Invasive monitoring

1. An indwelling Teflon arterial cannula (20 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 heparinized or plain 0.9% normal saline at a pressure of 300 mmHg, incorporating 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.

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.

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.

3. The flushing device allows 3–4 mL per hour 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 contractility, vascular tone (SVR) and stroke volume.

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 per 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.

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.

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.

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:

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 h 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. 11.9).

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

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 catheter.

5. These catheters are made of polyurethane. This provides good tensile strength, allowing larger lumens for smaller internal diameter.

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. 11.17).

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.

1. These central catheters are made of polyurethane or silicone and are usually inserted into the subclavian vein. The catheter can have one, two or three lumens (Fig. 11.18).

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. 11.19). 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 anti-microbial 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.

Problems in practice and safety features