INVASIVE TECHNIQUES
If an artery is cannulated a direct measurement of blood pressure can be obtained with the help of an infusion system, transducer and recorder. Ideally, for arterial cannulation, a peripheral artery should be chosen
so that the whole limb is not threatened if a clot or haematoma forms.
Although some physicians use the brachial artery, the radial artery is usually the first choice but, before cannulation, a modified Allen test is carried out. The patient's hand is clenched into a fist and the doctor occludes both the radial and ulnar arteries with his fingers. Then the patient relaxes his clenched fist and the doctor releases the pressure on the ulnar artery. The patient's hand should then flush within 5 seconds. If flushing does not occur or is delayed, then this indicates that there are poor blood vessel collaterals between the radial and ulnar arteries and, therefore, another artery should be used. If the ulnar artery is to be cannulated the test is performed similarly,
but pressure is released over the radial artery first. If there are adequate connections between the two arteries, a plastic
cannula may be inserted into the radial artery. A local anaesthetic is required in the conscious patient and the percutaneous route is usually satisfactory, open exposure rarely being necessary. Jntra-arterial cannulation may rarely cause thrombosis to occur and to minimize this risk the cannula should be constructed of Teflon, be short with parallel sides and be able to be sutured in place to avoid being dislodged. After insertion, the cannula and its connections must be fixed securely so
that there is no possibility of a leak occurring with the risk of severe or even fatal blood loss. To prevent clotting in the cannula, intermittent flushing
with heparinized saline through a three-way tap may be satisfactory. However, the high pressures generated by small syringes (Chapter 1, Fig. 1.1) can damage arterial walls or the diaphragm of the pressure transducer. Care should be taken when catheters are being flushed, and syringes smaller than 5 ml should not be used. For longer term recording a continuous flushing system (e.g. 'Intraflo'), as shown in Fig. 17.7, is more satisfactory.
The heparinized saline used is kept in a pressurized container at a
pressure of 300 mmHg (40 kPa). It then passes through a drip chamber to a constriction, adjusted so that the flow cannot exceed about 4 ml per hour. This flow continuously flushes the tubings and the arterial cannula. The diagram also indicates the pressure transducer with its amplifier and recorder, and such transducers are considered in Chapter 14.
THE ARTERIAL PRESSURE RECORDING
The final waveform produced may be displayed on an oscilloscope or a recorder tracing. Figure 17.8 shows typical arterial pressure recordings. It can be seen that the form of the pressure wave alters as blood flows to the periphery. The blood pressure wave becomes narrower and increases in amplitude in peripheral arteries so that, even with the patient supine, the systolic pressure in the dorsalis pedis artery is higher than in the radial which, in turn, is higher than that in the aorta. This modification of the wave pattern is caused by the change in diameter of the vessels and their elasticity, and possibly also because of the reflection of the wave pattern from the vessel walls.
the systolic and diastolic pressures are easily identified on a tracing and
the systolic pressure is found to be an average of 5 mmhg higher with direct measurement at the radial artery than with indirect techniques, while the diastolic pressure is about 8 mmhg lower. the recording also shows the dicrotic notch caused by intra-aortic vibrations. the frequency range of various biological signals is considered in chapters 13 and 15. in the case of the arterial pressure wave, the frequency range is 0 hz to about 40 hz. the apparatus used for arterial pressure measurement must be able to respond adequately to this range of frequencies. usually, the amplifiers and recorders have no difficulty in dealing with this frequency range, but problems may arise in relation to the transducer itself and its connections to the cannula..
RESONANCE AND DAMPING
Movement of the diaphragm of the pressure transducer converts the blood pressure change into an electrical signal. This movement of the diaphragm is associated with a very small movement of saline to and fro along the catheter with changes of pressure. Just as a weight on the end of a spring will oscillate at a particular frequency (known as the resonant frequency), so the pressure measuring system consisting of the transducer diaphragm, catheter and saline column possesses a resonant frequency at which oscillations can occur. If this is less than 40 Hz, it falls within the range of frequencies present in the blood pressure waveform. Oscillations occurring at the resonant frequency produce a sine wave which is superimposed on the blood pressure waveform, giving distortion from resonance as shown on Fig. 17.9. .
This problem can be avoided if the resonant frequency is outside the
range of frequencies present in the blood pressure waveform, although this can be difficult to achieve. The resonant frequency of the combination of catheter and transducer can be raised most easily by using a shorter, wider or stiffer catheter, so the problem is generally worse when a long catheter is used. If there is any restriction to the transmission of the blood pressure from
the artery to the transducer diaphragm, the displayed blood pressure waveform will be damped or smoothed out so that sharp changes are not displayed (Fig. 17.9). Damping of this sort can be produced by air bubbles in the catheter or in the transducer chamber which absorb the pressure change in the saline column. It is also caused by clot formation in the cannula which restricts the movement of the saline column. Both these effects reduce the deflection of the transducer diaphragm, and hence the size of the measured waveform.
COMPARISON OF INVASIVE AND NON-INVASIVE TECHNIQUES The main advantage of the direct invasive technique of arterial pressure measurement is its potential accuracy. It not only gives greater accuracy by indicating the exact form of the blood pressure trace, but it can also give reliable pressure readings even in the most hypotensive or shocked patient, whereas non-invasive indirect techniques usually fail to record blood pressure below a certain minimum. The invasive method gives a continuous record of the pressure in contrast to the intermittent record
provided by most non-invasive systems. Hence, it also gives better reliability than the non-invasive techniques if the pressure is continuously varying as, for example, in patients with an irregular or fluctuating pulse rate.
There are, however, disadvantages to the direct technique. There is
some risk of arterial damage, whereas the non-invasive methods are harmless to the patient. Direct systems are also more costly than the simpler indirect systems and there is a need for technical skill when using direct methods, while indirect techniques may be carried out by paramedical and even non-medical staff.
CENTRAL VENOUS PRESSURE
The term blood pressure is normally used to refer to arterial blood pressure. However, the venous pressure is also of importance to anaesthetists. Accurate non-invasive measurement of this is impossible, although distended jugular veins in the absence of thoracic inlet obstruction indicate a raised venous pressure. To measure central venous pressure a catheter is inserted via an arm
or neck vein and advanced so that it reaches the superior vena cava. Attached to this catheter are a saline drip, T-piece and a manometer as shown in Fig. 17.10. The techniques used are beyond the scope of this book, but it should be noted that faulty techniques can increase the risk of pneumothorax, haemothorax or hydrothorax and of infection.
Before a central venous pressure reading is taken from the manometer
the patient is placed in a horizontal position. Using a spirit level on a rod attached to the manometer, the zero of the scale is set to the level of the
midaxillary line, this being taken as representative of the level of the right atrium. The saline drip should be turned off when readings are made and slight movements of the saline level with respiration indicate correct positioning of the catheter. Readings are usually recorded as cmH20 pressure, but conversion to SI units is simple as 1 cmH20 is about 100 Pa. As an alternative to a simple saline manometer, a pressure transducer
may be used and this also gives a continuous recording and allows identification of the central venous pressure waveform.
VENOUS PRESSURE AND CARDIAC OUTPUT
The filling of the heart depends on adequate venous pressure. If venous pressure rises, the normal heart fills to a greater extent and automatically increases its output even in the absence of autonomie innervation. The action is described by Starling's law of the heart, and is explained at the beginning of this chapter. After appreciable blood loss, venous pressure falls and may be used as
a guide to transfusion, but care is needed in interpreting venous pressure values as they can be modified by venous tone. As shown in Fig. 17.11 both the right heart and pulmonary blood vessels
are interposed between the central venous pressure recording point and the left heart. Consequently, in a patient with left heart disease excessive intravenous transfusion can give pulmonary oedema before the warning sign of a high central venous pressure is seen. On the other hand, in a patient with lung disease a high central venous pressure may be caused by failure of the right heart. Ideally, both left atrial and central venous pressures should therefore be monitored. Measurement of pulmonary wedge pressure gives an indication of left
atrial pressure. The pulmonary wedge pressure is obtained by advancing
a special venous catheter through the right heart and pulmonary artery until it wedges into a small branch of the pulmonary artery, where it may be used to measure the pressure in the pulmonary capillaries. The special catheter used has a small balloon at its tip which can be inflated temporarily to facilitate its carriage into the pulmonary artery by the blood stream. It is also inflated during wedge pressure measurement. Continuous pressure recording helps to identify the position of the catheter and ensure that it is positioned in the pulmonary vessels before making the measurement.