To modify a diagram from Gardner , which describes the relationship between damping coefficient and resonant frequency on art line waveform interpretation:. So: taking an otherwise adequate arterial line trace and increasing the damping by increasing the elasticity of the tubing will take it into an unuseable overdamped territory. Overdamping results in a slurred waveform with overestimation of the diastolic and underestimation of systolic; however, the MAP value is usually preserved. In contrast, a kinked or clogged art line will see MAP systolic and diastolic all trending towards zero.
The fluid-filled system has a certain natural frequency of resonance. In an ideal system with some sort of perfectly inelastic tubing, the major determinant of this natural frequency is the length of the tubing: the longer the tubing, the lower the natural frequency. The patient's pulse oscillation is usually a fairly low-frequency phenomenon, and as the tubing length increases, the natural frequency approaches the patient's pulse wave frequency.
The system then resonates, amplifying the signal. Thus, the longer the tubing, the more resonance in the system, and consequently the system will be underdamped.
For the same reason, the tube lumen should always be no smaller than 1. To use the same diagram from Gardner:. So, as you can see, increasing the length of this ideal inelastic tubing will also pull the transducer set into a territory where it loses its clinical utility.
However, no tubing is "ideal" - it cannot be a rigid glassy pipe in order for it to remain clinically usable. So: what is the net effect of increasing tubing length in the real world? Yes, length of tubing can increase the resonance, and it can also cause overdamping. The longer the tubing, the lower the natural frequency of the transducer system, so theoretically the more resonance.
On the other hand, an excessively elastic length of tubing will cause damping by absorbing the energy of the pressure wave. In other words, longer tubing increases the damping coefficient and lowers the natural frequency. To put this on Gardner's diagram:. Zeroing and levelling are occasionally used interchangeably, but they are not the same thing. They tend to occur together in the clinical setting, but the terms describe different processes.
The canonical college definition is "a process which confirms that atmospheric pressure results in a zero reading by the measurement system". Atmospheric pressure varies little between the intensivists' eye level and the supine patients' aortic root level, and so strictly speaking the zeroing of an arterial line can take place with the transducer lying anywhere.
Re-zeroing must occasionally take place as both the transducer and the atmospheric pressure will gradually drift away from the calibration point. The canonical college definition is "a process which determines the position on the patient you wish to be considered to be your zero.
The system is conventionally "levelled" at the phlebostatic axis, which is a reference level we have used since probably The phlebostatic axis corresponds roughly with the position of the right atrium and aortic root, and his level has generally been accepted as the ideal reference level for measure the pressure of the blood returning to the heart.
It was therefore adopted as the reference level for CVP measurement. Critical Care Trauma Centre. Arterial Line. Top Stories. Getting the data on homelessness. Hospital News. MRT Week has started: Let's celebrate and recognize this essential role in health care.
Ensuring patients and Essential Care Partners are heard in palliative care. Donor Impact. Miracle Treat Day is Thursday October 28! BP monitoring is mandatory in patients having surgery with anesthesia [ 1 ] and patients with circulatory shock [ 2 ].
In perioperative and intensive care medicine, BP measurements have a significant impact on patient management, especially for the timely and correct identification and treatment of hypotension [ 3 , 4 ]. The choice of the BP monitoring method is important as it directly impacts clinical decision-making. Intermittent non-invasive measurements using oscillometry show clinically significant discrepancies compared to continuous invasive measurements using an arterial catheter and especially overestimate low BP [ 7 , 8 ].
In addition, continuous invasive BP monitoring detects twice as much intraoperative hypotension than intermittent non-invasive measurements using oscillometry and triggers vasopressor therapy in adults having non-cardiac surgery [ 9 ]. Continuous non-invasive BP monitoring—compared to intermittent non-invasive measurements using oscillometry—also reduces the amount of intraoperative hypotension [ 10 , 11 ].
However, validation studies comparing continuous non-invasive BP measurements to continuous invasive BP measurements using an arterial catheter revealed contradictory results regarding the interchangeability of the methods [ 12 , 13 ]. In addition, although continuous non-invasive BP monitoring using finger-cuff technologies is a promising approach in patients having surgery, it is not recommended in critically ill patients with circulatory shock [ 14 ].
Therefore, invasive BP monitoring remains to be the clinical reference method to measure BP in perioperative and intensive care medicine. Indications for the insertion of an arterial catheter include the need for continuous BP monitoring, the impracticality of non-invasive BP measurements, or the need for repeated arterial blood sampling.
Additionally, advanced invasive hemodynamic monitoring pulse wave analysis, transpulmonary thermodilution requires an arterial catheter. A major—but underestimated—risk of invasive BP monitoring using an arterial catheter is that wrong therapeutic actions are taken based on erroneous BP readings caused by unrecognized artifacts or measurement problems [ 15 ]. BP and advanced hemodynamic variables can only be reliably measured when BP waveforms are correctly recorded, transmitted, and analyzed.
Therefore, we systematically describe how to place an arterial catheter, correctly measure BP, and identify and solve common pitfalls.
To correctly measure BP using an arterial catheter, we propose a systematic 5-step approach that helps to 1 choose the catheter insertion site, 2 choose the type of arterial catheter, 3 place the arterial catheter, 4 level and zero the transducer, and 5 check the quality of the BP waveform.
Commonly used anatomical sites for arterial catheter placement are the radial, brachial, and femoral arteries. Less frequently used insertion sites are the ulnar, axillary, temporal, posterior tibial, and dorsal pedis arteries [ 16 ].
Catheter insertion in the radial artery is most commonly used because it is technically easy and rarely associated with major complications [ 17 ]. For radial artery cannulation, the wrist and hand should be carefully immobilized and secured with the wrist resting across a soft support and mildly dorsiflexed to keep the artery in position.
Cannulation should be started as distally as possible, as one can move to a more proximal puncture site after unsuccessful cannulation. The ulnar artery may be safely cannulated even following failed attempts to access the ipsilateral radial artery [ 20 , 21 ]. The brachial artery, even though it is the main artery of the arm, can also be used for BP monitoring [ 22 ].
Although the placement of an arterial catheter into the brachial artery has a low overall complication rate of 0. The brachial artery is best palpated medial to the biceps tendon in the antecubital fossa, when the shoulder is slightly abducted, the elbow extended, and the forearm supinated.
The femoral artery is the largest artery used for arterial catheter placement, and the complication rate of arterial catheter placement in the femoral artery is comparable to those of other sites [ 17 ]. Puncture of the femoral artery should be performed distally to the inguinal ligament to minimize the risk of hemorrhage into the pelvis or retroperitoneum. The axillary artery is the only alternative to the femoral artery for central BP measurement.
In some situations, the axillary artery may be preferred over the femoral artery e. The axillary artery can be palpated best when the arm is abducted and externally rotated. The cannulation site should be as high into the apex of the axilla as possible. The morphology of the BP waveform changes when the BP wave moves from the aorta to a more peripheral artery due to pulse wave amplification phenomena.
In the periphery, the BP waveform shows a higher systolic BP, a steeper slope of the systolic upstroke, a lower diastolic BP, and a lower and later dicrotic notch compared to BP waveforms recoded at the aortic root [ 25 ]. The decrease in diastolic BP is less pronounced than the increase in systolic BP [ 25 ].
The choice of the type of the arterial catheter depends on several factors, including the artery to be cannulated and expected cannulation problems. The catheter length should be chosen primarily depending on the cannulation site.
When using the brachial, femoral, or axillary artery, a longer catheter is recommended because of the distance between the surface of the skin and the lumen of the artery; using longer arterial catheters reduces the risk of dislocation. The length and inner diameter of the catheter influence the damping properties of the BP measurement system for details regarding the dynamic response of the pressure transducer, see step 5.
Twenty gauge catheters have been shown to be less affected by underdamping than G catheters [ 15 , 26 ] and can be generally recommended for radial cannulation. Complications occur less often with G catheters compared to larger catheters [ 16 ]. In difficult situations e. Before cannulation, the equipment for arterial catheter placement needs to be carefully prepared.
It includes sterile gloves and drapes, surgical mask, alcohol-based antiseptic skin prep solutions based on chlorhexidine gluconate [ 29 , 30 ], arterial catheter, adhesive tape, tubing system, and transducer kit.
This pressure prevents the backflow of blood from the cannulated artery into the catheter and the transducer system and maintains a continuous column of fluid from the system into the artery. Heparinized solutions are not recommended because heparin exposure might promote antibody formation leading to heparin-induced thrombocytopenia [ 31 ].
The insertion of the arterial catheter has to be performed under sterile conditions. Therefore, the skin is prepared with an antiseptic solution and local anesthetic should be subcutaneously administered above the artery in conscious patients. Different techniques can be used to place the catheter with or without the use of US [ 32 ], namely the separate guidewire approach, integral guidewire approach, and direct puncture. As soon as the artery is punctured, indicated by pulsatile blood flow through the needle, the guidewire is introduced through the lumen of the needle.
After the needle is removed, the catheter is advanced over the guidewire. The guidewire is then removed, leaving only the catheter in place. As soon as the artery is punctured, the blood fills the hub of the catheter. The needle-catheter is then advanced slightly through the vessel, the needle is completely removed, and the catheter is slowly withdrawn until pulsatile blood flow is observed.
Then, the separate guidewire is advanced into the vessel through the catheter. As a next step, the catheter is advanced over the wire, and the guidewire is removed, leaving only the catheter in place. This approach uses an integral guidewire that is inseparable from the catheter kit. Then, the angle of the needle-guidewire-catheter unit is decreased, bringing it more parallel to the skin.
The guidewire tab is advanced into the artery through the needle and catheter. Then, the catheter is advanced into the artery over the needle and guidewire, and the needle-guidewire component of the unit is removed. This has to be done, as the needle is slightly longer than the catheter and the backflow of blood just indicates that the needle tip—and not implicitly the catheter—is in the vessel.
Then, the catheter is advanced into the artery and the needle is removed. After placement of the catheter, it is connected to the transducer system and secured with suture or in a sutureless fashion with an adhesive dressing.
All of the arterial cannulation techniques described above can be performed under US guidance. Although several medical societies distinctly recommend the use of US for central venous catheterization [ 33 , 34 ], current guidelines do not yet recommend routine use of US for arterial catheterization [ 35 , 36 ]. Two recently published meta-analyses of randomized controlled trials comparing radial arterial cannulation using the landmark technique with US-guided techniques in adults provided evidence that US techniques offer advantages with regard to first-pass success and failure rate [ 37 , 38 ].
It seems obvious that—after education and training—arterial catheter placement under real-time visualization has advantages over the landmark technique. In specific situations, the use of US can facilitate successful arterial access e. US-guided arterial catheter placement has to be performed under sterile conditions with a sterile cover for the US probe and a sterile conductive medium [ 40 ].
Different US-guided arterial catheter placement techniques have been described [ 41 ]. The static or indirect technique is applied to identify the target artery before puncture i. Short- and long-axis views depending on the orientation of the US probe relative to the vessel can be used for arterial catheter placement.
For the short-axis out-of-plane technique, the US probe is placed orthogonal to the artery, so that the cross-sectional area of the arterial lumen is visualized. To circumvent this disadvantage, a modified short-axis technique dynamic needle tip positioning can be performed, in which the needle is gradually advanced with stepwise adjustment of the US probe following the needle tip until it is visible in the vessel lumen [ 42 ].
This technique may be more difficult to learn, but once it is mastered, it is superior to the short-axis approaches [ 39 ].
Common complications of arterial catheter placement include local pain and paresthesia, hematoma, and minor bleeding. The risk of ischemic complications is less than 0. Major, but less common, complications of arterial catheter placement are major bleeding, embolism of air or thrombotic material, vascular thrombosis and occlusion, vessel injury, pseudoaneurysm formation, and local nerve injury. It is less frequently compared to the incidence of radial artery occlusion 1.
Permanent occlusion of the radial artery however appears to be rare mean incidence, 0. Pseudoaneurysm formation of the femoral artery due to arterial cannulation occurs similarly often 0. The use of an arterial catheter bears the risk of unintentional intra-arterial injection of medications, disconnection of the tubing system resulting in massive blood loss, and catheter-related bloodstream infections [ 43 ]. The rates of catheter-related bloodstream infections are higher for femoral artery cannulation compared to radial artery cannulation relative risk, 1.
The pressure transducer where the mechanical signal is transduced into an electrical signal [ 44 ] must be leveled and zeroed to ensure that BP measurements are accurate. It needs to be distinguished between a measurement using a transducer alone without a zero line or a transducer with a zero line, as the leveling and zeroing procedures differ between the two methods. When using a transducer without a zero line, the transducer—or more precisely the stopcock of the transducer opening towards atmospheric pressure—needs to be leveled to the level of the vessel of interest Fig.
For instance, if a patient has surgery in a beach-chair position i. The level of the right atrium—that is very close to the level of the aortic root—is conventionally used as the reference level for most hemodynamic measurements [ 45 ]. If, however, the pressure at the circle of Willis should be monitored, the transducer must be elevated to the level of the base of the brain external acoustic meatus. Before the measurement starts, the transducer has to be zeroed using the zeroing function of the monitor.
For zeroing, the stopcock of the pressure transducer has to be opened towards the atmosphere while activating the zeroing function on the monitor. After this procedure, the stopcock of the pressure transducer needs to be closed to the atmosphere. Whenever the vessel of interest of the patient is moving relative to the pressure transducer, a leveling maneuver has to be performed.
Further zeroing maneuvers during the measurement are not required anymore. Leveling and zeroing of the pressure transducer. Especially in non-supine positions e.
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