Invasive arterial pressure monitoring
Invasive arterial pressure monitoring provides beat-to-beat real-time information with sustained accuracy.
Components
- 1.
An indwelling Teflon arterial cannula (20G or 22G) is used ( Fig. 11.1 ). The cannula has parallel walls to minimize the effect on blood flow to the distal parts of the limb. Cannulation can be achieved by directly threading the cannula (either by direct insertion method or a transfixation technique) or by using a modified Seldinger technique with a guidewire to assist in the insertion as in some designs ( Fig. 11.2 ).
Fig. 11.1
BD Flowswitch arterial cannula. Note the on–off switch valve.
(Courtesy BD Medical.)
Fig. 11.2
Argon Careflow arterial cannula with its guidewire.
(Courtesy Argon Medical Devices.)
- 2.
A column of bubble-free heparinized or plain 0.9% normal saline at a pressure of 300 mmHg incorporates a flushing device.
- 3.
Via the fluid column, the cannula is connected to a transducer ( Figs 11.3–11.5 ). This in turn is connected to an amplifier and oscilloscope. A strain gauge variable resistor transducer is used.
Fig. 11.3
Components of a pressure measuring system.
Fig. 11.4
Smiths Medex single-use disposable integrated pressure transducer.
Fig. 11.5
Smiths Medex reusable pressure transducer.
- 4.
The diaphragm (a very thin membrane) acts as an interface between the transducer and the fluid column.
- 5.
The pressure transducer is a device that changes either electrical resistance or capacitance in response to changes in pressure on a solid-state device. The moving part of the transducer is very small and has little mass.
Mechanism of action
- 1.
The saline column moves back and forth with the arterial pulsation causing the diaphragm to move. This causes changes in the resistance and current flow through the wires of the transducer.
- 2.
The transducer is connected to a Wheatstone bridge circuit ( Fig. 11.6 ). This is an electrical circuit for the precise comparison of resistors. It uses a null-deflection system consisting of a very sensitive galvanometer and four resistors in two parallel branches: two constant resistors, a variable resistor and the unknown resistor. Changes in resistance and current are measured, electronically converted and displayed as systolic, diastolic and mean arterial pressures. The Wheatstone bridge circuit is ideal for measuring the small changes in resistance found in strain gauges. Most pressure transducers contain four strain gauges that form the four resistors of the Wheatstone bridge.
Fig. 11.6
The Wheatstone bridge circuit where null deflection of the galvanometer implies R1/R2 = R3/R4.
- 3.
The flushing device allows 3–4 mL/h of saline (or heparinized saline) to flush the cannula. This is to prevent clotting and backflow through the catheter. Manual flushing of the system is also possible when indicated.
- 4.
The radial artery is the most commonly used artery because the ulnar artery is the dominant artery in the hand. The ulnar artery is connected to the radial artery through the palmar arch in 95% of patients. The brachial, femoral, ulnar or dorsalis pedis arteries are used occasionally.
- 5.
The information gained from invasive arterial pressure monitoring includes heart rate, pulse pressure, the presence of a respiratory swing, left ventricular (LV) contractility, vascular tone (systematic vascular resistance [SVR]) and stroke volume.
The arterial pressure waveform ( Fig. 11.7 )
- 1.
This can be characterized as a complex sine wave that is the summation of a series of simple sine waves of different amplitudes and frequencies.
- 2.
The fundamental frequency (or first harmonic) is equal to the heart rate, so a heart rate of 60 beats/min = 1 beat/s or 1 cycle/s or 1 Hz. The first 10 harmonics of the fundamental frequency contribute to the waveform.
- 3.
The system used to measure arterial blood pressure should be capable of responding to a frequency range of 0.5–40 Hz in order to display the arterial waveform correctly.
- 4.
The dicrotic notch in the arterial pressure waveform represents changes in pressure because of vibrations caused by the closure of the aortic valve.
- 5.
The rate of rise of the upstroke part of the wave (dP/dt) reflects the myocardial contractility. A slow rise upstroke might indicate a need for inotropic support. A positive response to the inotropic support will show a steeper upstroke. The maximum upward slope of the arterial waveform during systole is related to the speed of ventricular ejection.
- 6.
The position of the dicrotic notch on the downstroke of the wave reflects the peripheral vascular resistance. In vasodilated patients, e.g. following an epidural block or in septic patients, the dicrotic notch is positioned lower on the curve. The notch is higher in vasoconstricted patients.
- 7.
The downstroke slope indicates resistance to outflow. A slow fall is seen in vasoconstriction.
- 8.
The stroke volume can be estimated by measuring the area from the beginning of the upstroke to the dicrotic notch. Multiply that by the heart rate and the cardiac output can be estimated.
- 9.
Systolic time indicates the myocardial oxygen demand. Diastolic time indicates myocardial oxygen supply.
- 10.
Mean blood pressure is the average pressure throughout the cardiac cycle. As systole is shorter than diastole, the mean arterial pressure (MAP) is slightly less than the value half way between systolic and diastolic pressures. An estimate of MAP can be obtained by adding a third of the pulse pressure (systolic − diastolic pressure) to the diastolic pressure. MAP can also be determined by integrating a pressure signal over the duration of one cycle, divided by time.
The natural frequency
This is the frequency at which the monitoring system itself resonates and amplifies the signal by up to 20–40%. This determines the frequency response of the monitoring system. The natural frequency should be at least 10 times the fundamental frequency. The natural frequency of the measuring system is much higher than the primary frequency of the arterial waveform which is 1–2 Hz, corresponding to a heart rate of 60–120 beats/min. Stiffer (low compliance) tubing or a shorter length of tubing (less mass) produce higher natural frequencies. This results in the system requiring a much higher pulse rate before amplification.
The natural frequency of the monitoring system is:
- 1.
directly related to the catheter diameter
- 2.
inversely related to the square root of the system compliance
- 3.
inversely related to the square root of the length of the tubing
- 4.
inversely related to the square root of the density of the fluid in the system.
Problems in practice and safety features
- 1.
The arterial pressure waveform should be displayed ( Fig. 11.8 ) in order to detect damping or resonance. The monitoring system should be able to apply an optimal damping value of 0.64.
- a.
Damping is caused by dissipation of stored energy. Anything that takes energy out of the system results in a progressive diminution of amplitude of oscillations. Increased damping lowers the systolic and elevates the diastolic pressures with loss of detail in the waveform. Damping can be caused by an air bubble (air is more compressible in comparison to the saline column), clot or a highly compliant, soft transducer diaphragm and tube.
- b.
Resonance occurs when the frequency of the driving force coincides with the resonant frequency of the system. If the natural frequency is less than 40 Hz, it falls within the range of the blood pressure and a sine wave will be superimposed on the blood pressure wave. Increased resonance elevates the systolic and lowers the diastolic pressures. The mean pressure should stay unchanged. Resonance can be due to a stiff, non-compliant diaphragm and tube. It is worse with tachycardia.
Fig. 11.8
Arterial pressure waveform. (A) Correct, optimally damped waveform. (B) Underdamped waveform. (C) Overdamped waveform.
- a.
- 2.
To determine the optimum damping of the system, a square wave test (fast flush test) is used ( Fig. 11.9 ). The system is flushed by applying a pressure of 300 mmHg (compress and release the flush button or pull the lever located near the transducer). This results in a square waveform, followed by oscillations:
- a.
in an optimally damped system, there will be two or three oscillations before settling to zero
- b.
an overdamped system settles to zero without any oscillations
- c.
an underdamped system oscillates for more than three to four cycles before settling to zero.
Fig. 11.9
The square wave test (fast flush test). (A) Optimally damped system. (B) Overdamped system. (C) Underdamped system.
- a.
- 3.
The transducer should be positioned at the level of the right atrium as a reference point that is at the level of the midaxillary line. Raising or lowering the transducer above or below the level of the right atrium gives error readings equivalent to 7.5 mmHg for each 10 cm.
- 4.
Ischaemia distal to the cannula is rare but should be monitored for. Multiple attempts at insertion and haematoma formation increase the risk of ischaemia.
- 5.
Arterial thrombosis occurs in 20–25% of cases with very rare adverse effects such as ischaemia or necrosis of the hand. Cannulae in place for less than 24 hours very rarely cause thrombosis.
- 6.
The arterial pressure wave narrows and increases in amplitude in peripheral vessels. This makes the systolic pressure higher in the dorsalis pedis than in the radial artery. When compared to the aorta, peripheral arteries contain less elastic fibres so they are stiffer and less compliant. The arterial distensibility determines the amplitude and contour of the pressure waveform. In addition, the narrowing and bifurcation of arteries leads to impedance of forward blood flow, which results in backward reflection of the pressure wave.
- 7.
There is risk of bleeding due to disconnection.
- 8.
Inadvertent drug injection causes distal vascular occlusion and gangrene. An arterial cannula should be clearly labelled.
- 9.
Local infection is thought to be less than 20%. Systemic infection is thought to be less than 5%. This is more common in patients with an arterial cannula for more than 4 days with a traumatic insertion.
- 10.
Arterial cannulae should not be inserted in sites with evidence of infection and trauma or through a vascular prosthesis.
- 11.
Periodic checks, calibrations and re-zeroing are carried out to prevent baseline drift of the transducer electrical circuits. Zero calibration eliminates the effect of atmospheric pressure on the measured pressure. This ensures that the monitor indicates zero pressure in the absence of applied pressure, so eliminating the offset drift (zero drift). To eliminate the gradient drift , calibration at a higher pressure is necessary. The transducer is connected to an aneroid manometer using a sterile tubing, through a three-way stopcock and the manometer pressure is raised to 100 and 200 mmHg. The monitor display should read the same pressure as is applied to the transducer ( Fig. 11.10 ).
Fig. 11.10
Calibration of invasive pressure monitor.
- •
Consists of an arterial cannula, a heparinized saline column, a flushing device, a transducer, an amplifier and an oscilloscope.
- •
In addition to blood pressure, other parameters can be measured and estimated such as myocardial contractility, vascular tone and stroke volume.
- •
The waveform should be displayed to detect any resonance or damping.
- •
The measuring system should be able to cover a frequency range of 0.5–40 Hz.
- •
The monitoring system should be able to apply an optimal damping value of 0.64.
Exam tip: invasive arterial pressure is a very popular topic in exams. Know and understand the components of an invasive pressure system, how it works, factors that can affect its accuracy and the different waveforms. To be able to draw and explain a Wheatstone bridge circuit is also important.
Central venous catheterization and pressure
The central venous pressure (CVP) is the filling pressure of the right atrium. It can be measured directly using a central venous catheter. The catheter can also be used to administer fluids, blood, drugs, parenteral nutrition and sample blood. Specialized catheters can be used for haemofiltration, haemodialysis (see Chapter 13 , Haemofiltration) and transvenous pacemaker placement.
The tip of the catheter is usually positioned in the superior vena cava at the entrance to the right atrium. The internal jugular, subclavian and basilic veins are possible routes for central venous catheterization. The subclavian route is associated with the highest rate of complications but is convenient for the patient and for the nursing care.
The Seldinger technique is the common and standard method used for central venous catheterization ( Fig. 11.11 ) regardless of catheter type. The procedure should be done under sterile conditions:
- 1.
Introduce the cannula or needle into the vein using the appropriate landmark technique or an ultrasound-locating device.
- 2.
A J-shaped soft tip guidewire is introduced through the needle (and syringe in some designs) into the vein. The needle can then be removed. The J-shaped tip is designed to minimize trauma to the vessels’ endothelium.
- 3.
After a small incision in the skin has been made, a dilator is introduced over the guidewire to make a track through the skin and subcutaneous tissues and is then withdrawn.
- 4.
The catheter is then railroaded over the guidewire into its final position before the guidewire is withdrawn.
- 5.
Blood should be aspirated easily from all ports which should then be flushed with saline or heparin solution. All the port sites that are not intended for immediate use are sealed. A port should never be left open to air during insertion because of the risk of air embolism.
- 6.
The catheter is secured onto the skin and covered with a sterile dressing.
- 7.
A chest X-ray is performed to ensure correct positioning of the catheter and to detect pneumothorax and/or haemothorax.
- 8.
The use of ultrasound guidance should be routinely considered for the insertion of central venous catheters ( Fig. 11.12 ). Evidence shows that its use during internal jugular venous catheterization reduces the number of mechanical complications, the number of catheter-placement failures and the time required for insertion.
Fig. 11.12
Ultrasound image showing a needle in the internal jugular vein. The carotid artery is at the left lower corner.
The CVP is read using either a pressure transducer or a water manometer.
Pressure transducer
- 1.
A similar measuring system to that used for invasive arterial pressure monitoring (catheter, heparinized saline column, transducer, diaphragm, flushing device and oscilloscope system). The transducer is positioned at the level of the right atrium.
- 2.
A measuring system of limited frequency range is adequate because of the shape of the waveform and the values of the CVP.
Fluid manometer ( Fig. 11.13 )
- 1.
A giving set with either normal saline or 5% dextrose is connected to the vertical manometer via a three-way tap. The latter is also connected to the central venous catheter.
- 2.
The manometer has a spirit level side arm positioned at the level of the right atrium (zero reference point). The upper end of the column is open to air via a filter. This filter must stay dry to maintain direct connection with the atmosphere.
- 3.
The vertical manometer is filled to about the 20-cm mark. By opening the three-way tap to the patient, a swing of the column should be seen with respiration. The CVP is read in cm H 2 O when the fluid level stabilizes.
- 4.
The manometer uses a balance of forces: downward pressure of the fluid (determined by density and height) against pressure of the central venous system (caused by hydrostatic and recoil forces).
In both techniques, the monitoring system has to be zeroed at the level of the right atrium (usually at the midaxillary line). This eliminates the effect of hydrostatic pressure on the CVP value.
Catheters
Different types of catheters are used for central venous cannulation and CVP measurement. They differ in their lumen size, length, number of lumens, the presence or absence of a subcutaneous cuff and the material they are made of. The vast majority of catheters are designed to be inserted using the Seldinger technique although some are designed as ‘long’ intravenous cannulae (cannula over a needle) ( Fig. 11.14 ).
Antimicrobial-coated catheters have been designed to reduce the incidence of catheter-related bloodstream infection. These can be either antiseptic coated (e.g. chlorhexidine/silver sulfadiazine, benzalkonium chloride, platinum/silver) or antibiotic coated (e.g. minocycline/rifampin) on either the internal or external surface or both. The antibiotic-coated central lines are thought to be more effective in reducing the incidence of infection ( Fig. 11.15 ).
Multilumen catheter
- 1.
The catheter has two or more lumens of different sizes, e.g. 16G and 18G ( Fig. 11.16 ). Paediatric sizes also exist ( Fig. 11.17 ).
Fig. 11.16
An adult triple lumen catheter.
(Courtesy Vygon [UK] Ltd. © Vygon [UK] Ltd.)
Fig. 11.17
A paediatric triple lumen catheter.
(Courtesy Vygon [UK] Ltd. © Vygon [UK] Ltd.)
- 2.
The different lumens should be flushed with heparinized saline before insertion.
- 3.
Single and double lumen versions exist.
- 4.
Simultaneous administration of drugs and CVP monitoring is possible. It does not allow the insertion of a pulmonary artery (PA) catheter.
- 5.
These catheters are made of polyurethane. This provides good tensile strength, allowing larger lumens for smaller internal diameter.
Long central catheters/peripherally inserted central catheters (PICC)
- 1.
These catheters, 60 cm in length, are designed to be inserted through an introducing cannula via an antecubital fossa vein, usually the basilic vein ( Fig. 11.18 ).
Fig. 11.18
Double lumen PICC line.
(Courtesy Teleflex Medical.)
- 2.
They are used when a central catheter is required in situations when it is undesirable to gain access via the internal jugular or the subclavian veins, for example during head and neck surgery or prolonged antibiotic therapy. They are made of soft flexible polyurethane or silicone.
Hickman catheters
- 1.
These central catheters are made of polyurethane or silicone and are usually inserted into the internal jugular or subclavian vein. The catheter can have one, two or three lumens ( Fig. 11.19 ).
Fig. 11.19
A double lumen long-term Hickman catheter. Note the Dacron cuff.
(Courtesy Vygon [UK] Ltd. © Vygon [UK] Ltd.)
- 2.
The proximal end is tunnelled under the skin for a distance of about 10 cm.
- 3.
A Dacron cuff is positioned 3–4 cm from the site of entry into the vein under the skin. It induces a fibroblastic reaction to anchor the catheter in place ( Fig. 11.20 ). The cuff also reduces the risk of infection as it stops the spread of infection from the site of entry to the skin. Some catheters also have a silver impregnated cuff that acts as an antimicrobial barrier.
Fig. 11.20
Final position of a tunnelled Hickman catheter.
(Reproduced with permission from Viggo-Spectramed, a division of BOC Healthcare, Manchester, UK.)
- 4.
They are used for long-term chemotherapy, parenteral nutrition, blood sampling or as a readily available venous access especially in children requiring frequent anaesthetics during cancer treatment.
- 5.
These lines are designed to remain in situ for several months unless they become infected but require some degree of daily maintenance.
Dialysis catheters
These are large-calibre catheters designed to allow high flow rates of at least 300 mL/min. They are made of silicone or polyurethane. Most of them are dual lumen with staggered end and side holes to prevent admixture of blood at the inflow and outflow portions reducing recirculation.
Problems in practice and safety features
- 1.
Inaccurate readings can be due to catheter blockage, catheter inserted too far or using the wrong zero level.
- 2.
Pneumohaemothorax (with an incidence of 2–10% with subclavian vein catheterization and 1–2% with internal jugular catheterization), trauma to the arteries (carotid, subclavian and brachial), air embolism, haematoma and tracheal puncture are complications of insertion.
- 3.
Sepsis and infection are common complications with an incidence of 2.8–20%. Staphylococcus aureus and Enterococcus are the most common organisms.
. Guidelines for the prevention of intravascular catheter-related infections.)
Guidelines for reduction in sepsis and infection rates with the use of central venous catheters
- •
Education and training of staff who insert and maintain the catheters.
- •
Use the maximum sterile barrier precautions during central venous catheter insertion.
- •
Use of > 0.5% chlorhexidine preparation with alcohol preparations for skin antisepsis. If there is a contraindication to chlorhexidine, tincture of iodine, an iodophor or 70% alcohol can be used as alternative. Antiseptics should be allowed to dry according to the manufacturer’s recommendation prior to placing the catheter.
- •
Use a subclavian site, rather than a jugular or a femoral site, in adult patients to minimize infection risk for non-tunnelled central vascular catheter (CVC) placement.
- •
Use ultrasound guidance to place central venous catheters (if this technology is available) to reduce the number of cannulation attempts and mechanical complications. Ultrasound guidance should only be used by those fully trained in its technique.
- •
Use a CVC with the minimum number of ports or lumens essential for the management of the patient.
- •
Promptly remove any intravascular catheter that is no longer essential.
- •
When adherence to aseptic technique cannot be ensured (i.e. catheters inserted during a medical emergency), replace the catheter as soon as possible, i.e. within 48 hours.
- •
Use either sterile gauze or sterile, transparent, semipermeable dressing to cover the catheter site.
- •
Use a chlorhexidine/silver sulfadiazine or minocycline/rifampin-impregnated CVC in patients whose catheter is expected to remain in place > 5 days.
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