudden, inappropriately low blood pressure readings may suggest kinking in the line.¹
• The largest risk posed by arterial line use is the risk of ischemia, often caused by thrombosis. Cases of hemorrhage, skin necrosis, paresthesia, and infection have also been documented.¹
• The incidence of arterial catheter-related bloodstream infections for arterial lines is low. However, data indicate that femoral access sites have a higher risk of the above-mentioned complications compared to radial sites.⁵
• If air is not “burped” from the fluid bag, keeping the line patent, and/or the tubing is not primed, there is potential for the formation of an air embolism.¹
• Cardiac output calculations rely on the assumption that stroke volume remains constant beat-to-beat. This is not necessarily always the case during irregular heart rhythms such as atrial fibrillation, resulting in inaccuracies. Inaccurate cardiac output calculations may also occur during periods of negative regurgitant flow from arteries back into the left ventricle in patients with aortic insufficiency.¹

Use in Hypotensive Patients

• The use of ultrasound can help to identify the radial artery in hypotensive patients quickly. The artery can be identified from the veins, as the veins are easily compressed. By placing the center of the ultrasound transducer over the visualized radial artery, tissue tenting can often be seen when pressure is applied by the needle and catheter, helping to guide them into place.¹
• The cardiac output calculated by arterial lines is most useful for assessing volume status and responsiveness to fluid resuscitation. When vasopressors and/or inotropes are given, the calculated cardiac output may be less reliable due to alterations in vascular tone that are not accounted for.¹
• Prediction of fluid responsiveness is a cornerstone of resuscitation in critically ill patients. Patients are considered fluid-responsive if both ventricles are preload-responsive, meaning their cardiac output increases in response to a fluid bolus. It is important to note that giving fluid to non-responders may cause harm. As a result, ventricular response to preload can be estimated using PPV in mechanically ventilated patients. The variability in cardiac contractility between patients is what largely affects the degree of fluid responsiveness. Mechanical ventilation induces cyclic changes in preload; during mechanical insufflation, intrathoracic pressure increases, resulting in decreased venous return to the heart. Thus, right ventricle preload decreases, ultimately decreasing right ventricular and left ventricular stroke volume. On expiration, preload and stroke volume return to baseline in patients with preload-responsive ventricles. Thus, high PPV is seen. This is demonstrated in Figure 6. Many arterial line monitors continuously calculate and display PPV. For patients who are entirely mechanically ventilated with no spontaneous breathing or cardiac arrhythmias, a PPV >13% predicts that fluid responsiveness is likely.⁷
• Please see the OA summary on fluid responsiveness for more details. Link