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 ).

  
  

  
  

  
  

  

  
  

  
  

  BD Flowswitch arterial cannula. Note the on–off switch valve.

  
  

  (Courtesy BD Medical.)

  
  

  
  

  
  

  

  
  

  
  

  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.

  
  

  
  

  
  

  

  
  

  
  

  Components of a pressure measuring system.

  
  

  
  

  
  

  

  
  

  
  

  Smiths Medex single-use disposable integrated pressure transducer.

  
  

  
  

  
  

  

  
  

  
  

  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.

  
  

  
  

  
  

  

  
  

  
  

  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.

Arterial pressure waveform showing systolic, diastolic and mean pressures.

(Courtesy Philips.)

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.

  
  

  
  

  
  

  

  
  

  
  

  Arterial pressure waveform. (A) Correct, optimally damped waveform. (B) Underdamped waveform. (C) Overdamped waveform.

  
  
  
  
• 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.

  
  

  
  

  
  

  

  
  

  
  

  The square wave test (fast flush test). (A) Optimally damped system. (B) Overdamped system. (C) Underdamped system.

  
  
  
  
• 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 ).

  
  

  
  

  
  

  

  
  

  
  

  Calibration of invasive pressure monitor.

  
  

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 reduction in sepsis and infection rates with the use of central venous catheters

  
  

  
  
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    Education and training of staff who insert and maintain the catheters.

    
    
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    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.

    
    
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    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.

    
    
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    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.

    
    
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    Use a CVC with the minimum number of ports or lumens essential for the management of the patient.

    
    
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    Promptly remove any intravascular catheter that is no longer essential.

    
    
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    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.

    
    
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    Use either sterile gauze or sterile, transparent, semipermeable dressing to cover the catheter site.

    
    
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    Use a chlorhexidine/silver sulfadiazine or minocycline/rifampin-impregnated CVC in patients whose catheter is expected to remain in place > 5 days.

  
  

  
  

  . Guidelines for the prevention of intravascular catheter-related infections.)
  
  
  
  
• 4.
  

  A false passage may be created if the guidewire or dilator is advanced against resistance. The insertion should be smooth.

  
  
• 5.
  

  There may be cardiac complications such as self-limiting arrhythmias due to the irritation caused by the guidewire or catheter. Gradual withdrawal of the device is usually adequate to restore normal rhythm. More serious but unusual complications such as venous or cardiac perforation can be lethal.

  
  
• 6.
  

  Catheter-related venous thrombosis is thought to be up to 40% depending on the site, the duration of placement, the technique and the condition of the patient.

  
  
• 7.
  

  Microshock may occur. A central venous catheter presents a direct pathway to the heart muscle. Faulty electrical equipment can produce minute electrical currents (less than 1 ampere) which can travel via this route to the myocardium. This can produce ventricular fibrillation (VF) if the tip of the catheter is in direct contact with the myocardium (see Chapter 14 ). This very small current does not cause any adverse effects if applied to the body surface, but if passed directly to the heart, the current density will be high enough to cause VF, hence the name microshock.