Invasive monitoring

Components

• 1.

  An indwelling Teflon arterial cannula (20-G or 22-G) 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 ).

  
  

• 2.

  A column of bubble-free 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 and 11.4 ). This in turn is connected to an amplifier and oscilloscope. A strain gauge variable resistor transducer is used.

  
  

• 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. 11.5 ). 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.

  

• 3.

  The flushing device allows 3–4 mL/h of 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. 11.6 )

• 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 halfway between systolic and diastolic pressures. An estimate of MAP can be obtained by adding a third of the pulse pressure (systolic pressure − 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

The natural frequency of an object is that frequency at which it vibrates/oscillates after being moved or strummed. In the case of the pressure monitoring system, it 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. Therefore it is important for the monitoring system to have a significantly different frequency from the patient’s pulse rate. The natural frequency should be at least 10 times the fundamental frequency, which is the patient’s pulse rate. The natural frequency of the measuring system is much higher than the fundamental or primary frequency of the arterial waveform, which is 1–2 Hz, corresponding to a pulse rate of 60–120 beats/min. The maximum pulse rate a patient could have might be 180–240 beats/min or 3–4 Hz. Therefore the monitoring system should have a natural frequency of at least 40 Hz. Stiffer (low compliance) tubing or a shorter length of tubing (less mass) produces 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. 11.7 ) 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, noncompliant diaphragm and tube. It is worse with tachycardia.

  

• 2.

  To determine the optimum damping of the system, a square wave test (fast flush test) is used ( Fig. 11.8 ). 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.

  

• 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 a risk of bleeding due to disconnection.

• 8.

  An arterial cannula should be clearly labelled. This alerts staff to what could be fatal errors. Inadvertent air injection results in air embolus. Inadvertent drug injection causes distal vascular occlusion and necrosis.

• 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 rezeroing 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. 11.9 ).

  

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.

Multilumen catheter

• 1.

  The catheter has two or more lumens of different sizes ( Fig. 11.15 ), e.g. 16G and 18G ( Fig. 11.16 ). Paediatric sizes also exist ( Fig. 11.17 ).

  
  
  

• 2.

  The different lumens should be flushed with 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, usually around 60 cm in length, are designed to be inserted through an introducing peel-away sheath via an upper arm vein, usually the basilic ( Fig. 11.18 ).

  

• 2.

  They are used when a central catheter is required in situations when it is unnecessary or undesirable to gain access via the internal jugular or the subclavian veins. Most PICCs are sited for long-term cancer chemotherapy or antibiotic treatment, for example treating osteomyelitis or endocarditis.

• 3.

  They are made of soft flexible polyurethane or silicone. Single (4 Fr), double (5 Fr) and triple (5 or 6 Fr) versions exist. Some designs are suitable for power injection needed for contrast during computed tomography scans.

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 ).

  

• 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 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.

  

• 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, so 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.

  Pneumo and/or haemothorax (with an incidence of 2%–10% with subclavian vein catheterization and 1%–2% with internal jugular catheterization), accidental arterial puncture and associated trauma to the arteries (carotid, subclavian and brachial) with potential arteriovenous fistula formation, 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.

• 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.